Minimize GAIA (Global Astrometric Interferometer for Astrophysics)

GAIA (Global Astrometric Interferometer for Astrophysics) Mission

Spacecraft   Launch   Mission Status   Payload Module   Ground Segment   Big Data Archive   References

Gaia (mother Earth in Greek mythology) is an ESA cornerstone space astrometric mission, part of the Horizon 2000 Plus long-term scientific program, with the goal to compile a 3D space catalog of > 1000 million stars, or roughly 1% of the stars in our home galaxy, the Milky Way. Gaia will monitor each of its target stars about 70 times to a magnitude of G=20 over a period of 5 years. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission will also study about 500,000 distant quasars and will provide stringent new tests of Albert Einstein's General Theory of Relativity. 1) 2) 3) 4) 5)

Cataloguing the night sky is an essential part of astronomy. Before astronomers can investigate a celestial object, they must know where to find it. Without this knowledge, astronomers would wander helplessly in what Galileo once termed a ‘dark labyrinth'.

During the satellite's expected lifetime of five years, Gaia will observe each star about 70 times, each time recording its brightness, color and, most importantly, its position. The precise measurement of a celestial object's position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers.

By comparing Gaia's series of precise observations, today's astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia's highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies.

The primary mission objectives are:

• Measure the positions and velocity of approximately one billion stars in our Galaxy

• Determine their brightness, temperature, composition and motion through space

• Create a three-dimensional map of the Galaxy.

Additional discoveries expected:

- hundreds of thousands of asteroids and comets within our Solar System

- seven thousand planets beyond our Solar System

- tens of thousands of ‘failed' stars, called brown dwarfs

- twenty thousand exploding stars, called supernovae

- hundreds of thousands of distant active galaxies, called quasars.

Gaia objective is to provide a very accurate dynamical 3D map of our Galaxy by using global astrometry from space, complemented with multi-color multi-epoch photometric measurements. The aim is to produce a catalog complete for star magnitudes up to 20, which corresponds to more than one billion stars or about 1% of the stars of our Galaxy. The instrument sensitivity is such that distances beyond 20-100 kiloparsec (kpc) will be covered, therefore including the Galaxy bulge (8.5 kpc) and spiral arms. The measurements will not be limited to the Milky Way stars. These include the structure, dynamics and stellar population of the Magellanic Clouds, the space motions of Local Group Galaxies and studies of supernovae, galactic nuclei and quasars, the latter being used for materializing the inertial frame for Gaia measurements.

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Figure 1: Gaia measurements objectives (image credit: ESA, Airbus Defence and Space) 6)

 

Background: Gaia is ESA's second space mission dedicated to astrometry. It builds on the legacy of the successful Hipparcos mission (1989-1993). 7) Like Hipparcos, Gaia's observation strategy is based on detecting stellar positions in two fields of view separated by a 'basic angle', which for Gaia is 106.5º. This strategy allows astronomers to establish a coherent reference frame over the entire sky, yielding highly accurate measurements of stellar positions.

After a detailed concept and technology study during 1998–2000, Gaia was selected as a confirmed mission within ESA's scientific program in October 2000. It was confirmed by ESA's Science Program Committee following a re-evaluation of the science program in June 2002, and reconfirmed following another re-evaluation of the program in November 2003. The project entered Phase-B2/C/D in February 2006. As of the summer 2012, Gaia is in Phase-D (Qualification and Production) and will be launched in the second half of 2013. 8) 9) 10)

• In June 2013, ESA's billion-star surveyor, Gaia, has completed final preparations in Europe and is ready to depart for its launch site in French Guiana. The Gaia spacecraft arrived in Cayenne, French Guiana, on August 23, 2013 on board the Antonov 124 aircraft.

• On Oct. 23, 2013, ESA postponed the launch of the Gaia mission. The decision was taken due to a technical issue that was identified in another satellite already in orbit. The issue concerns components used in two transponders on Gaia that generate ‘timing signals' for downlinking the science telemetry. To avoid potential problems, they will be replaced.

The transponders were removed from Gaia at Kourou and returned to Europe, where the potentially faulty components were replaced and verified. After the replacements have been made, the transponders will be refitted to Gaia and a final verification test made. As a consequence of these precautionary measures, it will not be possible to launch Gaia within the window that includes the previously targeted launch date of 20 November. The next available launch window is 17 December to 5 January 2014. 11)

• Update Oct. 20, 2013: The upcoming launch manifest of Arianespace has now been established. Gaia is scheduled for launch on 20 December.

• Update Nov. 22, 2013: The checks on the Gaia satellite are proceeding well, enabling the launch to take place on December 19, 2013 (Ref. 11).

 

Some astrometry basics:

The precise measurement of a celestial object's position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers (Ref. 21). 12)

By comparing Gaia's series of precise observations, today's astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia's highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies. Gaia will be a discovery machine.

Stars as individuals and collectives:

To understand fully the physics of a star, its distance from Earth must be known. This is more difficult than it sounds because stars are so remote. Even the closest one is 40 trillion km away, and we cannot send spacecraft out to them to measure as they go. Nor can we bounce radar signals off them, which is the method used to measure distances within the Solar System. Instead, astronomers have developed other techniques for measuring and estimating distances.

The most reliable and only direct way to measure the distance of a star is by determining its 'parallax'. By obtaining extremely precise measurements of the positions of stars, Gaia will yield the parallax for one billion stars; more than 99% of these have never had their distances measured accurately. Gaia will also deliver accurate measurements of other important stellar parameters, including the brightness, temperature, composition and mass. The observations will cover many different types of stars and many different stages of stellar evolution.

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Figure 2: Distance to a star can be calculated with simple trigonometry from the measured parallax angle (1 a.u. is 1 Astronomical Unit, or 149.6 million km), image credit: ESA/Medialab

The principles of Gaia:

At its heart, Gaia is a space telescope – or rather, two space telescopes that work as one. These two telescopes use ten mirrors of various sizes and surface shapes to collect, focus and direct light to Gaia's instruments for detection. The main instrument, an astrometer, precisely determines the positions of stars in the sky, while the photometer and spectrometer spread their light out into spectra for analysis.

Gaia's telescopes point at two different portions of the sky, separated by a constant 106.5º. Each has a large primary mirror with a collecting area of about 0.7 m2. On Earth we are used to round telescope mirrors, but Gaia's will be rectangular to make the most efficient use of the limited space within the spacecraft. These are not large mirrors by modern astronomical standards, but Gaia's great advantage is that it will be observing from space, where there is no atmospheric disturbance to blur the images. A smaller telescope in space can yield more accurate results than a large telescope on Earth.

Gaia is just 3.5 m across, so three curved mirrors and three flat ones are used to focus and repeatedly fold the light beam over a total path of 35 m before the light hits the sensitive, custom-made detectors. Together, Gaia's telescopes and detectors will be powerful enough to detect stars up to 400,000 times fainter than those visible to the naked eye.

Gaia uses the global astrometry concept demonstrated by Hipparcos. The principle is to link stars with large angular distances in a network where each star is connected to a large number of other stars in every direction. The condition of closure of the network ensures the reduction of the position errors of all stars. This is achieved by the simultaneous observation of two fields of views separated by a very stable basic angle. The spacecraft is slowly rotating at a constant angular rate of 1º/min around a spin axis perpendicular to both fields of view, which describe a great circle on the sky in 6 hours. The spacecraft rotation axis makes an angle of 45º with the Sun direction (Figure 3). A slow precession around the Sun-to-Earth direction, with a 63.12 days period, enables to repeat the observation of sky objects with 86 transits on average over the 5 years of mission.

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Figure 3: Illustration of the sky scanning principle (image credit: ESA)

The resulting performance will enable a breakthrough in the astrometry field, as well regarding star position and velocity performance as for the number of objects observed.

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Figure 4: Gaia will improve the accuracy of astrometry measurements by several orders of magnitude compared with previous systems and observations (image credit: ESA)

 


 

Spacecraft:

Gaia is an exceptionally complex space observatory. ESA awarded Airbus Defence and Space (former Astrium SAS,Toulouse, France) the prime contract in May 2006 to develop and build the spacecraft. Together with the German and British branches of Astrium, more than 50 industrial subcontractor companies from across Europe are involved in building this discovery machine. The Gaia DPAC (Data Processing and Analysis Consortium) will process the raw data to be published in the largest stellar catalog ever made. 13) 14) 15) 16) 17) 18) 19) 20)

The Gaia spacecraft is composed of two sections: the Payload Module and the Service Module. The Payload Module is housed inside a protective dome and contains the two telescopes and the three science instruments. They are all mounted on a torus made of a ceramic material (silicon carbide). The extraordinary measurement accuracy required from Gaia calls for an extremely stable Payload Module that will barely move or deform once in space; this is achieved thanks to the extensive use of this material. 21)

Underneath the Payload Module, the Service Module contains electronic units to run the instruments, as well as the propulsion system, communications units and other essential components. These components are mounted on CFRP (CarbonFiber Reinforced Plastic) panels in a conical framework.

Finally, beneath the Service Module, a large sunshield keeps the spacecraft in shadow, maintaining the Payload Module at an almost constant temperature of around -110ºC, to allow the instruments to take their precise and sensitive readings. The sunshield measures about 10 m across, too large for the launch vehicle fairing, so it comprises a dozen folding panels that will be deployed after launch. Some of the solar array panels that are needed to generate power are fixed on the sunshield, with the rest on the bottom of the spacecraft.

The Gaia spacecraft configuration is driven by the required very high thermo-mechanical stability of the entire spacecraft. A low disturbance cold gas micro-propulsion is used for fine attitude control. The astrometric instrument is used for precise rate sensing in fine pointing operating mode.

Spacecraft launch mass

~2034 kg (dry mass = 1392 kg)

Spacecraft power

1.91 kW (EOL)

Spacecraft dimensions

Height = 3.5 m, deployed diameter = 10 m

Mission duration

5 years

Table 1: Parameters of the Gaia spacecraft

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Figure 5: Artist's rendition of the deployed GAIA spacecraft (image credit: ESA)

 

SVM (Service Module):

The SVM is generally referred to as 'the platform'. The SVM in turn is comprised of MSM (Mechanical Service Module) and an ESM (Electrical Service Module). 22) 23) 24)

MSM (Mechanical Service Module): The spacecraft main structure is of hexagonal conical shape. It is a sandwich panel structure with CFRP (Carbon Fiber Reinforced Plastic) face sheets, and a central cone supporting the propellant tanks. The MSM houses instruments needed for the basic control and operation of the satellite; this includes all mechanical, structural and thermal elements that support the instrument payload and spacecraft electronics. It also includes the chemical & micro propulsion systems, the deployable sunshield with solar arrays, the payload thermal tent and harness. The module consists of a central tube that is about 1.17 m long and hosts six radial panels to create a hexagonal spacecraft shape.

The service module also houses the communication subsystem, central computer and data handling subsystem, the high rate data telemetry, attitude control and star trackers. For telemetry and telecommand, low gain antenna uplink and downlink with a few kbit/s capacity are employed. The high gain antenna used for the science telemetry downlink will be used during each ground station visibility period of an average of about 8 hours per day.

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Figure 6: Photo of the SVM integration (image credit: EADS Astrium)

ESM (Electrical Service Module): The ESM design is driven by the science performance (attitude control laws with the hybridization of star tracker and payload measurements, high rate data telemetry, and regulated power bus for thermal stability). It houses the AOCS units, the communication subsystem, central computer and data handling subsystem, and the power subsystem.

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Figure 7: Diagram of the ESM (image credit: EADS Astrium)

AOCS (Attitude and Orbit Control Subsystem). The AOCS subsystem is characterized by:

- High precision 3-axis control

- The ASTRO (Astrometric) instrument is used for precise rate sensing during the fine pointing operational mode

- A high precision gyroscope is used for quick and efficient transitions during the fine pointing operational mode. Three FOGs (Fiber Optics Gyroscopes) use the interference of light to detect mechanical rotation. Each unit contains four closed-loop gyroscope channels to provide built-in redundancy.

- Rugged flight-proven initial acquisition and safe modes

- Three sun acquisition sensors plus one gyroscope provide spin-axis stabilization during the L2 transfer phase of the mission

- One large field of view star sensor plus use of the main instrument SM (Sky Mapper) for the 3-axis controlled operational phase.

Gaia AOCS architecture is based on a fully redundant set of equipment. Moving parts on board are strictly minimized (e.g. no reaction wheels, no mechanically steerable antenna). The data is downlinked through a novel electromagnetically steerable phased array antenna and attitude control is provided by a micro propulsion system that has its first flight use with Gaia. An atomic clock is used for precise time-stamping.

Two Autonomous Star Trackers are used in cold redundancy three FSS (Fine Sun Sensors) are used in hot redundancy through triple majority voting. Three Gyro packages provide coarse rate measurements where each gyro package comprises two fully independent co-aligned channels (i.e. a fiber optic gyroscope sensor plus associated electronics per channel), with the channels being used in cold redundancy. As mentioned above, the payload module provides very precise rate measurements when the spacecraft is operated in fine pointing science modes. 25)

For actuators, a bi-propellant Chemical Propulsion System (2 x 8 10 N thrusters used in cold redundancy) is used for orbit maintenance and attitude control in coarse AOCS modes (circa 350 kg MON + MMH).

The Micro-propulsion System (2 x 6 proportional micro-thrusters) can provide a range of 0 - 1000 µN at a resolution of 0.1 µN. The individual thrusters are driven by the micro-propulsion electronic, which is internally redundant and used in cold redundancy (circa 57 kg of GN2). The nominal science AOCS mode uses the cold gas micro-propulsion system.

C&DMS (Command & Data Management Subsystem). The C&DMS is characterized by:

- An ERC-32 based central computer and distinct input/output units for efficient software development

- Two segregated MIL-STD-1553 B data buses: one for the payload module and one for the service module

- SpaceWire data links for high-speed payload data

- FDIR architecture aiming at preserving payload integrity, with built-in autonomy for increased availability.

The PDHU (Payload Data Handling Unit) is, among other things, the 'hard-disk' of Gaia, responsible for temporary storage of science data received from the telescope before transmission back to Earth. It will receive thousands of compressed images per second from the observing system; this data will be sorted and stored. The individual star data objects will be prioritized based on the magnitude of the star. A complex file management system allows deletion of low-priority data in the event of data rates or volumes that exceed the capacity of the storage or transmission systems.

The solid-state storage subsystem of the PDHU has a capacity of 960 GB which, while not impressive by terrestrial standards, is extremely large for a space system. It uses a total of 240 SDRAM modules, each with a capacity of 4 GB, which populate six memory boards. The PDHU controller board is responsible for communication with the other spacecraft subsystems, file system management and the management of telemetry and telecommands. 26) 27)

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Figure 8: The PDHU (Payload Data Handling Unit), image credit: ESA

The PDHU communicates with the gigapixel focal plane over seven redundant 40 Mbit/s SpaceWire channels to acquire the scientific data coming from the seven VPUs (Video Processing Units) of the camera. The unit's controller sorts the incoming data according to star magnitude and manages deletion of low priority data should this become necessary. It sends data for transmission to Earth under the control of the CDMU (Command and Data Management Unit). The PDHU communicates with the CDMU via a MIL-STD-1553 data bus and delivers the science data over two 10 Mbit/s PacketWire channels.- The PDHU consumes only 26 W, has a mass of 14 kg, and occupies a volume of 2.3 liter.

EPS (Electrical Power Subsystem): The spacecraft is equipped with a 12.8 m2 high-efficiency triple-junction GaAs (Gallium-Arsenide) cell solar array, of which 7.3 m2 is in the form of a fixed solar array and 5.5 m2 is covered by 6 panels mechanically linked to deployable sunshield assembly.

For the launch, the deployable sunshield is folded against the payload module. After separation from the launch vehicle, it is deployed around the fixed solar array, in the same plane. During LEOP (Launch and Early Operations Phase), power is supplied by a 60 Ah mass-efficient Lithium-ion battery.

Optimum power supply during all phases of the mission is ensured by a PCDU (Power Control and Distribution Unit) with maximum power point tracking. The PCDU performs power management by generating a 28 V primary power bus that supplies power to all spacecraft subsystems. It also controls the battery state of charge and generates pyrotechnic commands as well as heater actuation as commanded by the C&DMS (Command & Data Management Subsystem).

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Figure 9: Photo of the battery (image credit: ABSL)

Propulsion: After injection into the L2 transfer orbit by the Soyuz-Fregat launcher, a chemical bi-propellant propulsion system (8 x 10 N) is used for the transfer phase. It will cover attitude acquisition, spin control, mid-course corrections, L2 orbit injection, and safe mode.

After arriving at L2, one redundant set of micro-propulsion thrusters will control the spin and precession motion of the spacecraft. Regular orbit maintenance will be performed by using the chemical propulsion thrusters. - The spacecraft uses a cold gas micropropulsion system for fine attitude control.

CPS (Chemical Propulsion Subsystem): CPS is a bi-propellant system using two tanks of Herschel/Planck heritage filled with with a total of ~400 kg of propellant featuring a blowdown ratio of 4:3. Use of monomethylhydrazine as fuel and nitrogen tetroxide as oxidizer. The 10 N thrusters are manufactured by Astrium consisting of a platinum alloy combustion chamber and nozzle that tolerates the operational temperature of 1,500°C. The thruster can be operated in a thrust range of 6 to 12.5 N with a nominal thrust of 10 N which generates a specific impulse of 291 seconds.

MPS (Micro Propulsion Subsystem): The MPS is being used for fine attitude pointing and spin rate management. A total of 12 cold gas thrusters are installed on the spacecraft being grouped in three clusters each featuring four cold gas thrusters. The thruster system uses high-pressure nitrogen propellant to provide very small impulses with a thrust range of 1 - 500 µN. The system uses two nitrogen tanks, each containing 28.5 kg of N2, stored at a pressure of 310 bar (Ref. 19). The CG-MPS (Cold Gas-Micro Propulsion Subsystem) was developed by TAS-I and Selex ES S.p.A., Italy. 28)

RF communications: All communication with the Gaia spacecraft is done using the X-band. For TT&C (Tracking Telemetry and Command), a low gain antenna uplink and downlink with a few kbit/s capacity and an omnidirectional coverage are employed. The science telemetry X-band downlink is based on a set of electronically-scanned phased array antennae accommodated on the service module bottom panel. This high gain antenna is used during each ground station visibility period of about 8 hours per day.

The X-band payload downlink rate is 10 Mbit/s from L2. To achieve this, Gaia uses a specially designed on-board phased array antenna to beam the payload data to Earth (a conventional steerable antenna would disturbed the very precise measurements).

Gaia is equipped with a total of three communication antennas – two LGAs (Low Gain Antennas) and a single X-band Medium Gain Phased Array Antenna. One LGA is located pointing in the +X direction while the other points to –X being located on the Thermal Tent and the base of the spacecraft, respectively. The two LGAs build an omni-direction communications system for housekeeping telemetry downlink and command uplink with data rates of a few kbit/s.

The MGA (Medium Gain Antenna) is located on the base of the Payload Module, protruding the DSA(Deployable Sunshield Assembly) . This directional antenna can achieve data rates of up to 10 Mbit/s for science data and telemetry downlink and telecommand reception.

Demodulation of the uplink signal is completed by the transponder units before the data flow is passed on to the CDMU (Command & Data Management Unit). The downlink data is encoded by the CDMU and modulated in X-band within the transponders before being amplified by the SSPA (Solid State Power Amplifier). The signal is combined in the phased array of the active antenna in order to orient the beam towards the Earth.

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Figure 10: Photo of the phased array antenna (image credit: EADS Astrium)

CCSDS Image Data Compression ASIC: In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers', such as bright stars and planets, and which are marred by the momentary ‘hot pixels' due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution, enabling all Gaia mission data to reach their home planet. 29)

CWICOM (CCSDS Wavelet Image COMpression ASIC) is a very high-performance image compression ASIC that implements the CCSDS 122.0 wavelet-based image compression standard, to output compressed data according to the CCSDS output source packet protocol standard. This integrated circuit was developed by Airbus DS through an ESA contract.

CWICOM offers dynamic, large compressed-rate range and high-speed image compression potentially relevant for compression of any 2D image with bi-dimensional data correlation (such as a hyperspectral data cube). Its highly optimized internal architecture allows lossless and lossy image compression at very high data rates (up to 60 Mpixels/s) without any external memory by taking advantage of its on-chip memory – almost 5 Mbit of embedded internal memory).

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Figure 11: The CWICOM ASIC is a customized microchip for imaging data compression (image credit: Airbus DS, ESA)

CWICOM is implemented using the largest matrix of the Atmel ATC18RHA ASIC family, and is provided within a standard surface mount package (CQFP 256). CWICOM offers a low-power, cost-effective and highly integrated solution for any image compression application, performing CCSDS image compression treatments without requiring any external memory. The simplicity of such a standalone implementation is achieved thanks to a very efficient internal embedded memory organization that removes any need for extra memory chip procurement and the potential obsolescence threatened by being bound to a specific external memory interface.

Change in data compression implementation:

In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms [which includes CCSDS standards, including the 122.0 implemented by CWICOM] was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers', such as bright stars and planets, and which are marred by the momentary ‘hot pixels' due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution,enabling all Gaia mission data to reach their home planet.

Instead of CWICOM, Gaia applies a tailored data compression algorithm (using a heavily tailored pre-processing stage followed by a variant of the Rice coder), using a software implementation running on VPUs (Video Processing Units). The GOCA (GAIA Optimum Compression Algorithm) project was entrusted by ESA to GTD System & Software Engineering (project prime) and IEEC (Institut d'Estudis Espacials de Catalunya), scientific partner), aimed in providing a deep understanding of the GAIA compression problem and offering a complete data compression system, both at an algorithm and implementation level. The main objectives of GOCA not only encompassed the review and evaluation of the already proposed compression scheme but the design of new algorithms for the mission. 30) 31) 32)

The CCDs in the focal plane are commanded by video-processing units (VPUs). Gaia has seven identical VPUs, each one dealing with a dedicated row of CCDs. Each CCD row, contains in order, two SM CCDs (one for each telescope), 9 AF CCDs, 1 BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven CCD rows). The VPUs run seven identical instances of the video-processing algorithms (VPAs), not necessarily with exactly the same parameter settings though. This (mix of some hardware and mostly) software is responsible for object detection (after local background subtraction), object windowing (see below), window conflict resolution, data binning, data prioritization, science-packet generation, data compression, etc.) 33)

TCS (Thermal Control Subsystem): A deployable sunshield with optimal thermoelastic behavior, made of multi-layer insulation sheets, is attached to the service module and folded against the payload module for the launch. After separation of the Gaia spacecraft from the launch vehicle, the Sun shield is deployed around the fixed solar array, in the same plane. - A thermal tent covers the payload, offering extra protection against micrometeoroids and radiation.

The very high stability thermal control is mostly passive and is achieved through optical surface reflector material, multilayer insulation sheets on the outer faces of the service module, and a black painted cavity, supplemented by heaters where required. Thermal stability is guaranteed by a constant solar aspect angle and the avoidance (as far as possible) of any equipment switch-ON/OFF cycles during nominal operation.

 

DSA (Deployable Sunshield Assembly): The bottom floor of the SVM is a dodecagonal-shaped panel to comply with the 12 frames of the DSA The main structure consists of carbon-fiber reinforced plastic face sheets.

DSA is folded up during launch and is deployed early in the flight. It is required to shade the payload unit and protect it from direct sunlight that could compromise instrument accuracy. Keeping the instrument at a constant temperature prevents expansion and contraction during temperature variations which would alter the instrument geometry ever so slightly with a large effect on data quality. The DSA is 10 m in diameter.

The DSA is an umbrella-type structure that consists of MLI (Multilayer Insulation) as the primary shield material and six rigid deployment booms as well as six secondary stiffeners. These booms have a single articulation on the base of the Service Module for easy deployment in the radial direction by a spring system. Spacing cables link the booms to the others to ensure a synchronized deployment sequence. The booms and strings are located on the cold side of the cover to limit thermoelastic flexing.

Attached to the DSA are six rectangular solar panels (with triple-junction solar GaAs cells) that are constantly facing the sun once the shield is deployed. They provide 1910 W of EOL (End of Life) power.

 

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Figure 12: Photo of Gaia's DSA deployment (image credit: Astrium SAS)

Legend to Figure 12: The DSA during deployment testing at Astrium Toulouse. Since the DSA will operate in microgravity, it is not designed to support its own weight in the one-g environment at Earth's surface. During deployment testing, the DSA panels are attached to a system of support cables and counterweights that bears their weight, preventing damage and providing a realistic test environment. The flight model thermal tent is visible inside the deploying sunshield and the mechanically representative dummy payload can be seen through the aperture in the tent.

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Figure 13: Photo of the Gaia SVM in the EMC chamber at Intespace, Toulouse, during launcher EMC compatibility testing (image credit: Astrium SAS)

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Figure 14: Exploded view of the Gaia spacecraft (image credit: EADS Astrium)

Propulsion systems

- 8 x 10 N bipropellant thrusters for coarse attitude control, orbit injection and trajectory corrections
- Low disturbance cold gas micro-propulsion for fine attitude control

Command and data management system

- ERC 32 based central computer with embedded solid state mass memory, complemented by a distinct input / output management unit
- Two segregated MIL-STD-1553B data bus, one for Payload Module and one for Service Module
- SpaceWire data links for high speed payload data

3-axes AOCS (Attitude and Orbit Control Subsystem)

- Payload instrument used for precise rate sensing in science mode
- High precision gyroscope to support transition to fine science pointing with instrument in the loop
- 3-axes autonomous star sensor for inertial attitude measurement
- Fine sun sensor for independent attitude monitoring

TT&C (Telemetry, Tracking and Command Subsystem)

- All X-band system
- Low Gain Antenna for spacecraft commanding, used also for spacecraft telemetry during critical phases
- Electrically steerable high gain antenna with built in solid state amplifiers for science downlink up to 8.7 Mbit/s from L2 orbit without mechanical disturbances

EPS (Electrical Power Subsystem)

- 60 Ah Li-ion battery
- 13 m2 triple junction cell solar array
- Power control and distribution with maximum power point tracking system

Payload data handling

- 7 Video Processing Units in parallel, each provided with 1 GIPS (Giga Instruction/s) processing capability
- 1 Terabit mass memory
- Atomic rubidium clock for accurate sequencing of the focal plane and star measurement dating

TCS (Thermal Control Subsystem)

- Fully passive thermal control
- Thermal tent for payload instrument, providing extra protection against space environment
- 100 m2 sunshield featuring a 0.1º flatness

Structure

- All Silicium Carbide instrument: structure & mirrors
- CFRP (Carbon Fiber Reinforced Plastic) SVM structure

Table 2: Summary of spacecraft subsystems (Ref. 6)

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Figure 15: Alternate exploded view of the Gaia spacecraft elements (image credit: EADS Astrium)

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Figure 16: The Gaia flight model spacecraft undergoing final electrical tests at Astrium Toulouse in June 2013 (image credit: EADS Astrium)

 

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Figure 17: Photo of the Gaia spacecraft in Nov. 2013 with an Astrium AIT engineer installing the transponders at the launch site (image credit: ESA)

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Figure 18: Photo of the Gaia spacecraft, tucked up inside the Soyuz fairing, ready to be mated with the Soyuz lower stages (image credit: ESA, M. Pedoussaut) 34)

 

Launch: The GAIA spacecraft was launched on December 19, 2013 (09:12:19 UTC) from Kourou by Arianespace, Europe's Spaceport in French Guiana. The launch vehicle was a Soyuz-STB with a Fregat-MT upper stage The launch is designated as Soyuz flight VS06. 35) 36) 37)

- About ten minutes later, after separation of the first three stages, the Fregat upper stage ignited, delivering Gaia into a temporary parking orbit at an altitude of 175 km.

- A second firing of the Fregat 11 minutes later took Gaia into its transfer orbit, followed by separation from the upper stage 42 minutes after liftoff. Ground telemetry and attitude control were established by controllers at ESOC (European Space Operation Centre) in Darmstadt, Germany, and the spacecraft began activating its systems.

- The sunshield, which keeps Gaia at its working temperature and carries solar cells to power the satellite, was deployed in a 10 minute automatic sequence, completed around 88 minutes after launch. Gaia is now en route towards L2 (Ref. 35).

Orbit: Large Lissajous orbits around L2 (Lagrangian Point 2), about 1.5 million km from Earth. L2 offers a stable thermal environment because the sunshield will protect Gaia from the Sun, Earth and Moon simultaneously, allowing the satellite to keep cool and enjoy a clear view of the Universe from the other side. In addition, L2 provides a moderate radiation environment, which benefits the longevity of the instrument detectors.

• The critical LEOP (Launch and Early Orbit Phase) will last approximately four days. In this phase, Gaia will perform the first activations – transmitter switch ON, priming of the chemical thrusters, first attitude control and finding of the sun position – followed by the sun shield deployment. Engineers on ground will perform orbit determination, then prepare and execute the critical 'Day 2' maneuver to inject Gaia into its final transfer trajectory toward the L2 Lagrange point (Ref. 159).

• LEOP will be followed by the transfer cruise phase, lasting up to 30 days, an L2 orbit injection maneuver, then the in-orbit commissioning phase, during which all operations to prepare for the routine operational phase are performed. In particular, the scientific FPA (Focal Plane Assembly) and related avionics will be thoroughly tested and calibrated. The commissioning phase is expected to last four months.

• The insertion into the final 300,000 x 200,000 km Lissajous orbit around L2 was performed one month after launch.

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Figure 19: Gaia mission scenario, from launch to in-orbit operations (image credit: ESA)

 


 

Mission status:

• May 9, 2019: A team led by researchers of the Institute of Cosmos Sciences of the University of Barcelona (ICCUB, UB-IEEC) and the Besançon Astronomical Observatory have analyzed data from the Gaia satellite and found that a heavy star formation burst occurred in the Milky Way about 3,000 million years ago. During this process, more than 50 percent of the stars that created the galactic disc may have been born. These results are derived from the combination of the distances, colors and magnitude of the stars that were measured by Gaia with models that predict their distribution in our galaxy. The study has been published in the journal Astronomy & Astrophysics. 38) 39)

- Like a flame fades when there is no gas in the cylinder, the rhythm of the stellar formation in the Milky Way, fueled by deposited gas, is predicted to decrease slowly and in a continuous way as the existing gas is extinguished. The results of the study show that although this process took place over the first 4,000 million years of Milky Way disc formation, a severe star formation burst, or "stellar baby boom," inverted this trend. A gas-rich satellite galaxy merged with the Milky Way, and could have introduced new fuel and reactivated the process of stellar formation. This mechanism would explain the distribution of distances, ages and masses that are estimated from the data taken from the European Space Agency Gaia satellite.

- "The time scale of this star formation burst, together with the great stellar mass involved in the process—thousands of millions of solar masses—suggests the disc of our galaxy did not have a steady and paused evolution. It may have suffered an external perturbation that began about 5 billion years ago," said Roger Mor, ICCUB researcher and first signer of the article.

- "We have been able to find this out by analyzing the precise distances for more than 3 million stars in the solar environment," says Roger Mor. "Thanks to these data, we could discover the mechanisms that controlled the evolution more than 8 to 10 billion years ago in the disc of our galaxy, which is not more than the bright band we see in the sky on a dark night and with no light pollution." Like in many research fields these days, these findings are possible thanks to the availability of the combination of a great amount of unprecedented precision data, and many hours of computing.

- Cosmologic models predict our galaxy would have been growing due the merging with other galaxies, a fact that has been stated by other studies using Gaia data. One of these merges could be the cause of the severe star formation burst that was detected in this study.

- "Actually, the peak of star formation is so clear, unlike predictions from before Gaia data availability, that we thought it necessary to treat its interpretation together with experts on cosmological evolution of external galaxies," notes Francesca Figuerars, lecturer at the Department of Quantum Physics and Astrophysics of the UB, ICCUB member and signer of the article.

- Santi Roca-Fàbrega from the Complutense University of Madrid, an expert in stellar modeling and co-author, said, "The obtained results match with what the current cosmological models predict, and what is more, our galaxy seen from Gaia's eyes is an excellent cosmological laboratory where we can test and confront models at a bigger scale in the universe."

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Figure 20: The region of the stellar formation Rho Ophiuchi observed by ESA Gaia satellite. The shining dots are stellar clusters with the massive and youngest stars of the region. The dark filaments track the gas and dust distribution, where the new stars are born. This is not a conventional photographic image but the result of the integration of all the received radiation by the satellite during the 22 months of continuous measurements through different filters on the spacecraft (image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO)

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Figure 21: Distribution of 3 million stars used in this study to detect the star formation burst from 2-3 billion years ago. Gaia provided the distance for each of these objects on the galactic disc. Shown is a scheme of the spiral arms of the Milky Way (image credit: University of Barcelona)

• May 2, 2019: While ESA's Gaia mission has been surveying more than one billion stars from space, astronomers have been regularly monitoring the satellite's position in the sky with telescopes across the world, including the European Southern Observatory in Chile, to further refine Gaia's orbit and ultimately improve the accuracy of its stellar census. 40) 41)

- One year ago, the Gaia mission released its much-awaited second set of data, which included high-precision measurements – positions, distance indicators and proper motions – of more than one billion stars in our Milky Way galaxy.

- The catalog, based on less than two years of observations and almost four years of data processing and analysis by a collaboration of about 450 scientists and software engineers, has enabled transformational studies in many fields of astronomy, generating more than 1000 scientific publications in the past twelve months.

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Figure 22: Pinpointing Gaia from Earth. A sequence of images taken as part of the Ground Based Optical Tracking campaign of ESA's Gaia satellite with the European Southern Observatory's (ESO) 2.6 m VLT Survey Telescope (VST) in Chile. This image combines ten observations performed on 14 April 2019: Gaia is visible as a line of ten faint dots just below the image center. The stars in the image appear as slightly elongated, since the telescope is following Gaia rather than the stars. The observations have been stacked using the stars as reference to show the movement of Gaia across the sky. The images were obtained using the OMEGACAM instrument with the SDSS-r filter on the VST, with an exposure time of 60 seconds for each individual observation; the whole sequence covers about 17 minutes. Astrometric determination of Gaia's position is conducted on each frame, using as reference the coordinates of the background stars as provided in the second Gaia data release. The results are then averaged and the resulting value for the position of Gaia has a precision of 20 mas (milliarcseconds) or better (one arcsecond is equivalent to the size of a Euro coin seen from a distance of about 4 km), image credit: ESO; CC BY 4.0

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Figure 23: Pinpointing Gaia from Earth – annotated (image credit: ESO; CC BY 4.0)

- Meanwhile in space, Gaia keeps scanning the sky and gathering data that is being crunched for future releases to achieve even higher precision on the position and motion of stars and enable ever deeper and more detailed studies into our place in the cosmos. But to reach the accuracy expected for Gaia's final catalogue, it is crucial to pinpoint the position and motion of the satellite from Earth.

- To this aim, the flight dynamics experts at ESA's operations center make use of a combination of techniques, from traditional radio tracking and ranging to simultaneous observing using two radio antennas – the so-called delta-DOR method.

Figure 24: Gaia scanning the sky. This animation shows the Gaia spacecraft spinning in space scanning the sky. Gaia's mission relies on the systematic and repeating observation of star positions in two fields of view. As the detectors repeatedly measure the position of each celestial object, they will detect any changes in the object's motion through space. To achieve its mission the spacecraft is spinning slowly, sweeping its two telescopes across the entire celestial sphere to make four complete rotations per day (video credit: ESA – C. Carreau)

Legend to Figure 24: Gaia's telescopes point at two different portions of the sky, separated by a constant 106.5°. Therefore, objects arrive in the second field of view 106.5 minutes after they are observed in the first. Meanwhile its spin axis precesses around the Sun with a period of about 63 days, allowing different parts of the sky to be scanned. This scanning strategy builds up an interlocking grid of positions, providing absolute – rather than relative – values of the stellar positions and motions. The spacecraft spin axis makes an angle of 45° with the Sun direction, ensuring that the payload is shaded from the Sun, but that the solar arrays can still produce electricity efficiently.

- In a unique and novel approach for ESA, the ground-based tracking of Gaia also includes optical observations provided by a network of medium-size telescopes across the planet.

- The European Southern Observatory's (ESO) 2.6 m VLT Survey Telescope (VST) in Chile records Gaia's position in the sky for about 180 nights every year.

- "This is an exciting ground-space collaboration, using one of ESO's world-class telescopes to anchor the trailblazing observations of ESA's billion star surveyor," says Timo Prusti, Gaia project scientist at ESA.

- "The VST is the perfect tool for picking out the motion of Gaia," adds Ferdinando Patat, head of the ESO's Observing Programs Office. "Using one of ESO's first-rate ground-based facilities to bolster cutting-edge space observations is a fine example of scientific cooperation."

- In addition, the 2 m Liverpool telescope located on La Palma, Canary Islands, Spain, and the Las Cumbres Optical Global Telescope Network, which operates 2 m telescopes in Australia and the US, have also been observing Gaia over the past five years as part of the Ground Based Optical Tracking (GBOT) campaign.

- "Gaia observations require a special observing procedure," explains Monika Petr-Gotzens, who has coordinated the execution of ESO's observations of Gaia since 2013. "The spacecraft is what we call a ‘moving target', as it is moving quickly relative to background stars – tracking Gaia is quite the challenge!"

- In these images Gaia is a mere dot of light among the many stars that the satellite itself has been measuring, so painstaking calibration is needed to transform this body of observations into meaningful data that can be included in the determination of the satellite's orbit.

- This required using data from Gaia's second release to identify the stars in each of the images collected over the past five years and calculate the satellite's position in the sky with a precision of 20 mas or better (one arcsecond is equivalent to the size of a Euro coin seen from a distance of about 4 km).

- The ground-based observations also provide key information to improve the determination of Gaia's velocity through space, which must be known to the precision of a few mm/s. This is necessary to correct for a phenomenon known as aberration of light – an apparent distortion in the direction of incoming light due to the relative motion between the source and an observer – in a way similar to tilting one's umbrella while walking through the rain.

- "After careful and lengthy data processing, we have now achieved the accuracy required for the ground-based observations of Gaia to be implemented as part of the orbit determination," says Martin Altmann, lead of the GBOT campaign from the Astronomisches Rechen-Institut, Center for Astronomy of Heidelberg University, Germany, who works in close collaboration with colleagues from the Paris Observatory in France.

- The GBOT information will be used to improve our knowledge of Gaia's orbit not only in observations to come, but also for all the data that have been gathered from Earth in the previous years, leading to improvements in the data products that will be included in future releases.

• April 29, 2019: While scanning the sky to chart a billion stars in our Milky Way galaxy, ESA's Gaia satellite is also sensitive to celestial bodies closer to home, and regularly observes asteroids 42) in our Solar System. 43)

Figure 25: Gaia's first asteroid survey. Animated view of the 14,099 asteroids in our Solar System, as viewed by ESA's Gaia satellite using information from the mission's second data release. The orbits of the 200 brightest asteroids are also shown, as determined using Gaia data. - In future data releases, Gaia will also provide asteroid spectra and enable a complete characterization of the asteroid belt. The combination of dynamical and physical information that is being collected by Gaia provides an unprecedented opportunity to improve our understanding of the origin and the evolution of the Solar System (video credit: Gaia Data Processing and Analysis Consortium (DPAC); Orbits: Gaia Coordinating Unit 4; P. Tanga, Observatoire de la Côte d'Azur, France; F. Spoto, IMCCE, Observatoire de Paris, France; Animation: Gaia Sky; S. Jordan / T. Sagristà, Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Germany)

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Figure 26: This view shows the orbits of more than 14,000 known asteroids (with the Sun at the center of the image) based on information from Gaia's second data release, which was made public in 2018. The majority of asteroids depicted in this image, shown in bright red and orange hues, are main-belt asteroids, located between the orbits of Mars and Jupiter; Trojan asteroids, found around the orbit of Jupiter, are shown in dark red. In yellow, towards the image center, are the orbits of several tens of near-Earth asteroids observed by Gaia: these are asteroids that come to within 1.3 astronomical units (AU) to the Sun at the closest approach along their orbit. The Earth circles the Sun at a distance of 1 AU (around 150 million km) so near-Earth asteroids have the potential to come into proximity with our planet (image credit: ESA/Gaia/DPAC)

- Most asteroids that Gaia detects are already known, but every now and then, the asteroids seen by ESA's Milky Way surveyor do not match any existing observations. This is the case for the three orbits shown in grey in this view: these are Gaia's first asteroid discoveries.

- The three new asteroids were first spotted by Gaia in December 2018, and later confirmed by follow-up observations performed with the Haute-Provence Observatory in France, which enabled scientists to determine their orbits. Comparing these informations with existing observations indicated the objects had not been detected earlier.

- While they are part of the main belt of asteroids, all three move around the Sun on orbits that have a greater tilt (15 degrees or more) with respect to the orbital plane of planets than most main-belt asteroids.

- The population of such high-inclination asteroids is not as well studied as those with less tilted orbits, since most surveys tend to focus on the plane where the majority of asteroids reside. But Gaia can readily observe them as it scans the entire sky from its vantage point in space, so it is possible that the satellite will find more such objects in the future and contribute new information to study their properties.

- Alongside the extensive processing and analysis of Gaia's data in preparation for subsequent data releases, preliminary information about Gaia's asteroid detections are regularly shared via an online alert system so that astronomers across the world can perform follow-up observations. To observe these asteroids, a 1-m or larger telescope is needed.

- Once an asteroid detected by Gaia has been identified also in ground-based observations, the scientists in charge of the alert system analyze the data to determine the object's orbit. In case the ground observations match the orbit based on Gaia's data, they provide the information to the Minor Planet Center, which is the official worldwide organization collecting observational data for small Solar System bodies like asteroids and comets.

- This process may lead to new discoveries, like the three asteroids with orbits depicted in this image, or to improvements in the determination of the orbits of known asteroids, which are sometimes very poorly known. So far, several tens of asteroids detected by Gaia have been observed from the ground in response to the alert system, all of them belonging to the main belt, but it is possible that also near-Earth asteroids will be spotted in the future.

- A number of observatories across the world are already involved in these activities, including the Haute-Provence Observatory, Kyiv Comet station, Odessa-Mayaki, Terskol, C2PU at Observatoire de la Côte d'Azur and Las Cumbres Observatory Global Telescope Network. The more that join, the more we will learn about asteroids – known and new ones alike.

- Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); Gaia Coordinating Unit 4; B. Carry, F. Spoto, P. Tanga (Observatoire de la Côte d'Azur, France) & W. Thuillot (IMCCE, Observatoire de Paris, France); Gaia Data Processing Center at CNES, Toulouse, France.

• March 7, 2019: In a striking example of multi-mission astronomy, measurements from the NASA/ESA Hubble Space Telescope and the ESA Gaia mission have been combined to improve the estimate of the mass of our home galaxy the Milky Way: 1.5 trillion (1.5 x 1012) solar masses. 44)

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Figure 27: This artist's impression shows a computer generated model of the Milky Way and the accurate positions of the globular clusters used in this study surrounding it. Scientists used the measured velocities of these 44 globular clusters to determine the total mass of the Milky Way, our cosmic home. Satellite: Hubble Space Telescope (image credit: ESA/Hubble, NASA, L. Calçada)

- The mass of the Milky Way is one of the most fundamental measurements astronomers can make about our galactic home. However, despite decades of intense effort, even the best available estimates of the Milky Way's mass disagree wildly. Now, by combining new data from the European Space Agency (ESA) Gaia mission with observations made with the NASA/ESA Hubble Space Telescope, astronomers have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129,000 light-years from the galactic center.

- Previous estimates of the mass of the Milky way ranged from 500 billion (500 x 109) to 3 trillion (3 x 1012) times the mass of the Sun. This huge uncertainty arose primarily from the different methods used for measuring the distribution of dark matter – which makes up about 90% of the mass of the galaxy.

- "We just can't detect dark matter directly," explains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis. "That's what leads to the present uncertainty in the Milky Way's mass – you can't measure accurately what you can't see!"

- Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the velocities of globular clusters – dense star clusters that orbit the spiral disc of the galaxy at great distances.
Note: Globular clusters formed prior to the construction of the Milky Way's spiral disk, where our Sun and the Solar System later formed. Because of their great distances, globular star clusters allow astronomers to trace the mass of the vast envelope of dark matter surrounding our galaxy far beyond the spiral disk.

- "The more massive a galaxy, the faster its clusters move under the pull of its gravity" explains N. Wyn Evans (University of Cambridge, UK). "Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated."
Note: The total velocity of an object is made up of three motions – a radial motion plus two defining the sideway motions. However, in astronomy most often only line-of-sight velocities are available. With only one component of the velocity available, the estimated masses depend very strongly on the assumptions for the sideway motions. Therefore measuring the sideway motions directly significantly reduces the size of the error bars for the mass.

- The group used Gaia's second data release as a basis for their study. Gaia was designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and to track their motions. Its second data release includes measurements of globular clusters as far as 65,000 light-years from Earth.

- "Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy" said Tony Sohn of STScI (Space Telescope Science Institute), Baltimore, MD, USA, who led the Hubble measurements.

- The team combined these data with Hubble's unparalleled sensitivity and observational legacy. Observations from Hubble allowed faint and distant globular clusters, as far as 130,000 light-years from Earth, to be added to the study. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well.

- "We were lucky to have such a great combination of data," explained Roeland P. van der Marel of STScI. "By combining Gaia's measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way's mass in a way that would be impossible without these two space telescopes."

- Until now, not knowing the precise mass of the Milky Way has presented a problem for attempts to answer a lot of cosmological questions. The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe. Accurately determining the mass for the Milky Way gives us a clearer understanding of where our galaxy sits in a cosmological context. 45)

Figure 28: Hubblecast 117 Light: Hubble & Gaia weigh the Milky Way. Measurements from the NASA/ESA Hubble Space Telescope and the ESA Gaia mission have been combined to improve the estimate of the mass of our home galaxy the Milky Way: 1.5 trillion solar masses (video credit: HubbleESA)

• February 26, 2019: ESA's Gaia satellite is on a mission: to map and characterize more than one billion of the stars in the Milky Way. Many of these stars reside in complex, eye-catching clusters scattered throughout our Galaxy and, by studying these stellar groupings, Gaia is revealing much about the formation and evolution of stars in our cosmic home and surroundings. 46)

- The Milky Way is full of stars. Our Galaxy contains over a hundred billion of them, from dwarf to giant, populating its crowded center and its spiralling disc.

- Many of these stars are thought to have formed in the same way: from huge clouds of cool, condensing molecular gas, which collapse under the influence of gravity and fragment to form groups of hundreds to thousands of stars, known as star clusters. Some of these clusters last thousands of millions of years, while others disperse rapidly, releasing their stellar residents into the Milky Way's disc.

- It is likely that also our Sun formed in a cluster some 4.5 billion years ago, and the quest for solar siblings – stars that were born in the same cluster as the Sun and then went on different paths – will provide important information on the birth of our parent star.

- Despite our growing knowledge, many open questions remain. For instance, how many clusters exist, how many are currently being formed, how many are falling apart – and at what pace? The incredible diversity of stars and their birth clusters is currently being explored by ESA's Gaia satellite.

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Figure 29: Gaia's all-sky view of our Milky Way Galaxy and neighboring galaxies, based on measurements of nearly 1.7 billion stars and displayed in an equirectangular projection. It has been obtained by projecting the celestial sphere onto a rectangle and is suitable for full-dome presentations. The map shows the total brightness and color of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016. Brighter regions indicate denser concentrations of especially bright stars, while darker regions correspond to patches of the sky where fewer bright stars are observed. The color representation is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky (image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO)

• February 7, 2019: ESA's Gaia satellite has looked beyond our Galaxy and explored two nearby galaxies to reveal the stellar motions within them and how they will one day interact and collide with the Milky Way – with surprising results. 47)

- Our Milky Way belongs to a large gathering of galaxies known as the Local Group and, along with the Andromeda and Triangulum galaxies – also referred to as M31 and M33, respectively – makes up the majority of the group's mass.

- Astronomers have long suspected that Andromeda will one day collide with the Milky Way, completely reshaping our cosmic neighborhood. However, the three-dimensional movements of the Local Group galaxies remained unclear, painting an uncertain picture of the Milky Way's future.

- "We needed to explore the galaxies' motions in 3D to uncover how they have grown and evolved, and what creates and influences their features and behavior," says lead author Roeland van der Marel of the STScI (Space Telescope Science Institute) in Baltimore, USA. "We were able to do this using the second package of high-quality data released by Gaia." 48)

- Gaia is currently building the most precise 3D map of the stars in the nearby Universe, and is releasing its data in stages. The data from the second release, made in April 2018, was used in this research.

- Previous studies of the Local Group have combined observations from telescopes including the NASA/ESA Hubble Space Telescope and the ground-based Very Long Baseline Array to figure out how the orbits of Andromeda and Triangulum have changed over time. The two disc-shaped spiral galaxies are located between 2.5 and 3 million light-years from us, and are close enough to one another that they may be interacting.

- Two possibilities emerged: either Triangulum is on an incredibly long six-billion-year orbit around Andromeda but has already fallen into it in the past, or it is currently on its very first infall. Each scenario reflects a different orbital path, and thus a different formation history and future for each galaxy.

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Figure 30: The future orbital trajectories of three spiral galaxies: our Milky Way (blue), Andromeda, also known as M31 (red), and Triangulum, also known as M33 (green). The circle indicates the current position of each galaxy, and their future trajectories have been calculated using data from the second release of ESA's Gaia mission. The Milky Way is shown as an artist's impression, while the images of Andromeda and Triangulum are based on Gaia data. Arrows along the trajectories indicate the estimated direction of each galaxy's motion and their positions, 2.5 billion years into the future, while crosses mark their estimated position in about 4.5 billion years. Approximately 4.5 billion years from now, the Milky Way and Andromeda will make their first close passage around one another at a distance of approximately 400,000 light-years. The galaxies will then continue to move closer to one another and eventually merge to form an elliptical galaxy (image credit: Orbits: E. Patel, G. Besla (University of Arizona), R. van der Marel (STScI); Images: ESA (Milky Way); ESA/Gaia/DPAC (M31, M33)

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Figure 31: This gigantic image of the Triangulum Galaxy — also known as Messier 33 — is a composite of about 54 different pointings with Hubble's Advanced Camera for Surveys. With a staggering size of 34,372 x 19,345 pixels, it is the second-largest image ever released by Hubble. It is only dwarfed by the image of the Andromeda Galaxy, released in 2015. The mosaic of the Triangulum Galaxy showcases the central region of the galaxy and its inner spiral arms. Millions of stars, hundreds of star clusters and bright nebulae are visible (image credit: NASA, ESA, and M. Durbin, J. Dalcanton, and B. F. Williams (University of Washington); CC BY 4.0)

- While Hubble has obtained the sharpest view ever of both Andromeda and Triangulum, Gaia measures the individual position and motion of many of their stars with unprecedented accuracy.

- "We combed through the Gaia data to identify thousands of individual stars in both galaxies, and studied how these stars moved within their galactic homes," adds co-author Mark Fardal, also of Space Telescope Science Institute.

- "While Gaia primarily aims to study the Milky Way, it's powerful enough to spot especially massive and bright stars within nearby star-forming regions – even in galaxies beyond our own."

- The stellar motions measured by Gaia not only reveal how each of the galaxies moves through space, but also how each rotates around its own spin axis.

- A century ago, when astronomers were first trying to understand the nature of galaxies, these spin measurements were much sought-after, but could not be successfully completed with the telescopes available at the time.

- "It took an observatory as advanced as Gaia to finally do so," says Roeland.

- "For the first time, we've measured how M31 and M33 rotate on the sky. Astronomers used to see galaxies as clustered worlds that couldn't possibly be separate ‘islands', but we now know otherwise.

- "It has taken 100 years and Gaia to finally measure the true, tiny, rotation rate of our nearest large galactic neighbor, M31. This will help us to understand more about the nature of galaxies."

- By combining existing observations with the new data release from Gaia, the researchers determined how Andromeda and Triangulum are each moving across the sky, and calculated the orbital path for each galaxy both backwards and forwards in time for billions of years.

- "The velocities we found show that M33 cannot be on a long orbit around M31," says co-author Ekta Patel of the University of Arizona, USA. "Our models unanimously imply that M33 must be on its first infall into M31."

- While the Milky Way and Andromeda are still destined to collide and merge, both the timing and destructiveness of the interaction are also likely to be different than expected.

- As Andromeda's motion differs somewhat from previous estimates, the galaxy is likely to deliver more of a glancing blow to the Milky Way than a head-on collision. This will take place not in 3.9 billion years' time, but in 4.5 billion – some 600 million years later than anticipated.

- "This finding is crucial to our understanding of how galaxies evolve and interact," says Timo Prusti, ESA Gaia Project Scientist.

- "We see unusual features in both M31 and M33, such as warped streams and tails of gas and stars. If the galaxies haven't come together before, these can't have been created by the forces felt during a merger. Perhaps they formed via interactions with other galaxies, or by gas dynamics within the galaxies themselves."

- "Gaia was designed primarily for mapping stars within the Milky Way — but this new study shows that the satellite is exceeding expectations, and can provide unique insights into the structure and dynamics of galaxies beyond the realm of our own. The longer Gaia watches the tiny movements of these galaxies across the sky, the more precise our measurements will become."

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Figure 32: A view of the Andromeda galaxy, also known as M31, with measurements of the motions of stars within the galaxy. This spiral galaxy is the nearest large neighbor of our Milky Way. The background image, obtained with NASA's Galex satellite at near-ultraviolet wavelengths, highlights regions within the galaxy where stars are forming. Blue symbols mark the locations of bright young stars that were used to measure the motion of the galaxy, and yellow arrows indicate the average stellar motions at various locations, based on data from the second release of ESA's Gaia satellite. A counter-clockwise rotation of the spiral galaxy's disc is evident. The precision of these measurements is expected to improve with the future Gaia data releases [image credit: ESA/Gaia (star motions); NASA/Galex (background image); R. van der Marel, M. Fardal, J. Sahlmann (STScI)]

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Figure 33: Sharpest ever view of the Andromeda Galaxy [image credit: NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler]

Legend to Figure 33: This image, captured with the NASA/ESA Hubble Space Telescope, is the largest and sharpest image ever taken of the Andromeda galaxy – otherwise known as M31. This is a cropped version of the full image and has 1.5 billion pixels. You would need more than 600 HD television screens to display the whole image. It is the biggest Hubble image ever released and shows over 100 million stars and thousands of star clusters embedded in a section of the galaxy's pancake-shaped disc stretching across over 40,000 light-years.

• January 10, 2019: How old are each of the stars in our roughly 13-billion-year-old galaxy? A new technique for understanding the star-forming history of the Milky Way in unprecedented detail makes it possible to determine the ages of stars at least two times more precisely than conventional methods, Embry-Riddle Aeronautical University (ERAU, Daytona Beach, USA) researchers reported 10 January at the American Astronomical Society (AAS) meeting in Seattle, WA. 49)

- Current star-dating techniques, based on assessments of stars in the prime or main sequence of their lives that have begun to die after exhausting their hydrogen, offer a 20%, or at best a 10% margin of error, explained Embry-Riddle Physics and Astronomy Professor Dr. Ted von Hippel. Embry-Riddle's approach, leveraging burnt-out remnants called white dwarf stars, reduces the margin of error to 5% or even 3%, he said.

- For this method to work, von Hippel and his team must measure the star's surface temperature, whether it has a hydrogen or helium atmosphere, and its mass. The surface temperature can be determined from a star's color and atmospheric constituents.

- "The star's mass matters because objects with greater mass have more energy and take longer to cool," said von Hippel. "This is why a cup of coffee stays hot longer than a teaspoon of coffee. Surface temperature, like spent coals in a campfire that's gone out, offer clues to how long ago the fire died. Finally, knowing whether there is hydrogen or helium at the surface is important because helium radiates heat away from the star more readily than hydrogen."

- Determining the precise masses of stars, particularly for large samples of white dwarfs, is very difficult. Now, astronomers have a new method to determine white dwarf masses.

- Leveraging Gaia Satellite Data: The method takes advantage of data captured by the European Space Agency's Gaia satellite, an ambitious mission to create a three-dimensional map of the Milky Way. Von Hippel, with recent Embry-Riddle graduate Adam Moss, current students Isabelle Kloc, Jimmy Sargent and Natalie Moticksa, and instructor Elliot Robinson, used highly precise Gaia measurements of the distance of stars.

- Just as a car's speedometer may appear to give two different readings from the driver's perspective versus the passenger's seat, celestial objects can appear to be in different locations, depending upon the viewer's vantage point. The Gaia measurements, based on the geometry of two different lines of site or "parallaxes" to objects, helped Embry-Riddle researchers determine the radius of stars based on their brightness. They could then use existing information on the star's mass-to-radius ratio — a calculation driven by the physical behavior of electrons — to fill in the last ingredient for determining the age of the star, its mass.

- Finally, by figuring out the abundance of different elements within the star, or its metallicity, researchers can further refine the age of the object, Moss and Kloc reported in two separate AAS poster presentations. Moss focused on pairs of stars with one white dwarf and one main sequence star similar to our Sun, while Kloc's research looked at two white dwarf stars in the same binary system.

- "The next level of study will be to determine as many of the elements in the periodic table as possible for the main sequence star within these pairs," von Hippel said. "That would tell us more about Galactic chemical evolution, based on how different elements built up over time as stars formed in our galaxy, the Milky Way."

- Though he emphasized that the current work remains preliminary, the team ultimately hopes to publish the ages of all white dwarf stars within the Gaia dataset: "That could allow researchers to significantly advance our understanding of star-formation within the Milky Way."

- Within the field of archaeology, von Hippel noted, carbon-dating methods made it possible to determine the age of structures, fossils, Stone Age sites and much more, thereby providing deeper insights into the evolution of life on Earth. "For today's astronomers, without knowing the age of different components of our galaxy, we don't have context. We've had techniques for dating celestial objects, but not precisely."

- The Embry-Riddle team's research collaborators were David Stenning and David van Dyk of Imperial College London; Elizabeth Jeffery of California Polytechnic State, San Luis Obispo; Kareem El-Badry of the University of California, Berkeley; and William Jeffery of the University of Texas, Austin.

• January 9, 2019: Data captured by ESA's galaxy-mapping spacecraft Gaia has revealed for the first time how white dwarfs, the dead remnants of stars like our Sun, turn into solid spheres as the hot gas inside them cools down. 50)

- This process of solidification, or crystallization, of the material inside white dwarfs was predicted 50 years ago but it wasn't until the arrival of Gaia that astronomers were able to observe enough of these objects with such a precision to see the pattern revealing this process.

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Figure 34: Illustration of a white dwarf, the dead remnant of a star like our Sun, with a crystallized, solid core. Once these stars have burnt all the nuclear fuel in their core, they shed their outer layers, leaving behind a hot core that starts cooling down. Data captured by ESA's galaxy-mapping spacecraft Gaia has revealed for the first time how white dwarfs turn into solid spheres as the originally hot matter inside their core starts crystallizing, becoming solid (image credit: University of Warwick/Mark Garlick)

- "Previously, we had distances for only a few hundred of white dwarfs and many of them were in clusters, where they all have the same age," says Pier-Emmanuel Tremblay from the University of Warwick, UK, lead author of the paper describing the results, published today in Nature. 51)

- "With Gaia we now have the distance, brightness and color of hundreds of thousands of white dwarfs for a sizeable sample in the outer disc of the Milky Way, spanning a range of initial masses and all kinds of ages."

- It is in the precise estimate of the distance to these stars that Gaia makes a breakthrough, allowing astronomers to gauge their true brightness with unprecedented accuracy.

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Figure 35: Stellar evolution: Artist impression of some possible evolutionary pathways for stars of different initial masses. Some proto-stars, brown dwarfs, never actually get hot enough to ignite into fully-fledged stars, and simply cool off and fade away. Red dwarfs, the most common type of star, keep burning until they have transformed all their hydrogen into helium, turning into a white dwarf. Sun-like stars swell into red giants before puffing away their outer shells into colorful nebula while their cores collapse into a white dwarf. The most massive stars collapse abruptly once they have burned through their fuel, triggering a supernova explosion or gamma-ray burst, and leaving behind a neutron star or black hole (image credit: ESA)

- White dwarfs are the remains of medium-sized stars similar to our Sun. Once these stars have burnt all the nuclear fuel in their core, they shed their outer layers, leaving behind a hot core that starts cooling down.

- These ultra-dense remnants still emit thermal radiation as they cool, and are visible to astronomers as rather faint objects. It is estimated that up to 97% of stars in the Milky Way will eventually turn into white dwarfs, while the most massive of stars will end up as neutron stars or black holes.

- The cooling of white dwarfs lasts billions of years. Once they reach a certain temperature, the originally hot matter inside the star's core starts crystallizing, becoming solid. The process is similar to liquid water turning into ice on Earth at zero degrees Celsius, except that the temperature at which this solidification happens in white dwarfs is extremely high – about 10 million degrees Celsius.

- In this study, the astronomers analyzed more than 15,000 stellar remnant candidates within 300 light years of Earth as observed by Gaia and were able to see these crystallizing white dwarfs as a rather distinct group.

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Figure 36: This diagram, known as Hertzsprung-Russell diagram (after the astronomers who devised it in the early 20th century to study stellar evolution) combines information about the brightness, color and distance of more than 15,000 white dwarfs within 300 light years of Earth. The data, shown as black dots, are from the second release of ESA's Gaia satellite (image credit: Pier-Emmanuel Tremblay, et al.)

- "We saw a pile-up of white dwarfs of certain colors and luminosities that were otherwise not linked together in terms of their evolution," says Pier-Emmanuel. "We realized that this was not a distinct population of white dwarfs, but the effect of the cooling and crystallization predicted 50 years ago."

- The heat released during this crystallization process, which lasts several billion years, seemingly slows down the evolution of the white dwarfs: the dead stars stop dimming and, as a result, appear up to two billion years younger than they actually are. That, in turn, has an impact on our understanding of the stellar groupings these white dwarfs are a part of.

- White dwarfs are traditionally used for age-dating of stellar populations such as clusters of stars, the outer disc, and the halo in our Milky Way," explains Pier-Emmanuel. "We will now have to develop better crystallization models to get more accurate estimates of the ages of these systems."

- Not all white dwarfs crystallize at the same pace. More massive stars cool down more rapidly and will reach the temperature at which crystallization happens in about one billion years. White dwarfs with lower masses, closer to the expected end stage of the Sun, cool in a slower fashion, requiring up to six billion years to turn into dead solid spheres.

- The Sun still has about five billion years before it becomes a white dwarf, and the astronomers estimate that it will take another five billion years after that to eventually cool down to a crystal sphere.

- "This result highlights the versatility of Gaia and its numerous applications," says Timo Prusti, Gaia project scientist at ESA. "It's exciting how scanning stars across the sky and measuring their properties can lead to evidence of plasma phenomena in matter so dense that cannot be tested in the laboratory."

• December 13, 2018: Launched in December 2013, ESA's Gaia satellite has been scanning the sky to perform the most precise stellar census of our Milky Way galaxy, observing more than one billion stars and measuring their positions, distances and motions to unprecedented accuracy. — Launched in December 2013, ESA's Gaia satellite has been scanning the sky to perform the most precise stellar census of our Milky Way galaxy, observing more than one billion stars and measuring their positions, distances and motions to unprecedented accuracy. 52)

Figure 37: The Universe of Gaia (video credit: ESA / CNES / Arianespace; ESA / Gaia / DPAC; Gaia Sky / S. Jordan / T. Sagristà; Koppelman, Villalobos and Helmi; Marchetti et al.; 13 December 2018; NASA / ESA / Hubble; ESO, M. Kornmesser, L. Calçada)

November 14, 2018: The SPC (Science Program Committee) of ESA has confirmed the continued operations of ten scientific missions in the Agency's fleet up to 2022. After a comprehensive review of their scientific merits and technical status, the SPC has decided to extend the operation of the five missions led by ESA's Science Program: Cluster, Gaia, INTEGRAL, Mars Express, and XMM-Newton. The SPC also confirmed the Agency's contributions to the extended operations of Hinode, Hubble, IRIS, SOHO, and ExoMars TGO. 53)

- This includes the confirmation of operations for the 2019–2020 cycle for missions that had been given indicative extensions as part of the previous extension process, and indicative extensions for an additional two years, up to 2022.
Note: Every two years, all missions whose approved operations end within the following four years are subject to review by the advisory structure of the Science Directorate. Extensions are granted to missions that satisfy the established criteria for operational status and science return, subject to the level of financial resources available in the science program. These extensions are valid for the following four years, subject to a mid-term review and confirmation after two years.

- The decision was taken during the SPC meeting at ESA/ESAC (European Space Astronomy Center) near Madrid, Spain, on 14 November.

- ESA's science missions have unique capabilities and are prolific in their scientific output. Cluster, for example, is the only mission that, by varying the separation between its four spacecraft, allows multipoint measurements of the magnetosphere in different regions and at different scales, while Gaia is performing the most precise astrometric survey ever realized, enabling unprecedented studies of the distribution and motions of stars in the Milky Way and beyond.

- Many of the science missions are proving to be of great value to pursue investigations that were not foreseen at the time of their launch. Examples include the role of INTEGRAL and XMM-Newton in the follow-up of recent gravitational wave detections, paving the way for the future of multi-messenger astronomy, and the many discoveries of diverse exoplanets by Hubble.

- Collaboration between missions, including those led by partner agencies, is also of great importance. The interplay between solar missions like Hinode, IRIS and SOHO provides an extensive suite of complementary instruments to study our Sun; meanwhile, Mars Express and ExoMars TGO are at the forefront of the international fleet investigating the Red Planet.

- Another compelling factor to support the extension is the introduction of new modes of operation to accommodate the evolving needs of the scientific community, as well as new opportunities for scientists to get involved with the missions.

Table 3: Extended life for ESA's science missions 53)

• October 31, 2018: ESA's Gaia mission has made a major breakthrough in unravelling the formation history of the Milky Way. Instead of forming alone, our Galaxy merged with another large galaxy early in its life, around 10 billion years ago. The evidence is littered across the sky all around us, but it has taken Gaia and its extraordinary precision to show us what has been hiding in plain sight all along. 54) 55)

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Figure 38: Artist's impression of the merger between the Gaia-Enceladus galaxy and our Milky Way, which took place during our Galaxy's early formation stages, 10 billion years ago (image credit: ESA (artist's impression and composition); Koppelman, Villalobos and Helmi (simulation); NASA/ESA/Hubble (galaxy image), CC BY-SA 3.0 IGO)

Legend to Figure 38: Astronomers uncovered this major event in the formation history of the Milky Way after discovering an ‘odd collection' of stars that move along elongated trajectories in the opposite direction to the majority of the Galaxy's other hundred billion stars, including the Sun. The discovery was possible thanks to the second data release of ESA's Gaia mission and its extraordinary precision. The positions and motions of the stars in Gaia-Enceladus (represented with yellow arrows) in this early phase of the merger are based on a computer simulation that models a similar encounter to that uncovered by Gaia.

- Using the first 22 months of observations, a team of astronomers led by Amina Helmi, University of Groningen, The Netherlands, looked at seven million stars – those for which the full 3D positions and velocities are available – and found that some 30,000 of them were part of an ‘odd collection' moving through the Milky Way. The observed stars in particular are currently passing by our solar neighborhood.

- We are so deeply embedded in this collection that its stars surround us almost completely, and so can be seen across most of the sky.

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Figure 39: Debris of galactic merger: Artist's impression of debris of the Gaia-Enceladus galaxy. Gaia-Enceladus merged with our Milky Way galaxy during its early formation stages, 10 billion years ago, and its debris can now be found throughout the Galaxy (image credit: ESA (artist's impression and composition); Koppelman, Villalobos and Helmi (simulation), CC BY-SA 3.0 IGO) 56)

- Even though they are interspersed with other stars, the stars in the collection stood out in the Gaia data because they all move along elongated trajectories in the opposite direction to the majority of the Galaxy's other hundred billion stars, including the Sun.

- They also stood out in the so-called Hertzprung-Russell diagram – which is used to compare the color and brightness of stars – indicating that they belong to a clearly distinct stellar population.

- The sheer number of odd-moving stars involved intrigued Amina and her colleagues, who suspected they might have something to do with the Milky Way's formation history and set to work to understand their origins.

- In the past, Amina and her research group had used computer simulations to study what happens to stars when two large galaxies merge. When she compared those to the Gaia data, the simulated results matched the observations.

- "The collection of stars we found with Gaia has all the properties of what you would expect from the debris of a galactic merger," says Amina, lead author of the paper published today in Nature. 57)

- In other words, the collection is what they expected from stars that were once part of another galaxy and have been consumed by the Milky Way. The stars now form most of our Galaxy's inner halo – a diffuse component of old stars that were born at early times and now surround the main bulk of the Milky Way known as the central bulge and disc.

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Figure 40: An artist's impression of our Milky Way galaxy, a roughly 13 billion-year-old 'barred spiral galaxy' that is home to a few hundred billion stars (image credit: Left: NASA/JPL-Caltech; right: ESA; layout: ESA/ATG medialab)

Legend to Figure 40: On the left, a face-on view shows the spiral structure of the Galactic Disc, where the majority of stars are located, interspersed with a diffuse mixture of gas and cosmic dust. The disc measures about 100,000 light-years across, and the Sun sits about half way between its center and periphery. - On the right, an edge-on view reveals the flattened shape of the disc. Observations point to a substructure: a thin disc some 700 light-years high embedded in a thick disc, about 3000 light-years high and populated with older stars.
The edge on view also shows the Galactic Bulge, located in the central portion of the Milky Way and hosting about 10 billion stars, which are mainly old and red. The bulge, also visible in the face-on view on the left, has an overall elongated shape that resembles that of a peanut-shaped bar, with a half-length of about 10,000 light-years, making the Milky Way a barred spiral galaxy.
Beyond the disc and bulge is the stellar halo, a roughly spherical structure with a radius of about 100 000 light-years, containing isolated stars as well as many globular clusters – large, compact conglomerations of some of the most ancient stars in the Galaxy. On a grander scale, the Milky Way is embedded in an even larger halo of invisible dark matter.

- The Galactic disc itself is composed of two parts. There is the thin disc, which is a few hundred light years deep and contains the pattern of spiral arms made by bright stars. And there is the thick disc, which is a few thousand light years deep. It contains about 10–20 percent of the Galaxy's stars yet its origins have been difficult to determine.

- According to the team's simulations, as well as supplying the halo stars, the accreted galaxy could also have disturbed the Milky Way's pre-existing stars to help form the thick disc.

- "We became only certain about our interpretation after complementing the Gaia data with additional information about the chemical composition of stars, supplied by the ground-based APOGEE survey," says Carine Babusiaux, Université Grenoble Alpes, France, and second author of the paper.

- Stars that form in different galaxies have unique chemical compositions that match the conditions of the home galaxy. If this star collection was indeed the remains of a galaxy that merged with our own, the stars should show an imprint of this in their composition. And they did.

- The astronomers called this galaxy Gaia-Enceladus after one of the Giants in ancient Greek mythology, ­who was the offspring of Gaia, the Earth, and Uranus, the Sky.

- "According to the legend, Enceladus was buried under Mount Etna, in Sicily, and responsible for local earthquakes. Similarly, the stars of Gaia-Enceladus were deeply buried in the Gaia data, and they have shaken the Milky Way, leading to the formation of its thick disc," explains Amina.

- Even though no more evidence was really needed, the team also found hundreds of variable stars and 13 globular clusters in the Milky Way that follow similar trajectories as the stars from Gaia-Enceladus, indicating that they were originally part of that system. Globular clusters are groups of up to millions of stars, held together by their mutual gravity and orbiting the center of a galaxy. The fact that so many clusters could be linked to Gaia-Enceladus is another indication that this must have once been a big galaxy in its own right, with its own entourage of globular clusters.

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Figure 41: Gaia-Enceladus stars across the sky (image credit: ESA/Gaia/DPAC; A. Helmi et al. 2018)

Legend to Figure 41: All-sky distribution of an 'odd collection' of stars detected in the second data release of ESA's Gaia mission. These stars move along elongated trajectories in the opposite direction to the majority of our Milky Way's other hundred billion stars and have a markedly different chemical composition, indicating that they belong to a clearly distinct stellar population.
From these clues, astronomers inferred that these stars are the debris of a galaxy that merged with our Milky Way during its early formation stages, 10 billion years ago, and named this galaxy Gaia-Enceladus.
The stars of Gaia-Enceladus are represented with different colors depending on their parallax – a measure of their distance – with purple hues indicating the most nearby stars and yellow hues the most distant ones. White circles indicate globular clusters that were observed to follow similar trajectories as the stars from Gaia-Enceladus, indicating that they were originally part of that system; cyan star symbols indicate variable stars that are also associated as Gaia-Enceladus debris.

- Further analysis revealed that this galaxy was about the size of one of the Magellanic Clouds – two satellite galaxies roughly ten times smaller than the current size of the Milky Way.

- Ten billion years ago, however, when the merger with Gaia-Enceladus took place, the Milky Way itself was much smaller, so the ratio between the two was more like four to one. It was therefore clearly a major blow to our Galaxy.

- "Seeing that we are now starting to unravel the formation history of the Milky Way is very exciting," says Anthony Brown, Leiden University, The Netherlands, who is a co-author of the paper and also chair of the Gaia Data Processing and Analysis Consortium Executive.

- Since the very first discussions about building Gaia 25 years ago, one of the mission's key objectives was to examine the various stellar streams in the Milky Way, and reconstruct its early history. That vision is paying off.

- "Gaia was built to answer such questions," says Amina. "We can now say this is the way the Galaxy formed in those early epochs. It's fantastic. It's just so beautiful and makes you feel so big and so small at the same time."

- "By reading the motions of stars scattered across the sky, we are now able to rewind the history of the Milky Way and discover a major milestone in its formation, and this is possible thanks to Gaia," concludes Timo Prusti, Gaia project scientist at ESA.

• October 02, 2018: A team of astronomers using the latest set of data from ESA's Gaia mission to look for high-velocity stars being kicked out of the Milky Way were surprised to find stars instead sprinting inwards – perhaps from another galaxy. 58)

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Figure 42: The positions and reconstructed orbits of 20 high-velocity stars, represented on top of an artistic view of our Galaxy, the Milky Way. These stars were identified using data from the second release of ESA's Gaia mission. The seven stars shown in red are sprinting away from the Galaxy and could be travelling fast enough to eventually escape its gravity. Surprisingly, the study revealed also thirteen stars, shown in orange, that are racing towards the Milky Way: these could be stars from another galaxy, zooming right through our own [image credit: ESA (artist's impression and composition); Marchetti et al 2018 (star positions and trajectories); NASA/ESA/Hubble (background galaxies), CC BY-SA 3.0 IGO]

- In April, ESA's stellar surveyor Gaia released an unprecedented catalogue of more than one billion stars. Astronomers across the world have been working ceaselessly over the past few months to explore this extraordinary dataset, scrutinizing the properties and motions of stars in our Galaxy and beyond with never before achieved precision, giving rise to a multitude of new and intriguing studies.

- The Milky Way contains over a hundred billion stars. Most are located in a disk with a dense, bulging center, at the middle of which is a supermassive black hole. The rest are spread out in a much larger spherical halo.

- Stars circle around the Milky Way at hundreds of km/s, and their motions contain a wealth of information about the past history of the Galaxy. The fastest class of stars in our Galaxy are called hypervelocity stars, which are thought to start their life near the Galactic center to be later flung towards the edge of the Milky Way via interactions with the black hole.

- Only a small number of hypervelocity stars have ever been discovered, and Gaia's recently published second data release provides a unique opportunity to look for more of them.

- Several groups of astronomers jumped into the brand-new dataset in search of hypervelocity stars immediately after the release. Among them, three scientists at Leiden University, the Netherlands, were in for a big surprise.

- For 1.3 billion stars, Gaia measured positions, parallaxes – an indicator of their distance – and 2D motions on the plane of the sky. For seven million of the brightest ones, it also measured how quickly they move towards or away from us.

- "Of the seven million Gaia stars with full 3D velocity measurements, we found twenty that could be travelling fast enough to eventually escape from the Milky Way," explains Elena Maria Rossi, one of the authors of the new study. 59)

- Elena and colleagues, who had already discovered a handful of hypervelocity stars last year in an exploratory study based on data from Gaia's first release, were pleasantly surprised, as they were hoping to find at most one star breaking loose from the Galaxy among these seven million. And there is more.

- "Rather than flying away from the Galactic center, most of the high velocity stars we spotted seem to be racing towards it," adds co-author Tommaso Marchetti. — "These could be stars from another galaxy, zooming right through the Milky Way."

- It is possible that these intergalactic interlopers come from the Large Magellanic Cloud, a relatively small galaxy orbiting the Milky Way, or they may originate from a galaxy even further afield.

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Figure 43: The Large Magellanic Cloud (LMC), one of the nearest galaxies to our Milky Way, as viewed by ESA's Gaia satellite using information from the mission's second data release. This view is not a photograph but has been compiled by mapping the total amount of radiation detected by Gaia in each pixel, combined with measurements of the radiation taken through different filters on the spacecraft to generate color information [image credit: ESA/Gaia/DPAC (Data Processing and Analysis Consortium); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal]

Legend to Figure 43: The image is dominated by the brightest, most massive stars, which greatly outshine their fainter, lower-mass counterparts. In this view, the bar of the LMC is outlined in great detail, along with individual regions of star formation like the giant 30 Doradus, visible just above the center of the galaxy.

- If that is the case, they carry the imprint of their site of origin, and studying them at much closer distances than their parent galaxy could provide unprecedented information on the nature of stars in another galaxy – similar in a way to studying martian material brought to our planet by meteorites.

- "Stars can be accelerated to high velocities when they interact with a supermassive black hole," Elena explains. "So the presence of these stars might be a sign of such black holes in nearby galaxies. But the stars may also have once been part of a binary system, flung towards the Milky Way when their companion star exploded as a supernova. Either way, studying them could tell us more about these kinds of processes in nearby galaxies."

- An alternative explanation is that the newly identified sprinting stars could be native to our Galaxy's halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way during its build-up history. Additional information about the age and composition of the stars could help the astronomers clarify their origin.

- "A star from the Milky Way halo is likely to be fairly old and mostly made of hydrogen, whereas stars from other galaxies could contain lots of heavier elements," says Tommaso. - "Looking at the colors of stars tells us more about what they are made of."

- New data will help nail down the nature and origin of these stars with more certainty, and the team will use ground-based telescopes to find out more about them. In the meantime, Gaia continues to make observations of the full sky, including the stars analyzed in this study.

- While investigating the nature of these possible stellar interlopers, the team is also busy digging into the full dataset from Gaia's second release, searching for more high-speed stars and looking forward to the future. At least two more Gaia data releases are planned in the 2020s, and each will provide both more precise and new information on a larger set of stars.

- "We eventually expect full 3D velocity measurements for up to 150 million stars," explains co-author Anthony Brown, chair of the Gaia Data Processing and Analysis Consortium Executive. "This will help find hundreds or thousands of hypervelocity stars, understand their origin in much more detail, and use them to investigate the Galactic center environment as well as the history of our Galaxy," he adds.

- "This exciting result shows that Gaia is a true discovery machine, providing the ground for completely unexpected discoveries about our Galaxy," concludes Timo Prusti, Gaia project scientist at ESA (Ref. 58).

• September 25, 2018: Using data from ESA's Gaia stellar surveyor, astronomers have identified four stars that are possible places of origin of ‘Oumuamua, an interstellar object spotted during a brief visit to our Solar System in 2017. 60)

- The discovery last year sparked a large observational campaign: originally identified as the first known interstellar asteroid, the small body was later revealed to be a comet, as further observations showed it was not slowing down as fast as it should have under gravity alone. The most likely explanation of the tiny variations recorded in its trajectory was that they are caused by gasses emanating from its surface, making it more akin to a comet.

- But where in the Milky Way did this cosmic traveller come from?

- Comets are leftovers of the formation of planetary systems, and it is possible that ‘Oumuamua was ejected from its home star's realm while planets were still taking shape there. To look for its home, astronomers had to trace back in time not only the trajectory of the interstellar comet, but also of a selection of stars that might have crossed paths with this object in the past few million years.

- "Gaia is a powerful time machine for these types of studies, as it provides not only star positions but also their motions," explains Timo Prusti, Gaia project scientist at ESA.

- To this aim, a team of astronomers led by Coryn Bailer-Jones at the Max Planck Institute for Astronomy in Heidelberg, Germany, dived into the data from Gaia's second release, which was made public in April. 61)

- The Gaia data contain positions, distance indicators and motions on the sky for more than a billion stars in our Galaxy; most importantly, the data set includes radial velocities – how fast they are moving towards or away from us – for a subset of seven million, enabling a full reconstruction of their trajectories. The team looked at these seven million stars, complemented with an extra 220,000 for which radial velocities are available from the astronomical literature.

- As a result, Coryn and colleagues identified four stars whose orbits had come within a couple of light years of ‘Oumuamua in the near past, and with relative velocities low enough to be compatible with likely ejection mechanisms.

- All four are dwarf stars – with masses similar to or smaller than our Sun's – and had their ‘close' encounter with the interstellar comet between one and seven million years ago. However, none of them is known to either harbor planets or to be part of a binary stellar system; a giant planet or companion star would be the preferred mechanism to have ejected the small body.

- While future observations of these four stars might shed new light on their properties and potential to be the home system of ‘Oumuamua, the astronomers are also looking forward to future releases of Gaia data. At least two are planned in the 2020s, which will include a much larger sample of radial velocities, enabling them to reconstruct and investigate the trajectories of many more stars.

- "While it's still early to pinpoint ‘Oumuamua's home star, this result illustrates the power of Gaia to delve into the history of our Milky Way galaxy," concludes Timo.

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Figure 44: Artist impression of the interstellar object ‘Oumuamua. Observations since its discovery in 2017 show that the object is slightly deviating from the trajectory it would be following if it were only influenced by the gravity of the Sun and the planets. Researchers assume that venting material from its surface due to solar heating is responsible for this behavior. This outgassing can be seen in this artist's impression as a subtle cloud being ejected from the side of the object facing the Sun (image credit: ESA/Hubble, NASA, ESO, M. Kornmesser)

• September 19, 2018: ESA's star mapping mission, Gaia, has shown our Milky Way galaxy is still enduring the effects of a near collision that set millions of stars moving like ripples on a pond. The close encounter likely took place sometime in the past 300–900 million years. It was discovered because of the pattern of movement it has given to stars in the Milky Way disk – one of the major components of our Galaxy. 62)

- The pattern was revealed because Gaia not only accurately measures the positions of more than a billion stars but also precisely measures their velocities on the plane of the sky. For a subset of a few million stars, Gaia provided an estimate of the full three-dimensional velocities, allowing a study of stellar motion using the combination of position and velocity, which is known as ‘phase space'.

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Figure 45: Artist's impression of a perturbation in the velocities of stars in our Galaxy, the Milky Way, that was revealed by ESA's star mapping mission, Gaia. Scientists analyzing data from Gaia's second release have shown our Milky Way galaxy is still enduring the effects of a near collision that set millions of stars moving like ripples on a pond (image credit: ESA, CC BY-SA 3.0 IGO)

- In phase space, the stellar motions revealed an interesting and totally unexpected pattern when the star's positions were plotted against their velocities. Teresa Antoja from Universitat de Barcelona, Spain, who led the research couldn't quite believe her eyes when she first saw it on her computer screen.

- One shape in particular caught her attention. It was a snail shell-like pattern in the graph that plotted the stars' altitude above or below the plane of the Galaxy against their velocity in the same direction. It had never been seen before. "At the beginning the features were very weird to us," says Teresa. "I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear."

Figure 46: Snail shell pattern in the velocity of stars. This graph shows the altitude of stars in our Galaxy above or below the plane of the Milky Way against their velocity in the same direction, based on a simulation of a near collision that set millions of stars moving like ripples on a pond (image credit: T. Antoja et al. 2018)

Legend to Figure 46: The snail shell-like shape of the pattern reproduces a feature that was first seen in the movement of stars in the Milky Way disk using data from the second release of ESA's Gaia mission, and interpreted as an imprint of a galactic encounter. - The close encounter revealed by the Gaia data likely took place sometime in the past 300–900 million years, and the culprit could be the Sagittarius dwarf galaxy, a small galaxy containing a few tens of millions of stars that is currently in the process of being cannibalized by the Milky Way.

- But the Gaia data had undergone multiple validation tests by the Gaia Data Processing and Analysis Consortium teams all over Europe before release. Also, together with collaborators, Teresa had performed many tests on the data to look for errors that could be forcing such shapes on the data. Yet no matter what they checked, the only conclusion they could draw was that these features do indeed exist in reality.

- The reason they had not been seen before was because the quality of the Gaia data was a huge step up from what had come before. "It looks like suddenly you have put the right glasses on and you see all the things that were not possible to see before," says Teresa.

- With the reality of the structure confirmed, it came time to investigate why it was there. "It is a bit like throwing a stone in a pond, which displaces the water as ripples and waves," explains Teresa.

- Unlike the water molecules, which settle again, the stars retain a ‘memory' that they were perturbed. This memory is found in their motions. After some time, although the ripples may no longer be easily visible in the distribution of stars, they are still there when you look in their velocities.

- The researchers looked up previous studies that had investigated such ‘phase mixing' in other astrophysical settings and in quantum physics situations. Although no one had investigated this happening in the disk of our Galaxy, the structures were clearly reminiscent of each other.

- "I find this really amazing that we can see this snail shell shape. It is just like it appears in text books," says Amina Helmi, University of Groningen, The Netherlands, a collaborator on the project and the second author on the resulting paper. 63)

- So the next question was what had ‘hit' the Milky Way to cause this behavior in the stars. We know that our Galaxy is a cannibal. It grows by eating smaller galaxies and clusters of stars that then mix in with the rest of the Galaxy. But that didn't seem to be the case here.

- Then Amina recalled her own and others studies of the Sagittarius dwarf galaxy. This small galaxy contains a few tens of millions of stars and is currently in the process of being cannibalized by the Milky Way.

- Its last close pass to our Galaxy was not a direct hit – it passed close by. This would have been enough so that its gravity perturbed some stars in our Galaxy like a stone dropping into water.

- The clincher was that estimates of Sagittarius's last close encounter with the Milky Way place it sometime between 200 and 1000 million years ago, which is almost exactly what Teresa and colleagues calculated as an origin for the beginning of the snail shell-like pattern.

- So far, however, the association of the snail shell feature with the Sagittarius dwarf galaxy is based on simple computer models and analyses. The next step is to scrutinize the phenomenon more fully to gain knowledge of the Milky Way.

- The scientists plan to investigate this galactic encounter as well as the distribution of matter in the Milky Way by using the information contained in the snail shell shape. One thing is certain. There is a lot of work to do.

- "The discovery was easy; the interpretations harder. And the full understanding of its meaning and implications might take several years." said Amina.

- Gaia is one of ESA's cornerstone missions and was designed primarily to investigate the origin, evolution and structure of the Milky Way. In April, it made available its second data release, which is the data that made this discovery possible.

- "This is exactly the kind of discovery we hoped would come from the Gaia data," says Timo Prusti, Gaia project scientist at ESA. "The Milky Way has a rich history to tell, and we are starting to read that story."

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Figure 47: The Sagittarius dwarf galaxy in Gaia's all-sky view. The Sagittarius dwarf galaxy, a small satellite of the Milky Way that is leaving a stream of stars behind as an effect of our Galaxy's gravitational tug, is visible as an elongated feature below the Galactic center and pointing in the downwards direction in the all-sky map of the density of stars observed by ESA's Gaia mission between July 2014 to May 2016 (image credit: ESA/Gaia/DPAC)

• August 20,2018: The mass of a very young exoplanet has been revealed for the first time using data from ESA's star mapping spacecraft Gaia and its predecessor, the quarter-century retired Hipparcos satellite. 64)

- Astronomers Ignas Snellen and Anthony Brown from Leiden University, the Netherlands, deduced the mass of the planet Beta Pictoris b from the motion of its host star over a long period of time as captured by both Gaia and Hipparcos.

- The planet is a gas giant similar to Jupiter but, according to the new estimate, is 9 to 13 times more massive. It orbits the star Beta Pictoris, the second brightest star in the constellation Pictor.

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Figure 48: The planet Beta Pictoris b is visible orbiting its host star in this composite image from the European Southern Observatory's (ESO) 3.6 m telescope and the NACO (Nasmyth Adaptive+CONICA Optics) instrument on ESO's 8.2-m VLT (Very Large Telescope). The Beta Pictoris system is only about 20 million years old, roughly 225 times younger than the Solar System. Observing this dynamic and rapidly evolving system can help astronomers shed light on the processes of planet formation and early evolution (image credit: ESO/A-M. Lagrange et al.)

- The planet was only discovered in 2008 in images captured by the VLT (Very Large Telescope) at the European Southern Observatory in Chile. Both the planet and the star are only about 20 million years old – roughly 225 times younger than the Solar System. Its young age makes the system intriguing but also difficult to study using conventional methods.

- "In the Beta Pictoris system, the planet has essentially just formed," says Ignas. "Therefore we can get a picture of how planets form and how they behave in the early stages of their evolution. On the other hand, the star is very hot, rotates fast, and it pulsates."

- This behavior makes it difficult for astronomers to accurately measure the star's radial velocity – the speed at which it appears to periodically move towards and away from the Earth. Tiny changes in the radial velocity of a star, caused by the gravitational pull of planets in its vicinity, are commonly used to estimate masses of exoplanets. But this method mainly works for systems that have already gone through the fiery early stages of their evolution.

- In the case of Beta Pictoris b, upper limits of the planet's mass range had been arrived at before using the radial velocity method. To obtain a better estimate, the astronomers used a different method, taking advantage of Hipparcos' and Gaia's measurements that reveal the precise position and motion of the planet's host star in the sky over time.

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Figure 49: Astronomers can measure the mass of exoplanets by looking at tiny deviations in the trajectories of their host stars caused by the gravitational pull of the orbiting planets. These can be observed either along the line of sight, looking for small changes in a star's radial velocity, or on the plane of the sky, using astrometric measurements. To be able to make accurate assessments, the astrometric observations need to cover a period of many years. In this picture, the white dashed spiral shows the evolution of a star's trajectory observable from the Earth, caused by the combination of parallax and proper motion. The brown band shows the range of deviations of the star's trajectory caused by a possible planet orbiting it (image credit: ESA) 65)

- "The star moves for different reasons," says Ignas. "First, the star circles around the center of the Milky Way, just as the Sun does. That appears from the Earth as a linear motion projected on the sky. We call it proper motion. And then there is the parallax effect, which is caused by the Earth orbiting around the Sun. Because of this, over the year, we see the star from slightly different angles."

- And then there is something that the astronomers describe as ‘tiny wobbles' in the trajectory of the star across the sky – minuscule deviations from the expected course caused by the gravitational pull of the planet in the star's orbit. This is the same wobble that can be measured via changes in the radial velocity, but along a different direction – on the plane of the sky, rather than along the line of sight.

- "We are looking at the deviation from what you expect if there was no planet and then we measure the mass of the planet from the significance of this deviation," says Anthony. "The more massive the planet, the more significant the deviation."

- To be able to make such an assessment, astronomers need to observe the trajectory of the star for a long period of time to properly understand the proper motion and the parallax effect.

- The Gaia mission, designed to observe more than one billion stars in our Galaxy, will eventually be able to provide information about a large amount of exoplanets. In the 22 months of observations included in Gaia's second data release, published in April, the satellite has recorded the star Beta Pictoris about thirty times. That, however, is not enough.

- "Gaia will find thousands of exoplanets, that's still on our to-do list," says Timo Prusti, ESA's Gaia project scientist. "The reason that the exoplanets can be expected only late in the mission is the fact that to measure the tiny wobble that the exoplanets are causing, we need to trace the position of stars for several years."

- Combining the Gaia measurements with those from ESA's Hipparcos mission, which observed Beta Pictoris 111 times between 1990 and 1993, enabled Ignas and Anthony to get their result much faster. This led to the first successful estimate of a young planet's mass using astrometric measurements.

- "By combining data from Hipparcos and Gaia, which have a time difference of about 25 years, you get a very long term proper motion," says Anthony. "This proper motion also contains the component caused by the orbiting planet. Hipparcos on its own would not have been able to find this planet because it would look like a perfectly normal single star unless we had measured it for a much longer time. Now, by combining Gaia and Hipparcos and looking at the difference in the long term and the short term proper motion, we can see the effect of the planet on the star."

- The result represents an important step towards better understanding the processes involved in planet formation, and anticipates the exciting exoplanet discoveries that will be unleashed by Gaia's future data releases. 66)

• August 20, 2018: The second data release of ESA's Gaia mission, made in April, has marked a turning point in the study of our Galactic home, the Milky Way. With an unprecedented catalog of 3D positions and 2D motions of more than a billion stars, plus additional information on smaller subsets of stars and other celestial sources, Gaia has provided astronomers with an astonishing resource to explore the distribution and composition of the Galaxy and to investigate its past and future evolution. 67)

- The majority of stars in the Milky Way are located in the Galactic disc, which has a flattened shape characterized by a pattern of spiral arms similar to that observed in spiral galaxies beyond our own. However, it is particularly challenging to reconstruct the distribution of stars in the disc, and especially the design of the Milky Way's arms, because of our position within the disc itself. — This is where Gaia's measurements can make the difference.

- The image of Figure 50 shows a 3D map obtained by focusing on one particular type of object: OB stars, the hottest, brightest and most massive stars in our Galaxy. Because these stars have relatively short lives – up to a few tens of million years – they are mostly found close to their formation sites in the Galactic disc. As such, they can be used to trace the overall distribution of young stars, star formation sites, and the Galaxy's spiral arms.

- The map, based on 400,000 of this type of star within less than 10,000 light-years from the Sun, was created by Kevin Jardine, a software developer and amateur astronomer with an interest in mapping the Milky Way using a variety of astronomical data.

- It is centered on the Sun and shows the Galactic disc as if we were looking at it face-on from a vantage point outside the Galaxy.

- To deal with the massive number of stars in the Gaia catalog, Kevin made use of so-called density isosurfaces, a technique that is routinely used in many practical applications, for example to visualize the tissue of organs of bones in CT (Computer Tomography) scans of the human body. In this technique, the 3D distribution of individual points is represented in terms of one or more smooth surfaces that delimit regions with a different density of points.

- Here, regions of the Galactic disc are shown with different colors depending on the density of ionizing stars recorded by Gaia; these are the hottest among OB stars, shining with ultraviolet radiation that knocks electrons off hydrogen atoms to give them their ionized state.

- The regions with the highest density of these stars are displayed in pink/purple shades, regions with intermediate density in violet/light blue, and low-density regions in dark blue. Additional information from other astronomical surveys was also used to map concentrations of interstellar dust, shown in green, while known clouds of ionized gas are depicted as red spheres.

- The appearance of ‘spokes' is a combination of dust clouds blocking the view to stars behind them and a stretching effect of the distribution of stars along the line of sight.

- An interactive version of this map is also available as part of Gaia Sky, a real-time, 3D astronomy visualization software that was developed in the framework of the Gaia mission at the Astronomisches Rechen-Institut, University of Heidelberg, Germany.

- Further details including annotated version of the map: Mapping and visualizing Gaia DR2.

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Figure 50: Star density map, showing the 3D distribution of the most massive stars in our Galactic neighborhood, based on the latest data from ESA's Gaia mission (image credit: Galaxy Map / K. Jardine)

• April 25, 2018: Gaia's second release of the star catalog. ESA's Gaia mission has produced the richest star catalog to date, including high-precision measurements of nearly 1.7 billion stars and revealing previously unseen details of our home Galaxy. A multitude of discoveries are on the horizon after this much awaited release, which is based on 22 months of charting the sky. The new data includes positions, distance indicators and motions of more than one billion stars, along with high-precision measurements of asteroids within our Solar System and stars beyond our own Milky Way Galaxy. 68) 69)

Note 1: A series of scientific papers describing the data contained in the release and their validation process will appear in a special issue of Astronomy & Astrophysics.

Note 2: A series of 360-degree videos and other Virtual Reality visualization resources are available at http://sci.esa.int/gaia-vr

- Preliminary analysis of this phenomenal data reveals fine details about the make-up of the Milky Way's stellar population and about how stars move, essential information for investigating the formation and evolution of our home Galaxy. -"The observations collected by Gaia are redefining the foundations of astronomy," says Günther Hasinger, ESA Director of Science.

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Figure 51: Gaia's all-sky view of our Milky Way Galaxy and neighboring galaxies, based on measurements of nearly 1.7 billion stars. The map shows the total brightness and color of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016 (image credit: ESA/Gaia/DPAC) 70)
Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal.

Legend to Figure 51: Brighter regions indicate denser concentrations of especially bright stars, while darker regions correspond to patches of the sky where fewer bright stars are observed. The color representation is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky.

The bright horizontal structure that dominates the image is the Galactic plane, the flattened disc that hosts most of the stars in our home Galaxy. In the middle of the image, the Galactic center appears vivid and teeming with stars. Darker regions across the Galactic plane correspond to foreground clouds of interstellar gas and dust, which absorb the light of stars located further away, behind the clouds. Many of these conceal stellar nurseries where new generations of stars are being born.

Sprinkled across the image are also many globular and open clusters – groupings of stars held together by their mutual gravity, as well as entire galaxies beyond our own.

The two bright objects in the lower right of the image are the Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way.

In small areas of the image where no color information was available – to the lower left of the Galactic center, to the upper left of the Small Magellanic Cloud, and in the top portion of the map – an equivalent greyscale value was assigned.

The second Gaia data release was made public on 25 April 2018 and includes the position and brightness of almost 1.7 billion stars, and the parallax, proper motion and color of more than 1.3 billion stars. It also includes the radial velocity of more than seven million stars, the surface temperature of more than 100 million stars, and the amount of dust intervening between us and of 87 million stars. There are also more than 500,000 variable sources, and the position of 14,099 known Solar System objects – most of them asteroids – included in the release.

A complementary image showing Gaia's density map of the stars is available here.

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Figure 52: Gaia's all-sky view of our Milky Way Galaxy and neighboring galaxies. The maps show the total brightness and color of stars (top), the total density of stars (middle) and the interstellar dust that fills the Galaxy (bottom). These images are based on observations performed by the ESA satellite in each portion of the sky between July 2014 and May 2016, which were published as part of Gaia second data release on 25 April 2018 (image credit:ESA/Gaia/DPAC). 71)
Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); Top and middle: A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal;Bottom: Gaia Coordination Unit 8; M. Fouesneau / C. Bailer-Jones, Max Planck Institute for Astronomy, Heidelberg, Germany.

- "Gaia is an ambitious mission that relies on a huge human collaboration to make sense of a large volume of highly complex data. It demonstrates the need for long-term projects to guarantee progress in space science and technology and to implement even more daring scientific missions of the coming decades."

- Gaia was launched in December 2013 and started science operations the following year. The first data release, based on just over one year of observations, was published in 2016; it contained distances and motions of two million stars.

- The new data release, which covers the period between 25 July 2014 and 23 May 2016, pins down the positions of nearly 1.7 billion stars, and with a much greater precision. For some of the brightest stars in the survey, the level of precision equates to Earth-bound observers being able to spot a Euro coin lying on the surface of the Moon.

- The new catalog lists the parallax and velocity across the sky, or proper motion, for more than 1.3 billion stars. From the most accurate parallax measurements, about ten per cent of the total, astronomers can directly estimate distances to individual stars.

- "The second Gaia data release represents a huge leap forward with respect to ESA's Hipparcos satellite, Gaia's predecessor and the first space mission for astrometry, which surveyed some 118 000 stars almost thirty years ago," says Anthony Brown of Leiden University, The Netherlands. Anthony is the chair of the Gaia Data Processing and Analysis Consortium Executive, overseeing the large collaboration of about 450 scientists and software engineers entrusted with the task of creating the Gaia catalogue from the satellite data.

- As well as positions, the data include brightness information of all surveyed stars and color measurements of nearly all, plus information on how the brightness and color of half a million variable stars change over time. It also contains the velocities along the line of sight of a subset of seven million stars, the surface temperatures of about a hundred million and the effect of interstellar dust on 87 million.

- Gaia also observes objects in our Solar System: the second data release comprises the positions of more than 14 000 known asteroids, which allows precise determination of their orbits. A much larger asteroid sample will be compiled in Gaia's future releases.

- Further afield, Gaia closed in on the positions of half a million distant quasars, bright galaxies powered by the activity of the supermassive black holes at their cores. These sources are used to define a reference frame for the celestial coordinates of all objects in the Gaia catalog, something that is routinely done in radio waves but now for the first time is also available at optical wavelengths.

- Major discoveries are expected to come once scientists start exploring Gaia's new release. An initial examination performed by the data consortium to validate the quality of the catalog has already unveiled some promising surprises – including new insights on the evolution of stars.

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Figure 53: The second data release from ESA's Gaia mission contains a high-precision catalog of the entire sky, covering celestial objects near and far. It includes objects such as asteroids in our Solar System as well as the stellar population of our Milky Way Galaxy and its satellites – globular clusters and nearby galaxies. It also extends to distant quasars that are being used to define a new celestial reference system. -This infographic summarizes the cosmic scales covered by this comprehensive dataset, which provides a wide range of topics for the astronomy community (image credit: ESA, CC BY-SA 3.0 IGO) 72)

Galactic archeology (Ref. 68)

- "The new Gaia data are so powerful that exciting results are just jumping at us," says Antonella Vallenari from the Istituto Nazionale di Astrofisica (INAF) and the Astronomical Observatory of Padua, Italy, deputy chair of the data processing consortium executive board.

- "For example, we have built the most detailed Hertzsprung-Russell diagram of stars ever made on the full sky and we can already spot some interesting trends. It feels like we are inaugurating a new era of Galactic archeology."

- Hertzsprung-Russell diagram: Named after the two astronomers who devised it in the early twentieth century, the Hertzsprung-Russell diagram compares the intrinsic brightness of stars with their color and is a fundamental tool to study populations of stars and their evolution.

- A new version of this diagram, based on four million stars within five thousand light-years from the Sun selected from the Gaia catalog, reveals many fine details for the first time. This includes the signature of different types of white dwarfs – the dead remnants of stars like our Sun – such that a differentiation can be made between those with hydrogen-rich cores and those dominated by helium.

- Combined with Gaia measurements of star velocities, the diagram enables astronomers to distinguish between various populations of stars of different ages that are located in different regions of the Milky Way, such as the disc and the halo, and that formed in different ways. Further scrutiny suggests that the fast-moving stars thought to belong to the halo encompass two stellar populations that originated via two different formation scenarios, calling for more detailed investigations.

- "Gaia will greatly advance our understanding of the Universe on all cosmic scales," says Timo Prusti, Gaia project scientist at ESA. "Even in the neighborhood of the Sun, which is the region we thought we understood best, Gaia is revealing new and exciting features."

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Figure 54: More than four million stars within five thousand light-years from the Sun are plotted on this diagram using information about their brightness, color and distance from the second data release from ESA's Gaia satellite. It is known as a Hertzsprung-Russell diagram after the astronomers who devised it in the early 20th century, and it is a fundamental tool to study populations of stars and their evolution (image credit: ESA/Gaia/DPAC) 73)
Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); Carine Babusiaux, IPAG – Université Grenoble Alpes, GEPI – Observatoire de Paris, France.

Legend to Figure 54: This Hertzsprung-Russell diagram, obtained by a selection of stars in Gaia's second release catalog, is the most detailed to date made by mapping stars over the entire sky, containing roughly a hundred times more stars than the one obtained using data from ESA's Hipparcos mission, the predecessor of Gaia, in the 1990s. This new diagram contains so much highly accurate information that astronomers have been able to identify fine details that were never before seen.

The Hertzsprung-Russell diagram can be imagined as a stellar family portrait: stars are plotted according to their color (on the horizontal axis) and brightness (on the vertical axis) and are grouped in different regions of the diagram depending mainly on their masses, chemical composition, ages, and stages in the stellar life cycle. Information about stellar distances is fundamental to calculate the true brightness, or absolute magnitude, of stars.

Brighter stars are shown in the top part of the diagram, while fainter stars are in the lower part. Bluer stars, which have hotter surfaces, are on the left, and redder stars, with cooler surfaces, on the right. The color scale in this image does not represent the color of stars but is a representation of how many stars are plotted in each portion of the diagram: black represents lower numbers of stars, while red, orange and yellow correspond to increasingly higher numbers of stars.

The large diagonal stripe across the center of the graph is known as the main sequence. This is where fully-fledged stars that are generating energy by fusing hydrogen into helium are found. Massive stars, which have bluer or whiter colors, are found in the upper left end of the main sequence, while intermediate-mass stars like our Sun, characterized by yellow colors, are located mid-way. Redder, low-mass stars are found towards the lower right.

As stars age they swell up, becoming brighter and redder. Stars experiencing this are shown on the diagram as the vertical arm leading off the main sequence and turning to the right. This is known as the red giant branch.

While the most massive stars swell into red giants and explode as powerful supernovae, stars like our Sun end their days in a less spectacular fashion, eventually turning into white dwarfs – the hot cores of dead stars. These are found in the lower left of the diagram.

The huge leap forward from Hipparcos to Gaia is especially visible in the white dwarf region of the diagram. While Hipparcos had obtained reliable distance measurements to only a handful of white dwarfs, more than 35,000 such objects are included in this diagram based on Gaia data. This allows astronomers to see the signature of different types of white dwarfs such that a differentiation can be made between those with hydrogen-rich cores and those dominated by helium.

Galaxy in 3D (Ref. 68)

- For a subset of stars within a few thousand light-years of the Sun, Gaia has measured the velocity in all three dimensions, revealing patterns in the motions of stars that are orbiting the Galaxy at similar speeds.

- Future studies will confirm whether these patterns are linked to perturbations produced by the Galactic bar, a denser concentration of stars with an elongated shape at the center of the Galaxy, by the spiral arm architecture of the Milky Way, or by the interaction with smaller galaxies that merged with it billions of years ago.

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Figure 55: The Large Magellanic Cloud (LMC), one of the nearest galaxies to our Milky Way, as viewed by ESA's Gaia satellite using information from the mission's second data release (image credit: ESA/Gaia/DPAC) 74)
Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); P. McMillan, Lund Observatory, Sweden; A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal

Legend to Figure 55: This image combines the total amount of radiation detected by Gaia in each pixel with measurements of the radiation taken through different filters on the spacecraft to generate color information. Information about the proper motion of stars – their velocity across the sky – is represented as the texture of the image.

Measuring the proper motion of several million stars in the LCM, astronomers were able to see an imprint of the stars rotating clockwise around the center of the galaxy. The image processing technique used to create this image is called Line Integral Convolution.

• April 3, 2018: What is the first creature that comes to mind when you look at the dark cloud in this image of Figure 56? Perhaps a dark kitten with a vivid white nose, front paws stretching towards the right of the frame and tail up towards the left? Or perhaps a fox, running with its mouth open and looking ahead, its vigilant eyes pointing to the right? 75) 76)

- In fact, this animal-themed shape belongs to a dark nebula, a dense cloud of gas and dust in the constellation of Orion, the Hunter, with the cat's nose (or fox's eye) corresponding to the Orion Nebula Cluster, a star cluster near the famous Orion Nebula, M42. The image is based on data from the first release of ESA's Gaia satellite, and shows the density of stars observed while scanning that region of the sky.

- While this particular nebula is not visible to the naked eye, similar clouds can be seen against the bright background of the Milky Way from dark locations in the southern hemisphere. Finding shapes in these dark nebulas is part of the astronomical tradition of various cultures, from South America to Australia, that include ‘dark cloud constellations' resembling a variety of creatures in their firmaments.

- Launched in 2013, Gaia has been charting more than a billion stars to unprecedented accuracy. This information is extremely valuable to astronomers who are studying the distribution of stars across our Galaxy.

- Even in the dark patches where fewer stars are observed, Gaia's meticulous census provides important information to study the interstellar material that blocks starlight. It is in these dark clouds of gas and dust that new generations of stars come to life.

- The first data release from Gaia, published in 2016, contained the position on the sky of more than a billion stars, as well as the distance and motions of about two million stars. Astronomers worldwide are now looking forward to the next data release, planned for 25 April, which will include the distance and motions for the full sample of stars, greatly extending the reach of the previous survey.

- So far, Gaia data have been used to study only the most nearby regions of star formation, within several hundred light-years of us. With the new data, it will be possible to investigate in great detail regions that are much farther away, like the Orion star-forming complex, located some 1500 light-years from us, and to estimate the 3D distribution not only of stars but also of the dusty dark clouds where stars are born.

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Figure 56: Space Science Image of the Week: Some interstellar clouds take on interesting shapes, like in this animal-themed Gaia image of the Orion constellation (image credit: ESA/Gaia/DPAC)

• March 21, 2018: Last month, ESA's Gaia satellite experienced a technical anomaly followed by a 'safe mode' event. After thorough examination, the spacecraft was successfully recovered and resumed normal scientific operations, while the mission team keeps investigating the exact cause of the anomaly. 77)

- On 18 February, errors of two electrical units on the service module of Gaia led the spacecraft to trigger an automatic safe mode. Safe modes occur when certain spacecraft parameters fall out of their normal operating ranges and the spacecraft automatically takes measures to preserve its safety. During this safe mode, the science instruments were disabled in order to protect them, and telecommunication with Earth took place through the spacecraft's low-gain antenna.

- Following the anomaly, the mission team conducted an initial inquiry into what caused the spacecraft to activate the safe mode. They quickly identified the problem as deriving from a failure in one of the two transponders on board Gaia, but the root cause of the malfunction is still being investigated. After an in-depth inquiry, the team recovered the satellite, which went back to its normal scientific operations on 28 February using the second identical back-up transponder.

- The team is still investigating the origin of the anomaly and its possible relation to the lifetime of the second transponder. Meanwhile, Gaia has been collecting data since it resumed operations at the end of last month.

- Scientists worldwide are looking forward to the second data release of Gaia, which will take place on 25 April and is based on observations performed between mid 2014 and mid 2016. The mission has already collected all data needed for its third release; these data will be processed and analyzed over the next few years.

• February 27, 2018: Last year, ESA's Gaia mission helped astronomers make unique observations of Neptune's largest moon, Triton, as it passed in front of a distant star. This is a preview of the superb quality and versatility of the Gaia data that will be released in April. 78)

- When a small Solar System body such as a moon or an asteroid passes in front of a star and temporarily blocks its light, the occultation is an extraordinary chance for astronomers to study the properties of the foreground object. And, of course, the more accurate the prediction of both objects' positions on the sky, the better the observations.

- This is why, when a group of astronomers were planning to observe the rare occultation of a distant star by Neptune's moon Triton on 5 October 2017, they made a special request to the Gaia team.

- The astronomers, led by Bruno Sicardy from Pierre and Marie Curie University and the Observatory of Paris, France, had used all available observations to compute the path that the moon's shadow would sweep across our planet. Within less than three minutes, the occultation would first cross Europe and North Africa, rapidly moving towards North America.

- They knew that somewhere, within this couple of thousand kilometer-wide stretch, would lie a very special thin strip, only about 100-km across. Observers situated on this strip would be perfectly aligned with both Triton and the distant star, and therefore able to see the so-called central flash.

- This sharp brightening of the star happens half way through the occultation, and is caused by focussing of the starlight by deep layers in the moon's atmosphere – about 10 km above surface. The central flash contains all-important information to study the profile of Triton's atmosphere and the possible presence of haze in it.

- To narrow down the best locations to observe the occultation, and possibly the flash, the astronomers turned to Gaia and its unprecedentedly accurate measurements of the positions of more than a billion stars.

- They obtained a first estimate using the star's position from the first batch of Gaia data (Gaia DR1), which were publicly released in 2016. However, knowledge of the star's proper motion – how it moves across the sky over the years – would substantially improve their estimate.

- So they approached the Gaia team, asking to receive extra information about the star's motion across the sky from the upcoming second release of Gaia data (Gaia DR2), which is planned for 25 April 2018.

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Figure 57: Improved estimate of the ground trace of Triton occultation (image credit: Google, INEGI, ORION-ME; annotation: ERC Lucky Star project)

- Having recognized the importance of these observations, the Gaia team published not only the preliminary position and proper motion of the occulted star from DR2, but also the positions of 453 other stars that could be used to refine the estimate of Triton's orbit. With this additional information, they computed again the location of the thin strip where the central flash would be observed, shifting it roughly 300 km southwards of the earlier prediction.

- Come 5 October, a large collaboration of professional as well as amateur astronomers scattered across three continents were ready to observe Triton's occultation at more than a hundred stations.

- Almost eighty of them were able to monitor the phenomenon, and the improved prediction of observing locations based on the specially released, preliminary data from Gaia DR2 led to twenty-five successful detections of the central flash, from Spain and Portugal to the south of France and the north of Italy. The astronomers are now busy analyzing the data collected during this campaign to learn more about the atmosphere of Triton.

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Figure 58: Observations of Triton occultation (image credit: Google, INEGI, ORION-ME; annotation: ERC Lucky Star project)

- This occultation was a rare opportunity to detect possible changes in Triton's atmospheric pressure almost thirty years after the flyby of NASA's Voyager probe in August 1989. Observations of the central flash can also provide unique information to detect possible winds near Triton's surface; current analysis of the data indicates a quiet and still atmosphere.

- With the release of the position, parallax and proper motion of more than 1.3 billion stars measured with unprecedented accuracy, Gaia will provide an invaluable resource for all branches of astrophysics. It will also be of great help to professional and amateur astronomers who will be planning the observation of stellar occultations by Solar System bodies in the future, including that of another star by Triton on 6 October 2022.

- The star that was occulted by Triton on 5 October 2017 is UCAC4 410-143659, a 12.7 V-magnitude (12.2 G-magnitude) star, situated in the constellation of Aquarius. — Observations of this occultation by Triton are coordinated by Bruno Sicardy (Université Pierre et Marie Curie and Observatoire de Paris), leader of the ERC Lucky Star project.

• January 29, 2018: If you gazed at the night sky over the past few weeks, it is possible that you stumbled upon a very bright star near the Orion constellation. This is Sirius, the brightest star of the entire night sky, which is visible from almost everywhere on Earth except the northernmost regions. It is, in fact, a binary stellar system, and one of the nearest to our Sun – only eight light-years away. 79)

- Known since antiquity, this star played a key role for the keeping of time and agriculture in Ancient Egypt, as its return to the sky was linked to the annual flooding of the Nile. In Ancient Greek mythology, it represented the eye of the Canis Major constellation, the Great Dog that diligently follows Orion, the Hunter.

- Dazzling stars like Sirius are both a blessing and a curse for astronomers. Their bright appearance provides plenty of light to study their properties, but also outshines other celestial sources that happen to lie in the same patch of sky.

- This is why Sirius has been masked in this picture taken by amateur astronomer Harald Kaiser on 10 January from Karlsruhe, a city in the southwest of Germany.

- Once the glare of Sirius is removed, an interesting object becomes visible to its left: the stellar cluster Gaia 1, first spotted last year using data from ESA's Gaia satellite.

- Gaia 1 is an open cluster – a family of stars all born at the same time and held together by gravity – and it is located some 15,000 light-years away. Its chance alignment next to nearby, bright Sirius kept it hidden to generations of astronomers that have been sweeping the heavens with their telescopes over the past four centuries. But not to the inquisitive eye of Gaia, which has been charting more than a billion stars in our Milky Way galaxy.

- Mr. Kaiser heard about the discovery of this cluster during a public talk on the Gaia mission and zealously waited for a clear sky to try and image it using his 30 cm diameter telescope. After covering Sirius on the telescope sensor – creating the dark circle on the image – he succeeded at recording some of the brightest stars of the Gaia 1 cluster.

- Gaia 1 is one of two previously unknown star clusters that have been discovered by counting stars from the first set of Gaia data, which was released in September 2016. Astronomers are now looking forward to Gaia's second data release, planned for 25 April, which will provide vast possibilities for new, exciting discoveries.

- More information about opportunities for amateur astronomers to follow up on Gaia observations here.

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Figure 59: Gaia 1 cluster image taken from Karlsruhe (Germany) by Harald Kaiser using a 30 cm telescope. The bright, central blob in the center of the image is Sirius (image credit: Harald Kaiser)

• January 25, 2018: Following the release of the first catalog in 2016, ESA's Gaia mission will publish its long-awaited map of more than 1 billion stars in our Galaxy on 25 April 2018. 80)

- Launched on 19 December 2013, and in routine science operations since 25 July 2014, Gaia has been mapping the positions and motions of stars at an accuracy level never before achieved, far below one thousandth of an arcsecond. Such an accuracy relies on the satellite's extraordinary equipment, long stretches of uninterrupted observations, and a collaboration of about 450 scientists and software experts who are entrusted with the complex task of analyzing and processing the data.

- The first release, which included the positions on the sky for 1 billion stars plus an estimate of the distance (parallax) and proper motion for a subset of 2 million bright stars, has already had a profound impact on the global astronomical community and generated hundreds of scientific publications.

- But it is the second release of Gaia data that astronomers across the world are eagerly looking forward to. - Based on 22 months of data collected between 25 July 2014 and 23 May 2016, it will contain the positions, parallaxes, and proper motions for more than 1.3 billion stars, as well as G magnitudes for all of them. It will also contain a measure of each star's color – the BP magnitude, recorded by the blue photometer, and the RP magnitude, recorded by the red photometer – for the vast majority of sources.

- And there is more: the second Gaia release will also include: radial velocities for more than six million stars; temperature estimates for about 150 million stars; the light curves of more than half a million variable stars, including Cepheids and RR Lyrae; and the positions of more than 13,000 objects in the Solar System – mostly asteroids – based on more than 1.5 million observations.

• January 17, 2018: ESA's Gaia observatory was launched in December 2013, and is now surveying our Milky Way, creating one of the most accurate-ever maps of the stars in our home galaxy and helping to answer questions about its origin and evolution. 81)

- The spacecraft is operated by teams at ESA's mission control in Darmstadt, Germany, who send commands, download data and status information and ensure the health and functioning of this marvelous explorer. They also plan and conduct the routine maneuvers needed to keep it in its orbital position, 1.5 million km from Earth at L2 in the Sun-Earth system.

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Figure 60: In this picture, operations engineers can be seen working around part of the Gaia Avionics Model (image credit: ESA, J. Mai)

- Avionics are the devices, hardware and software on a spacecraft that enable it to be controlled from the ground, include propulsion, attitude control, communication, computers and navigation.

- "This model contains engineering copies of some of Gaia's onboard units," says spacecraft operations manager Dave Milligan. "It is used for high-fidelity simulations allowing our team to test procedures and software on the ground before they are executed on the real thing, 1.5 million km away. It is was recently used to test an automatic micropropulsion recovery process, now successfully used in flight."

• January 5, 2018: The Gaia spacecraft – which is busy creating the most detailed 3D map of the Milky Way – could also be instrumental when it comes to gravitational wave research. That's what a team of astronomers from the University of Cambridge recently claimed. According to their study, the Gaia satellite has the necessary sensitivity to study ultra-low frequency gravitational waves that are produced by supermassive black hole mergers. 82) 83) 84)

- GWs (Gravitational Waves) are generated by violent events, such as the collisions of black holes or neutron stars, objects whose extreme density strongly warps the surrounding spacetime. LIGO and Advanced Virgo detect a GW by the stretching and squeezing it produces, but a passing GW would also cause the Earth to oscillate in its location with respect to the stars. So distant stars, viewed from Earth or from an orbiting space telescope, would seem to temporarily shift position in a predictable way.

- Detection of GWs by observing extra-galactic objects, rather than stars, was suggested in the 1990s. 85) And large networks of astronomers are already gearing up to detect GWs via their effects on radio emissions from pulsars, for which the highly regular stream of fast pulses would be slightly shifted in frequency . Now astronomer Christopher Moore of the University of Cambridge in the UK and his co-workers have predicted the fingerprint that a GW would leave in the massive field of stars observed by ESA's Gaia space telescope. The observatory was launched in late 2013 to make high-precision observations of the positions of stars in our Galaxy.

- The data from Gaia will be released in the early 2020s. Then, says Moore, "Gaia will make measuring this effect a realistic prospect for the first time." While Gaia was being designed, researchers recognized that GW detection would be one potential use of the instrument. 86) Many factors contribute to the feasibility of the approach, including the precision and long duration of the astrometric measurements. Gaia will observe about a billion stars over 5–10 years, locating each one of them at least 80 times during that period. Observing so many stars is the major advance provided by Gaia, says Moore.

- To this end, Moore and his colleagues simulated gravitational waves produced by a binary SMBH (SuperMassive Black Hole) – i.e. two SMBHs orbiting one another. What they found was that by compressing the data sets by a factor of more than 106 (measuring 100,000 stars instead of a billion at a time), GWs could be recovered from Gaia data with an only 1% loss of sensitivity.

- This method would be similar to that used in Pulsar Timing Arrays, where a set of millisecond pulsars are examined to determine if gravitational waves modify the frequency of their pulses. However, in this case, stars are being monitored to see if they are oscillating with a characteristic pattern, rather than pulsing. By looking at a field of 100,000 stars at a time, researchers would be able to detect induced apparent motions (Figure 61).

- Success is not guaranteed, however. "It could be that all the waves at these ultralow frequencies are too weak even for Gaia to detect," says Moore. He says that the most likely source of GWs visible in the Gaia data will be a pair of supermassive black holes bound in a tight orbit with a period of roughly a year and located in the center of some large, relatively nearby galaxy. Such objects could have between 10 million and 100 million times the Sun's mass.

- "This is a nifty idea and is worth an effort," says astrophysicist Carl Gwinn of the University of California at Santa Barbara, although he says that it will be challenging to eliminate conflicting signals from instrumental effects such as fluctuations in the spacecraft orientation.

- Jos de Bruijne, deputy project scientist for the Gaia mission at ESA, confirms that the proposals and results of simulations are consistent with the capabilities of the instrument. According to Gwinn, a space-based instrument is required to see such effects, as refraction in the Earth's atmosphere would prevent observation by ground-based telescopes.

- De Bruijne says the approach might reveal GW sources not easily visible to interferometric detectors like LIGO, including primordial GWs produced in the inflationary epoch of the big bang. Gwinn adds that Gaia should also be able to see GWs emitted when stars from a host galaxy merge with the central supermassive black hole, which should be a fairly common occurrence.

 

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Figure 61: A Gaia star field, with red and black lines indicating induced apparent motions of the stars within a hemisphere (image credit: Kavli Institute for Cosmology, Cambridge)

• December 20, 2017: Four years ago, on 19 December 2013, Gaia was launched from Europe's Spaceport in Kourou, French Guiana. The mission's operations teams led by Spacecraft Operations Manager David Milligan (Figure 62), followed the event from the control room in Darmstadt, Germany. 87)

- Measuring stellar positions and motions to the required accuracy is not easy, requiring extreme precision in the stability of the spacecraft, detailed knowledge of its position and unprecedented accuracy in timing.

- This marvel of technology has executed over 1.6 million commands, completed 29 maneuvers, performed some 1108 billion total measurements, made more than 3100 ground-station contacts and accumulated 47.5 TB of science data.

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Figure 62: Operations image of the week: ESA's galaxy surveyor Gaia was launched four years ago this week. David Milligan is based at ESOC (image credit: ESA)

• December 18, 2017: This colorful streamer is created from real data that represents the viewing direction of ESA's Gaia satellite as it scans across the sky (Figure 63). 88)

- Gaia's mission is to make the largest, most precise 3D map of our Galaxy by surveying more than a thousand million stars. This goal relies on the systematic and repeating observation of star positions in two fields of view to detect any changes in an object's motion through space. As such, Gaia is rotating slowly, sweeping its two telescopes, which are separated by 106.5º, across the entire celestial sphere to make four complete rotations per day.

- At the same time, its spin axis changes orientation around the Sun with a period of about 63 days, allowing different parts of the sky to be scanned. This approach builds up an interlocking grid of stellar positions and motions.

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Figure 63: Gaia sky scan. While star maps have provided the focus of previous image releases, this image represents the motion of the satellite itself across the full sky (image credit: ESA/Gaia)

- Gaia's attitude is plotted every second with two dots: one to represent each of the two telescopes at that moment in time. The positions are plotted in celestial coordinates: right ascension (horizontal axis) and declination (vertical axis), which can be imagined as lines of longitude and latitude printed on the inside of the celestial sphere.

- The colors indicate the direction of the scan, the key for which is indicated in the border of the image. The direction of the scan with respect to north (up) can be found by starting in the center of the image and moving towards the border to the appropriate color. Each color around the border covers a range of 18º.

- Zooming in reveals first a cross weave-like pattern, and ultimately the individual dots. The highest density of overlapping pattern represents the areas of sky that have been scanned multiple times in different directions already, while the lower density regions were scanned fewer times and in fewer different directions. Eventually the gaps will close, as Gaia makes more and more repeated scans of the sky.

- Gaia was launched on 19 December 2013 and began routine operations in July 2014. The first catalogue of more than a billion stars, based on the first 14 months of scanning, was published in September 2016. The image shown here was created from Gaia's movements between 1 October 2014 and 31 May 2016. This corresponds to the period that includes the second data release, which is scheduled for April 2018.

• December 13, 2017: Measuring the positions and motions of more than a billion stars, ESA's Gaia mission will refine our knowledge about our place in the Universe, providing the best ever star chart of our Milky Way and its neighboring galaxies. 89)

- One of the nearest galaxies to our Galaxy is the LMC (Large Magellanic Cloud), located around 166,000 light-years away and visible to the naked eye at intermediate and southern latitudes.

- With a mass roughly equivalent to ten billion times the mass of our Sun – about one tenth of the Milky Way – the LMC is home to an intense star-forming activity, forming stars five time faster than in our Galaxy. Different aspects of the galaxy's stellar population are depicted in these two images, based on data collected by the Gaia satellite during its first 14 months of operations.

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Figure 64: LMC, one of the nearest galaxies to our Milky Way, as viewed by ESA's Gaia satellite after its first 14 months of operations. These views are not photographs but were compiled by mapping the total density of stars (left) and the total amount of radiation, or flux (right), detected by Gaia in each pixel (image credit: ESA/Gaia/DPAC)

- The view on the left, compiled by mapping the total density of stars detected by Gaia in each pixel of the image, shows the large-scale distribution of stars in the LMC, delineating the extent of the spiral arms. The image is peppered with bright dots – these are faint clusters of stars. A series of diagonal stripes, visible along the central thick structure, or bar, are an artefact caused by Gaia's scanning procedure. These will gradually decrease as more data are gathered throughout the lifetime of the mission.

- On the right, a different image provides a complementary view that reveals other aspects of this galaxy and its stars. Created by mapping the total amount of radiation, or flux, recorded per pixel by Gaia, this image is dominated by the brightest, most massive stars, which greatly outshine their fainter, lower-mass counterparts. In this view, the bar of the LMC is more clearly delineated, alongside individual regions of star formation like the sparkling 30 Doradus, visible just above the center of the galaxy.

- The images of Figure 65, also obtained using data from the first 14 months of Gaia science operations, depict two nearby spiral galaxies: Andromeda (also known as M31), which is slightly more massive than the Milky Way and, at roughly 2.5 million light-years away, the largest galaxy in our vicinity; and its neighbor, the Triangulum galaxy (also known as M33, Figure 66) home to some fifty billion stars and located about 2.8 million light-years away.

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Figure 65: The Andromeda galaxy, or M31, the largest galactic neighbor to our Milky Way, as viewed by ESA's Gaia satellite after its first 14 months of operations. These views are not photographs but were compiled by mapping the total density of stars (left) and the total amount of radiation, or flux (right), detected by Gaia in each pixel (image credit: ESA/Gaia/DPAC)

- On the left of Figure 65, the image based on the stellar density shows where stars of all types are located within the galaxy. - On the right, the image based on the total flux mainly depicts the bright end of the stellar population of the galaxy, tracing out the regions of most intense star formation.

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Figure 66: The Triangulum galaxy, or M33, a galactic neighbor to our Milky Way, as viewed by ESA's Gaia satellite after its first 14 months of operations. These views are not photographs but were compiled by mapping the total density of stars (left) and the total amount of radiation, or flux (right), detected by Gaia in each pixel (image credit: ESA/Gaia/DPAC)

- For all these stars and more, the second release of Gaia data – planned for April 2018 – will also contain measurements of their parallax, which quantifies a star's distance from us, and of their motion across the sky. Astronomers are eagerly awaiting this unprecedented data set to delve into the present and past mysteries of our Galaxy and its neighbors.

- By analyzing the motions of individual stars in external galaxies like the LMC, Andromeda, or Triangulum, it will be possible to learn more about the overall rotation of stars within these galaxies, as well as the orbit of the galaxies themselves in the swarm they are part of, known as the Local Group.

- In the case of the LMC, a team of astronomers have already attempted to do so by using a subset of data from the first Gaia release, the TGAS (Tycho–Gaia Astrometric Solution), for which parallaxes and proper motions had also been provided by combining the new data with those from ESA's first astrometry mission, Hipparcos. In the TGAS data set, consisting of two million stars, they identified 29 stars in the LMC with good measurements of proper motions and used them to estimate the rotation of the galaxy, providing a taster of the studies that will become possible with future releases of Gaia data.

- Observations of the LMC and its neighbor, the SMC (Small Magellanic Cloud), with Gaia are extremely important also for studying variable stars like Cepheids and RR Lyrae. These stars can be used as indicators of cosmic distances in galaxies beyond our own as long as they are first calibrated in a 'local' laboratory, such as the LMC and SMC, where it is possible to obtain a more direct estimate of their distance using parallax determined with Gaia.

- Astronomers in the Gaia Data Processing and Analysis Consortium, or DPAC, tested this method on hundreds of LMC variable stars from the TGAS sample as part of the validation of the data from the first release. Their results, which are promising even though preliminary, are an exciting example of the rich scientific harvest that will be possible with future releases of the data that are being gathered by Gaia.

• December 7, 2017: ESA's SPC (Science Program Committee) has approved indicative extensions, up to 2019-2020, for the operation of eight scientific missions. 90)

- During its meeting at ESA Headquarters in Paris, on 21-22 November, the SPC approved indicative extensions for the continued operation of five ESA-led missions: Gaia, INTEGRAL, Mars Express, SOHO, and XMM-Newton. This followed a comprehensive review of the current operational status and outlook of the missions and their expected scientific returns during the extension period. The decision will be subject to confirmation towards the end of 2018.

- The lifetime of Gaia, ESA's billion star surveyor, was extended by eighteen months, from 25 July 2019 to 31 December 2020. This is the first time that Gaia, which was launched in 2013 and originally funded for a five-year mission, has been subject to the extension process. — Mars Express, SOHO, and XMM-Newton each received extensions of two years, so their operations will continue at least until the end of 2020.

- The SPC extended the operations of the high-energy observatory INTEGRAL by one year, until 31 December 2019. A proposal to extend INTEGRAL until the end of 2020, as well as a proposal concerning a two-year extension of the magnetospheric plasma mission, Cluster, will be presented to the next meeting of the SPC in February 2018.

- The go-ahead was also given to continue ESA's contributions to the operations of three international collaborative missions: the HST (Hubble Space Telescope), and IRIS (Interface Region Imaging Spectrograph), which are both led by NASA, as well as the Japanese-led mission Hinode.

- Every two years, all missions whose approved operations end within the following four years are subject to review by the advisory structure of the Science Directorate. Extensions are granted to missions that satisfy the established criteria for operational status and science return, subject to the level of financial resources available in the science program. These extensions are valid for the following four years, subject to a mid-term review and confirmation after two years. Extensions for operations in the period 2017-2018 were approved by the SPC in November 2016, but the indicative extension, for 2019-2020, had been deferred until the November 2017 meeting to allow the SPC to evaluate the outcome of the ESA Ministerial Council meeting in December 2016.

• November 27, 2017: By pinning down, for the first time, the three-dimensional motions of individual stars in the nearby Sculptor dwarf galaxy, astronomers have shed new light on the distribution of invisible dark matter that pervades the galaxy. This study combined the positions of stars measured by ESA's Gaia mission with observations from the NASA/ESA Hubble Space Telescope taken twelve years earlier. 91)

- Our Milky Way galaxy, home to hundreds of billions of stars, is orbited by more than a dozen smaller satellite galaxies. The best known in this swarm, the Large and Small Magellanic Clouds, are visible with the naked eye from middle and southern latitudes, while the others are much fainter and can only be observed with the aid of telescopes.

- These satellite galaxies are classed as dwarfs, since they contain only a small fraction of the stars hosted in regular galaxies. They are all engaged in gravitational interactions with the Milky Way, and these dynamical processes might leave telltale traces in their shape.

- These satellite galaxies are classed as dwarfs, since they contain only a small fraction of the stars hosted in regular galaxies. They are all engaged in gravitational interactions with the Milky Way, and these dynamical processes might leave telltale traces in their shape.

- A new study has now brought one of these galaxies, the dwarf Sculptor, into unprecedented focus.

- "For the first time, we've been able to determine how individual stars are moving through a dwarf satellite of our Milky Way," says Davide Massari from the Kapteyn Astronomical Institute in Groningen, The Netherlands, who is lead author of the paper published today in Nature Astronomy. 92) 93)

- "This was possible by combining the exceptionally accurate measurements of stellar positions from Gaia's first data release with equally outstanding observations taken over twelve years earlier by the Hubble Space Telescope."

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Figure 67: The Sculptor dwarf galaxy, a satellite of our Milky Way, imaged with the Wide Field Imager camera on the 2.2 m MPG/ESO telescope at ESO's La Silla Observatory in Chile (image credit: ESO)

Legend to Figure 67: Located around 300,000 light years away, it was the first dwarf galaxy to be discovered around the Milky Way by astronomer Harlow Shapley in 1937. - Astronomers have been looking at stars in dwarf galaxies for decades, striving to reconstruct their origin and to uncover the past history of our Galaxy and its environment.

Based on the combination of data taken twelve years apart by ESA's Gaia mission and the NASA/ESA Hubble Space Telescope, a team of astronomers have pinned down for the first time the three-dimensional motions of individual stars in the Sculptor dwarf galaxy, shedding new light on the underlying distribution of invisible dark matter that pervades the galaxy.

Figure 68: Exaggerated view of the minuscule motions of stars in the Sculptor dwarf galaxy (image credit: ESA/Gaia; ESA/Hubble, NASA; D. Massari et al. (2017))

- As expected, the shifts in the star positions as observed on the plane of the sky – called proper motions – are tiny, even over the long time baseline between 2002, when the Hubble observations were performed, and the first set of publicly released Gaia data, gathered between 2014 and 2015.

- "We couldn't wait to see whether the Gaia observations could be used to measure individual proper motions in another galaxy, so we were thrilled to see this was indeed possible," says Amina Helmi, a co-author of the study, also based at Kapteyn Astronomical Institute in Groningen.

- Massari, Helmi and colleagues measured the proper motions of roughly a hundred stars in the Sculptor dwarf galaxy. For a smaller subset of ten stars, chosen among those with the smallest errors, the astronomers could also retrieve from the literature an estimate of the radial velocity, which quantifies the stellar motion along the line of sight.

- Using the proper motion and radial velocity measurements, they were able to reconstruct how these stars move in three dimensions – the first time this was done for a dwarf galaxy.

- Last week, ESA approved an extension of Gaia operations for an additional 18 months, so it will keep scanning the sky until at least 2020. One of the key science drivers for the extension was the study of proper motions of stars in dwarf galaxies, which requires observations taken over as long a time baseline as possible.

- "With Gaia, we are looking at our Galaxy and its neighbors 'in action', delving into the motions of stars, and unveiling their secrets," says Timo Prusti, Gaia project scientist at ESA. "This study provides us with an amazing preview of the exciting science that can be done with the full Gaia catalog."

• November 15, 2017: It's the perfect meeting of old and new. Astronomers have combined the latest data from ESA's Gaia mission with a simple analysis technique from the 18th century to discover a massive star cluster that had previously escaped detection. Now, subsequent investigations are helping reveal the star-forming history of our Galaxy, the Milky Way. 94)

- In the latter years of the 18th century, astronomers William and Caroline Herschel began to count stars. William called the technique "star gaging" and his aim was to determine the shape of our Galaxy.

- Ever since 1609, when Galileo lifted his telescope to the misty patch of light known as the Milky Way and saw that it was composed of myriad faint stars whose light all blurred together, we have known that there are different numbers of stars in different directions throughout space. This means that our local collection of stars, the Galaxy, must have a shape to it. Herschel set out to find out what that shape was.

- He used a large telescope, 6.10 m in length, mounted between tall wooden frames to sweep out a large circle in the sky that passed through the Milky Way at right angles. He then split this circle into more than 600 regions and counted or estimated the number of stars in each.

- With this simple technique the Herschels produced the first shape estimate for the Galaxy. Fast-forward to the 21st century and now researchers use star counts to search for hidden star clusters and satellite galaxies. They look for regions where the density of stars rises higher than expected. These patches are called stellar over-densities.

- Back in 1785, Herschel's circular track passed close to the brightest star in the night sky Sirius. Now, scientists mining the first data released from the ESA spacecraft Gaia have revisited that particular area of the sky and made a remarkable discovery.

- They have revealed a large star cluster that could have been discovered more than a century and a half ago had it not been so close to Sirius.

- The cluster was spotted by Sergey E. Koposov, then at the University of Cambridge (UK) and now at Carnegie Mellon University in Pittsburgh, PA, USA, and his colleagues. They have been looking for star clusters and satellite galaxies in various surveys for the past decade. It was natural for them to do this with the Gaia mission's first data release.

- Gaia is the European Space Agency's astrometric mission. Collecting positions, brightnesses and additional information for more than a billion sources of light, its data allows nothing less than the most precise 'star gaging' ever.

- These days the laborious task of counting the stars is done by computers but the results still have to be scrutinized by humans. Koposov was combing the list of over-densities when he saw the massive cluster. At first it seemed too good to be true.

- "I thought it must be an artefact related to Sirius," he says. Bright stars can create false signals, termed artefacts, that astronomers must be careful not to mistake for stars. An early paper from the Gaia team had even discussed artefacts around Sirius using a nearby patch of sky to the one Koposov was looking at.

- Although he moved on and found another over-density that looked promising, his mind kept wanting to return to the first one. "I thought, 'That's strange, we shouldn't have that many artefacts from Sirius.' So I went and looked at it again. And I realized that it too was a genuine object," he says.

- These two objects were named: Gaia 1 for the object located near Sirius, and Gaia 2, which is close to the plane of our Galaxy, and both were duly published. 95) Gaia 1 in particular contains enough mass to make a few thousand stars like the Sun, is located 15 thousand light years away, and spread across 30 light years. This means it is a massive star cluster.

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Figure 69: The brightest star in this WISE (Wide-Field Infrared Survey Explorer) image is Sirius, the brightest star in the night sky. To the left of Sirius, and centered on this image, is Gaia 1, a massive star cluster discovered by scientists mining Gaia data (image credit: Sergey Koposov; NASA/JPL; D. Lang, 2014; A.M. Meisner et al. 2017)

- Collections of stars like Gaia 1 are called open clusters. They are families of stars that all form together and then gradually disperse around the Galaxy. Our own Sun very likely formed in an open cluster. Such assemblies can tell us about the star formation history of our Galaxy. Finding a new one that can be easily studied is already paying dividends.

- "The age is of great interest," says Jeffrey Simpson, Australian Astronomical Observatory, who conducted follow-up observations with colleagues using the 4 m-class Anglo-Australian telescope at Siding Springs Observatory, Australia.

- Identifying 41 members of the cluster, Simpson and colleagues found that Gaia 1 is unusual in at least two ways. Firstly, it is about 3 billion years old. This is odd because there are not many clusters with this age in the Milky Way.

- Typically clusters are either younger than a few hundred million years – these are the open clusters – or older than 10 billion years – these are a distinct class called globular clusters, which are found beyond the main bulk of stars in our Galaxy. Being of intermediate age, Gaia 1 might represent an important bridge in our understanding between the two populations.

- Secondly, its orbit through the galaxy is unusual. Most open clusters lie close to the plane of the Galaxy but Simpson found that Gaia 1 flies high above it before ducking down and passing underneath. "It might go as much as a kiloparsec (more than 3000 light years) above and below the plane," he says. About 90% of clusters never go more than a third of this distance.

- Simulations of clusters with orbits like Gaia 1 find that they are stripped of stars and dispersed by these high velocity 'plane passages'. That puts it at odds with the age estimate.

- "Our finding that Gaia 1 is three billion years old is curious as the models would have it not surviving anywhere near as long. More research is required to try and reconcile this," says Simpson.

- The next Gaia data release will give accurate proper motions and distances to an unprecedented number of stars, which can be used to more efficiently find star clusters that were buried too deep in the stellar field or were too diffuse or too distant to be seen before.

- For the astronomers ready to explore the Gaia data, the adventure has only just begun. Gaia's second data release is scheduled for April 2018. Subsequent data releases are scheduled for 2020 and 2022.

• October 26, 2017: ESA's Gaia mission, in orbit since December 2013, is surveying more than a thousand million stars in our Galaxy, monitoring each target star about 70 times over a five-year period and precisely charting their positions, distances, movements and brightness. 96)

- Although Gaia is not equipped with a dedicated radiation monitor, it can provide information about the space weather – and the solar particles and radiation – that it encounters at its unique orbital position at L2 (Lagrangian Point 2), about 1.5 million km from Earth.

- In September, Gaia unexpectedly detected a large quantity of protons – subatomic particles – emitted by a solar flare as shown in Figure 70.

- This proton storm was also reported by NOAA's GOES weather satellite, which is equipped with a particle-sensing instrument.

- The solar flare that gave rise to these protons took place on 10 September 2017, and the peak flow of protons streaming past Gaia occurred at about midnight on 11 September.

- "Gaia is designed to withstand these space weather storms and it was able to continue as normal throughout the period of increased radiation," says spacecraft operations engineer Ed Serpell. "During the days of the increased radiation, the amount of ground contact with the ESA deep-space network was increased to provide more realtime information about the spacecraft performance. This additional visibility confirmed how well Gaia was performing and no intervention was necessary."

- The storm had some minor, temporary effect on Gaia's attitude and orbit control system. The excess protons also caused the main science instrument to generate much more data than usual, which had to be offloaded from the onboard memory.

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Figure 70: In this image, captured by Gaia's Wave Front Sensor – a sort of ‘camera within a camera' in its main star-sensing instrument – the streaks of ‘snow' are trails of individual protons. - During normal space weather conditions, the image would only include one or two proton trails. The long trail running horizontally across the image indicates a particularly energetic proton (image credit: ESA / E. Serpell)