GAIA (Global Astrometric Interferometer for Astrophysics)
GAIA (Global Astrometric Interferometer for Astrophysics) MissionSpacecraft 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.
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.
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.
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.
Figure 4: Gaia will improve the accuracy of astrometry measurements by several orders of magnitude compared with previous systems and observations (image credit: ESA)
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.
Table 1: Parameters of the Gaia spacecraft
Figure 5: Artist's rendition of the deployed GAIA spacecraft (image credit: ESA)
SVM (Service Module):
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.
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.
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)
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).
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.
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).
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.
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.
Figure 13: Photo of the Gaia SVM in the EMC chamber at Intespace, Toulouse, during launcher EMC compatibility testing (image credit: Astrium SAS)
Figure 14: Exploded view of the Gaia spacecraft (image credit: EADS Astrium)
Table 2: Summary of spacecraft subsystems (Ref. 6)
Figure 15: Alternate exploded view of the Gaia spacecraft elements (image credit: EADS Astrium)
Figure 16: The Gaia flight model spacecraft undergoing final electrical tests at Astrium Toulouse in June 2013 (image credit: EADS Astrium)
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)
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. 100).
• 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.
Figure 19: Gaia mission scenario, from launch to in-orbit operations (image credit: ESA)
• On October 5, 2017, the largest moon of Neptune will pass in front of a distant star. This rare event will temporarily block the star's light from Earth and provide an excellent opportunity to study the moon's intriguing atmosphere. Data from ESA's Gaia mission is allowing astronomers to precisely plan their observations. 38)
- Alignments of this kind are called occultations. This particular one will be visible from Earth along a track that just skims the southern states of the USA, crosses the Atlantic and then passes through Europe. Observations are planned from Belgium, France, Germany, Greece, Italy, Spain, Switzerland and the UK, by both professional and amateur observers.
- Triton is the largest moon of Neptune. In terms of size it is an almost identical twin to Pluto. Also like Pluto, Triton possesses a tenuous nitrogen atmosphere, which the astronomers are keen to study.
Figure 20: Occultation of a star by Triton (image credit: ESA/ATG medialab)
- As the moon drifts in front of the star, some of the starlight passes through the atmosphere and is diverted through a process called refraction. This results in a dimming of the star, from which density, pressure and temperature profiles of the atmosphere can be derived.
- To get the best data from the occultation, astronomers have to be on the central track. This is where the alignment between the Earth, Triton and the star is perfect. At this point, astronomers get to see a 'central flash'. This momentary brightening occurs because the atmosphere of Triton acts like a lens that focuses the starlight to Earth. This gives access to the deepest possible atmospheric layers, just a few kilometers above the surface.
- However, from the initial prediction of the occultation, the central path's track across the Earth's surface was uncertain by about 1200-1500 km. This made it easy to be in the wrong place and miss the flash completely. This is where Gaia came in.
Figure 21: Ground trace of Triton occultation (image credit: Google, INEGI, ORION-ME; annotation: ERC Lucky Star project)
- Data from the publicly available first Gaia release (Gaia DR1) could already give a more accurate ground track but Gaia DR1 only provides this star's position. Knowing its motion on the sky, a quantity called the proper motion, would help to improve the accuracy even more. However, despite Gaia DR1 containing 2 million proper motions, this particular star's motion would only become available in April 2018 with the Gaia second data release (Gaia DR2).
- Since the forthcoming Triton occultation presents a rare opportunity to study the atmosphere, Josselin Desmars, an astronomer from the Observatoire de Paris in France, decided to approach the Gaia team and make a special request on behalf of the ERC Lucky Star project. The project, led by Bruno Sicardy from Université Pierre et Marie Curie and Observatoire de Paris, studies the poorly-known, but important, Trans-Neptunian Objects. Desmars coordinates the predictions of this Triton event using recent astrometric measurements of Neptune's moon made in Brazil.
- "In April 2017, we asked the Gaia team if we could get some stars from their upcoming second data release in order to refine the prediction and gather observing stations along the central flash," says Desmars. The Gaia team agreed on the importance of the observation and released the preliminary Gaia DR2 position and proper motion to help the campaign.
- Armed with this information, a much more accurate track has been calculated. Now the various observing stations across Europe can be placed at well-defined offsets to ensure that at least one of them sees the central flash.
- The first definitive measurements of Triton's atmosphere were made in 1989 by the Voyager 2 spacecraft during its flyby. It discovered that the atmosphere was extremely tenuous (just one 70, 000th of the surface pressure of Earth), extended up to 800 km from the moon's surface, and possessed a temperature of around 38 Kelvin (-235°C).
- Eight years later, an occultation took place that allowed astronomers using the Hubble Space Telescope to measure the atmosphere again. They saw something surprising. Triton's atmospheric temperature had increased by 5%, which could indicate that Triton was warming during the Neptunian summer.
- The occultation on 5 October is the first favorable one since 1997 and astronomers want to see if the warming trend has continued. "We hope to get a lot of observations from professionals and amateurs to let us really study the atmosphere," says Desmars.
- This is not the first time Gaia data have been provided for such an event. Desmars also used preliminary Gaia DR2 data for occultations of Chariklo, a distant asteroid. This asteroid is known to have a ring system surrounding it, like a mini Saturn. The Gaia data helped to improve the predicted occultation paths, leading to very successful observational campaigns in the summer of this year.
- In May, the Gaia team released the preliminary data for the star that will be occulted by Triton. This week, the Gaia team made public preliminary astrometric data for an additional few hundred stars in the field surrounding Triton to allow astronomers to further improve the occultation predictions. The team cautions that for all of these preliminary data full validation is not yet done and therefore some caution is required when interpreting the results.
• September 25, 2017: A team of astronomers led by Andreas Koch of the Lancaster University, U.K., has recently conducted a detailed chemical abundance study of the recently discovered star cluster Gaia 1. The research, published Sept. 12 on arXiv.org, offers new insights into the nature of this newly found cluster. 39) 40)
- Gaia 1 is a large, massive, nearby star cluster first identified in May 2017 in the Data Release 1 (DR1) from the Gaia space observatory operated by ESA (European Space Agency). Located some 15,000 light years away from the Earth, this cluster has a half-light radius of about 29 light years and a mass of approximately 22,000 solar masses.
- Although fundamental parameters of Gaia 1 are known, the researchers are still uncertain about its age, metallicity and origin. The cluster was initially classified as an intermediate-age (6.3 billion years old), and moderately metal-rich system. However, further studies found that Gaia 1 is about 3.3 billion years younger and more metal-rich than previously thought. Moreover, some astronomers have suggested the possibility of extragalactic origin for Gaia 1, given that its orbit leads it up to 5,500 light-years above the Milky Way's disk.
- Now, Koch's team has performed a new chemical analysis of Gaia 1 with the aim of clarifying these uncertainties. The researchers have measured detailed chemical abundances of 14 elements in four red giant members of the cluster.
- "This work focuses on a detailed chemical abundance analysis of four red giant members of Gaia 1, based on high-resolution spectroscopy, which we complement by an investigation of the orbital properties of this transition object," the authors wrote in the paper.
- The four stars were observed with the Echelle spectrograph at the 2.5 m Du Pont telescope at the Las Campanas Observatory, Chile. Besides the previously estimated iron content, this instrument allowed the researchers to obtain abundances of lithium, carbon, oxygen, magnesium, aluminum, silicon, calcium, titanium, scandium, vanadium, chromium, cobalt, nickel and zinc.
- The analysis revealed that Gaia 1 is associated with the Milky Way's thick disk and is more metal-poor than previously estimated. Moreover, they suggest that the system is a massive open cluster rather than a low-mass globular cluster.
- "Our abundance analysis revealed alpha abundances that are consistent with the thick disk (...) Despite the presence of mild light element variations in Na, Mg, and Al no obvious (anti-) correlations are seen between those elements as would be expected in galactic globular clusters. Since its stellar mass of a few ten thousand solar masses places Gaia 1 at the boundary between very low-mass globular clusters and very luminous open clusters, our analysis rather comforts the latter view," the paper reads.
• September 4, 2017: Gaia, ESA's billion-star surveyor, is detecting stars and measuring their properties in order to build up the most precise 3D map of the Milky Way. By accurately measuring the motion of each star, astronomers will be able to peer back in time to understand the Milky Way's history, its evolution and its destiny. 41)
- In general, as Gaia registers stars, only data covering the object of interest are transmitted to the ground. However, in the densest regions on the sky there are more stars close to each other than the detection and processing system of Gaia can cope with, which could result in a less complete census in these crowded areas.
- To help mitigate this, a scientific selection of high-density regions is made to cover them in a special imaging mode, as illustrated here. These types of observations are carried out routinely every time Gaia scans over these regions.
- The image of Figure 22, taken on 7 February 2017, covers part of the Sagittarius I Window (Sgr-I) located only two degrees below the Galactic Center. Sgr-I has a relatively low amount of interstellar dust along the line of sight from Earth, giving a 'window' to stars close to the Galactic Center.
- The stellar density here is an incredible 4.6 million stars per square degree. The image covers about 0.6 square degrees, making it conceivable that there are some 2.8 million stars captured in this image sequence alone.
- The image appears in strips, each representing a sky mapper CCD). The image has been lightly processed to bring out the contrast of the bright stars and darker traces of gas and dust. Zooming in reveals some imaging artifacts relating to the CCDs, including some vertical striping, as well as short bright streaks indicating cosmic rays. Analysis of these images will only start once the effort required by the routine data processing allows.
- Gaia's first catalog of more than a billion stars, based on the first 14 months of data collection, was released in September 2016. The next release is targeting April 2018, with subsequent releases foreseen for 2020 and 2022.
Figure 22: Gaia sky mapper image near the Galactic center, acquired on 7 February 2017, covers part of the Sagittarius I Window (Sgr-I), image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO
• August 31, 2017: The movements of more than 300,000 stars surveyed by ESA's Gaia satellite reveal that rare close encounters with our Sun might disturb the cloud of comets at the far reaches of our Solar System, sending some towards Earth in the distant future. 42)
- As the Solar System moves through the Galaxy, and as other stars move on their own paths, close encounters are inevitable – though 'close' still means many trillions of kilometers.
- A star, depending on its mass and speed, would need to get within about 60 trillion kilometers before it starts to have an effect on the Solar System's distant reservoir of comets, the Oort Cloud, which is thought to extend out to 15 trillion kilometers from the Sun, 100 000 times the Sun–Earth distance. — For comparison, the outermost planet Neptune orbits at an average distance of about 4.5 billion kilometers, or 30 Sun–Earth distances.
- The gravitational influence of stars that pass near the Oort Cloud could perturb the paths of comets residing there, jolting them onto orbits that bring them in to the inner Solar System. While this is thought to be responsible for some of the comets that appear in our skies every hundred to thousand years, it also has the potential to put comets on a collision course with Earth or other planets.
- Understanding the past and future motions of stars is a key goal of Gaia as it collects precise data on stellar positions and motions over its five-year mission. After 14 months, the first catalogue of more than a billion stars was recently released, which included the distances and the motions across the sky for more than two million stars.
- By combining the new results with existing information, astronomers began a detailed, large-scale search for stars passing close to our Sun. So far, the motions relative to the Sun of more than 300,000 stars have been traced through the Galaxy and their closest approach determined for up to five million years in the past and future. Of them, 97 stars were found that will pass within 150 trillion kilometers, while 16 come within about 60 trillion km.
- While the 16 are considered reasonably near, a particularly close encounter of one star, Gliese 710, in 1.3 million years' time, stands out. It is predicted to pass within just 2.3 trillion km or about 16,000 Earth–Sun distances, well within the Oort Cloud.
- The star is already well-documented, and thanks to the Gaia data, the estimated encounter distance has recently been revised. Previously, there was a 90% degree of certainty that it would come within 3.1–13.6 trillion kilometers. Now, the more accurate data suggest that it will come within 1.5–3.2 trillion km, with 2.3 trillion km most likely.
- Furthermore, although Gliese 710 has a mass of 60% that of our Sun, it travels much slower than most stars: nearly 50,000 km/h at closest approach, compared with the average 100,000 km/h. The speed of its passage means it will have plenty of time to exert its gravitational influence on bodies in the Oort Cloud, potentially sending showers of comets into the Solar System. - Despite its slower pace, it will still appear as the brightest, fastest object in the night sky at closest approach.
- Importantly, the latest study used Gaia measurements to make a general estimate of the rate of stellar encounters, taking into account uncertainties such as stars that might not have been observable in the existing catalog. 43)
• August 16, 2017: While surveying the positions of over a billion stars, ESA's Gaia mission is also measuring their color, a key diagnostic to study the physical properties of stars. A new image provides a preview of Gaia's first full-color all-sky map, which will be unleashed in its highest resolution with the next data release in 2018. 44)
Figure 23: This map is a preview of Gaia's measurements of the sky in color, generated using preliminary data from the photometric instrument on board (image credit: ESA/Gaia/DPAC/CU5/DPCI/CU8/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones)
- Stars come in a variety of colors that depend on their surface temperature, which is, in turn, determined by their mass and evolutionary stage. Massive stars are hotter and therefore shine most brightly in blue or white light, unless they are approaching the end of their life and have puffed up into a red supergiant. Lower-mass stars, instead, are cooler and tend to appear red.
- Measuring stellar colors is important to solve a variety of questions, ranging from the internal structure and chemical composition of stars to their evolution.
- Gaia, ESA's astrometry mission to compile the largest and most precise catalogue of stellar positions and motions to date, has also been recording the color of the stars it observes. The satellite was launched in December 2013 and has been collecting scientific data since July 2014.
- A special effort in the Gaia DPAC (Data Processing and Analysis Consortium) is dedicated to the challenging endeavour of extracting stellar colors from the satellite data. While these measurements will be published with Gaia's second data release in April 2018, a preview of the Gaia sky map in color demonstrates that the ongoing work is progressing well.
- The new map, based on preliminary data from 18.6 million bright stars taken between July 2014 and May 2016, shows the middle value (median) of the colors of all stars that are observed in each pixel. It is helpful to look at it next to its companion map, showing the density of stars in each pixel, which is higher along the Galactic Plane – the roughly horizontal structure that extends across the image, corresponding to the most densely populated region of our Milky Way galaxy – and lower towards the poles.
Note: The preliminary color map shows a sample of stars that have been selected randomly from all Gaia stars with G-band magnitudes brighter than 17 and for which both color measurements (from the blue and the red channels of Gaia's photometric instrument) are available.
Figure 24: Gaia's density map (companion to the preliminary color sky map). This map shows the distribution on the sky of the stars that have been used to generate a preliminary map of Gaia's sky in color. It is meant as a visual aid to better appreciate the content of the view of the sky in color (image credit: ESA/Gaia/DPAC/CU5/DPCI/CU8/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones)
- Even though this map is only meant as an appetizer to the full treat of next year's release, which will include roughly a hundred times more stars, it is already possible to spot some interesting features.
- The reddest regions in the map (Figure 23), mainly found near the Galactic Center, correspond to dark areas in the density map: these are clouds of dust that obscure part of the starlight, especially at blue wavelengths, making it appear redder – a phenomenon known as reddening.
- It is also possible to see the two Magellanic Clouds – small satellite galaxies of our Milky Way – in the lower part of the map.
- The task of measuring colors is performed by the photometric instrument on Gaia. This instrument contains two prisms that split the starlight into its constituent wavelengths, providing two low-resolution spectra for each star: one for the short, or blue, wavelengths (330-680 nm) and the other for the long, or red, ones (640-1050 nm). Scientists then compare the total amount of light in the blue and red spectra to estimate stellar colors.
- To precisely calibrate these spectra, however, it is necessary to know the position of each source on Gaia's focal plane to very high accuracy – in fact, to an accuracy that only Gaia itself can provide.
- As part of the effort to extract physical parameters from the data sent back by the satellite, scientists feed them to an iterative algorithm that compares the recorded images of stars to models of how such images should look: as a result, the algorithm provides a first estimate of the star's parameters, such as its position, brightness, or color. By collecting more data and feeding them to the algorithm, the models are constantly improved and so are the estimated parameters for each star.
- The first Gaia data release, published in September 2016, was based on less than a quarter of the total amount of data that will be collected by the satellite over its entire five-year mission, which is expected to observe each star an average of 70 times. This first release, listing unprecedentedly accurate positions on the sky for 1.142 billion stars, along with their brightness, contained no information on stellar colors: by then, it had not been possible to run enough iterations of the algorithm to accurately estimate additional parameters.
- As the satellite continues to observe more stars, scientists have now had more time to feed data to the iterative algorithm to obtain estimates of stellar colors, like the ones shown in the new map. These estimates will be validated, over the coming months, as part of the overall data processing effort leading to the second Gaia data release.
- Since the first data release, scientists across the world have been using Gaia's brightness measurements – which are obtained over the full G-band, from 330 to 1050 nm – along with datasets from other missions to estimate stellar colors. These studies have been applied to a variety of subjects, from variable stars and stellar clusters in our Galaxy to the characterization of stars in the Magellanic Clouds.
- Next year, the second release of Gaia data will include not only the position and G-band brightness, but also the blue and red color for over a billion stars – in addition to the long-awaited estimates of stellar parallaxes and proper motions based on Gaia measurements for all the observed stars. This extraordinary dataset will allow scientists to delve into the secrets of our Galaxy, investigating its composition, formation and evolution to an unparalleled degree of detail.
Note: Gaia's goal is to measure the parallax (a small, periodic change in the apparent position of a star caused by Earth's yearly revolution around the Sun, which depends on the star's distance from us) and proper motion (the motion of stars across the plane of the sky caused by their physical movement through the Galaxy) for over one billion stars. In the process, Gaia will measure also the brightness and color of these stars, take spectra for a subset of them, and observe a variety of other celestial objects, from asteroids in our own Solar System to distant galaxies beyond the Milky Way.
• June 26, 2017: With the help of software that mimics a human brain, ESA's Gaia satellite spotted six stars zipping at high speed from the center of our Galaxy to its outskirts. This could provide key information about some of the most obscure regions of the Milky Way. 45)
- Our galactic home, the Milky Way, houses more than a hundred billion stars, all kept together by gravity. Most are located in a flattened structure – the Galactic disc – with a bulge at its center, while the remaining stars are distributed in a wider spherical halo extending out to about 650,000 light-years from the center.
- Stars are not motionless in the Galaxy but move around its center with a variety of velocities depending on their location – for example, the Sun orbits at about 220 km/s, while the average in the halo is of about 150 km/s.
- Occasionally, a few stars exceed these already quite impressive velocities.
- Some are accelerated by a close stellar encounter or the supernova explosion of a stellar companion, resulting in runaway stars with speeds up to a few hundred km/s above the average.
- A new class of high-speed stars was discovered just over a decade ago. Swooping through the Galaxy at several hundred of km/s, they are the result of past interactions with the supermassive black hole that sits at the center of the Milky Way and, with a mass of four million Suns, governs the orbits of stars in its vicinity.
- "These hypervelocity stars are extremely important to study the overall structure of our Milky Way," says Elena Maria Rossi from Leiden University in the Netherlands, who presented Gaia's discovery of six new such stars today at the European Week of Astronomy and Space Science in Prague, Czech Republic. 46)
- "These are stars that have travelled great distances through the Galaxy but can be traced back to its core – an area so dense and obscured by interstellar gas and dust that it is normally very difficult to observe – so they yield crucial information about the gravitational field of the Milky Way from the center to its outskirts."
- Unfortunately, fast-moving stars are extremely difficult to find in the stellar haystack of the Milky Way, as current surveys list the speed of at most a few hundred thousand stars.
- To find them, scientists have been looking for young, massive stars that would stand out as interlopers in the old stellar population of the Galactic halo. Given away by their out-of-place age, these stars are likely to have received an extra kick to reach the halo. Further measurements of their speeds and estimates of their past paths can confirm if they are indeed hypervelocity stars that were shoved away from the center of the Milky Way.
Figure 25: Artist's impression of two stars speeding from the center of our Galaxy, the Milky Way, to its outskirts. These hypervelocity stars move at several hundred of km/s, much faster than the galactic average. Their high speeds are the result of a past interaction with the supermassive black hole that sits at the center of the Milky Way and, with a mass of four million Suns, governs the orbits of stars in its vicinity (image credit: ESA, CC BY-SA 3.0 IGO)
• April 20, 2017: While compiling an unprecedented census of one billion stars in our Galaxy, ESA's Gaia mission is also surveying stars beyond our Milky Way. A new image of M33, also known as the Triangulum galaxy, shows tens of thousands of stars detected by Gaia, including a small stellar census in its star-forming region NGC 604. This is a striking example of the mission's potential to detect and characterize stars in nearby galaxies (Figure 26). 47)
- Even though the majority of objects detected by Gaia are Milky Way stars, the satellite has also been observing a wealth of stars that belong to other galaxies, providing an important database to study the stellar populations of galaxies beyond our own. Observations of the M33 galaxy provide a clear demonstration of Gaia's prospects for extragalactic studies. — In addition to stars in the Milky Way and nearby galaxies, Gaia has been detecting distant galaxies and quasars – the bright cores of distant galaxies with an active black hole at their center – as well as hundreds of thousands of asteroids and other small bodies in the Solar System.
- Located some 2.4 million light-years away, M33 is one of our neighboring galaxies and is part of the Local Group, the assembly of galaxies that includes our Milky Way and the Andromeda galaxy. A spiral galaxy with less than one tenth the mass of the Milky Way or Andromeda, M33 is the third largest member of the Local Group and is thought to be a satellite galaxy of Andromeda.
- Gaia's detections correspond to about 40 000 of the galaxy's brightest stars, a small fraction of the total stellar population of M33, which amounts to roughly 40 billion stars. The high stellar density of this region means that Gaia can only detect a fraction of these extragalactic stars each time it scans this portion of the sky.
- Very dense stellar fields – like external galaxies, as well as the densest portions of our Galaxy, such as Baade's Window (Note: Baade's Window is an area on the sky toward the center of the Milky Way with little contamination by interstellar dust, where the density of stars reaches up to three million per square degree) — are particularly challenging for Gaia. Even if very powerful, the data processing units on the satellite have a limit to the number of objects that can be observed per second without saturating the onboard computer. As time goes by, Gaia will survey these regions on many more occasions and with different scanning configurations, eventually recording more of the detected stars.
Figure 26: A view of M33, also known as the Triangulum galaxy, obtained by ESA's Gaia satellite. A new image of the patch of sky where M33 is found, based on 26 scans performed between 7 and 9 December 2016, shows all points where Gaia detected one or more sources (image credit: ESA/Gaia/DPAC)
- M33 hosts a well-known star-forming region, NGC 604, which is home to many newborn stars. With a density of about one million stars per square degree, NGC 604 is one such challenging region to observe (Figure 27).
- A comparison between the Gaia detections in that region and a high-resolution image of this stellar nursery from the NASA/ESA Hubble Space Telescope shows that Gaia has detected about 150 individual stars in the NGC 604 region, which spans only about 0.55 square arcminutes in the sky. This number might not sound big in absolute terms, but it is impressive when considering that the observed field is about 2000 times smaller than the area covered by the full Moon in the sky, revealing that Gaia is able to detect a significant number of extragalactic stars.
- This is even more remarkable given that Gaia's observed stars are automatic detections obtained by the satellite's onboard computing system while scanning over the region, whereas the stars detected in astronomical images, such as those from Hubble, are the result of dedicated observations obtained by pointing at specific regions on the sky, which are later analyzed by scientists.
- With this quality of data, observations of stellar fields in Local Group galaxies like M33 will yield estimates of the position, parallax, and proper motion for a wealth of extragalactic stars – these will be published in the future Gaia data releases (Note: Due to the great distance of these extragalactic stars, they are mostly among the faintest sources detected by the satellite, requiring an especially meticulous analysis of the data in order to interpret them correctly).
Figure 27: The star forming region NGC 604 viewed by Gaia (left) and Hubble (right), image credit: ESA/Gaia/DPAC (left); ESA/Hubble & NASA (right)
• April 10, 2017: Figure 28 may look like a brightly decorated Easter egg wrapping, but it actually represents how ESA's Gaia satellite scanned the sky during its first 14 months of science operations, between July 2014 and September 2015. 48)
- The oval represents the celestial sphere, with the colors indicating how frequently the different portions of the sky were scanned. Blue represents the regions scanned most frequently in that time period; the lighter colors lesser so.
- The satellite scans great circles on the sky, with each lasting about six hours. During the first month, the scanning procedure was such that the ecliptic poles were always included. This meant that Gaia observed the stars in those regions many times, providing an invaluable database for the initial calibration of the observations.
- Then, the satellite started its main survey, scanning in such a way to achieve the best possible coverage of the whole sky.
- Over its five-year mission, Gaia will survey one billion stars in our Galaxy and local galactic neighborhood, measuring their position and motion at unprecedented accuracy, in order to build the most precise 3D map of the Milky Way and answer questions about its structure, origin and evolution.
Figure 28: The initial 14 months of the Gaia mission provided the first catalog of the brightness and precise position of more than a billion stars, the largest all-sky survey of celestial objects to date (image credit: ESA/Gaia/DPAC; acknowledgement: B. Holl (University of Geneva, Switzerland) on behalf of DPAC)
• February 13, 2017: Using a novel method and Gaia data, University of Toronto astronomers have estimated that the speed of the Sun and its distance from the center of the Milky Way Galaxy is approximately 240 km/second. In turn, they have used that result to calculate that the Sun is approximately 7.9 kiloparsec from the Galaxy's center — or almost 26,000 light-years. 49)
- Using data from the Gaia space telescope and the RAVE (RAdial Velocity Experiment) survey 50), Jason Hunt and his colleagues determined the velocities of over 200,000 stars relative to the Sun. Hunt is a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics, University of Toronto. The collaborators found an unsurprising distribution of relative velocities: there were stars moving slower, faster and at the same rate as the Sun.
- But they also found a shortage of stars with a Galactic orbital velocity of approximately 240 km/s slower than the Sun's. The astronomers concluded that the missing stars had been stars with zero angular momentum; i.e. they had not been circling the Galaxy like the Sun and the other stars in the Milky Way Galaxy. "Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there," says Hunt. "This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighborhood. ...By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun," says Hunt, "we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving."
- Hunt and his colleagues then combined this finding with the proper motion of the supermassive blackhole known as Sagittarius A* ("A-star") that lies at the center of the Galaxy to calculate the 7.9 kiloparsec distance. 51)
- Proper motion is the motion of an object across the sky relative to distant background objects. They calculated the distance in the same way a cartographer triangulates the distance to a terrestrial landmark by observing it from two different positions a known distance apart. The result was published in Astrophysical Journal Letters in December 2017. 52)
- The method was first used by Hunt's co-author, current chair of the Department of Astronomy & Astrophysics at the University of Toronto, Prof. Ray Calberg, and Carlberg's collaborator, Prof. Kimmo Innanen. But the result Carlberg and Innanen arrived at was based on less than 400 stars.
- Gaia is creating a dynamic, three-dimensional map of the Milky Way Galaxy by measuring the distances, positions and proper motion of stars. Hunt and his colleagues based their work on the initial data release from Gaia which included hundreds of thousands of stars. By the end of its 5 year mission, the space mission will have mapped well over 1 billion stars.
- The velocity and distance results are not significantly more accurate than other measurements. But according to Hunt, "Gaia's final release in late 2017 should enable us to increase the precision of our measurement of the Sun's velocity to within approximately one km/sec, which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center."
• January 24, 2017: While best known for its surveys of the stars and mapping the Milky Way in three dimensions, ESA's Gaia has many more strings to its bow. Among them, its contribution to our understanding of the asteroids that litter the Solar System. Now, for the first time, Gaia is not only providing information crucial to understanding known asteroids, it has also started to look for new ones, previously unknown to astronomers. 53)
Figure 29: Asteroid Gaia-606 on 26 October 2016 (image credit: Observatoire de Haute-Provence & IMCCE)
- Since it began scientific operations in 2014, Gaia has played an important role in understanding Solar System objects. This was never the main goal of Gaia — which is mapping about a billion stars, roughly 1% of the stellar population of our Galaxy — but it is a valuable side effect of its work. Gaia's observations of known asteroids have already provided data used to characterize the orbits and physical properties of these rocky bodies more precisely than ever before.
- "All of the asteroids we studied up until now were already known to the astronomy community," explains Paolo Tanga, Planetary Scientist at Observatoire de la Côte d'Azur, France, responsible for the processing of Solar System observations.
- These asteroids were identified as spots in the Gaia data that were present in one image and gone in one taken a short time later, suggesting they were in fact objects moving against the more distant stars.
Figure 30: Gaia's asteroid detections [ESA/Gaia/DPAC/CU4, L. Galluccio, F. Mignard, P. Tanga (Observatoire de la Côte d'Azur)]
- Once identified, moving objects found in the Gaia data are matched against known asteroid orbits to tell us which asteroid we are looking at. "Now," continues Tanga, "for the first time, we are finding moving objects that can't be matched to any cataloged star or asteroid."
- The process of identifying asteroids in the Gaia data begins with a piece of code known as the IDT (Initial Data Processing) software – which was largely developed at the University of Barcelona and runs at the Data Processing Center at ESAC (European Space Astronomy Center), ESA's establishment in Spain.
- This software compares multiple measurements taken of the same area and singles out objects that are observed but cannot be found in previous observations of the area. These are likely not to be stars but, instead, Solar System objects moving across Gaia's field of view. Once found, the outliers are processed by a software pipeline at the CNES (Centre National d'Etudes Spatiales) data center in Toulouse, France, which is dedicated to Solar System objects. Here, the source is cross matched with all known minor bodies in the Solar System and if no match is found, then the source is either an entirely new asteroid, or one that has only been glimpsed before and has never had its orbit accurately characterized.
- Although tests have shown Gaia is very good at identifying asteroids, there have so far been significant barriers to discovering new ones. There are areas of the sky so crowded that it makes the IDT's job of matching observations of the same star very difficult. When it fails to do so, large numbers of mismatches end up in the Solar System objects pipeline, contaminating the data with false asteroids and making it very difficult to discover new ones.
- "At the beginning, we were disappointed when we saw how cluttered the data were with mismatches," explains Benoit Carry, Observatoire de la Côte d'Azur, France, who is in charge of selecting Gaia alert candidates. "But we have come up with ways to filter out these mismatches and they are working! Gaia has now found an asteroid barely observed before."
Figure 31: Asteroid Gaia-606 on 26 October 2016 (image credit: Observatoire de Haute-Provence & IMCCE)
Legend to Figure 31: These six images show the asteroid Gaia-606 (indicated by an arrow) on 26 October 2016. The images, spanning a period of a little more than 18 minutes, were taken at the Observatoire de Haute Provence in southern France by William Thuillot, Vincent Robert and Nicolas Thouvenin (Observatoire de Paris/IMCCE).
- The asteroid in question, nicknamed Gaia-606, was found in October 2016 when Gaia data showed a faint, moving source. Astronomers immediately got to work and were able to predict the new asteroid's position as seen from the ground over a period of a few days. Then, at the Observatoire de Haute Provence (southern France), William Thuillot and his colleagues Vincent Robert and Nicolas Thouvenin (Observatoire de Paris/IMCCE) were able to point a telescope at the positions predicted and show this was indeed an asteroid that did not match the orbit of any previously catalogued Solar System object.
- However, despite not being present in any catalog, a more detailed mapping of the new orbit has shown that some sparse observations of the object do already exist. This is not uncommon with new discoveries where, as with Gaia-606 (now renamed 2016 UV56), objects that first appear entirely new transpire to be re-sightings of objects whose previous observations were not sufficient to map their orbits.
- Gaia-606 was found in the main asteroid belt, which is not surprising given how many asteroids exist there. However, Gaia also provides data from swathes of the sky not extensively observed by existing ground-based surveys giving it the potential to find asteroids in areas where others would not look. One such area is a region close to the Sun as seen from Earth. Observations are made from the Earth during the night when the angle between any source and the Sun is fairly large, whilst Gaia can make observations at any time and so observe objects much closer to the Sun. This gives Gaia the exciting potential to observe asteroids that orbit within Earth's orbit – these are known as Atira asteroids and only sixteen are currently known.
- Gaia also has the potential to make discoveries at high ecliptic latitudes. Not because ground-based surveys of Solar System objects cannot observe there, but because they tend not to. The vast majority of asteroids exist in the ecliptic plane and, as a result, it is here that most surveys concentrate their efforts. Gaia has no such prejudices and scans the entire sky, giving it the potential to discover new asteroids in the less crowded areas missed by other surveys (Ref. 53).
• December 23, 2016: The size of the Gaia DR1 (Data Release 1) datasets requires sophisticated analysis and plotting tools. For example, a simple plot of the positions of the stars on the sky in Gaia DR1 miserably fails, even if only 10 000 points or 1 000 000 points (top left and right panels of Figure 32, respectively) are shown. However, a density plot as that in the bottom adequately reveals the rich structure of the data, including all 1 142 679 769 sources in Gaia DR1, and can be generated in less than a second with VAEX. This density plot reveals, for example, structure in the galactic disk and artefacts due to the scanning nature of the observations performed with Gaia. 54)
With the arrival of large catalogs such as the Gaia DR1, which contains more than a billion objects, new methods of handling and visualising these data volumes are needed. For many science cases, as well as for quality checks of the data, one needs to visualise all or large parts of the data. While scatter plots would suffice for small catalogues, it would not work for the full Gaia catalog. Apart from the long time it takes to render each individual point as a glyph, overplotting makes the plot useless, as demonstrated in Figure 32, presented by Maarten Breddels at the Astro-informatics IAU Symposium in 2016 at Sorrento, Italy. 55)
This figure demonstrates how plotting a random subset of 10,000 stars (0.001% of the data, top left panel) shows structure in the galactic disk, while plotting just a million stars (0.1% of the data, top right panel), already starts to hide many of the structures present in the data. Instead, in the bottom panel of this figure, showing all data (more than a billion stars) in the form of a density plot reveals much more structure: dust lanes are clearly visible in the disk, our neighboring galaxies (the Large and Small Magellanic Clouds) stand out clearly against the background, but also artefacts in the data due to the scanning law of the satellite become visible. In this plot, low densities correspond to black, and high densities to white, where the scaling is logarithmic.
To visualize and explore large catalogues such as Gaia DR1, Maarten Breddels from the Kapteyn Astronomical Institute (University of Groningen, The Netherlands) developed a software package to perform the calculations needed for these visualizations efficiently. The calculations to compute the number of stars in each pixel take only about a second on a high-end desktop machine. Statistics, such as minimum, maximum, mean, moments, etc., can also be calculated efficiently in any number of dimensions.
The software packaged ,called VAEX, exists of two parts. The first is a Python package, allowing fast calculations of statistics for any property of the data (or any mathematical operations on them), and their visualization in, for instance, the Jupyter notebook. Built on top of this, is a graphical program for Linux and Mac OS X, enabling interactive exploration of the data including zooming, panning, and screen selections. VAEX is open source, available under a MIT License.
This work has been carried out in collaboration with Amina Helmi. It has been funded by a grant from the Netherlands Research School for Astronomy (NOVA), and a Vici grant from the Netherlands Organization for Scientific Research (NWO).
Figure 32: Interactive and statistical visualization of Gaia DR1 with VAEX (image credit: ESA/Gaia/DPAC/CU9, Maarten Breddels, Amina Helmi)
• December 19, 2016: Since the launch of Gaia on 19 December 2013 Gaia performed an impressive number of astrometric, photometric and spectroscopic measurements. More than 60 billion times an object moved through the Gaia focal plane. 56)
• November 10, 2016: ESA's Gaia is surveying stars in our Galaxy and local galactic neighborhood in order to build the most precise 3D map of the Milky Way and answer questions about its structure, origin and evolution. 57)
- Where there are more stars, as in the Galactic center, the map is brighter; where there are fewer, the map is darker. The map includes brightness data corresponding to several million stars.
Figure 33: These engineering data have been accumulated over 18 months and combined to create a 'map' of the observed star densities, from which a beautiful and ghostly virtual image of our magnificent Milky Way galaxy can be discerned, showing the attendant globular clusters and Magellanic clouds (image credit: ESA)
• October 27, 2016: While scanning the sky to measure the position of over one billion stars in our Galaxy, ESA's Gaia satellite has detected two rare instances of stars whose light was temporarily boosted by other celestial objects passing across their lines of sight. One of these stars is expected to brighten again soon. Gaia's measurements will be instrumental to learn more about the nature of these 'cosmic magnifying glasses'. 58)
Figure 34: Variations of brightness of the star Gaia 16aye caused by a microlensing event, as a massive object passed across its line of sight. The Gaia Photometric Science Alerts Team nicknamed this star Ayers Rock, after the famous landmark in Australia [image credit: ESA/Gaia/DPAC, P. Mroz, L. Wyrzykowski, K. A. Rybicki (Warsaw)]
- After the initial discovery of this magnitude 14.5 star with Gaia (dark circles), astronomers started observing it with different telescopes on the ground (colored circles), revealing a rather peculiar pattern of brightness variations.
- Instead of a single rise and fall, the star has undergone two consecutive brightness peaks of roughly two magnitudes, then became fainter for a few weeks. It later exhibited a sharp increase to magnitude 12 and rapidly declined again.
- The intricate pattern of variations suggests that the star is not being lensed by a single object but rather by a binary system. The black line shows the expected brightness variation from a microlensing model with a binary lens, which most likely consists of two stars, but might also involve a planet or even a black hole.
• Sept. 14, 2016: An all-sky view of stars in our Galaxy – the Milky Way – and neighboring galaxies, based on the first year of observations from ESA's Gaia satellite, from July 2014 to September 2015.The annotated map (Figure 35) shows the density of stars observed by Gaia in each portion of the sky. Brighter regions indicate denser concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed. 59)
- The Milky Way is a spiral galaxy, with most of its stars residing in a disc about 100,000 light-years across and about 1000 light-years thick. This structure is visible in the sky as the Galactic Plane – the brightest portion of this image –which runs horizontally and is especially bright at the center. Darker regions across the Galactic Plane correspond to dense clouds of interstellar gas and dust that absorb starlight along the line of sight.
- Many globular and open clusters – groupings of stars held together by their mutual gravity – are also sprinkled across the image. Globular clusters, large assemblies of hundreds of thousands to millions of old stars, are mainly found in the halo of the Milky Way, a roughly spherical structure with a radius of about 100,000 light-years, and so are visible across the image. Open clusters are smaller assemblies of hundreds to thousands of stars and are found mainly in the Galactic Plane.
- 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. Other nearby galaxies are also visible, most notably Andromeda (also known as M31), the largest galactic neighbor to the Milky Way, in the lower left of the image. Below Andromeda is its satellite, the Triangulum galaxy (M33).
Figure 35: Gaia's first sky map, annotated (image credit: ESA/Gaia/DPAC)
- On its way to assembling the most detailed 3D map ever made of our Milky Way galaxy, Gaia has pinned down the precise position on the sky and the brightness of 1142 million stars. As a taster of the richer catalog to come in the near future, today's release also features the distances and the motions across the sky for more than two million stars.
Figure 36: Gaia's first sky map (image credit: ESA/Gaia/DPAC; Acknowledgement: A. Moitinho & M. Barros (CENTRA – University of Lisbon), F. Mignard (Observatoire de la Côte d'Azur), on behalf of DPAC)
- "Gaia is at the forefront of astrometry, charting the sky at precisions that have never been achieved before," says Alvaro Giménez, ESA's Director of Science. "Today's release gives us a first impression of the extraordinary data that await us and that will revolutionize our understanding of how stars are distributed and move across our Galaxy."
- Launched 1000 days ago, Gaia started its scientific work in July 2014. This first release is based on data collected during its first 14 months of scanning the sky, up to September 2015. "The beautiful map we are publishing today shows the density of stars measured by Gaia across the entire sky, and confirms that it collected superb data during its first year of operations," says Timo Prusti, Gaia project scientist at ESA.
- The stripes and other artefacts in the image reflect how Gaia scans the sky, and will gradually fade as more scans are made during the five-year mission. "The satellite is working well and we have demonstrated that it is possible to handle the analysis of a billion stars. Although the current data are preliminary, we wanted to make them available for the astronomical community to use as soon as possible," adds Timo Prusti.
- Transforming the raw information into useful and reliable stellar positions to a level of accuracy never possible before is an extremely complex procedure, entrusted to a pan-European collaboration of about 450 scientists and software engineers: the Gaia DPAC (Data Processing and Analysis Consortium). "Today's release is the result of a painstaking collaborative work over the past decade," says Anthony Brown from Leiden University in the Netherlands, and consortium chair.
- "Together with experts from a variety of disciplines, we had to prepare ourselves even before the start of observations, then treated the data, packaged them into meaningful astronomical products, and validated their scientific content."
- In addition to processing the full billion-star catalog, the scientists looked in detail at the roughly two million stars in common between Gaia's first year and the earlier Hipparcos and Tycho-2 Catalogs, both derived from ESA's Hipparcos mission, which charted the sky more than two decades ago.
- By combining Gaia data with information from these less precise catalogs, it was possible to start disentangling the effects of 'parallax' and 'proper motion' even from the first year of observations only. Parallax is a small motion in the apparent position of a star caused by Earth's yearly revolution around the Sun and depends on a star's distance from us, while proper motion is due to the physical movement of stars through the Galaxy.
- This new catalog is twice as precise and contains almost 20 times as many stars as the previous definitive reference for astrometry, the Hipparcos Catalog. As part of their work in validating the catalog, DPAC scientists have conducted a study of open stellar clusters – groups of relatively young stars that were born together – that clearly demonstrates the improvement enabled by the new data.
- The data from Gaia's first release can be accessed at http://archives.esac.esa.int/gaia. The content of this first release was presented today during a media briefing at ES/ESAC (European Space Astronomy Center) in Villanueva de la Cañada, Madrid, Spain. Fifteen scientific papers describing the data contained in the release and their validation process will appear in a special issue of Astronomy & Astrophysics.
• August 16, 2016: Operating in the depths of space, far beyond the Moon's orbit, ESA's Gaia spacecraft has now completed two years of a planned five-year survey of the sky. Despite a series of unexpected technical challenges, the mission is on track to complete the most detailed and complex mapping of the heavens ever undertaken. 61)
- Located at the L2 Lagrange point, 1.5 million km from Earth, Gaia surveys the entire sky as it spins on its axis. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia is providing a huge treasure trove of data that enables scientists to tease out their distances and motions through our Galaxy.
- "More than 50 billion focal plane transits, 110 billion photometric observations and 9.4 billion spectroscopic observations have been successfully processed to date," notes Fred Jansen, ESA's mission manager for Gaia. The immense volume of data and their complex nature have required a huge effort from the scientists and software developers, distributed across Europe, who make up Gaia's DPAC (Data Processing and Analysis Consortium). The first product of their efforts will be a public data release scheduled for 14 September.
- "The spacecraft is working well and the data processing is on the right track," says Timo Prusti, ESA's project scientist for Gaia. "Most of the spacecraft systems that are crucial for the success of our mission have behaved as well as, or even better than, expected. "These include the focal plane assembly, onboard data handling system, onboard detection of sources, the phased array antenna, and the attitude control and micro-propulsion subsystems."
- Nevertheless, the first two years of Gaia operations have not been without their challenges. A few largely unforeseen problems have occurred, requiring detailed, ongoing investigations by teams from ESA, industry and the DPAC.
- One problem detected early in the commissioning phase was associated with water freezing on some parts of the optics, causing a reduction in the sensitivity of the telescopes. The mission team expected some water to be trapped in the spacecraft before launch, followed by freezing once the spacecraft reached orbit and started to outgas. Gaia's mirrors were already equipped with heaters to take care of this. However, the amount of ice was much more than expected and the ice deposits survived the heating procedure that took place early in the mission.
- A second complication is caused by stray light infiltrating into Gaia's focal plane at a level higher than predicted before launch. Light from very bright stars and the Milky Way is reflected onto the focal plane, but the more serious stray light is coming from sunlight being scattered by minute fibers around the edge of the insulating blanket that covers Gaia's 10 m diameter sunshield.
- "This results in higher background 'noise' in the data," explains Prusti. "This is irrelevant for observations of brighter stars, but more significant for fainter stars. We can still easily detect stars of magnitude 20, but the stray light means that we can pinpoint locations of faint stars to an accuracy of 500 µs instead of the planned 300 µs. However, it is important to stress that, despite this 'noise', Gaia is still the most accurate star mapper ever built."
- Another surprise is a series of minor mechanical vibrations which Timo Prusti dubs 'micro-clanks'. "Thermal expansion and contraction effects are seen on all spacecraft, which results in so-called 'clanks'," he says. "The mechanical changes on Gaia are much smaller than normal, and the only reason we can detect them is because its attitude control system is so sensitive. "We have discovered that the micro-clanks cause minute structural changes within the spacecraft which lead to tiny discontinuities in the rotation angle of the spacecraft. They result in small inaccuracies in the timing and positions of the stars as they pass across Gaia's array of CCDs. - "However, we are not too worried about this because we can see when it is happening and we know how to fix it. New software will be implemented next year to remove the effect from the data."
- A similar effect is caused by micrometeorites, specks of space dust that strike the spacecraft at high speed. However, this environmental hazard was well understood and predicted before launch. "Fairly large impacts may occur a few times each month," says Jansen. "If they change the spacecraft's rotation rate by more than 10 milliarcseconds per second, they can adversely affect the science data, but such events are rare and their impact on the overall science data return is very small."
- Finally, the Gaia team has been analyzing data from a laser device called the 'basic angle monitor'. The instrument is designed to measure, with an extremely high level of accuracy, any variations in the angle of separation between Gaia's two telescopes. This is necessary in order to correct for expected periodic variations in the separation angle caused by thermal changes in the payload as Gaia spins. — Knowing this basic angle to an accuracy of 0.5 microarcseconds every 15 minutes is crucial in order to achieve absolute global astrometry – mapping of stellar positions - and it is an essential requirement in the data analysis process. However, during commissioning, it was noted that the basic angle fluctuates periodically, at a level more than 100 times greater than expected. Careful investigation has concluded that the variation is probably caused by thermal expansion and contraction resulting from some solar heat reaching the payload via the service module.
- "Although the mission has experienced a number of technical challenges, all of these have either been mitigated by additional software and analysis, or the mitigating mechanism is known and tested but full implementation is pending," says Jansen. "Overall, through the extensive efforts of ESA, DPAC, and prime contractor Airbus Defence and Space, these effects have had a limited impact on our ability to achieve the mission's original science objectives." — "We are confident that we shall still be able to analyze more than one billion stars, measuring each star's position and motion up to 100 times more accurately than Gaia's predecessor Hipparcos," says Prusti.
Figure 37: Illustration of the GAIA BAM (Basic Angle Monitor) bar (image credit: TNO, Fred Kamphues, Ref. 61)
• July 4, 2016: ESA's billion-star surveyor Gaia, launched on 19 December 2013, and in routine science operations since 25 July 2014, will release the first mission data on 14 September 2016. 62)
- Gaia is designed to map more than 1 billion stars in our Galaxy, and to provide positions, parallaxes and proper motions at an unprecedented accuracy level, far below a milliarcsecond. These accuracies can only be achieved after a complex data processing that requires observations taken throughout the 5-year nominal mission. For this reason, the final Gaia results will not be available until the early 2020s, but a number of early releases have been foreseen, based on increasingly longer stretches of observations.
- The first Gaia data release, which will be available online on 14 September, will include the positions and G magnitude for about one billion stars using observations taken between 25 July 2014 and 16 September 2015.
- In addition, for a subset of data – about 2 million stars in common between the Tycho-2 Catalog and Gaia – there will be a five-parameter astrometric solution, giving the positions, parallaxes, and proper motions for those objects. This is referred to as the TGAS ( Tycho-Gaia Astrometric Solution).
- Photometric data for RR Lyrae and Cepheid variable stars that were observed frequently during a special scanning mode that repeatedly covered the ecliptic poles will also be made public.
• May 2016: Automation is being gradually introduced into the Gaia mission in a stepwise fashion. - After commissioning and the first full sky survey it was seen that Gaia's Science return could be increased. This involved a significant increase of data volume (circa 45%) to be downlinked (due to various factors, but mainly since Gaia is able to observe stars fainter than the original magnitude 20 limit). In order to downlink this data in an efficient manner, a stepwise approach has been implemented involving taking unmanned ground station passes to downlink this extra science data. A simple solution using open loop automation was quickly implemented, and this has gradually been improved over time adding robustness and closed loop functionality. Gaia is also the first ESOC mission to use the new infrastructure closed loop automation software 'MATIS'. 63)
- Early in the routine phase it became clear that Gaia was performing so well that the pre-launch data downlink estimates used to plan the ground station time were insufficient. A number of factors were at work, but the primary factor was that Gaia can reach fainter magnitudes than required. In order to recover this valuable extra science data, the ESOC FCT (Flight Control Team) quickly investigated the possibility of obtaining extra ground station time for the mission through ESA's 35 m network. Extra ground station time was possible but the increase in expected data was more than could be handled by the existing Spacecraft Controller team through shift work (i.e. the concept was for a single pass of ca. 8 hours with one human operator per day, with some extra shifts during galactic plane scans that occur approximately 20% of the time). Updated data estimates showed an increase of approximately 45%, which required two passes every day, with three passes during galactic plane scans.
- The operations concept was reviewed and a solution found that would allow all the data to be downlinked without increasing the team size. This concept, termed open loop ground automation, involved taking passes without human operators present via pre-planned science downlink operations. One pass per day is maintained with a human operator in control, with an additional pass dedicated purely to science downlink executed without an operator present. Science data is downlinked to the ground station and forwarded to ESOC automatically, via preprogrammed commanding sent from the mission control system to the ground stations (to enable the dedicated ground communication links for science data), and commanding executed from the spacecraft onboard timeline (to enable the transmission of science data on the spacelink).
- Although this system works well, there is increased risk of science data loss on the spacelink in comparison to a concept fully reliant on human operators, who have the possibility to intervene (e.g. by stopping the science downlink on-board in case of problems on the ground station downlink chain, or weather degrading the space to ground downlink). To mitigate this risk, an OBCP (On Board Control Procedure) was developed and activated onboard. This protects the Science downlink by only allowing it to continue in the presence of a good uplink carrier signal detected at spacecraft level from the ground station, i.e. if the uplink level is good at the spacecraft then generally the downlink level at the ground station is also good.
- This open loop automation concept was successfully validated and implemented in Q4 2014, becoming fully operational by November 2014 (ca. one month after the decision to investigate ways to increase ground station time was taken). Since then Science data return has been boosted by approximately 45%.
• March 7, 2016: While scanning the sky to measure the position and velocity of a billion stars, ESA's Gaia satellite also records many 'guest stars' – astronomical sources that, for a short period of time, are much brighter than usual. Some of these transient objects are stars undergoing major outburst events that suddenly boost their brightness, while others are supernovae, the powerful explosions at the end of a star's life. Among these detections, it is also possible to find entire galaxies, which might occasionally become brighter due to bursts of light caused by the accreting activity of the supermassive black holes at their core. 64)
- To further understand the physical properties of these transient sources, it is crucial to observe them for a longer period of time after the first detection. For this reason, the Gaia DPAC (Data Processing and Analysis Consortium) includes a team of scientists that are responsible of scrutinizing the data daily to look for unusual sources.
- Whenever they identify a transient object, the Gaia Science Alert Team, based at the Institute of Astronomy in Cambridge, UK, announces the detection to the astronomical community so that other scientists can follow up on the source with other telescopes, on Earth and in space.
- The team started to issue science alerts in September 2014, shortly after the beginning of the mission's routine scientific operations. However, the Science Alert Team was very soon overwhelmed with the number of potential transient sources, up to a million per day, which had to be heavily filtered to distinguish real guest stars from contaminants in the data stream. This data overload was due to the fact that, at the time, scientists in DPAC were still getting themselves acquainted with the complex nature of the observations performed by Gaia and the huge amount of information they contain.
- So, after about ten months and almost three hundred science alerts issued, publication of the alerts was paused for six months, in the second half of 2015. During this time, the Science Alert Team implemented a number of upgrades in the filtering and detection algorithms that are used to find transients. Eventually, the improved pipeline was switched on again in January 2016.
- The new filtering algorithm is now tuned to mask out very dense regions of the sky, especially towards the most crowded areas along the plane of our Galaxy, the Milky Way, where the false alarm rate is still too high. By combining this mask with an improved treatment of the Gaia data, the team managed to reduce the rate of potential transients from almost one million to a few hundreds per day.
- It still takes a lot of human work to identify the actual transient – about four per day – in the total number of candidate sources, but it is definitely more manageable.
- The team is looking forward to further improving the detection algorithms, and predictions suggest they might eventually find about ten transients per day. Many of these sources are being followed up on by observational programs in both the northern and southern hemispheres.
• November 3, 2015: A local cosmic celebrity was recently pictured among the multitude of stars and Solar System bodies surveyed by ESA's Gaia satellite: Comet 67P/Churyumov–Gerasimenko, currently accompanied by another ESA spacecraft, Rosetta. 65)
- While scanning the sky to map the positions and motions of a billion stars in our Galaxy, Gaia also picks up objects much closer to home, such as asteroids and comets in the Solar System. With its ability to detect faint and moving objects, Gaia has already identified tens of thousands of asteroids since routine science operations began in July 2014, and these data will be used to determine their orbits to unprecedented accuracy.
Figure 38: Rosetta comet seen by Gaia (image credit: ESA)
Legend to Figure 38: The image shows the comet's coma and tail. The nucleus and Rosetta, which was some 300 km from the surface at the time, are both hidden in the innermost pixel. A number of background stars are also sprinkled around the image, which measures about 4.5 arcminutes across – about one-seventh of the Moon's diameter. — While this image of Rosetta's comet has mainly a symbolic value – an ESA mission, 1.5 million km from Earth, looking at a fellow science mission and its object of study, both located over 260 million km away – scientific data were also collected during this observation. In fact, besides the special observing mode used to obtain the image, the comet was also caught by the onboard detection software as a 'suspected moving object'. Over the three-second observation, it appeared to move by some 100 km with respect to the background stars, as seen from a distance of 260 million km.
Flight Operations Experience from Gaia's first 1.5 years (Oct. 2015). 66)
1) Telescope contamination by water ice: Almost immediately after the L2 insertion maneuver and upon first operation of the telescope, it was noticed that the laser light level from the BAM (Basic Angle Monitor) was dropping (the BAM contains a laser interferometer that precisely measures the basic angle between the two fields of view). In parallel, the expected magnitude of the imaged stars was checked and the same effect seen (ruling out a laser problem). It was soon understood that the root cause was water ice sublimating onto the optical surfaces. Experts at Airbus DS, SOC (Science Operations Center) and DPAC (Data Processing and Analysis Consortium) could narrow down the problem to specific mirrors in the payload and targeted re-activation of decontamination heaters (primarily around mirror M2) was able to completely clear the problem.
- The impact of ice contaminated mirrors is two-fold. Firstly the apparent magnitude of the imaged stars gradually falls, which eventually means that the faintestend stars drop below the onboard magnitude limit and fail to be recorded and downlinked. Secondly, if left over longer time scales, the point spread function is eventually degraded. For the first problem a magnitude buffer was included onboard to allow fainter stars (than magnitude 20) to also be downlinked. This was possible due to the excellent performance of the telescope and FPA that could be used below the required magnitude limit of 20.
- After this targeted decontamination had cleared the problem a full decontamination was again performed part way through the commissioning, since the ice contamination was seen to be returning and also to rule out any connection with the Straylight problem. This procedure was seen to successfully clear the contamination problem and confirmed no link to the Straylight phenomena. The effect of ice build up and decontaminating recovery operations can be seen in Figure 39, which covers the initial operations.
Figure 39: The Gaia telescope response loss due to ice build-up and recovery using decontamination operations.
- The decontamination operations impact science due to the thermal disturbance and subsequent settling time required. Hysteresis implies that small mirror refocus operations may also need to be performed. These procedures have been optimized over time (to minimize science impact) and repeated three additional times in flight. Whilst the ice contamination has returned periodically, the rate at which it returns has dramatically reduced and the time between required decontamination operations has been increasing. The last decontamination was performed in June 2015, and since this time there has been no discernible return of the ice.
2) Telescope straylight: Also noticed early in the telescope commissioning, was an excess of straylight in particular parts of the FPA that varied with the spacecraft rotation phase. This data alone was not sufficient to disentangle the origin of the straylight (e.g. from diffracted sunlight or particular bright stellar/solar system objects), so it was decided to run two dedicated in-flight tests. In these tests, the spacecraft was reprogrammed to operate away from the nominal SAA (Sun Aspect Angle) of 45º (see Figure 3); with one test performed at 42º and one at 0º. Note that these attitudes could not be used for science due to the impact on end performance, but were purely for gathering engineering diagnostics. Additionally, operating at these non-nominal attitudes necessitated dedicated procedures to be developed and executed to perform various service module programming [e.g. the PAA (Phased Array Antenna) could not be used from this attitude].
- These tests were successful in that they unambiguously demonstrated the solar origin of the straylight, since it was dramatically reduced when operating at SAA= 0º.
- The Straylight and contamination problems occurred contemporaneously. At the time it was thought the two problems could be linked and that an ice layer may have formed on the inner tent roof, creating a bright surface that boosted straylight onto the FPA. This was disproved via the inflight full decontamination performed in commissioning (see above) and also via ground laboratory tests, which showed that thin ice layers on the roof would not be able increase straylight in line with the in-flight measurements.
- A working group was setup to investigate this issue and subsequently concluded that the extra straylight was caused by fibers protruding beyond the edge of the sunshield, which boosted diffracted light from the sun. - With the straylight understood and characterized, work commenced on developing a new onboard software version for the 7 VPUs (Video Processing Units). This new software contained updates from the commissioning phase, including functionality to mitigate the effect of straylight. This new software version was developed at Airbus DS during the second half of 2014 and delivered to ESOC Q1 2015. After validation campaign and coordination with the SOC/DPAC science segment, the new software was activated onboard and has been operational since April 2015.
3) Gaia's Big Data and Open Loop Ground Automation: With the commissioning successfully completed and recoveries and updates being worked on or in place, the routine phase of the mission started in June 2014. After an initial scanning law period where EPSL (Ecliptic Pole Scanning Law) was used, the nominal scan law was activated in September 2014. Since then the Gaia mission has been steadily acquiring the huge dataset required for the mapping products.
- Early in the routine phase it became clear that Gaia was performing so well that the pre-launch data downlink estimates used to plan the ground station time were insufficient. A number of factors were at work, but the primary factor was that Gaia can reach fainter magnitudes than required.
- In order to recover this valuable extra science data, the ESOC/FCT ( Flight Control Team) quickly investigated the possibility of obtaining extra ground station time for the mission through ESA's 35 m network. Extra ground station time was possible but the increase in expected data was more than could be handled by the existing Spacecraft Controller team through shift work (i.e. the concept was for a single pass with human operator per day, with some extra shifts during galactic plane scans that occur approximately 20% of the time). Updated data estimates showed an increase by around 45%, which required 2 passes every day, with three passes during galactic plane scans.
- The operations concept was reviewed and a solution found that would allow all the data to be downlinked without increasing the team size. This concept, termed open loop ground automation, involved taking passes without human operators present via pre-planned science downlink operations. One pass per day is maintained with a human operator in control, with an additional pass dedicated purely to science downlink executed without an operator present. Science data is downlinked to the ground station and forwarded to ESOC automatically, via pre-programmed commanding sent from the mission control system to the ground stations and commanding executed from the spacecraft onboard timeline.
- Although this system works well, there is increased risk of science data loss on the downlink in comparison to a concept fully reliant on human operators who have the possibility to intervene (e.g. by stopping the science downlink in case of problems on the ground station downlink chain or weather degrading the space to ground downlink). To mitigate this risk an OBCP (On Board Control Procedure) was developed and implemented onboard. This protects the science downlink by only allowing the downlink to continue in the presence of a good uplink carrier signal detected at spacecraft level from the ground station.
- This open loop automation concept was successfully validated and implemented in Q4 2014. Becoming fully operational by November 2014. Since then, science data return has been boosted by approximately 45%.
4) Outlook: After a successful commissioning campaign the Gaia mission is performing well and on course to deliver the expected dataset for the fields of Astrometry and Astronomy as a whole. The first map releases are planned for the summer of 2016. This time is required due to the enormous data processing task that is underway at SOC and DPAC and the necessity to obtain a minimum number of transits per star (Ref. 66).
• On August 21, 2015, ESA's billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode. 67)
- After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia can tease out their distances and motions through the Milky Way galaxy.
- For the first 28 days, Gaia operated in a special scanning mode that sampled great circles on the sky, but always including the ecliptic poles. This meant that the satellite observed the stars in those regions many times, providing an invaluable database for Gaia's initial calibration.
- At the end of that phase, on 21 August 2014, Gaia commenced its main survey operation, employing a scanning law designed to achieve the best possible coverage of the whole sky.
- Since the start of its routine phase, the satellite recorded 272 billion positional or astrometric measurements 54.4 billion brightness or photometric data points, and 5.4 billion spectra.
- The Gaia team has spent a busy year processing and analyzing these data, en route towards the development of Gaia's main scientific products, consisting of enormous public catalogues of the positions, distances, motions and other properties of more than a billion stars. Because of the immense volumes of data and their complex nature, this requires a huge effort from expert scientists and software developers distributed across Europe, combined in Gaia's DPAC (Data Processing and Analysis Consortium).
- "The past twelve months have been very intense, but we are getting to grips with the data, and are looking forward to the next four years of nominal operations," says Timo Prusti, Gaia project scientist at ESA. "We are just a year away from Gaia's first scheduled data release, an intermediate catalogue planned for the summer of 2016. With the first year of data in our hands, we are now halfway to this milestone, and we're able to present a few preliminary snapshots to show that the spacecraft is working well and that the data processing is on the right track."
- As one example of the ongoing validation, the Gaia team has been able to measure the parallax for an initial sample of two million stars.
- The nearer a star is to the Sun, the larger its parallax, and thus the parallax measured for a star can be used to determine its distance. In turn, the distance can be used to convert the apparent brightness of the star into its true brightness or 'absolute luminosity'. Astronomers plot the absolute luminosities of stars against their temperatures – which are estimated from the stars' colors – to generate a 'Hertzsprung-Russell diagram', named for the two early 20th century scientists who recognized that such a diagram could be used as a tool to understand stellar evolution.
- The graph of Figure 40 shows the absolute luminosity of almost one million stars observed by Gaia as a function of their color. The diagram is based on a combination of data from Gaia's first year of observations and earlier data from ground-and space-based telescopes. Gaia has made an average of roughly 14 measurements of each star on the sky thus far, but this is generally not enough to disentangle the parallax and proper motions. To overcome this, the scientists have combined Gaia data with positions extracted from the Tycho-2 catalog, based on data taken between 1989 and 1993 by Gaia's predecessor, the Hipparcos satellite.
- The luminosity measurements are based on data from Hipparcos and ground-based telescopes, and the color information comes also from ground-based observations. This preliminary diagram provides a taste of what the mission will deliver in the coming years. Later it will be possible to compile a 'Hertzsprung-Russell diagram' based on the Gaia data alone.
- The data points appear to populate some characteristic regions of the diagram, with most of them distributed along the diagonal running from the top left corner to the bottom right: this is called the main sequence of stars, identifying all stars that are burning hydrogen in their cores – a phase that takes up the majority of a star's lifetime. Along the main sequence, brighter and more massive stars are located towards the top left of the diagram, whereas stars with lower masses and brightnesses are found towards the lower right.
- The large clump of data points in the right half of the graph identifies red giant stars: these are evolved stars that have exhausted hydrogen in their cores. As their cores collapse under their own weight, the outer layers of these stars inflate, creating large and cool – thus red – envelopes.
Figure 40: Gaia's first Hertzsprung-Russell diagram (image credit: ESA/Gaia/DPAC/IDT/FL/DPCE/AGIS)
• July 3, 2015: The image of Figure 41, based on housekeeping data from ESA's Gaia satellite, is no ordinary depiction of the heavens. While the image portrays the outline of our Galaxy, the Milky Way, and of its neighboring Magellanic Clouds, it was obtained in a rather unusual way. 68)
- As Gaia scans the sky to measure positions and velocities of a billion stars with unprecedented accuracy, for some stars it also determines their speed across the camera's sensor. This information is used in real time by the attitude and orbit control system to ensure the satellite's orientation is maintained with the desired precision. These speed statistics are routinely sent to Earth, along with the science data, in the form of housekeeping data. They include the total number of stars, used in the attitude-control loop, that is detected every second in each of Gaia's fields of view.
- It is the latter – which is basically an indication of the density of stars across the sky – that was used to produce this uncommon visualization of the celestial sphere. Brighter regions indicate higher concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed.
- A few globular clusters – large assemblies up to millions of stars held together by their mutual gravity – are also sprinkled around the Galactic Plane. Globular clusters, the oldest population of stars in the Galaxy, sit mainly in a spherical halo extending up to 100 ,000 light-years from the center of the Milky Way.
- The globular cluster NGC 104 is easily visible in the image, to the immediate left of the Small Magellanic Cloud. Other globular clusters are highlighted in an annotated version of this image (Figure 42).
- Interestingly, the majority of bright stars that are visible to the naked eye and that form the familiar constellations of the sky are not accounted for in this image because they are too bright to be used by Gaia's control system. Similarly, the Andromeda galaxy – the largest galactic neighbor of the Milky Way – also does not stand out here.
- Counterintuitively, while Gaia carries a billion-pixel camera, it is not a mission aimed at imaging the sky: it is making the largest, most precise 3D map of our Galaxy, providing a crucial tool for studying the formation and evolution of the Milky Way.
Figure 41: Stellar density map (image credit: ESA/Gaia – CC BY-SA 3.0 IGO, Edmund Serpell)
Legend to Figure 41: The outline of our Galaxy, the Milky Way, and of its neighboring Magellanic Clouds, in an image based on housekeeping data from ESA's Gaia satellite, indicating the total number of stars detected every second in each of the satellite's fields of view. The brighter regions indicate higher concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed.
- The plane of the Milky Way, where most of the Galaxy's stars reside, is evidently the brightest portion of this image, running horizontally and especially bright at the center. Darker regions across this broad strip of stars, known as the Galactic Plane, correspond to dense, interstellar clouds of gas and dust that absorb starlight along the line of sight.
- The Galactic Plane is the projection on the sky of the Galactic disc, a flattened structure with a diameter of about 100, 000 light-years and a vertical height of only 1000 light-years. Beyond the plane, only a few objects are visible, most notably the Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way, which stand out in the lower right part of the image. A few globular clusters – large assemblies up to millions of stars held together by their mutual gravity – are also sprinkled around the Galactic Plane.
Figure 42: Annotated stellar density map (image credit: ESA/Gaia – CC BY-SA 3.0 IGO, Edmund Serpell)
• June 30, 2015: A full-size working model of Gaia's internal systems arrived at ESA/ESOC in Darmstadt this week. The Avionics Model is mounted in a circular setup (~4 m in diameter) representing the systems on the actual satellite, now orbiting the Sun–Earth Lagrangian point L2 . With this model, the ESA flight control specialists responsible for Gaia now have access to a fully functional test bench of the inner workings of the billion-star surveyor. 69)
- The model will remain at ESOC for the rest of the mission, with the team trained to use and maintain it with the support of Airbus Defence and Space, Toulouse, the prime contractor during Gaia's development.
Figure 43: Photo of the Gaia avionics model at ESOC to be used by flight control personnel. In the image (L-R): Sonia Perez, Andreas Rudolph, Kevin Kewin, Guillermo Lorenzo (image credit: ESA/L. Guilpain - CC BY-SA IGO 3.0)
• April 15, 2015: Using telescopes on Earth, Gaia's GBOT (Ground Based Optical Tracking) campaign has been delivering daily datasets, which are used to determine the satellite's position, since launch. Making use of data that are generated for its primary purpose, the tracking of Gaia, the campaign has now found an additional science application: the detection of asteroids. Using the astrometric pipeline, several tens of known and unknown asteroids are being detected every night. 70) 71)
Figure 44: Asteroid 0012820 "Robin Williams" discovered by the Tautenburg Observatory in 1978 and re-observed by GBOT with the VST (VLT Survey Telescope) on February 13, 2015 (image credit: GBOT office, Heidelberg)
• April 9, 2015: 2015 is the International Year of Light and marks an important milestone in the history of physics with the one-hundredth anniversary of Einstein's Theory of General Relativity. Having the ability to test some of its aspects to an unprecedented accuracy, Gaia will probe the tiny deviations predicted by General Relativity in our solar system. However, the satellite will also see other evidence at work such as gravitational lensing shown in the images of the so-called Einstein Cross, discovered in 1985 (Q2237+030), and of a very similar one (HE0435-1223), discovered in 2002. 72)
- Gravitational lensing was postulated by Einstein as a consequence of light bending in a gravitational field, although it was not seen until 1979 with the observation of two identical quasars, the Twin Quasars, located in the same direction with the same redshift.
- The two images of Figure 45 show the individual measurements of two outstanding instances of gravitational lensing, each with four lensed images of a distant quasar. The data have been collected by the Gaia astrometric detector over the last few months and have been processed with the nominal pipeline, without paying particular attention to the peculiarities of the sources. In both cases, the four images, closely packed in a square of less than two arcsec side, have been recorded as four independent sources at every passage of the system in the Gaia FOVs (Fields of View). The core of the foreground galaxy is also seen near the center of the Einstein Cross and was measured as if it were a star, while it was not detected in HE0435-1223. However, we know that since the on-board detection was done successfully, more data has been sent to the ground and small field images are available, but have not yet been analyzed.
- In both systems, the distant light source is a quasar, a very compact galaxy located at a distance of about 10 billion light years (z=1.7) while the lensing galaxy is much closer to us, but still several 100 Mpc (Megaparsec) away. The quasar, the galaxy and the observer are almost perfectly aligned (within 50 marcsec), causing the quasar light ray to pass through the galactic bulge that bends the rays, eventually producing the four images when reaching the Earth.
- Using the coordinates of the quasars, the orbit of Gaia and the nominal attitude, it was possible to predict accurately (within 0.5 s) the crossing times through the Gaia FOVs and then locate the relevant observations among the 40 million similar observations acquired by Gaia on an average day. The Initial Data Treatment developed by a DPAC team in Barcelona (and run at ESAC) produces every day a very preliminary, and crude for Gaia standards, astrometry and photometry of all the sources detected on-board. These are the positions plotted in the images with the corresponding HST images in the background. The magnitudes of the individual images range from 17 to 19. As we can see, the repeated observations collected over several months are very consistent and a straight combination yields already an absolute position of each image with an accuracy of about 50 marcsec for the Einstein Cross and better for the second source. This will improve when more data are received and with the global astrometric solution and the final spacecraft attitude. By the end of 2015 there will be 9 new observations of Q2237+030 and 16 for HE0435-1223.
- This early inspection of Gaia data proves that the on-board detector performs very well and can distinguish individual faint point sources within a second of arc of each other and carry out independent measurements at nominal accuracy.
- It is expected that Gaia will observe most of the about 100 known multi-imaged lenses and discover more around known and newly discovered quasars. All the images of these sources will be combined and systematically screened over an area of a few arcseconds square to detect further small images, possibly too faint to be seen in Gaia's standard acquisition mode.
Figure 45: The Einstein Cross (left) and HE0435-1223 (right) with Gaia astrometric positions placed over HST images (image credit: ESA/Gaia/DPAC/Christine Ducourant, Jean-Francois Lecampion (LAB/Observatoire de Bordeaux), Alberto Krone-Martins (SIM/Universidade de Lisboa, LAB/Observatoire de Bordeaux), Laurent Galluccio, Francois Mignard (Observatoire de la Côte d'Azur, Nice)
Legend to Figure 45: Gaia's on-board system was able to detect four images of the distant quasar in both cases and the intervening lens at the middle of the Einstein Cross. The positions are supplied by the Gaia Initial Data Treatment in a routine mode, with a very preliminary attitude determination. The magnitude of the images ranges from 17 to 19 and the astrometric accuracy of each position in this preliminary reduction is around 100 marcsec. It will be much improved during the global astrometric processing where the spacecraft attitude will also be solved together with the source astrometry.
• March 17, 2015: Gaia's FPA (Focal Plane Assembly) with its 106 CCDs is the biggest radiation monitor in space. The interactions with space radiation are detected on the CCDs in form of: 73)
5) Permanent performance degradations caused by:
- Flatband voltage shifts as result of the Total Ionizing Dose (TID)
- Increased CTI (Charge Transfer Inefficiency) as result of newly created charge trapping sites through displacement damage under NIEL (Non-Ionizing Energy Loss).
6) Transient effects leaving charge cloud tracks in the CCD pixels, when high-energy particles create electron-hole pairs along their trajectory through the silicon lattice. Different particles can leave these tracks, but for simplification the appearance of the deposited signal track on the CCD images will be called "Cosmic Rays" (CR) throughout this document.
While the permanent performance degradations are subject to other studies, the interest for this data release is the detection of cosmic rays. Gaia has no shutter and therefore no dark images are taken to be able to assess the radiation background without any other external signal sources. Furthermore nearly all CCDs are read out in windowing mode with the windows centered on the on-board detected objects, which makes cosmic ray analysis on the received science data a difficult task.
The high volume of object detections and the limited transmission bandwidth between L2 and Earth lead to optimization of the generated on-board data volume and due to Gaia's self-calibrating scheme, only few additional calibration data needs to be acquired in addition to the science objects.
One of the most suitable sets of data items for cosmic rays is the occasional engineering data from the readout process of the BAM (Basic Angle Monitor) CCDs acquired through the on-board SIF (Service Interface Function), which allows downlinking information otherwise discarded onboard before transmission. The BAM is Gaia's metrology system to measure the variations of the angle between both telescopes' line-of-sights based on laser interferometry. To be able to expose the static laser interference pattern, these are the only CCDs on board Gaia, which are operated in a stare mode with several seconds exposure time in addition to the TDI readout mode.
The BAM readout windows are centered on the highest signal interference fringes and therefore unsuitable for cosmic ray analysis. In regular intervals throughout the mission an extended length readout strip with the same BAM window width and location is recorded through the SIF mode. These window extensions contain a substantially reduced signal, which in the extreme areas of the window is completely dominated by the TDI transfer signal integration and not by exposure. Cutouts of these extreme areas are pre-processed for the purpose of cosmic ray assessment. This technical note describes the data format and content of the images.
The two BAM CCDs are located at one corner of the Gaia FPA (Figure 46, bottom left). They are divided into a nominal and redundant system marked BAM_N and BAM_R, respectively, both fed from its independent laser interferometry system. Only one system is working at a given time, and redundancy is meant here as duplication with the best performing system working during operations.
Apart from accumulated ionizing dose and proton fluency over the mission lifetime, transient events in form of cosmic ray background can give an indication of the radiation environment at Gaia's operational orbit around the Sun-Earth Lagrange point L2. Cosmic ray transient events on CCDs leave a characteristic charge trail in the images, which can be analyzed in terms of deposited charge, angle of incidence and track length. To support these studies, a small data set is being made available here (Ref. 73) together with a technical note that describes the data release content and format in more detail.
The 152 Flexible Image Transport System (FITS) image files are subsets of the occasional engineering data from the readout process of the BAM CCDs acquired through the on-board SIF (Service Interface Function), which allows downlinking information otherwise discarded on-board before transmission. These BAM CCD images provide the most suitable data set, because of two reasons: (1) these are the only CCDs on board Gaia, which are operated in a stare mode with several seconds exposure time in addition to the nominal TDI readout mode, and (2) the particular readout window subsets contain apart from the cosmic rays no other external signal source but a relatively low and constant background that has been removed in a pre-processing step.
The complete data set covering data acquisitions from May 2014 to January 2015 and the corresponding technical note are available for download from the links of Ref. 73); the data files are also in Ref. 73).
Figure 46: Location of the two BAM CCDs marked "BAM_N" and "BAM_R" in the context of the Gaia FPA (image credit: ESA/ESAC)
• January 2015: Routine operations continue. On average some 40 million stars cross the focal plane every day triggering astrometric, photometric and spectroscopic measurements. Owing to the nature of the astrometric measurements, no early data release is possible, but science alerts are issued regularly. In the meantime, the user community has been kept up to date with news items and data demonstrating the performance of Gaia. 74)
• January 2015: One year ago on January 8, 2014, the Gaia spacecraft had reached its operational orbit at L2 (launch Dec. 19, 2013). Nominal scanning started in September 2014. At this point, Gaia was working so well that it was producing more data than originally foreseen, since it was able to see stars fainter than required. Towards the end of the year, operators had to come up with a method to partially automate ground operations allowing Gaia to take advantage of more ground station time and to expand its mapping data set. 75)
- The science data are now coming down in huge quantities (11 billion camera transits were recorded by the one-year launch anniversary), with anticipation slowly building for what Gaia may find in the coming years.
• Dec. 19, 2014: Gaia was launched one year ago on Dec. 19, 2013. After an exciting year with a successful L2 orbit insertion, a challenging commissioning period and the start of routine operations, Gaia is now scanning the sky, mapping on average 40 million stars a day. Gaia's routine phase started on 25 July 2014, and the spacecraft's current state and usage of consumables indicate that the mission can be extended past its nominal 5-year lifetime.
- In the past year Gaia has recorded 11.1 billion transits with 120.5 billion astrometric, 22.2 billion photometric and 3.3 billion spectroscopic measurements with its 106 CCDs on board. In the routine phase the corresponding numbers are 6.76 billion transits with 73.4, 13.5 and 2 billion astrometric, photometric and spectroscopic measurements. The total data volume from Gaia so far is 9.2 TB. The whole sky has been observed at least once in the routine phase. The spacecraft is currently performing astrometry and photometry for stars brighter than G = 20.7 mag and spectroscopy till GRVS = 16.2 mag.
• September 2014: While scanning the sky to measure the positions and movements of stars in our Galaxy, Gaia has discovered its first stellar explosion in another galaxy far, far away (Figures 47 and 48). This powerful event, now named Gaia14aaa, took place in a distant galaxy some 500 million light-years away, and was revealed via a sudden rise in the galaxy's brightness between two Gaia observations separated by one month. 76)
- Gaia, which began its scientific work on 25 July, repeatedly scans the entire sky, so that each of the roughly one billion stars in the final catalogue will be examined an average of 70 times over the next five years.
- Discovery of supernova Gaia14aaa: In Figure 47, the light curve shows how the galaxy significantly brightened up between the two consecutive Gaia observations because of a stellar explosion, or supernova, which was named Gaia14aaa. This is the first supernova discovered with Gaia. 77)
Figure 47: Light curve of galaxy SDSS J132102.26+453223.8 obtained with Gaia [image credit: ESA/Gaia/DPAC/Z. Kostrzewa-Rutkowska (Warsaw University Astronomical Observatory) & G. Rixon (Institute of Astronomy, Cambridge)]
Legend to Figure 47: It shows the evolution in time of the galaxy's brightness. The brightness is indicated on the vertical axis; smaller magnitude values indicate a brighter source. The data points and error bars at the lower left corner are from the first observation, performed on 31 July 2014, and they are in line with previous observations of the same galaxy performed with other telescopes. The data points at the upper right corner are from the second observation, performed on 30 August 2014, and reveal a sudden rise in brightness of almost two magnitudes (roughly a factor of 6). - Using data from Gaia and other telescopes, astronomers confirmed that Gaia14aaa is a Type Ia supernova, the explosion of a white dwarf caused by the accretion of matter from a companion star in a binary system.
Figure 48: Artist's impression of a Type Ia supernova — the explosion of a white dwarf locked in a binary system with a companion star (image credit: ESA, ATG medialab, C. Carreau)
Legend to Figure 48: While other types of supernovas are the explosive demises of massive stars, several times more massive than the Sun, Type Ia supernovas are the end product of their less massive counterparts. Low-mass stars, with masses similar to the Sun's, end their lives gently, puffing up their outer layers and leaving behind a compact white dwarf. Due to their high density, white dwarfs can exert an intense gravitational pull on a nearby companion star, accreting mass from it until the white dwarf reaches a critical mass that then sparks a violent explosion.
• Routine operations started immediately after the commissioning period. At the beginning, the EPSL (Ecliptic Pole Scanning Law) was used, for 28 days, to guide the mapping of the sky. In this mode every scan crossed both the North and the South Ecliptic Pole. The advantage was that the stellar fields at the poles were studied in detail before the Gaia launch and could therefore be used for calibration. 78)
- During the second routine operations month, Gaia was commanded to follow the NSL (Nominal Scanning Law). The transition from EPSL to NSL was done in a continuous fashion to transfer the calibration over to the nominal mode. At the end of September, Gaia's spin axis orientation was optimized to catch a bright star close to the limb of Jupiter later in the mission for a light deflection experiment.
• August 4, 2014: Seven months after launch and following extensive commissioning, ESA's Gaia satellite is ready to start its scientific mission. The science phase formally began on July 18, 2014, meaning that Gaia is now under the responsibility of the Mission Manager, William O'Mullane, and the Science Operations Center team at ESA/ESAC (European Space Astronomy Center) in Madrid, Spain. 79)
On this occasion, the Gaia flag also changed home. The Gaia Project Manager, Giuseppe Sarri, handed it over to William O'Mullane, who is holding the flag in this photo (Figure 49, right) together with Payload Manager Philippe Garé (left). This symbolic handover marks a new chapter in the mission's story.
This is one of two flags that flew at Europe's Spaceport in Kourou, French Guiana during the weeks leading up to the launch on December 19, 2013. Shortly after, the other flag was sent to ESA/ESOC (European Space Operations Center) in Darmstadt, Germany, where the Mission Operations Center has since been taking care of Gaia's health.
While the Mission Operations Center receives the data from the satellite, the Science Operations Center coordinates their distribution to the Data Processing and Analysis Consortium, who will eventually produce the Gaia catalog.
The design on the flag represents the attempt of humankind to reach for the stars, an enduring fascination that resonates with the scientific goals of the Gaia mission. Originally designed for the launcher fairing, the logo will now fly at two of ESA's establishments in Europe, looking forward to Gaia's measurements and to the many discoveries to come.
Figure 49: Handover of the Gaia flag by ESA Teams to mark the start of science operations (image credit: ESA)
• July 29, 2014: The Gaia mission is operational to begin its five-year science phase. Following extensive in-orbit commissioning and several unexpected challenges, ESA's billion-star surveyor, Gaia, is now ready to begin its science mission. 80)
The commissioning phase uncovered some unexpected anomalies:
- One problem detected early in the commissioning was associated with water freezing on some parts of the optics, causing a temporary reduction in transmission of the telescopes. This water was likely trapped in the spacecraft before launch and emerged once it was in a vacuum. Heating the affected optics to remove the ice has now largely solved this problem, but it is likely that one or two more 'decontamination' cycles will be required during the mission to keep it in check.
- Another problem is associated with 'stray light' reaching Gaia's focal plane at a level higher than predicted before launch. This appears to be a mixture of light from the Sun finding its way past Gaia's 10 m diameter sunshield and light from other astronomical objects, both making their way to the focal plane as a diffuse background.
The commissioning of Gaia came to its formal end on July 18, 2014 when the board members of the MIOCR (Mission In-Orbit Commissioning Review) confirmed the readiness of the space and ground segments to start routine operations. The review summarized the commissioning activities both on ground and in orbit. New scientific performance estimates have been calculated since using in-orbit commissioning data. 81)
On July 25, 2014, Gaia started its routine phase by scanning the sky for 28 days using the so-called ecliptic-poles scanning law. This is useful to bootstrap the basic calibrations of the data. After these 28 days, the nominal scanning law will be used to determine how Gaia is scanning the sky. Although the commissioning phase has ended, some activities remain to be completed. The root causes of the stray light and the basic-angle variations have not been found yet. A dedicated working group will hence address these topics during the remainder of 2014. Nonetheless, Gaia has started its 5-year journey today to produce a map of the Galaxy in three dimensions for one billion stars with unprecedented precision!
• June 16, 2014: A series of exhaustive tests have been conducted over the past few months to characterize some anomalies that have been revealed during the commissioning of Gaia. Key among these are an increased background seen in Gaia's focal plane assembly due to stray light entering the satellite and reduced transmission of the telescope optics. In an effort to understand both problems, much of the diagnostic work has been focussed on contamination due to small amounts of water trapped in the spacecraft before launch that has been "outgassing" now that Gaia is in a vacuum. The Gaia payload module is illustrated in Figure 52. 82)
The water vapor freezes out as ice on cold surfaces and since Gaia's payload sits at temperatures between –100 and –150°C in the dark behind the big sunshield, that is where it ends up, including on the telescope mirrors. The ice initially led to a significant decrease in the overall transmission of the optics, but this problem was successfully dealt with by using heaters on Gaia's mirrors and focal plane to remove the ice, before letting them cool down to operational temperatures again.
Some ice on the mirrors was expected – that is why the mirrors are equipped with heaters – but the amount detected was higher than expected. As the spacecraft continues to outgas for a while, future 'decontamination' campaigns are foreseen to keep the transmission issue in check using a much lighter heating procedure to minimize any disturbing effect on the thermal stability of the spacecraft.
With regards to the stray light, our analysis of the test data indicates that it is a mixture of sunlight diffracting over the edge of the sunshield and brighter sources in the 'night sky' on the payload side, both being scattered into the focal plane. A model has been developed which goes some way to explaining the stray light seen in the focal plane, but not all aspects are yet understood.
One key working hypothesis was that ice deposits have built up on the ceiling of the thermal tent structure surrounding the payload, and that scattering off this ice might enhance the stray light. Although there is no way to directly confirm that this is indeed the situation, the Gaia project team nevertheless considered ways of removing any such ice.
Unlike the mirrors and focal plane, the thermal tent does not have any heaters, so alternative solutions had to be explored. One option analyzed in detail would involve altering the attitude of the spacecraft to allow sunlight to directly enter the thermal tent in order to remove any ice that might be there. The risks associated with this concept were assessed, and software and procedures developed to carry it out safely, but there is currently no plan to do so.
Under the assumption that the stray light cannot be completely eliminated, the team is investigating a variety of modified observing strategies to help reduce its impact over the course of the mission, along with modified on-board and ground software to best optimize the data that will be collected. Even if the team has to work with the stray light, it is already known that it will only affect the quality of the data collected for the faintest of Gaia's one billion stars.
Stray light increases the background detected by Gaia and thus the associated noise. The impact is largest for the faintest stars, where the noise associated with the stellar light itself is comparable to that from the background, but there is minimal impact on brighter ones, for which the background is an insignificant fraction of the total flux.
The stray light is variable across Gaia's focal plane and variable with time, and has a different effect on each of Gaia's science instruments and the corresponding science goals. Thus, it is not easy to characterize its impact in a simple way.
However, broadly speaking, the team's current analysis is that if the stray light remains as it is today, its impact will be to degrade the astrometric accuracy of a solar-type star at magnitude 20, the faint limit of Gaia, by roughly 50%, from 290 µarcsec to 430 µarcsec by the end of the mission. Things improve as one moves to progressively brighter stars, and by magnitude 15, the accuracy will remain unaltered at approximately 25 µarcsec.
It is important to realize that for many of Gaia's science goals, it is these relatively brighter stars and their much higher accuracy positions that are critical, and so it is good to see that they are essentially unaffected. Also, the total number of stars detected and measured will remain unchanged.
For brightness and low-resolution spectroscopic measurements made by Gaia's photometric instruments, current indications are that the faintest stars at magnitude 20 will have been measured to roughly the 6–8% level by the end of the mission, rather than a nominal 4%, while brighter stars will remain more accurate at about 0.4%.
The RVS (Radial Velocity Spectrometer) is most affected by the stray light and about 1.5 magnitudes of sensitivity could be lost, although the number of stars that that translates into will not be known until on-going data analysis is complete.
Finally, Gaia also contains a laser interferometer called the BAM (Basic Angle Monitor) system, designed to measure the angle of separation between Gaia's two telescopes to an accuracy of 5 µarcsec every few minutes. This is necessary in order to correct for variations in the separation angle caused by 'normal' thermal changes in the payload as Gaia spins. The system is working as planned, but is seeing larger-than-expected variations in the basic angle. The team is currently examining these data to discover if this issue will have any impact.
A comprehensive understanding of these issues will be given, when a thorough analysis of all engineering tests is complete. Gaia has nearly completed its performance verification data taking, and is about to start a month-long dedicated science observation run. Once the data have been fully analyzed, the team will be able to provide a detailed quantitative assessment of the scientific performance of Gaia (Ref. 82).
• February 2014: The Gaia observatory is slowly being brought into focus. A test calibration image (Figure 50), taken as part of commissioning the mission to 'fine tune' the behavior of the instruments, is one of the first proper 'images' to be seen from Gaia, but ironically, it will also be one of the last, as Gaia's main scientific operational mode does not involve sending full images back to Earth. — Once Gaia starts making routine measurements, it will generate truly enormous amounts of data. To maximize the key science of the mission, only small 'cut-outs' centered on each of the stars it detects will be sent back to Earth for analysis. 83)
In the commissioning phase, the telescopes must be aligned and focused, along with precise calibration of the instruments, a painstaking procedure that will take several months — to understand the full behavior and performance of the instruments — before Gaia is ready to enter its five-year operational phase. As part of that process, the Gaia team has been using a test mode to download sections of data from the camera, including the image of NGC1818 (Figure 50), a young star cluster in the Large Magellanic Cloud. The image covers an area less than 1% of the full Gaia field of view.
Figure 50: Gaia calibration image shows a dense cluster of stars in the Large Magellanic Cloud, a satellite galaxy of our Milky Way (image credit: ESA, DPAC, Airbus DS)
• With a final, modest, thruster burn on January 14, 2014, ESA's billion-star surveyor finalized its entry into its orbit around 'L2', a virtual point far out in space. L2 provides a moderate radiation environment, which helps extend the life of the instrument detectors in space. However, orbits around L2 are fundamentally unstable. 84)
- Lissajous orbit: In terms of the math, the thruster burns on January 2014 are moving Gaia onto what's known as a 'stable manifold' – a pathway in space that will lead the spacecraft to orbit around L2. Gaia is now moving in a so-called Lissajous orbit around L2, once every 180 days. - The name Lissajous refers to the shape of the path traced out by the orbit as seen from Earth, which will rise then fall above and below the ecliptic plane (the plane of Earth's orbit around the Sun) while sometimes leading and sometimes lagging the Earth. 85) 86)
Figure 51: Schematic view of Gaia's Lissajous orbit about L2 (image credit: ESA)
• January 08, 2014: The Gaia spacecraft is now in its operational orbit around the Lagrangian point L2, a gravitationally stable virtual region in space, 1.5 million km from Earth. 87)
- Entering orbit around L2 is a rather complex endeavor, achieved by firing Gaia's thrusters in such as way as to push the spacecraft in the desired direction whilst keeping the Sun away from the delicate science instruments.
- Once the spacecraft instruments have been fully tested and calibrated – an activity that started en route to L2 and will continue for another four months – Gaia will be ready to enter a five-year operational phase.
• Dec. 20, 2013: Gaia performed an important thruster burn to set course to its destination. The critical maneuver boosts Gaia into its 263,000 x 707,000 x 370,000 km, 180 day-long orbit around L2.
The first 2 days of operations were critical for the mission success. After separation from the launcher, the spacecraft starts an autonomous sequence that consisted in the main following steps:
- Telemetry initialization
- Payload bipods release
- Chemical propulsion system activation
- Sunshield (DSA) deployment
- Second sun acquisition.