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BepiColombo Mission

Spacecraft    MPO    Status   Launch   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)


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)


MPO (Mercury Planetary Orbiter)

MMO (Mercury Magnetospheric Orbiter)


3-axis stabilized

15 rpm spin-stabilized


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



Data volume (downlink)

1550 Gbit/year

160 Gbit/year

Equivalent average data rate

50 kbit/s

5 kbit/s


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


April 2018


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


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


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


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)



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)


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)


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.


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


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.


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.


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 35).

- 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.


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. 51).

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.



Development status:

• July 23, 2018: Last week the second of two solar arrays on the BepiColombo Mercury Transfer Module (MTM) underwent final inspections and deployment before being folded and stowed for launch. 24)


Figure 12: In this image, the solar array is attached to the MTM, which is out of view to the right, and engineers are carefully checking the alignment of the deployed array. Electrical tests and illumination tests were performed before folding the five-panel, 15 m long array and tensioning the cables ahead of one last deployment test (image credit: ESA)

- After a final inspection, the solar array was folded again and a temporary protective red cover installed, concluding a successful test phase of the transfer module's solar arrays.

- The MTM will carry the two science orbiters – ESA's Mercury Planetary Orbiter and JAXA's Mercury Magnetospheric Orbiter – to the innermost planet using solar electric propulsion along with gravity assist flybys at Earth, Venus and Mercury.

- Shortly before arriving at Mercury in 2025, the MTM will separate and the two science orbiters will be captured into orbit together, before separating and moving into their respective orbits. Together they will provide the most up-to-date investigation of the least explored planet in the inner Solar System to date.

• July 13, 2018: On 6 July, a test deployment of one of the two solar arrays of the BepiColombo MTM (Mercury Transfer Module) was performed during launch preparations at Europe's Spaceport. The MTM will use solar electric propulsion to take the two science orbiters of the BepiColombo mission to the innermost planet, along with gravity assist flybys at Earth, Venus and Mercury itself. 25)


Figure 13: Deployment of MTM solar wing

• July 9, 2018: The ESA-JAXA BepiColombo mission at Europe's Spaceport is undergoing intense preparations for launch. Here, sewing of the insulation blankets on ESA's MPO (Mercury Planetary Orbiter) is taking place, while JAXA's MMO (Mercury Magnetospheric Orbiter) can be seen in the background. 26)


Figure 14: Sewing MPO insulation blankets (image credit: ESA-B. Guillaume)

• June 28, 2018: The BepiColombo MPO (Mercury Planetary Orbiter) with the first part of the multi-layered insulation fitting is completed. The white outer layer is a high-temperature blanket to protect the spacecraft from the extreme thermal conditions that will be experienced in Mercury orbit. 27)

- The joint ESA-JAXA mission comprises two scientific orbiters – ESA's Mercury Planetary Orbiter and JAXA's MMO (Mercury Magnetospheric Orbiter) – that will be carried to the innermost planet by the MTM (Mercury Transfer Module). Launch preparations are taking place at Europe's Spaceport in Kourou. The launch window is open 5 October – 29 November, 2018.


Figure 15: The MPO spacecraft of ESA is shown with the multi-layered insulation (image credit: ESA-B. Guillaume)

• June 12, 2018: One of the main activities in recent weeks for the BepiColombo team at Europe's Spaceport in Kourou has been the installation of multi-layered insulation foils and sewing of high-temperature blankets on the Mercury Planetary Orbiter (Figure 16). 28)

- The white blankets are made from quartz fibers. Because the fabric is not electrically conductive, to control the build-up of electrostatic charge on the surface of the spacecraft, conducting threads have been woven through the outer layer every 10 cm. The edges of the outer blanket are hand-sewn together once installed on the module, as seen in this image.

- The face of the spacecraft the engineer is working on is the panel that will always look at Mercury's surface and as such many of the science instruments are focused here. This includes the orbiter's cameras and spectrometers, a laser altimeter and particle analyzer.

- The face of the spacecraft the engineer is working on is the panel that will always look at Mercury's surface and as such many of the science instruments are focused here. This includes the orbiter's cameras and spectrometers, a laser altimeter and particle analyzer.

- The panel also has fixtures to connect the module to the Transfer Module during the cruise to Mercury.


Figure 16: The insulation is to protect the spacecraft from the extreme thermal conditions that will be experienced in Mercury orbit. - While conventional multi-layered insulation appears gold-colored, the upper layer of the module's striking white high-temperature blanket provides the focus of this image (image credit: ESA–B. Guillaume)

- The face of the spacecraft pointing to the left in this orientation is the spacecraft radiator, which will eventually be fitted with ‘fins' designed to reflect heat directionally, allowing the spacecraft to fly at low altitude over the hot surface of the planet. Heat generated by spacecraft subsystems and payload components, as well as heat that comes from the Sun and Mercury and ‘leaks' through the blankets into the spacecraft, will be conducted to the radiator by heat pipes and ultimately radiated into space.

- The oval shapes correlate to star trackers, used for navigation, while a spectrometer is connected with ground support equipment towards the top. At the back of this face, the magnetometer boom can be seen folded against the spacecraft – it has now also been fitted with multi-layered insulation.

• June 5, 2018: The multi-month payload preparation phase is making progress for the upcoming BepiColombo mission to planet Mercury, which will be launched later this year on an Arianespace Ariane 5 flight from the Spaceport. 29)

- BepiColombo was developed in a joint effort of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It comprises three spacecraft modules and a sunshield, which were delivered along with ground support equipment and other essential hardware during a series of cargo flights in April and May.

- Following their arrival in French Guiana and subsequent transfer by road to the Spaceport's S5 payload preparation facility, the modules – including the Mercury Planetary Orbiter (MPO), Mercury Magnetospheric Orbiter (MMO) and Mercury Transfer Module (MTM) – were removed from their protective shipping containers for inspection. Afterward, they were switched on and battery tested, then fitted with mechanical and electrical ground support systems.


Figure 17: BepiColombo's MPO and MTM undergo electrical testing during activity at the Spaceport (image credit: Arianespace)

- Additional activities to be performed include attaching solar wings to the three modules and testing their deployment mechanisms, dressing the spacecraft in protective insulation, installing the sunshield, conducting pressure tests, fueling and integration.

• May 17, 2018: The Flight Control and Software Support Teams at ESA/ESOC in Germany are ensuring that the vital elements are ready for a simulation campaign ahead of the launch of BepiColombo, the ESA-JAXA mission to Mercury in October 2018. 30)

- The Flight Control Team rehearsed the post-launch deployment of the Medium Gain Antenna, switching on and checking out the MTM Electric Propulsion System and configuring the spacecraft for cruise mode. - Engineers also checked the Mission Control System, spacecraft simulator, flight control procedures and Operations Control Center facilities.

- The full-scale simulations start on 28 May 2018 and will involve the complete Mission Control Team, including Flight Dynamics colleagues, Project and Industry Support teams and experts from the Estrack network of ground stations.

• May 11, 2018: The spacecraft of the BepiColombo mission to Mercury have arrived safely at Europe's Spaceport in Kourou, French Guiana, marking the start of six months of preparation to ready the craft for launch. 31)

- The joint ESA-JAXA mission, which comprises three spacecraft modules and a sunshield, were transported from their home base in ESA's technical heart in the Netherlands by road to Amsterdam Schiphol airport. They were flown to Kourou, French Guiana, on a series of four Antonov cargo aircraft over the last two weeks.

- They were accompanied by approximately 40 containers of essential ground-support equipment, hardware, and other loose items needed for the pre-launch testing and configuring the spacecraft at the launch facility. Another 15 containers travelled by sea.

- Amongst the first tasks at the Spaceport were setting up offices and control rooms, and establishing network connections to support remote links for teams elsewhere in the world to interface with the test equipment that will be connected to the spacecraft.

- Once the spacecraft arrived, their containers were opened and the modules lifted out on their respective multi-purpose trolleys for inspection, to confirm that no problems arose from the transport. The spacecraft were also switched on and batteries tested, and the various mechanical and electrical ground support systems installed.

- Over the next six months activities will include attaching the solar wings to the modules and their deployment mechanisms tested, dressing the spacecraft in protective insulation to prepare for the harsh space environment and extreme temperatures they will experience operating close to the Sun, installing the sunshield, conducting pressure tests, fuelling, and connecting the three spacecraft together.

- The ESA MPO (Mercury Planetary Orbiter) and the JAXA MMO (Mercury Magnetospheric Orbiter) will travel together to the innermost planet, carried by ESA's MTM (Mercury Transfer Module).


Figure 18: The shipping container is lifted from ESA's MPO of the ESA–JAXA BepiColombo mission after arriving at Europe's Spaceport in Kourou for launch preparations (image credit: ESA/CNES/Arianespace/Optique video du CSG – P.Baudon)


Figure 19: JAXA's MMO (Mercury Magnetospheric Orbiter) of the ESA–JAXA BepiColombo mission is unpacked at Europe's Spaceport in Kourou (image credit: JAXA/ESA–M. Basile)

• April 23, 2018: When the MPO (Mercury Transfer Module) of the BepiColombo mission fires its electric propulsion thrusters an ion beam is extracted. This is created through the ionization of xenon propellant, generating the charged particles that can be accelerated further using an electric field. - Together with gravity assist flybys at Earth, Venus and Mercury, the thrust from the ion beam provides the means to travel to the innermost planet. 32)

- After escaping the pull of Earth's gravity with the Ariane 5 launcher, the spacecraft is on an orbit around the Sun. The transfer module then has to use its thrusters to brake against the mighty pull of the Sun's gravity. It also has to tune the shape of its orbit in order to make a series of nine gravity assist flybys at the planets before finally delivering the mission's two science spacecraft into Mercury orbit.

- This image is an excerpt from a supercomputer simulation that models the flow of plasma around the spacecraft just after the high energy ion beam is switched on. An outline of the composite spacecraft with its extended solar arrays is included for reference (Figure 20).

- The simulation was performed to demonstrate that the plasma produced by the thruster is not damaging to the spacecraft: its materials, including solar arrays or instruments, for example, or to the electric propulsion system itself. The simulations also confirmed there are no spurious or dangerous charging events.


Figure 20: The simulation tracks the particles in the beam as well as those that diffuse around the spacecraft, which are created by the interaction of the high energy beam ions with the neutral xenon atoms that also flow out of the thruster. It shows the density of the plasma flowing around the spacecraft and its evolution: red represents high density, blue is low density (image credit: ESA/Félicien Filleul)

- Inflight measurements will verify the simulation results and help improve ways in which the generated plasma, spacecraft and space environment interactions can be better modelled.

- BepiColombo is a joint endeavour between ESA and JAXA. After their seven-year interplanetary journey, the two science orbiters – the MPO (Mercury Planetary Orbiter) and the MMO (Mercury Magnetospheric Orbiter) – will start their main mission to provide the most in-depth study of mysterious Mercury to date.

- The spacecraft begin transferring to Europe's spaceport in Kourou this week, where an intensive period of preparations will ready the mission for launch later this year.

- The simulations were performed by Félicien Filleul as part of ESA's Young Graduate Trainee program.

• April 17, 2018: The MPO (Mercury Planetary Orbiter) of the ESA-JAXA BepiColombo mission to Mercury during the final stages of packing ahead of its journey from ESA in the Netherlands to Europe's spaceport in Kourou. It is pictured here with the shipping container in background, which will subsequently be lowered over the spacecraft and the platform it is sitting on. 33)


Figure 21: The spacecraft's 3.7m wide radiator side faces the viewer; this will be mounted with highly reflective fins to minimize absorption of heat radiated from Mercury, and to allow radiation towards deep space. The three oval shapes indicate startrackers, which will be used for navigation. The rectangular panel at the bottom close to center is the PHEBUS science instrument, an ultraviolet spectrometer. Small thruster units are also visible in each corner. The magnetometer and medium gain antenna booms are folded on the top; the high gain antenna is partially seen to the left (image credit: ESA–C.Carreau)

• On 14 March 2018,the BepiColombo flight control team at ESA/ESOC in Darmstadt, Germany, was joined by experts from the mission team at the Agency's technical center in the Netherlands (ESTEC) as well as industry to conduct a ‘system validation test'. Such tests are critical milestones in getting a spacecraft, its onboard software, the ground systems and the mission control team ready to handle the real flight. 34)

- This week, engineers connected their mission control systems to the actual spacecraft, which is now located at ESA/ESTEC, via telecom links, allowing them to ‘talk' to BepiColombo just as they will after launch when it is in space en route to mercury.

- A modern spacecraft has 42,000 telemetry parameters and 2,650 control parameters in its software – comparable to a mid-size jet aircraft – and hundreds of thousands of lines of code on board.

- BepiColombo, ESA's first mission to Mercury, has two science craft: ESA's Mercury Planetary Orbiter, with 11 experiments and instruments, and Japan's Mercury Magnetospheric Orbiter, with five experiments and instruments.

- The spacecraft, along with ground equipment and mission experts, are set to start the move from the Netherlands to Europe's Spaceport in Kourou, French Guiana at the end of next month. The launch window is open from 5 October until 29 November.


Figure 22: In the photo, in the foreground: Spacecraft Operations Engineer Emanuela Bordoni; center, Deputy Spacecraft Operations Manager Christoph Steiger; at rear, Susanne Fugger, responsible for BepiColombo operations at Airbus Defence and Space, Germany (image credit: ESA)

• March 9, 2018: Europe's first mission to Mercury will soon be ready for shipping to the spaceport to begin final preparations for launch. The mission passed major reviews yesterday, namely the QAR (Qualification Acceptance Review), consisting of the System Qualification Review (QR) and the Flight Acceptance Review (FAR). This means that the three BepiColombo spacecraft, along with ground equipment and mission experts, are confirmed to start the move from ESA/ESTEC in the Netherlands to Europe's Spaceport in Kourou, French Guiana at the end of next month. The launch window is open from 5 October until 29 November 2018. 35) 36)

- "It's been a long and occasionally bumpy road to this point, and there is still plenty to do until we are ready for launch," says Ulrich Reininghaus, ESA's BepiColombo project manager, "but we are extremely pleased to finally move our preparations to the launch site, and are grateful to everyone who has made this possible. - In parallel we are continuing with some long-duration firing tests on a replica transfer module thruster, under space-like conditions, to be best prepared for our journey to Mercury."

- Once at Kourou, an intensive six months of essential preparations are needed, including more review checkpoints.

- The work includes dressing the spacecraft in protective insulation to prepare for the harsh space environment and extreme temperatures they will experience operating close to the Sun, attaching and testing the solar wings and their deployment mechanisms, installing the sunshield, fuelling, and connecting the three spacecraft together.

- The final weeks will see the spacecraft stack inside the Ariane 5 rocket fairing, and preparing the launch vehicle itself, ready to blast the mission on a seven-year journey around the inner Solar System to investigate Mercury's mysteries.


Figure 23: Timeline of flybys during BepiColombo's 7.2 year journey to Mercury, starting with the opening of the nearly two month long launch window in October 2018 (image credit: ESA)

- The MTM (Mercury Transfer Module) will carry the two science orbiters, MPO and MMO, to the innermost planet, using a combination of solar power, electric propulsion and nine gravity-assist flybys of Earth, Venus and Mercury to set it on course.

- The two orbiters will make complementary measurements of the innermost planet and its environment from different orbits, from its deep interior to its interaction with the solar wind, to provide the best understanding of Mercury to date, and how the innermost planet of a solar system forms and evolves close to its parent star.


Figure 24: Artist's impression of BepiColombo in cruise configuration, approaching Mercury. On its 7.2 year journey to the innermost planet, BepiColombo will fly by Earth once, Venus twice and Mercury six times before entering into orbit (image credit — spacecraft: ESA/ATG medialab; Mercury: NASA/JPL)

Legend to Figure 24: The MTM (Mercury Transfer Module) is shown at the right with ion thrusters firing, and with its solar wings extended, spanning about 30 m from tip-to-tip. The 7.5 m-long solar wing of the MPO (Mercury Planetary Orbiter) in the middle is seen extending to the top. The MMO (Mercury Magnetospheric Orbiter) is hidden inside the sunshield towards the left in this orientation. The view of Mercury is based on imagery from NASA's Mariner 10 mission.

• On 6 March 2018, the BepiColombo engineering model was delivered to ESA's mission control center in Darmstadt, Germany. 37)

- BepiColombo – ESA's first mission to Mercury – is based on two spacecraft: the ESA-led MPO (Mercury Planetary Orbiter), with 11 experiments and instruments, and the JAXA-led MMO (Mercury Magnetospheric Orbiter), carrying five experiments and instruments.

- The engineering model delivered to Darmstadt comprises a 3D mock-up of the ESA's MPO module, plus a ‘flat-sat' mock-up of the MTM (Mercury Transfer Module), which ties the ESA and JAXA modules together during their cruise to Mercury.

- The engineering model is an electrically faithful replication of the most critical elements of the spacecraft's main platform and flight control systems, such as its computers, mass memory and power systems.

- Flight controllers will use the model throughout the mission to check software and procedures before uploading them to the real spacecraft. They will also train for flight events such as firing the electric thrusters, swinging by planets and separating the modules.


Figure 25: In this photo, Airbus technician Stanislaw Ballardt looks out from inside the ESA module during installation work on 7 March (image credit: ESA)

• December 7, 2017: The BepiColombo MTM (Mercury Transfer Module) has completed its final major test inside ESA's Large Space Simulator, proving it will be able to withstand the temperature extremes it will experience on its journey to Mercury. — On the one hand, the mission will be exposed to the cold vacuum of space. The other extreme is, the spacecraft will travel close to the Sun, receiving 10 times the solar energy than we do on Earth. This translates to about 11 kW/m2 at Mercury, with the spacecraft expected to endure heating to about 350ºC, similar to a pizza oven. 38)

- Inside the simulator, the largest of its kind in Europe at 15 m high and 10 m wide, pumps create a vacuum a billion times lower than the standard sea-level atmosphere, while the chamber's walls are lined with tubes pumped with liquid nitrogen to create low temperatures of about –180ºC. At the same time, a set of 25 kW IMAX projector-class lamps are used with mirrors to focus light onto the craft to generate the highest temperatures.

- During the latest tests, carried out between 24 November and 4 December 2017, the module was rotated through 13º to either side to monitor the heating and distribution. The ion engines were also activated – without creating thrust from an ion beam given the confines of the test chamber – to confirm that the module's electric propulsion system can operate in this challenging environment.


Figure 26: The MTM is seen here stacked on a replica interface to mimic the science orbiters that it will be attached to during launch and the 7.2 year journey to Mercury. The four ion thrusters are seen on the top of module in this orientation. Not present in this test, the module will also be equipped with two solar wings that will unfold to a span of 30 m (image credit: ESA, C. Carreau, CC BY-SA 3.0 IGO)

- The transfer module's job is to carry ESA's MPO (Mercury Planetary Orbiter) and Japan's MMO (Mercury Magnetospheric Orbiter) to the planet, where they will separate and enter their respective orbits. The craft will use a combination of gravity assist flybys at Earth, Venus and Mercury along with the transfer module's ion thrusters to reach its destination.

- The module will now be checked before the entire assembly is shipped to Europe's Spaceport in Kourou, French Guiana next year. With this last major test complete, the mission is on track to be launched in the two-month window opening on 5 October 2018.

• July 6, 2017: ESA's Mercury spacecraft has passed its final test in launch configuration, the last time it will be stacked like this before being reassembled at the launch site next year. 39) 40)

- BepiColombo's two orbiters, Japan's MMO (Mercury Magnetospheric Orbiter) and ESA's MPO (Mercury Planetary Orbiter), will be carried together by the MTM (Mercury Transport Module). The carrier will use a combination of electric propulsion and multiple gravity-assists at Earth, Venus and Mercury to complete the 7.2 year journey to the Solar System's mysterious innermost planet.

- Once at Mercury, the orbiters will separate and move into their own orbits to make complementary measurements of Mercury's interior, surface, exosphere and magnetosphere. The information will tell us more about the origin and evolution of a planet close to its parent star, providing a better understanding of the overall evolution of our own Solar System.

- To prepare for the harsh conditions close to the Sun, the spacecraft have undergone extensive testing both as separate units, and in the 6 m-high launch and cruise configuration.

- One set of tests carried out earlier this year at ESA's technical center in the Netherlands focused on deploying the solar wings, and the mechanisms that lock each panel in place. The 7.5 m-long array of the Mercury Planetary Orbiter and the two 12 m-long array of the Mercury Transport Module will be folded while inside the Ariane 5 rocket.


Figure 27: The complete BepiColombo spacecraft stack on 5 July 2017: From bottom to top: the Mercury Transfer Module (sitting on top of a cone-shaped adapter, and with one folded solar array visible to the right); the Mercury Planetary Orbiter (with the folded solar array seen towards the left, with red protective cover), and the MMO (Mercury Magnetospheric Orbiter). The Mercury Magnetospheric Orbiter's Sunshield and Interface Structure (MOSIF) that will protect the MMO during the cruise to Mercury is sitting on the floor to the right (image credit: ESA–C. Carreau , CC BY-SA 3.0 IGO)

• June 14, 2017: Media representatives are invited to a briefing on BepiColombo, ESA and JAXA's joint mission to Mercury, and to view the spacecraft before it leaves for Europe's Spaceport in Kourou, French Guiana, for launch next year. 41)

- Mercury is the least explored planet of the inner Solar System. BepiColombo is set to follow up on many of the intriguing results of NASA's Messenger mission, probing deeper into Mercury's mysteries than ever before. It will examine the peculiarities of its internal structure and magnetic field generation, and how it interacts with the Sun and solar wind. It will investigate surface features and chemistry, such as the ice in permanently shadowed craters at the poles. The mission's science will help revolutionize our understanding of the formation of our Solar System, and in the evolution of planets close to their parent stars.


Figure 28: The full BepiColombo stack seen in the Large European Acoustic Facility (LEAF) at ESA/ESTEC in June 2017. The walls of the chamber are fitted with powerful speakers that reproduce the noise during launch (image credit: ESA–C. Carreau, CC BY-SA 3.0 IGO) 42)

• March 6, 2017: The BepiColombo mission to Mercury is undergoing final testing at ESA/ESTEC in the Netherlands prior to its launch from Europe's Spaceport in Kourou, French Guiana in October 2018 (Figure 29). 43)

- The opening will be repeated after the spacecraft has been vibrated to simulate the conditions of launch, and again after it arrives at the launch site. - The wing will be folded against the body inside the Ariane 5 launch vehicle and will only open once in space.

- The MPO will be attached to Japan's MMO (Mercury Magnetospheric Orbiter), which will sit inside a protective sunshield. The two scientific spacecraft will be carried to the innermost planet by the MTM (Mercury Transport Module), using a combination of electric propulsion and multiple gravity-assists at Earth, Venus and Mercury.

- After the 7.2 year journey, the two will separate and make complementary measurements of Mercury's interior, surface, exosphere and magnetosphere. The data will tell us more about the origin and evolution of a planet located close to its parent star, providing a better understanding of the overall evolution of our own Solar System as well as exoplanet systems.


Figure 29: ESA's MPO (Mercury Planetary Orbiter) first saw the 7.5 m long three-panel solar wing being attached, and then unfurled. This was the first time the array had been deployed while attached to the orbiter. The panels were held from above to simulate the weightlessness of space (image credit: ESA)

Legend to Figure 29: In this view, the solar wing is partially unfolded. The ‘back' of the wing is facing the viewer, showing the cabling that will be connected to the main body, while the reflective Sun-facing side of the panels are not seen. One of the back panels is also reflective, to deflect stray light coming from the body.

• February 21, 2017: The French space agency CNES and Roscosmos, the Russian federal space agency, have signed an agreement concerning Russia's contribution to the PHEBUS ultraviolet spectrometer designed to study Mercury's exosphere as part of the science payload on the MPO (Mercury Planetary Orbiter) for the BepiColombo mission. 44)

- CNES, which is overseeing France's contribution to the BepiColombo mission, is leading development and system-level integration of the PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) instrument and will support French science activities throughout the operational phase of the mission. Roscosmos is developing the spectrometer scanning system for CNES and is involved in interface work on the instrument and associated testing.

- The Russian contribution is being led by IKI RAN, the Space Research Institute of the Russian Academy of Sciences, mandated by Roscosmos. In France, the LATMOS atmospheres, environments and space observations laboratory, part of the CNRS national scientific research center, has been selected for this mission.

• January 26, 2017: BepiColombo, Europe's first mission to Mercury, is currently being put through its paces at ESA/ESTEC (European Space Research and Technology Center) in the Netherlands. Mechanical and vibration tests will get underway in April with a view to a launch in October 2018. BepiColombo will arrive at Mercury, the smallest planet in our Solar System, in December 2025. 45)

- The ESA-led joint European and Japanese mission consists of two spacecraft -MPO (Mercury Planetary Orbiter) and MMO (Mercury Magnetospheric Orbiter) - as well as a sunshield and a Mercury Transfer Module, which will power its seven year journey using its solar electric propulsion engine. It will be a mission of further discovery after NASA's Messenger spacecraft uncovered a number of surprises - including evidence of water ice at the closest planet to the Sun and a magnetic dipole field.

• September 6, 2016: The MTM (Mercury Transfer Module) will carry Europe's MPO (Mercury Planetary Orbiter) and Japan's MMO (Mercury Magnetospheric Orbiter) together to the Sun's innermost planet. The four ion thrusters are positioned at the bottom of the spacecraft (Figure 30), known as the ‘engine bay', which provides the thrust during the mission's journey, including long firing periods lasting several months at a time. 46)

- "Completing the integration of the solar electric propulsion thruster floor is a major achievement for the BepiColombo project," says project manager Ulrich Reininghaus.

- By ionizing their propellant plume using electrical energy from the solar panels, the T6 thrusters can accelerate BepiColombo with an efficiency 15 times greater than a conventional chemical thruster.

- The work took place at ESA/ESTEC in the Netherlands, the largest spacecraft testing facility in Europe. The 22 cm diameter T6 was designed for ESA by QinetiQ in the UK, whose expertise in electric propulsion stretches back to the 1960s.


Figure 30: The base of the MTM (Mercury Transfer Module) with its four T6 ion thrusters is fully fitted for its 6.5 year journey to Mercury, along with the rest of the BepiColombo spacecraft (image credit: ESA)

• April 27, 2016: A quartet of highly efficient T6 thrusters is being installed on ESA's BepiColombo spacecraft to Mercury at ESA's ESTEC Test Center in Noordwijk, the Netherlands (Figure 31). The 22 cm diameter T6 was designed for ESA by QinetiQ in the UK, whose expertise in electric propulsion stretches back to the 1960s. It is an scaled-up version of the 10 cm T5 gridded ion thruster, which played a crucial role in ESA's GOCE gravity-mapping mission by continuously compensating for vestigial atmospheric drag along its extremely-low orbit. 47)

- The Mercury Transfer Module will carry Europe's Mercury Planetary Orbiter and Japan's Mercury Magnetospheric Orbiter together to Sun's innermost planet over the course of 6.5 years. "BepiColombo would not be possible in its current form without these T6 thrusters," explains ESA propulsion engineer Neil Wallace. "Standard chemical thrusters face a fundamental upper limit on performance, set by the amount of energy in the chemical reaction that heats the ejected propellant producing the thrust. "Ion thrusters can reach much higher exhaust speeds, typically an order of magnitude greater, because the propellant is first ionized and then accelerated using electrical energy generated by the solar panels. The higher velocity means less propellant is required.

- "The down side is that the thrust levels are much lower and therefore the spacecraft acceleration is also low – meaning the thrusters have to be operating for long periods. However, in space there is nothing to slow us down, so over prolonged periods of thrusting the craft's velocity is increased dramatically. Assuming the same mass of propellant, the T6 thrusters can accelerate BepiColombo to a speed 15 times greater than a conventional chemical thruster."


Figure 31: The eerie blue exhaust trail of a T6 ion thruster, a quartet of which will transport BepiColombo towards the innermost planet (image credit: NASA/JPL)

• January 27, 2016: The BepiColombo orbiter completed compatibility testing at ESTEC. The spacecraft was subjected to two tests by the project team: 48)

- First, the craft was checked for electrical compatibility with the electrical field generated by the Ariane-5 launcher that will deliver it into orbit, with no possibility of interference with BepiColombo's receivers.

- Secondly, incompatibility tests between the different subsystems of the spacecraft itself were performed when it orbits Mercury. In particular, the team checked that its trio of antennas on top can communicate properly with Earth.

- The orbiter was positioned to allow deployment of its medium-gain antenna in terrestrial gravity. The high-gain antenna reflector meanwhile was deployed in a worst-case position, supported by a dedicated fixture. The spacecraft was tilted by means of a large platform while the high-gain antenna was supported by a tower made of wood, transparent to radio waves. All test cables used were shielded to reduce potential interference.


Figure 32: The orbiter underwent ‘electromagnetic compatibility, radiated emission and susceptibility' testing last month inside the Maxwell chamber of ESA's ESTEC Test Center in Noordwijk, the Netherlands (image credit: ESA, G. Porter, CC BY-SA 3.0 IGO)

• July 22, 2015: The antenna that will connect Europe's BepiColombo with Earth is being tested for the extreme conditions it must endure orbiting Mercury. The trial is taking place over 10 days inside the LSS (Large Space Simulator) at ESA/ESTEC. The 1.5 m diameterHGA (High Gain Antenna), plus its boom and support structure, are subjected to a shaft of intense sunlight in vacuum conditions, while gradually rotated through 90º. 49)

- The mammoth chamber's high-performance pumps create a vacuum a billion times lower than standard sea-level atmosphere, while the chamber's black interior walls are lined with tubes pumped full of –190°C liquid nitrogen to mimic the extreme cold of deep space. At the same time, the hexagonal mirrors seen at the top of Figure 33, reflect simulated sunlight onto the satellite from a set of 25 kW bulbs more typically employed to project IMAX movies. In this case, the alignment of the 121 mirrors was adjusted to tighten the focus of their light beam, reproducing the intensity of sunlight experienced in Mercury orbit – around 10 times more intense than terrestrial illumination.


Figure 33: Photo of the BepiColombo antenna in LSS (image credit: ESA)

• May 15, 2015: The MIXS (Mercury Imaging X-ray Spectrometer), a UK-built instrument, has been shipped from the University of Leicester's Space Research Centre to the European Space Agency where it will be integrated with the BepiColombo spacecraft. 50)

• Fall 2014: Preparation of the MPO (Mercury Planetary Orbiter) FM spacecraft for the thermal system test continued as planned. The test configuration is complete and a final dress rehearsal test was conducted. Installation of the outer skin progressed on the spacecraft; loading in the test chamber occurred on 24 October. 51)

- The MTM (Mercury Transfer Module) had also been delivered to ESTEC in July 2014. The thruster floor with pre-integrated thruster pointing mechanisms was delivered and verified with the high-pressure regulator and electronics unit.


Figure 34: Photo of the BepiColombo spacecraft as it is moved into ESA's space simulator (image credit: ESA) 52)

• January 2014: The BepiColombo CDR (Critical Design Review) was completed. The MPO (Mercury Planetary Orbiter) PFM (Proto-Flight Model) AIT (Assembly, Integration and Test) continued at Thales Alenia Space Italy in Turin with Integrated System Tests (ISTs) on individual subsystems. ISTs for the DMS (Data Management Subsystem) and the Communication Subsystem were completed. Three FM and two QM payload instruments (MERMAG, MERTIS, MGNS, PHEBUS QM and BELA QM) were integrated and functionally verified on the MPO. The SERENA instrument was delivered, while electrical integration of the PCDU FM (Flight Model) is in progress. 53)

- Most of the spacecraft harness was integrated on the MTM (Mercury Transfer Module) PFM in Turin. Anomalies that occurred in MTM solar panel substrate manufacturing are being fixed with a dedicated sample test program. MPO panel manufacturing is on hold, pending agreement on whether additional process modifications are necessary in addition to the general improvements in process control identified for MTM panel production.


Figure 35: Illustration of the BepiColombo MCS (Mercury Composite Spacecraft), image credit: Airbus DS (Ref.22)


Launch: BepiColombo is scheduled to be launched in October 2018 with Ariane-5 from Europe's Spaceport in French Guiana. 54) 55) 56)

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 (Ref. 54).

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).


Figure 36: Cruise trajectory for April 2018 launch, showing the sun distance, SEP (Solar Electric Propulsion) usage, and planetary flybys (image credit: ESA)

Figure 37 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.


Figure 37: 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).


Figure 38: 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 38. 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.