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BepiColombo Mission

Space Segment    MPO    Launch   Status   MMO    MTM    Ground Segment    References 

BepiColombo is one of ESA’s (European Space Agency) cornerstone missions being conducted in cooperation with Japan, it will explore Mercury, the planet closest to the Sun. Europe's space scientists have identified the mission as one of the most challenging long-term planetary projects, because Mercury's proximity to the Sun makes it difficult for a spacecraft to reach the planet and to survive in the harsh environment found there. The scientific interest in going to Mercury lies in the valuable information that such a mission can provide to enhance our understanding of the planet itself as well as the formation of our Solar System; information which cannot be obtained with observations made from Earth. The overall goal is to study and understand the composition, geophysics, atmosphere, magnetosphere and history of Mercury, the least explored planet in the inner Solar System. In particular, the mission has the following scientific objectives: 1) 2) 3) 4) 5) 6) 7) 8)

- Investigate the origin and evolution of a planet close to the parent star

- Study Mercury as a planet: its form, interior structure, geology, composition and craters

- Examine Mercury's vestigial atmosphere (exosphere): its composition and dynamics

- Probe Mercury's magnetized envelope (magnetosphere): its structure and dynamics

- Determine the origin of Mercury's magnetic field

- Investigate polar deposits: their composition and origin

- Perform a test of Einstein's theory of general relativity

Set to arrive at Mercury in 2024, BepiColombo will investigate properties of the innermost planet of our Solar System that are still mysterious, such as its high density, the fact that it is the only planet with a magnetic field similar to Earth’s, the much higher than expected amount of volatile elements detected by NASA’s Messenger probe and the nature of water ice that may exists in the permanently shadowed areas at the poles.

The BepiColombo mission is named after Professor Giuseppe (Bepi) Colombo (1920-1984) from the University of Padova, Italy, a mathematician and engineer of astonishing imagination. He was the first to see that an unsuspected resonance is responsible for Mercury's habit of rotating on its axis three times for every two revolutions it makes around the Sun. He also suggested to NASA how to use a gravity-assist swing-by of Venus to place the Mariner 10 spacecraft in a solar orbit that would allow it to fly by Mercury three times in 1974-5.

ESA's Science Program Committee decided at its meeting in Naples in 1999 to name the Mercury cornerstone mission in honor of Giuseppe Colombo's achievements.

Mercury is small compared to the Earth, with a diameter of only 4878 km. It orbits the Sun in an elliptic orbit between 0.3 and 0.47 AU from the Sun. Mercury is difficult to observe from the Earth, due to its close proximity to the very bright Sun. For an in-depth study of the planet and its environment, it is therefore necessary to operate a spacecraft equipped with scientific instrumentation around the planet. It is, however, difficult for a spacecraft to reach Mercury, as even more energy is needed than sending a mission to Pluto. Departing from Earth, a spacecraft needs to decelerate to come closer to the Sun and as the solar gravitational force increases with the square of the distance, the required reverse thrust increases accordingly. Furthermore, the thermal environment close to the Sun and close to the hottest planet in the solar system is extremely aggressive, as the direct solar radiation is 10 times higher than at Earth’s distance. 9)

Despite the advances in space flight and the growth in planetary research over the last few decades, enabling detailed investigations of the Earth, Mars, Venus, the outer planets and several moons and asteroids, scientists have not been able to observe much of Mercury. For a very long time the data delivered by NASA’s (US National Aeronautics and Space Administration) Mariner 10, which visited Mercury in1974-1975, was among the best available. During these flybys Mariner 10 was able to image about 45% of the planet’s surface and to discover its unexpected magnetic field. Further discoveries by Mariner 10 are the existence of gaseous species forming an exosphere and the presence of a unique magnetosphere. However, little to nothing is known about Mercury’s interior structure or its elemental and mineralogical composition.

With the launch of the NASA Discovery class mission MESSENGER (Mercury Surface, Space ENvironment, Geochemistry and Ranging) in 2004, the first spacecraft was launched to orbit Mercury. MESSENGER already collected data from two Venus flybys and three Mercury flybys in 2008 and 2009. The orbital phase of the MESSENGER mission started in March 2011. The MESSENGER data will provide valuable discoveries of Mercury and its environment that can be used by the BepiColombo mission in tuning its observations to the most important investigations of planet Mercury.

NASA's MESSENGER mission came to a planned end on May 30, 2015 when it slammed into Mercury’s surface at about 14,000 km/h and created a new crater on the planet’s surface. MESSENGER ended up exceeding its planned mission timeline by three years, by which time the spacecraft had completely depleted its fuel. The last of the fuel was used to position it within the gravitational pull of Mercury and the Sun, so it could delay as long as possible its inevitable plummet towards the surface – while continuing to beam back images – and go out with a bang. 10)

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Figure 1: Messenger’s iridescent Mercury (image credit: NASA, JHU/APL, Carnegie Institution, Washington) 11)

Legend to Figure 1: The contrasting colors have been chosen to emphasize the differences in the composition of the landscape across the planet. The darker regions exhibit low-reflectance material, particularly for light at redder wavelengths. As a result, these regions take on a bluer cast.

- The crisscrossing streaks across the disc of the planet show up in shades of light blue, grey and white. These regions take on a light blue hue for a different reason: their youthfulness. As material is exposed to the harsh space environment around Mercury it darkens, but these pale ‘rays’ are formed from material excavated from beneath the planet’s surface and sent flying during comparatively recent impacts. For this reason, they have retained their youthful glow.

- The yellowish, tan-colored regions are “intermediate terrain”. Mercury also hosts brighter and smoother terrain known as high-reflectance red plains. One example can be seen towards the upper right, where there is a prominent patch that is roughly circular. This is the Caloris basin, an impact crater thought to have been created by an asteroid collision during the Solar System’s early days.

As the nearest planet to the Sun, Mercury has an important role in showing us how planets form. Mercury, Venus, Earth and Mars make up the family of terrestrial planets; each one carrying essential information to trace the history of the whole group.

The knowledge of how they originated and evolved is key to understanding how conditions supporting life arose in the Solar System, and possibly elsewhere. As long as Earth-like planets orbiting other stars remain inaccessible to astronomers, the Solar System is the only laboratory where scientists can test models applicable to other planetary systems.

Exploring Mercury is therefore fundamental to answering important astrophysical and philosophical questions such as 'Are Earth-like planets common in the Galaxy?'

A European mission to Mercury was first proposed in May 1993. Although an assessment showed it to be too costly for a medium-size mission, ESA made a Mercury orbiter one of its three new cornerstone missions when the Horizon 2000 science program was extended in 1994. Gaia competed with BepiColombo for the fifth cornerstone mission. In October 2000, ESA approved a package of missions for 2008–2013 and both BepiColombo and Gaia were approved.

In February 2007, the mission was approved as part of the Cosmic Vision program. Following an unavoidable increase in the mission’s mass during 2008, the launch vehicle was changed from Soyuz-Fregat to Ariane 5. Final approval for the redesigned mission was given by ESA’s Science Program Committee in November 2009.

BepiColombo represents the first time ESA and JAXA have joined forces for the implementation of a major space science mission.

BepiColombo’s mission is especially challenging because Mercury's orbit is so close to our star, the Sun. The planet is hard to observe from a distance, because the Sun is so bright. Furthermore, it is difficult to reach because a spacecraft must lose a lot of energy to ‘fall’ towards the planet from the Earth. The Sun’s enormous gravity presents a challenge in placing a spacecraft into a stable orbit around Mercury.

Only NASA's Mariner 10 and Messenger missions have visited Mercury so far. Mariner 10 provided the first-ever close-up images of the planet when it flew past three times in 1974-1975. En route to its final destination in orbit around Mercury in 18 March 2011, Messenger flew past the planet 3 times (14 January 2008, 6 October 2008, and 29 September 2009), providing new data and images. Once BepiColombo arrives in 2024, it will help reveal information on the composition and history of Mercury. It should discover more about the formation and the history of the inner planets in general, including Earth.

Mercury is a major Roman god. He is the patron god of financial gain, commerce, eloquence (and thus poetry), messages/communication (including divination), travelers, boundaries, luck, trickery and thieves; he is also the guide of souls to the underworld. In Greek mythology, Hermes is an Olympian god of transitions and boundaries. In the Roman adaptation of the Greek pantheon, Hermes is identified with the Roman god Mercury. — Hence, in an orbit around the planet Mercury, the point that is closest to Mercury is termed “periherm” while the farthest point of a spacecraft orbit is called “apoherm”.

Since Mercury is the closest planet to the sun (0.31 AU to 0.47 AU distant) a peak solar intensity of 11 solar constants (14,500 W/m2) is experienced which imposes enormous thermal challenges on the spacecraft modules and their external equipment.

Table 1: Some history and background 12)

The space segment design is driven essentially by the scientific payload requirements,the launch mass constraints and the harsh thermal and radiation environment at Mercury. Key technologies required for the implementation of this challenging mission include the following:

• High-temperature thermal control materials (coatings, adhesives,resins,MLI,OSR).

• Radiator design for high-infrared environment.

• High-temperature and high-intensity solar cells,diodes and substrates for the solar arrays.

• High-temperature steerable high-gain and medium-gain antennas.

• High specific impulse(Isp=4300 s)and high total impulse (23.7 mNs),to be provided by gridded ion engines.

• Payload technology,such as detectors,filters and laser technology.




Space segment:

The BepiColombo mission is based on two spacecraft:

1) MPO (Mercury Planetary Orbiter) to map the planet. MPO is a three-axis stabilized and a nadir pointing spacecraft with an instrument suite of 11 experiments and instruments. MPO is led by ESA. The MPO will focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere and magnetosphere. In addition, it will test Einstein’s theory of general relativity.

2) MMO (Mercury Magnetospheric Orbiter) to investigate its magnetosphere. MMO is a spinning spacecraft carrying a payload of five experiments and instruments. The MMO is led by JAXA.

Among several investigations, BepiColombo will make a complete map of Mercury at different wavelengths. It will chart the planet's mineralogy and elemental composition, determine whether the interior of the planet is molten or not, and investigate the extent and origin of Mercury’s magnetic field.

MPO is ESA's scientific contribution to the mission. JAXA/ISAS (Japan Aerospace Exploration Agency/Institute of Space and Astronautical Science) is providing the MMO. ESA is also building the MTM (Mercury Transfer Module), which will carry the two orbiters to their destination, and the MOSIF (MMO Sunshield and Interface Structure), which provides thermal protection and the mechanical and electrical interfaces for the MMO. The MCS (Mercury Composite Spacecraft) consists of the MPO, MMO, MTM and MOSIF. ESA is responsible for the overall mission design, the design, development and test of the MPO, MTM and MOSIF, the integration and test of the MCS and the launch. 13) 14)

Parameter

MPO (Mercury Planetary Orbiter)

MMO (Mercury Magnetospheric Orbiter)

Stabilization

3-axis stabilized

15 rpm spin-stabilized

Orientation

Nadir pointing

Spin axis at 90º to Sun

Orbit at Mercury

Polar orbit, period of 2.3 hr, 480 x 1500 km

Polar orbit, period of 9.3 hr, 590 x 11,640 km

Spacecraft mass

4100 kg (total mass at launch)
1150 kg (in Mercury orbit)


275 kg (in Mercury orbit)

Spacecraft size

3.9 x 2.2 x 1.7 m (excluding solar wings)

1.9 m ∅ x 1.1 m

Payload mass, power

80 kg, 100-150 W

45 kg, 90 W

Telemetry band

X/Ka-band

X-band

Data volume (downlink)

1550 Gbit/year

160 Gbit/year

Equivalent average data rate

50 kbit/s

5 kbit/s

Antenna

High-temperature resistant 1.0 m X/Ka-band high-gain steerable antenna

0.8 m X-band phased array high-gain antenna

Operational lifetime at Mercury

> 1 year

> 1 year

Table 2: Key parameters of the two spacecraft

Launch, journey and orbit:

The BepiColombo trajectory employs a solar electric propulsion system so that a combination of low-thrust arcs and flybys at Earth, Venus and Mercury are used to reach Mercury with low relative velocity. A brief summary of the key stages in the journey to Mercury are given here:

• Launch on Ariane 5 in April 2018 on escape trajectory to reach Venus

• Cruise trajectory with solar electric propulsion stage - the SEPM (Solar Electric Propulsion Module), up to 290 mN thrust - plus six gravity assists: Venus (twice) and Mercury (four times)

• Approximately 6.7 year cruise phase to Mercury

• Ion propulsion stage jettisoned shortly before arrival at Mercury

• Capture and insertion by chemical propulsion engines within the MPO

• On reaching MMO orbit the MMO is released

• MOSIF is released before further descending to the MPO orbit

• MPO is inserted into final orbit using thrust from chemical propulsion engines

• For MPO and MMO: one Earth-year (4 Mercury years) operations in Mercury orbit with optional one year extension.

Key mission dates

Event

April 2018

Launch

25 July 2019

First Venus flyby

20 May 2020

Second Venus flyby

09 April 2021

First Mercury flyby

27 March 2022

Second Mercury flyby

16 December 2023

Third Mercury flyby

24 January 2024

Fourth Mercury flyby

18 December 2024

Arrival at Mercury

27 March 2015

MPO in final orbit

01 May 2026

End of nominal mission

01 May 2027

End of extended mission

Table 3: Key mission dates for a 2018 launch into a heliocentric transfer orbit

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Figure 2: Artist's rendition of the BepiColombo MCS (Mercury Composite Spacecraft) in cruise configuration heading toward Mercury (image credit: ESA) 15) 16)

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Figure 3: Artist's rendition of BepiColombo's MPO and MMO spacecraft in their respective Mercury orbits (image credit: ESA, C. Carreau)

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Figure 4: Artist's view of the BepiColombo spacecraft MPO (ESA, foreground) and MMO (JAXA, background) at Mercury (image credit: ESA/ATG medialab. The Mercury image was taken by NASA's Messenger spacecraft, image credit: NASA, JHU/APL, and Carnegie Institution) 17)


Spacecraft

Industrial involvement:

• In November 2006, ESA awarded the prime contract for the Implementation Phase to Airbus Defence and Space, former EADS Astrium of Friedrichshafen, Germany. The PDR (Preliminary Design Review) was completed in October 2008.

This included the design and procurement of the 'cruise-composite' spacecraft, including the ESA’s MPO (Mercury Planetary Orbiter), the MTM (Mercury Transfer Module), the MMO’s sunshield and the interface between the MPO and the MMO. 18) Furthermore, the prime contractor provides the design and development of the data management and attitude and orbit control subsystems, and the integration of the engineering model (Ref. 3).

• In December 2012, TAS-I (Thales Alenia Space-Italia) signed a contract with Astrium GmbH for BepiColombo. TAS-I is part of the industrial Core Team, coordinating 35 European manufacturers within its workpackage. The contract concerns the telecommunications, thermal control and electric power distribution systems, along with satellite integration and testing, plus support during the launch campaign. In addition, TAS-I is developing the X- and Ka-band transponders, onboard computer, mass memory and high-gain antenna, a 1.1 m diameter dish antenna that will enable the satellite to communicate with Earth, while also carrying out a Radio Science experiment during the mission. 19)

• In the UK, Airbus DS (formerly Astrium Ltd.) is the co-prime contractor for the electrical and chemical propulsion systems, for the structure of all modules and for the thermal control of the MTM (Mercury Transfer Module). Airbus DS in France will develop the onboard software.

• The MMO and its scientific payload are designed and developed by JAXA. They are responsible for procuring the spacecraft from an industrial team led by NEC.

MCS (Mercury Composite Spacecraft):

The composite spacecraft, as shown in Figure 5, consists of four modules: the MPO (Mercury Planetary Orbiter), the MMO (Mercury Magnetospheric Orbiter) protected by the MOSIF (MMO SunShield and InterFace Structure) and the MTM (Mercury Transfer Module). The MPO,developed by ESA, is the scientific module characterized by a set of observation instruments to study the surface of Mercury and the gravity field of the planet. The MMO, developed by JAXA, is a spin stabilized scientific module aimed at the study of the magnetic field of Mercury. Both orbiters, the MPO and the MMO, will be carried on top of the transfer module MTM during the cruise phase. 20) 21) 22)

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Figure 5: Schematic view of the BepiColombo MCS (Mercury Composite Spacecraft), image credit: Airbus DS

As the mission evolves, then the number of modules decreases. These evolving configurations are a composite of x modules, hence the various configurations are known as MCSn (Mercury Composite Spacecraft), where “n” represents the states L for launch, C for Cruise, A for Approach and O for Orbit.

The MPO, whatever tasks it may perform during cruise, is ultimately a free-flying spacecraft containing all the capabilities needed to perform its scientific mission – for which careful optimization was necessary when considering the thermal environment. The MPO therefore contains most of the capabilities also needed during cruise. In order not to compromise the MPO design by taking unnecessary hardware into Mercury orbit, hardware needed solely for cruise is accommodated in a separate MTM (Mercury Transfer Module).

The MMO (Mercury Magnetospheric Orbiter) is eventually also a free-flying spacecraft containing all the capabilities needed to perform its scientific mission. However with the spacecraft capabilities controlled from the MPO during cruise, the MMO then remains passive throughout (apart from periodic check-outs). Since the MMO is a normally spinning spacecraft, it requires thermal protection during the 3-axis stabilized cruise.

The 4th module of the MCS derives from the MMO’s needs: the MOSIF (MMO SunShield and InterFace Structure) providing thermal protection as well as all the interfaces between MPO and MMO.

The MCSL (Mercury Composite Spacecraft -Launch) and MCSC (Mercury Composite Spacecraft-Cruise) are composed of:

1) MPO (Mercury Planetary Orbiter)

- Spacecraft optimized for its operational mission

- Performs command & control for MCS (with only minor hardware modification for MCS configurations, notably the size of the reaction wheels to control the MCS)

- CPS (Chemical Propulsion System) not used during cruise, however after MTM separation the MPO performs approach propulsion and apoherm lowering of Mercury orbit.

2) MMO (Mercury Magnetospheric Orbiter)

- Spins during operational mission

- Is passive during cruise – apart from checkouts.

3) MOSIF (MMO SunShield and InterFace Structure)

- Thermal protection for the MMO

- Mechanical interface for the MMO

- Harness routing between MPO and MMO

4) MTM (Mercury Transfer Module)

- Provides MEPS (MTM Electric Propulsion System) plus chemical propulsion (for cruise AOCS and navigation correction)

- Provides power for electric propulsion system and for MPO +MMO

- Separated before capture into Mercury orbit.

The MCSA (Mercury Composite Spacecraft-Approach) is created upon MTM separation. On reaching the MMO orbit, the MMO is released to create the MCSO (Mercury Composite Spacecraft-Orbit). The MOSIF is ejected shortly afterwards to leave the MPO.

Power Subsystem: Each of the 3 modules MPO, MMO and MTM contains power generation, storage and distribution hardware. The MPO and MMO power subsystems supply standard 28 V regulated power within the module, with the MMO being supplied by the MPO, as long as both modules are connected. During the cruise phase the power subsystem is a composite of all 3 modules (controlled from the MPO) whereby, all power is provided by the MTM solar array. The MTM generates both 100 V and 28 V supplies. 23)

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Figure 6: Composition of MCS (Mercury Composite Spacecraft), showing module functions and contributions to MCS (image credit: Airbus DS)




MPO (Mercury Planetary Orbiter) Spacecraft

The MPO design is optimized to meet the needs of the payload when the spacecraft is in its operational orbit. The payload components are mounted on the nadir side of the spacecraft, with certain instruments or sensors located directly at the main radiator, to achieve low detector temperatures (Ref. 13).

Structure: The spacecraft structure uses a double-H configuration, designed to harmonize with the single radiator plane necessitated by the Mercury orbit. Heat generated by spacecraft subsystems and payload components, as well as heat that is coming from the Sun and Mercury as it "leaks" through the blankets into the spacecraft, is carried to the radiator by panel-embedded heat pipes. The structural design provides free access to all equipment and instruments during the AIT (Assembly, Integration and Test) program. The design is mass efficient, with the primary structure serving as the mounting surface for all equipment; it will remain permanently assembled during AIT, avoiding the need for connector brackets and preventing alignment disturbances. MPO has four-point bolted interfaces to both the MTM (Mercury Transfer Module) and the MOSIF (MMO Sunshield and Interface Structure), which provides thermal protection and the mechanical and electrical interfaces for the MMO during the journey to Mercury.

The configuration and thermal design provide a classical thermal environment for internally mounted instrument equipment – avoiding costly development programs by re-use of available hardware – while employing dedicated high temperature technologies for external items such as antennas, the solar array, the sun sensors and MLI (Multi-Layer Insulation), which are exposed to the harsh thermal environment around Mercury.

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Figure 7: MPO – showing equipment panel perpendicular to radiator (image credit: Airbus DS)

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Figure 8: MPO spacecraft in deployed configuration (image credit: Airbus DS)

Power system generation: The MPO provides a 28 V regulated bus which also feeds the MMO during Cruise. The MPO includes a 96 Ah lithium-ion battery. The SA (Solar Array) uses both OSRs (Optical Solar Reflectors) and the control of the sun incidence angle to maintain the temperature below 190ºC. For most of the Mercury year, the solar array requires continuous rotation, in order to generate adequate power while at the same time limiting the temperature. The three-panel array has its rotation axis in an optimized direction, nevertheless, the so-called "artificial eclipses" (a condition in which the Sun vector is along the rotational axis of the solar array, i.e. no power is available) occur for short periods at certain times of the Mercury year due to the solar array mounting geometry. The solar array will provide up to 1000 W of electrical power during full science operations phases. During both the natural and artificial eclipses the battery in the MPO will provide electrical energy to the spacecraft in order to allow the scientific operations to continue without interruption (Ref. 23).

Solar Array Control: Because of the intense heat, the single-sided MPO solar array features a mix of solar cells and Optical Surface Reflectors (OSR) to keep its temperature below 200°C. The large MTM solar arrays (area of over 40 m2 in total) use the same high-temperature technology and can provide up to 13 kW power. During cruise, the entire composite is powered through the MTM SA, while the MPO SA is only required at Mercury (remaining edge on to the sun during cruise to limit degradation).

Both arrays can be rotated around their longitudinal axis using dedicated solar array drive mechanisms, under control of the AOCS. To maintain the temperature in the allowed range, both arrays require a special control approach, commanding an offpointing while still achieving sufficiently high power generation.

MTM solar array control in cruise: Down to a sun distance of 0.62 AU, the MTM solar array can be pointed straight at the sun with no thermal limitations. At sun distances smaller than that, the array must also be offpointed to not violate maximum operating temperatures.

MPO solar array control at Mercury: Due to Mercury albedo and infrared radiation, the maximum exposure of the MPO solar array towards the sun varies over the MPO operational orbit. The MPO SA hence has to be rotated continuously to avoid violation of temperature limits.

AOCS (Attitude and Orbit Control Subsystem): The AOCS equipment consists of:

• Three STRs (Star Trackers), each comprising a star tracker unit housing the optics and electronics, a shutter - which can be closed in the event of a major attitude control anomaly - and a baffle

• Two IMUs (Inertial Measurement Units), including four high-accuracy rate-integrating gyros and four accelerometers in a tetrahedral configuration, together with the processing electronics

• Two redundant sets of two FSS (Fine Sun Sensors)

• Four reaction wheel assemblies, controlled by two sets of wheel drive electronics

• Two redundant sets of four 22 N hydrazine / MON-3 (Mixed Oxides of Nitrogen), a mixture of nitrogen tetroxide and 3% of nitric oxide thrusters to provide the change in velocity (ΔV) needed for orbit capture and orbit lowering to the MMO and MPO operational orbits

• Two redundant sets of four 10 N monopropellant (hydrazine) thrusters for attitude control and reaction wheel momentum off-loading.

The reaction wheels are mounted in a tetrahedral configuration; attitude control can be achieved with four wheels operating simultaneously (the nominal operational scenario) or any combination of three wheels.

During science operations, at least two STRs will be used in combination. In the event of major system anomaly on the spacecraft and consequent loss of attitude control, dedicated shutters will protect the STR optical paths to prevent damage due to accidental sun pointing.

For MPO science operations the AOCS must provide continuous nadir pointing whilst meeting accuracy and stability requirements. Two Star Trackers (plus a 3rd for redundancy) and an IMU are co-mounted with instruments on an optical bench while 4 reaction wheels serve as actuators (with 5 N thrusters used for wheel offloading). The AOCS also controls the thermally critical orientation of the solar array and the 22 N thrusters for orbit maneuvers during MOI (Mercury Orbit Insertion).

This basic AOCS is enhanced with sun sensors for survival mode and is further enhanced for the MCS configuration when the MEPS thrusters and MTM 10 N thrusters serve as actuators. As for the MPO, MTM Solar Arrays are also thermally controlled by the appropriate orientation.

During the cruise phase, the AOCS controls the MEPS thruster orientation and corresponding MCS attitude as required by the uploaded mission timeline – with fine pointing of the MEPS thrusters minimizing momentum accumulation by the reaction wheels.

The thermal environment experienced in the MPO orbit and during cruise allows (for a number of thermally critical items) only deviations from nominal attitudes in the order of seconds before overheating and damage occurs. In the event of an OBC reboot, the Survival Mode will be entered and the AOCS control will be transferred to the FCE and a second IMU. In Survival Mode the AOCS uses Sun sensors as the attitude reference. Different survival attitudes apply for the various spacecraft configurations.

From the many changes of flight configuration, the number of actuators employed and the stringent safe and survival modes the AOCS consists of 17 operational modes.

AOCS operations in particular are impacted by S/C modularity. Preparations of attitude slew or orbit control maneuvers have to take into account the vastly different S/C characteristics. Different AOCS guidance contexts need to be maintained depending on the S/C configuration. For instance, the S/C attitude when entering safe mode is configuration-dependent (Figure 9). AOCS solar array guidance is entirely different between MCSC and MPO/MCSA/MCSO configurations. As a result, the interface between the Flight Control Team and the Flight Dynamics team for commanding the S/C is particularly sophisticated.

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Figure 9: Different safe mode attitudes depending on S/C configuration: +Y sun pointing for MCSC, sun close to +X for MCSA/O, sun close to –Z for MPO. Safe mode concept is to have a rotation around the sun line, which has to be in synch with the orbital motion around Mercury in MPO and MCSO configurations (image credit: ESA)

TCS (Thermal Control Subsystem): The MPO TCS must regulate the equipment temperatures (achieving standard equipment levels), transfer heat to the single radiator, shield the radiator from planet infrared illumination, reject 1200 W of dissipated heat from the payload and spacecraft equipments and reject up to 300 W of parasitic heat which enters the MPO body. These functions are achieved by means of (Ref. 22):

- Heatpipes embedded in the equipment mounting panels to collect and transfer the heat the radiator panel

- Spreader heatpipes in the radiator panel, thermally connected to the equipment panels by 90° linking heatpipes

- 97 heatpipes are used, of which a few are 3-dimensional hence difficult to test on ground

- Fixed louvers are mounted in front of the radiator to reflect the planet infrared radiation away from the radiator whilst allowing the radiator an extensive view to space

- The entire MPO body is covered with high temperature MLI developed for BepiColombo

- The outer heat shield comprises 2 layers of Nextel ceramic cloth followed by 11 aluminum layers. The Nextel layers reach 380°C

- Moving inwards to lower temperature, 26 layers of aluminized Upilex are followed by 10 layers of aluminized Mylar

- Spacers of glass fiber and AAerofoam are used to separate the layers in the 4 packets, while Kapton rosettes separate the packets

- The installed MLI has a thickness of 65 mm. The total MLI mass is 94 kg.

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Figure 10: Photo of a section through the MPO MLI (image credit: Airbus DS)

The MOSIF MLI must shade the MMO and limit the infrared heat load to the MMO. The MOSIF MLI is characterized by:

- A single Nextel outer layer

- 7 dimpled titanium layers separated by glass spacers

- It is freely supported over lengths of up to 2.5 m and must withstand the vibration and acoustic environments of the launch.

The MTM TCS must regulate the equipment temperatures, distribute heat in the radiators, reject 2000 W of dissipated heat equipments and reject up to 300 W of parasitic heat which enters the MTM body. These functions are achieved by means of:

- Heatpipes embedded in the radiator panels (which also serve for equipment mounting)

- The embedded heatpipe network is enhanced by surface heatpipes

- 63 heatpipes are used

- Derivatives of the high-temperature MLI are used.

Further MLI applications result from the stack configuration and the separation interfaces of the MPO:

- While the modules are protected as described above, solar illumination gaps between modules can not be tolerated

- Elaborate Gap Closure MLI is implemented between MTM-MPO and MPO-MOSIF. This MLI is in contact with the MPO and is attached to the separating modules (Figure 148).

- The 4-point mechanical interfaces between modules leave holes of Ø140 mm in MLI (to these add 2 x Ø170 mm holes for the connectors at each interface). These holes are closed by DTCs (Deployable Thermal Covers) containing MLI disks to drastically reduce the heat load. The 12 DTCs are mounted between the MLI layers with the cylinder and ring (white coated) protruding through the MLI heatshield.

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Figure 11: Photo of a DTC (Deployable Thermal Cover) to close separation apertures (image credit: Airbus DS, Ref. 22)

DHS (Data Management System): The basic MPO DHS comprises redundant OBCs (On-Board Computer) and an internally redundant SSMM (Solid State Mass Memory) for payload and spacecraft data storage. A MIL-STD-1553B bus is used for spacecraft telemetry and telecommand while all payload TM/TC and science data interfaces use SpaceWire. BepiColombo is the first spacecraft with a network application of SpaceWire interfaces. The MPO provides all the intelligence during cruise and is enhanced with additional data buses to the MMO and MTM for this purpose.

Permanent availability of a functioning processor to guarantee safe and prompt attitude control is provided in Survival Mode by redundant FCEs (Failure Control Electronics) which take over the control functions in the event of an OBC reboot. The FCEs retain control for 7 minutes after which it is taken over by the reconfigured OBC.

The SSMM (Solid State Mass Memory) is a stand-alone unit in the BepiColombo MPO DMS (Data Management System). The SSMM interfaces with the BepiColombo payload instruments and the OBC (On-Board Computer) via main and redundant SpaceWire links. The SSMM stores telemetry packets according to the CCSDS for later downlink via X- or Ka-band. The SSMM also routes telecommand (TC) packets from the OBC to the relevant payload instrument and the returning telemetry reports from instruments to OBC (Ref. 149).

SSMM has a capacity of 384 Gbit. This storage area is organized in packet stores (maximum 50 packet stores active in parallel) for telemetry data storage. There are two types of packet stores that can be created in the SSMM: cyclic packet stores - when the packet store is full, old data is overwritten; and non-cyclic packet store - when the packet store is full the data storage is interrupted (that means new data can not be stored and is lost) and an action from ground is necessary in order to free space by deleting old data via telecommand.

The telemetry science data packets are stored in the SSMM packet stores based on PIDs (Process ID). One PID can only be associated to one SSMM packet store at a time, but several PIDs can be routed to the same SSMM packet store. The instruments will generate low- and/or high-priority science data and store it in different packet stores based on the PIDs.

CPS (Chemical Propulsion Systems): The MPO CPS is tasked with the 15 MOI manoeuvres and attitude control, for which it is equipped with redundant 4 x 22 N and redundant 4 x 5 N thrusters. The 22 N thrusters are bipropellant while the 5 N thrusters are monopropellant: these are combined into the first dual-mode propulsion system implemented on a European spacecraft. The system uses hydrazine and MON (Mixed Oxides of Nitrogen). 669 kg of propellant are carried, giving a capability of 1000 m/s ΔV plus attitude control.

RF communications: The MPO is equipped with two fixed LGAs (Low Gain Antennas), a 2-axis steerable MGA (Medium Gain Antenna) and a 2-axis steerable 1.1 m diameter HGA (High Gain Antenna). The two X-band LGAs will provide omnidirectional coverage at small distances from Earth and can also be used for emergency commanding at any distance. The X-band MGA will be used primarily during the interplanetary cruise phase and in safe and survival modes. The HGA will provide X-band uplink and downlink and Ka-band downlink communications for spacecraft and science operations. The HGA will also be used during the cruise phase to enhance communications and data dump capabilities whenever needed. The X-band horn MGA is steerable around the MPO or MCS obstructions in order to view Earth and is the primary antenna during cruise.

The newly developed DST (Deep Space Transponder) supports telecommanding uplink in X-band with telemetry downlink in both X- and Ka-bands to enable the downlink of 1550 Gb/year of science data. The DST supports ranging in X/X-band and X/Ka-band while the Ka/Ka-band ranging is provided with the inclusion of the payload-provided MORE translator. This ranging strategy is related to the Radio Science Experiment and requires high stability of the HGA.

Power amplification is by TWTAs for both X- and Ka-bands. All antennas are exposed to the severe thermal environment and are based on titanium. The antenna pointing mechanisms for HGA and MGA are capable of operating at 250°C.

ESA's Cebreros 35 m ground station (Ávila, Spain) is planned to be the primary ground facility for communications during all mission phases. The ground stations at Kourou (LEOP), New Norcia (critical phases during cruise and Mercury capture), Perth (LEOP), Usuda (backup) and Uchinoura (backup) will be available for backup during critical flight phases and/or for use during special campaigns.

Ka-band Operations: To increase the scientific return without increasing the duration of the ground station contacts, the BepiColombo transponder includes a Ka-band transmitter in addition to the traditional X-band receiver/transmitter. Use of Ka-band on the downlink was technically validated on the Smart-1 mission of ESA, but this will be the first operational use on a scientific ESA mission.

The quality of a Ka-band link is strongly dependent on weather conditions at the ground station. This is addressed both in the S/C design and the operations approach:

- A space-to-ground closed-loop file transfer protocol is provided, allowing to automatically recover any lost data due to unpredictable Ka-band link variations, similar to a file transfer protocol used on the uplink for BepiColombo (as well as for previous ESA interplanetary missions).

- Selection of an adequate downlink rate for Ka-band operations depending on the expected weather conditions affects the scientific return: if the planning is too conservative, the advantages of Ka-band may not be fully exploited. If it is too optimistic, too much data will need to be retransmitted. While the precise operational concept is yet to be detailed (this will only be relevant for routine operations at Mercury, i.e. not before early 2025), ESOC is currently running studies dedicated to special tools that incorporate local weather forecasts for optimizing the downlink bit rate. Outcome of a first study activity was that by introducing a 1-day weather forecast in the operations concept, a potential advantage of up to 20% in data volume, could be achieved as compared to methods based on availability of seasonal or monthly statistics of the attenuation and brightness temperature.


Launch: The ESA-JAXA BepiColombo mission to Mercury was launched on 20 October 2018 (01:45:28 GMT) with Ariane-5 ECA (VA245) from Europe’s Spaceport in French Guiana. This was the third and final cornerstone mission of the Horizon 2000+ program. — Signals from the spacecraft, received at ESA’s control center in Darmstadt, Germany, via the New Norcia ground tracking station (Australia) at 02:21 GMT confirmed that the launch was successful. 24) 25) 26) 27)

The launch delay decision was made after a major electrical problem was detected during preparations for a thermal test of the MTM (Mercury Transfer Module), one of the major spacecraft elements of BepiColombo. The six-month postponement will have no impact on the science return of the mission. However, the new flight time to Mercury will be 7.2 years, and BepiColombo will now arrive in December 2025, one year later than previously anticipated. The seven-year cruise to the innermost planet of our Solar System will include 9 flybys of Earth, Venus and Mercury. The MTM, MPO and MMO are currently undergoing intensive tests in ESA/ESTEC (European Space Research and Technology Center) in the Netherlands. Everything is going well with the MPO and MMO. The last of the instrument flight models was installed recently on the MPO. 28)

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Figure 12: The ESA-JAXA BepiColombo mission to Mercury lifts off from Europe’s Spaceport in Kourou (image credit: 2018 ESA-CNES-Arianespace)

Orbit: The launch will be followed by a 7.2 years cruise phase, including planetary swingbys at Venus and Mercury, eventually achieving a weak capture by Mercury in December 2025 (1 year later than previously planned). During the cruise phase, electric propulsion will be used for extended periods of time. This is provided by the MTM module, which will be jettisoned at Mercury arrival. The seven-year cruise to the innermost planet of our Solar System will include 9 flybys of Earth, Venus and Mercury. — A set of complex maneuvers will deliver the MMO to its operational orbit, and finally the MPO will be put into a 1500 x 480 km polar orbit (orbital period of about 2.2 hr) to start its scientific mission, planned to last for one Earth year (with a 1 year extension capability).

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Figure 13: Cruise trajectory for April 2018 launch, showing the sun distance, SEP (Solar Electric Propulsion) usage, and planetary flybys (image credit: ESA)

Figure 14 shows the BepiColombo spacecraft. The combined stack can have the following configurations:

1) MCSC (Mercury Composite Spacecraft - Cruise): MTM, MPO, MMO sunshield (MOSIF) and MMO

2) MCSA (Mercury Composite Spacecraft - Approach): MPO, MOSIF and MMO following separation of the MTM

3) MCSO (Mercury Composite Spacecraft - Orbit): MPO and MOSIF following release of the MMO.

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Figure 14: Artist’s impression of BepiColombo in cruise configuration (top), with the various elements of the cruise stack in exploded view (bottom left), and the MPO at Mercury (bottom right), image credit: ESA

MAP (Mercury Approach Phase): The MAP starts after the last electric propulsion maneuver has been completed, approximately two months before the first Mercury orbit insertion maneuver. During this phase, the MTM will separate from the spacecraft stack. The remaining composite of MPO/MMO/MOSIF, the MCSA configuration, will drift into Mercury's sphere of influence, and will need only a small maneuver to get captured in an initial orbit of approximately 590 x 178,000 km ((April 2018 launch scenario). This process is known as a 'weak stability boundary' capture.

MOI (Mercury Orbit Insertion) phase: The MOI starts thereafter, including a series of chemical propulsion maneuvers with the aim of achieving the operational orbit firstly for the MMO (11,639 x 590 km, i=90º, RAAN=67.8º, ω=-2º) and eventually for the MPO (1500 x 480 km, i=90º, RAAN=67.8º, ω=16º).

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Figure 15: Schematic of the MOI sequence for launch in April 2018 (depicted in ecliptic J2000 frame), image credit: ESA

Operations in the MOI phase are driven by the following main constraints:

- Below a certain altitude, the S/C rotation around the sun line has to be synchronized with the orbital motion around Mercury, to ensure thermal limits are not violated.

- Maneuvers shall not take place around Mercury perihelion ±60 deg due to thermal constraints.

- The S/C undergoes eclipse seasons during MOI, which are power-critical in the higher orbits. Special operational measures like boost heating prior to eclipse entry are expected to be required for ensuring a positive power budget. It is imperative to sufficiently lower the orbit prior to the aphelion eclipse season. In particular, a failure to separate the MMO as planned prior to the eclipse season could be mission-critical in case the orbit has not been lowered sufficiently.

- Operational constraints on maneuver execution: a delta time of at least 3 days is observed between maneuvers. No maneuvers are allowed during solar conjunction periods (no ground contact possible) and as from 7 days before (a failed maneuver shortly before a solar conjunction may lead to the S/C using incorrect guidance and hence a violation of thermal constraints, with no ground intervention possible).

This leads to a rather constrained MOI timeline as shown in Figure 15. Five initial burns are performed to reduce the apoherm altitude to the MMO target value of 11,639 km. Following separation of the MMO, the MOSIF is separated shortly after, bringing the S/C into MPO configuration. Another 10 maneuvers are then required to achieve the MPO operational orbit. Duration of the MOI phase is about 3 months, with a total ΔV for the sequence shown in the figure of about 963 m/s.

Mercury Orbit Phase: Once the MPO mission orbit is reached, the final commissioning of the MPO and its payload is performed; this will last about one month. The MPO attitude follows a continuous nadir-pointing profile, providing optimum viewing conditions for the payload.

All MPO science data will be stored in the spacecraft's solid-state mass memory and downlinked during daily station passes with ESA's Cebreros ground station. Every half Mercury year, about every 44 Earth days, the attitude of the spacecraft will have to be reversed around the nadir direction to keep the radiator pointing away from the Sun.

The MMO will communicate with the JAXA/ISAS Sagamihara Space Operations Center via the Usuda Deep Space Center (UDSC) 64 m antenna in Nagano, Japan.

Nominal mission science operations are scheduled to be performed for one Earth year, with a planned extension of another year.




Mission status

• October 15, 2021: The magnetic and particle environment around Mercury was sampled by BepiColombo for the first time during the mission’s close flyby of the planet at 199 km on 1-2 October 2021, while the huge gravitational pull of the planet was felt by its accelerometers. 29)

- The magnetic and accelerometer data have been converted into sound files and presented here for the first time. They capture the ‘sound’ of the solar wind as it bombards a planet close to the Sun, the flexing of the spacecraft as it responded to the change in temperature as it flew from the night to dayside of the planet, and even the sound of a science instrument rotating to its ‘park’ position.

Figure 16: How a spacecraft ‘feels’ a planetary flyby. A spectrogram visualizing the effects of the 1-2 October 2021 Mercury flyby on the ESA/JAXA BepiColombo spacecraft, created from data recorded by the Italian Spring Accelerometer (ISA) onboard the Mercury Planetary Orbiter. Closest approach took place at 23:34:41 UTC on 1 October. ISA recorded the vibrations and the movements of the spacecraft as it flew past the planet. These detections are not audible to the human ear but have been ‘sonified’ and matched to a frequency plot to better visualize the different events (video credit: ESA/BepiColombo/ISA/ASI-INAF)

Unexplored territory

- “It may have been a fleeting flyby, but for some of BepiColombo’s instruments, it marked the beginning of their science data collection, and a chance to really start preparing for the main mission,” says Johannes Benkhoff, ESA’s BepiColombo project scientist. “These flybys also offer the chance to sample regions around Mercury that will not be accessible once we’re in orbit. In this case BepiColombo provided us insight into the particles present close to the planet, as well as the magnetic field boundaries as it traversed through the magnetosphere at greater distances.”

- The PHEBUS ultraviolet spectrometer collected data for an hour around the closest approach, focusing on the elements present in the planet’s extremely low-density atmosphere, or exosphere, which is generated either from the solar wind or from the planet’s surface. Clear peaks of hydrogen and calcium were recorded after the close approach, once BepiColombo exited the shadow of Mercury.

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Figure 17: The BepiColombo PHEBUS spectrometer made its first measurements of Mercury’s tenuous atmosphere (exosphere) during the 1-2 October 2021 close flyby of the planet. The two brightest detections are shown in this uncalibrated quick-look data plot, highlighting the presence of calcium and hydrogen. The emission is seen as a function of time, which peaked once BepiColombo exited the shadow of Mercury (dotted region), shortly after closest approach (image credit: ESA/BepiColombo/PHEBUS, LATMOS/CNES, IKI/Roscosmos, DESP/JAXA)

- Hydrogen and calcium are just two examples of what can be found in the exosphere; once in orbit around Mercury, PHEBUS will characterize Mercury’s exosphere composition and dynamics in great detail, watching how it changes with location and time. PHEBUS is one of several spectrometers that will study Mercury from orbit to understand its surface composition, including looking for ice in permanently shadowed regions of high-latitude craters.

- During the flyby, the Mercury Gamma-ray and Neutron Spectrometer (MGNS) was also operated, detecting bright fluxes of neutron and gamma rays. These emissions are known to be produced by the interaction of galactic cosmic rays with the uppermost surface layers of Mercury, and also provide information about the surface composition. A detailed analysis of the data – also from the Venus flyby – is currently in progress.

Magnetic boundaries

- Sensors on the magnetometer boom – the structure seen extending from the Mercury Planetary Orbiter (MPO) in some of the MCAM images – recorded details of the solar wind and magnetic field around Mercury. During this flyby, the magnetometer team were particularly excited to collect data from so close over the planet’s southern hemisphere; so far, only Mercury’s northern hemisphere has been magnetically surveyed by NASA's Messenger mission.

- “It’s like having just explored North America and seeing South America through binoculars, but unfortunately having to abort the expedition. As a researcher, you’re naturally curious and desperate to go back,” says Daniel Heyner from TU Braunschweig in Germany, who leads the MPO magnetometer researcher group. “That makes this flyby particularly interesting, as it is the first time that data from the planet’s southern hemisphere close to the surface is available – even if it is just a small sample.”

Figure 18: Sound of the solar wind at Mercury. . A sonification of the magnetic field data is presented here. Two synthesizers can be heard which are controlled by three characteristics of the magnetic field. The first synthesizer's pitch represents the slowly changing magnitude of the background magnetic field. First the solar wind is heard, then the increase and decrease of the planetary magnetic field, and then the solar wind again. The lower the pitch the more intense the magnetic field. The pitch of the second synthesizer is controlled by the variations of the magnetic field magnitude. A more turbulent magnetic field is represented by a faster change in pitch. The magnetosheath crossing after closest approach is very prominent. Using the amplitude envelope as a control for the volume of the second synthesizer emphasizes the different magnetospheric regions (video credit: ESA/BepiColombo/MPO-MAG/IGEP-IWF-IC-ISAS)

- The data has been converted into sound to be audible to the human ear. The resulting sonification captures the changing intensity of the magnetic field and solar wind, including the moment the spacecraft crossed the magnetosheath ­– the highly turbulent boundary region between the solar wind and the magnetosphere around the planet.

- Once in Mercury orbit, complementary magnetic field measurements made by both ESA’s MPO and JAXA’s Mercury Magnetospheric Orbiter (known as Mio) will lead to a detailed analysis of the planet’s magnetic field and its source, in order to better understand the origin, evolution and current state of the planet’s interior. Moreover, the two orbiters will travel through different areas of Mercury’s magnetosphere and on different timescales, measuring simultaneously how the magnetic field changes over time and in space, and its relationship to the powerful solar wind.

- In the meantime, Daniel and his colleagues will start to follow up on questions such as: can the characteristics of the magnetic field from the northern hemisphere be easily transferred to the southern hemisphere? Has the magnetic field generated by the dynamo perhaps even changed in the last six years after the Messenger mission – as it continuously does on Earth? The new BepiColombo flyby data – and eventually data from its main science mission – will be compared with global magnetic field models created from the Messenger mission to create the most accurate picture yet of Mercury’s magnetic field.

Feeling the crunch

- The Italian Spring Accelerometer (ISA) onboard the MPO recorded the accelerations measured by the spacecraft as it experienced the extreme gravitational pull of the planet during the flyby, and the response of the change in temperature as the spacecraft entered and exited the shadow of the planet. Furthermore, ISA detected the motion of the PHEBUS spectrometer as it clicked back into its ‘parking’ bracket after it completed its operations at Mercury. — This information has also been translated into an audio file (Figure 16).

- “On the acceleration plots that were appearing on our screens, we could see the tidal effects of Mercury on the BepiColombo structure, the drop of the solar radiation pressure during the transit in the shadow of the planet, and the movement of the center of mass of the spacecraft due to flexing of the large solar arrays,” says Carmelo Magnafico of the Italian National Institute for Astrophysics (INAF). “The real science begins now for us, because in the difference between those expected effects and the actually measured data stands the ISA scientific value. We are extremely happy.”

- ISA will support the study of Mercury’s internal structure and test Einstein's theory of General Relativity to an unprecedented level of accuracy. It will also be central to providing accurate orbit determination of the MPO around Mercury, and of Mercury’s center of mass as it orbits around the Sun.

- The October gravity assist maneuver was the first at Mercury and the fourth of nine flybys overall. During its seven-year cruise to the smallest and innermost planet of the Solar System, BepiColombo makes one flyby at Earth, two at Venus and six at Mercury to help steer it on course to arrive in Mercury orbit in 2025.

- Further science results from the October flyby may be reported in future scientific journals once the science teams have had time to fully analyze the data. All MCAM images are available via the Planetary Science Archive.

• October 2, 2021: The ESA/JAXA BepiColombo mission has captured its first views of its destination planet Mercury as it swooped past in a close gravity assist flyby last night. 30)

- The closest approach took place at 23:34 UTC on 1 October at an altitude of 199 km from the planet’s surface. Images from the spacecraft’s monitoring cameras, along with scientific data from a number of instruments, were collected during the encounter. The images were already downloaded over the course of Saturday morning, and a selection of first impressions are presented here.

- “The flyby was flawless from the spacecraft point of view, and it’s incredible to finally see our target planet,” says Elsa Montagnon, Spacecraft Operations Manager for the mission.

- The monitoring cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution, and are positioned on the Mercury Transfer Module such that they also capture the spacecraft’s structural elements, including its antennas and the magnetometer boom.

- Images were acquired from about five minutes after the time of close approach and up to four hours later. Because BepiColombo arrived on the planet’s nightside, conditions were not ideal to take images directly at the closest approach, thus the closest image was captured from a distance of about 1000 km.

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Figure 19: The joint European-Japanese BepiColombo mission captured this view of Mercury on 1 October 2021 as the spacecraft flew past the planet for a gravity assist maneuver. The image was taken at 23:41:12 UTC by the Mercury Transfer Module’s Monitoring Camera 2 when the spacecraft was 1410 km from Mercury. Closest approach of 199 km took place shortly before, at 23:34:41 UTC on 1 October. This image is one of the closest acquired during the flyby. The cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution. The magnetometer boom of the Mercury Planetary Orbiter and part of the body of the spacecraft are also visible in the image. Close to the edge of the image is the 342 km Raphael crater, which has smaller, younger craters on its floor. Nearby, the Flaubert crater has a cluster of central peaks rather than the single central peak typical of somewhat smaller craters. Central peaks are a result of ‘elastic rebound’ of the target area when hit by a high-speed impactor. Data from BepiColombo’s orbital tour of Mercury will enable us to better understand impact cratering (image credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO)

- “It was an incredible feeling seeing these almost-live pictures of Mercury,” says Valetina Galluzzi, co-investigator of BepiColombo’s SIMBIO-SYS imaging system that will be used once in Mercury orbit. “It really made me happy meeting the planet I have been studying since the very first years of my research career, and I am eager to work on new Mercury images in the future.”

- “It was very exciting to see BepiColombo’s first images of Mercury, and to work out what we were seeing,” says David Rothery of the UK’s Open University who leads ESA’s Mercury Surface and Composition Working Group. “It has made me even more enthusiastic to study the top quality science data that we should get when we are in orbit around Mercury, because this is a planet that we really do not yet fully understand.”

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Figure 20: The joint European-Japanese BepiColombo mission captured this view of Mercury on 1 October 2021 as the spacecraft flew past the planet for a gravity assist maneuver. This image was taken at 23:44:57 UTC by the Mercury Transfer Module’s Monitoring Camera 3, when the spacecraft was 2687 km from Mercury. Closest approach of 199 km took place shortly before, at 23:34:41 UTC. - Visible in the image is a portion of the southern hemisphere of Mercury. Extensive lava plains cover the surface. The largest clearly visible crater, albeit partly obscured by part of the spacecraft, is the 251 km diameter Haydn crater, named after the Austrian composer (1732-1809). The smoothness of its floor indicates that it has been partly flooded by lavas. Where the sunrise is hitting the surface close to the night side of the planet, the topography of the terrain is enhanced. One example where this is apparent is a feature called Astrolabe Rupes, below right of Haydn crater. This sunlit ‘lobate scarp’ is one of many thrust faults resulting from Mercury’s slow global contraction caused by interior cooling (image credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO)

- Although the cratered surface looks rather like Earth’s Moon at first sight, Mercury has a much different history. Once its main science mission begins, BepiColombo’s two science orbiters – ESA’s Mercury Planetary Orbiter and JAXA’s Mercury Magnetospheric Orbiter – will study all aspects of mysterious Mercury from its core to surface processes, magnetic field and exosphere, to better understand the origin and evolution of a planet close to its parent star. For example, it will map the surface of Mercury and analyze its composition to learn more about its formation. One theory is that it may have begun as a larger body that was then stripped of most of its rock by a giant impact. This left it with a relatively large iron core, where its magnetic field is generated, and only a thin rocky outer shell.

- Mercury has no equivalent to the ancient bright lunar highlands: its surface is dark almost everywhere, and was formed by vast outpourings of lava billions of years ago. These lava flows bear the scars of craters formed by asteroids and comets crashing onto the surface at speeds of tens of kilometers per hour. The floors of some of the older and larger craters have been flooded by younger lava flows, and there are also more than a hundred sites where volcanic explosions have ruptured the surface from below.

- BepiColombo will probe these themes to help us understand this mysterious planet more fully, building on the data collected by NASA’s Messenger mission. It will tackle questions such as: What are the volatile substances that turn violently into gas to power the volcanic explosions? How did Mercury retain these volatiles if most of its rock was stripped away? How long did volcanic activity persist? How quickly does Mercury’s magnetic field change?

- “In addition to the images we obtained from the monitoring cameras we also operated several science instruments on the Mercury Planetary Orbiter and Mercury Magnetospheric Orbiter,” adds Johannes Benkhoff, ESA’s BepiColombo project scientist. “I’m really looking forward to seeing these results. It was a fantastic night shift with fabulous teamwork, and with many happy faces.”

- BepiColombo’s main science mission will begin in early 2026. It is making use of nine planetary flybys in total: one at Earth, two at Venus, and six at Mercury, together with the spacecraft’s solar electric propulsion system, to help steer into Mercury orbit. Its next Mercury flyby will take place 23 June 2022.

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Figure 21: The joint European-Japanese BepiColombo mission captured this view of Mercury on 1 October 2021 as the spacecraft flew past the planet for a gravity assist maneuver. The image was taken at 23:44:12 UTC by the Mercury Transfer Module’s Monitoring Camera 2, when the spacecraft was about 2418 km from Mercury. Closest approach of about 199 km took place shortly before, at 23:34 UTC. In this view, north is towards the lower right. The cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution. - The region shown is part of Mercury’s northern hemisphere including Sihtu Planitia that has been flooded by lavas. A round area smoother and brighter than its surroundings characterizes the plains around the Calvino crater, which are called the Rudaki Plains. The 166 km-wide Lermontov crater is also seen, which looks bright because it contains features unique to Mercury called ‘hollows’ where volatile elements are escaping to space. It also contains a vent where volcanic explosions have occurred. BepiColombo will study these types of features once in orbit around the planet (image credit: ESA/BepiColombo/MTM , CC BY-SA 3.0 IGO)

• August 13, 2021: ESA’s Solar Orbiter and BepiColombo spacecraft made a historic Venus flyby earlier this week, passing by the planet within 33 hours of each other and capturing unique imagery and data during the encounter. 31)

- Solar Orbiter flew past Venus on 9 August at a distance of 7995 km, while BepiColombo skimmed past at just 552 km from the planet’s surface on 10 August. The flybys were needed to give the spacecraft a gravity assist to help them reach their next destinations. BepiColombo will make the first of six flybys at Mercury during the night of 1-2 October, before entering orbit in 2025. Solar Orbiter will make a close Earth flyby on 27 November, before further Venus slingshots will tilt its inclination in order to get the first-ever views of the Sun’s poles.

- The Venus flybys required extremely precise deep-space navigation work, ensuring that the spacecraft were on the correct approach trajectories accurate to within just a few kilometers at a distance of 187.7 million km from Earth.

Feeling the heat

- As expected during BepiColombo’s close flyby, the spacecraft modules felt a rapid increase of heat as it passed from the nightside to dayside of the planet. The JAXA Mercury Magnetospheric Orbiter (MMO), situated inside the sunshield, recorded an increase of 110 degrees Celsius on one of its eight solar panels, from -100ºC to +10ºC. Within the spacecraft itself only an increase of 2-3 degrees was observed, demonstrating the effectiveness of the insulation.

- On the European Mercury Transfer Module, a temperature increase of 50 degrees was observed on the spacecraft radiator, while the Mercury Planetary Orbiter (MPO) recorded a change of about 20 degrees.

Gravity tug

- Both Solar Orbiter and BepiColombo also felt the immense gravitational pull of the planet in the angular momentum of their reaction wheels, which are used to maintain spacecraft attitude, keeping it pointing on course.

- The Italian Spring Accelerometer (ISA) onboard the BepiColombo MPO recorded the accelerations measured by the spacecraft with great sensitivity. The ISA team then translated the acceleration data into frequency to make them audible to the human ear. The resulting sound is rich with interesting effects due to the planet’s gravity acting on the spacecraft structure, the response of the spacecraft to the rapid temperature changes, and the reaction wheels that are working hard to compensate for these effects.

Figure 22: The accelerometer also felt the tidal effects acting on the spacecraft as it flew at different distances past Venus. The very small difference in gravitational attraction between BepiColombo’s center of mass and ISA relative to Venus could be detected, the first time an accelerometer recorded this effect at another planet. The team is analyzing this precious data and will use the measurement as a reference to fine-tune the instrument ahead of the scientific phase at Mercury (video credit: ESA)

Multipoint science

- Many of the science instruments were on during the flybys, using the opportunity to collect data on the Venusian magnetic, plasma and particle environment around the spacecraft. Moreover, the unique aspect of the dual flyby is that the two datasets can be compared from locations not usually sampled by a planetary orbiter.

Figure 23: The variability of the total magnetic field during BepiColombo’s second Venus flyby as measured by the Mercury Planetary Orbiter’s Magnetometer (OB sensor). The timeframe covers 12:00 to 14:30 UTC on 10 August 2021, including the closest approach at 13:51 UTC (video credit: ESA)

- The BepiColombo MPO magnetometer team created a simple sonification of the variability of the total magnetic field as they flew past Venus. The audio captures low-frequency wind-like noises caused by the solar wind and its interaction with Venus. The sudden transition of the spacecraft into the very calm solar wind at the bow shock (the location where the planet’s magnetosphere meets the solar wind) is clearly recorded.

- The Solar Orbiter magnetometer team also describes the magnetic field increasing in magnitude due to the compression of the field as they travelled past the flanks of the planet, and then a sharp drop as they crossed the bow shock back into the solar wind again.

- And while Solar Orbiter crossed through the tail of the magnetosphere and out of the bow shock into the solar wind, BepiColombo was ‘upstream’, so the teams will know the input magnetic field conditions throughout the encounter to see how Venus has affected the solar wind downstream. It will take many weeks to make a detailed analysis of the two datasets.

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Figure 24: Flying through Venus’ magnetic environment. Quick look magnetic field strength data recorded by Solar Orbiter’s magnetometer during the 9 August 2021 Venus flyby. The field is seen increasing in magnitude due to the compression of the field as the spacecraft travels past the flank of the planet, and then the sharp drop as it passes the bowshock back into the solar wind (image credit: ESA/Solar Orbiter/MAG team)

• August 10, 2021: The joint European-Japanese BepiColombo mission captured this view of Venus on 10 August 2021 as the spacecraft passed the planet for a gravity assist maneuver. 32)

- The maneuver, the second at Venus and the third of nine flybys overall, helped steer the spacecraft on course for Mercury. During its seven-year cruise to the smallest and innermost planet of the Solar System, BepiColombo makes one flyby at Earth, two at Venus and six at Mercury to brake against the gravitational pull of the Sun in order to enter orbit around Mercury. Its first Mercury flyby will take place 1-2 October 2021.

- BepiColombo, which comprises ESA’s Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter of the Japan Aerospace Exploration Agency (JAXA), is scheduled to reach its target orbit around the smallest and innermost planet of the Solar System in 2025.

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Figure 25: The image was taken at 13:57:56 UTC by the Mercury Transfer Module’s Monitoring Camera 3, when the spacecraft was 1573 km from Venus. Closest approach of 552 km took place shortly before, at 13:51:54 UTC. The cameras provide black-and-white snapshots in 1024 x 1024 pixel resolution. The image has been lightly processed to enhance contrast and use the full dynamic range. A small amount of optical vignetting is seen in the bottom left of the image. The high-gain antenna of the Mercury Planetary Orbiter and part of the body of the spacecraft are visible in front of Venus, at top left (image credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO)