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BepiColombo Mission to Mercury: First Year of Flight
Journal Pre-proof BepiColombo Mission to Mercury: First Year of Flight Christoph Steiger, Elsa Montagnon, Andrea Accomazzo, Paolo Ferri PII:
S0094-5765(20)30065-5
DOI:
https://doi.org/10.1016/j.actaastro.2020.01.041
Reference:
AA 7869
To appear in:
Acta Astronautica
Received Date: 13 December 2019 Accepted Date: 31 January 2020
Please cite this article as: C. Steiger, E. Montagnon, A. Accomazzo, P. Ferri, BepiColombo Mission to Mercury: First Year of Flight, Acta Astronautica, https://doi.org/10.1016/j.actaastro.2020.01.041. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 IAA. Published by Elsevier Ltd. All rights reserved.
70th International Astronautical Congress (IAC), Washington D.C., United States, 21-25 October 2019. Copyright ©2019 by the International Astronautical Federation (IAF). All rights reserved.
IAC-19-A3.5.2 BepiColombo Mission to Mercury: First Year of Flight Christoph Steiger*, Elsa Montagnon, Andrea Accomazzo, Paolo Ferri Mission Operations Department, ESA/ESOC, Darmstadt, Germany * Corresponding Author ([email protected]) Abstract Launched in October 2018, the ESA/JAXA BepiColombo mission is set to arrive at Mercury in late 2025. The modular S/C consists of two scientific Mercury orbiters and a cruise module with electric propulsion to support the 7-years cruise phase with planetary flybys at Earth (1x), Venus (2x) and Mercury (6x). Flight operations of the composite spacecraft are performed by ESOC Darmstadt, Germany. In the first year of flight, extensive near-Earth commissioning operations as well as two electric propulsion thrust arcs have taken place, with the S/C in excellent shape and well on its way to Mercury. Starting with a mission overview, the paper will present operations in the first year of flight, the current status of BepiColombo, as well as providing an outlook on upcoming activities and events. Keywords: BepiColombo, Mercury, Deep Space Mission, Flight Operations, ESA Acronyms/Abbreviations AOCS CPS CSW EP EPCM ESOC ETB FCE FCESW FCT FD FDIR FSS HGA IMU ISA JAXA LEOP LGA MCAM MCS MCSA
Attitude and Orbit Control System Chemical Propulsion System Central Software (running on OBC) Electric Propulsion Electric Propulsion Control Mode European Space Operations Centre Engineering Test Bed Failure Control Electronics Failure Control Electronics Software Flight Control Team Flight Dynamics Failure Detection, Isolation and Recovery Fine Sun Sensor High Gain Antenna Inertial Measurement Unit Italian Spring Accelerometer Japan Aerospace Exploration Agency Launch and Early Orbit Phase Low Gain Antenna Monitoring Cameras System Mercury Composite Spacecraft Mercury Composite Spacecraft Approach (stack consisting of (MPO, MOSIF, MMO) MCSC Mercury Composite Spacecraft Cruise (stack consisting of MTM, MPO, MOSIF, MMO) MCSO Mercury Composite Spacecraft Orbit (stack consisting of MPO, MOSIF) MEPS Mercury Electric Propulsion System MGA Medium Gain Antenna MGNS Mercury Gamma-ray and Neutron Spectrometer MMO Mercury Magnetospheric Orbiter MOSIF MMO Sunshade and Interface Structure MPO Mercury Planetary Orbiter MPO-MAG Mercury Planetary Orbiter Magnetometer MTL Mission Timeline MTM Mercury Transfer Module
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NECP NM OBC OCM OD OSR PHEBUS RAM RM RIU RWU SA SAPP S/C SASM SBM SEP SEPS SERENA SGM SHM STR TPE TPM TT&C
Near Earth Commissioning Phase AOCS Normal Mode On-board Computer AOCS Orbit Control Mode Orbit Determination Optical Surface Reflector Probing of Hermean Exosphere by Ultraviolet Spectroscopy Random Access Memory Reconfiguration Module Remote Interface Unit Reaction Wheel Unit Solar Array Survival Attitude Pointing Phase (of SASM) Spacecraft AOCS Sun Acquisition and Survival Mode AOCS Standby Mode Solar Electric Propulsion Solar Electric Propulsion System Search for Exospheric Refilling and Emitted Natural Abundance Safeguard Memory AOCS Safe Hold Mode Star Tracker Thruster Pointing Electronics Thruster Pointing Mechanism Telemetry, Tracking & Command
1. Introduction ESA’s BepiColombo mission is a collaboration with the Japan Aerospace Exploration Agency (JAXA), with the objective to study the planet and its environment, in particular global characterization of Mercury through the investigation of its interior, surface, exosphere and magnetosphere [1]. BepiColombo was launched in Oct 2018 and is set to arrive at Mercury in late 2025.
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Fig. 1. Cruise trajectory, showing Sun distance, electric propulsion usage (“SEP”), and planetary flybys at Earth, Venus, Mercury 1.1 Mission Overview The BepiColombo mission consists of two scientific spacecraft, ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (MMO), launched together as a single composite including a dedicated propulsion module (MTM). MPO and MTM have been developed under ESA contract by an international consortium led by Airbus Defence and Space. Flight operations are performed by ESA/ESOC. BepiColombo was launched on 20th Oct 2018 with Ariane-5 from Kourou. Launch is followed by a 7 years cruise phase, including planetary swingbys at Earth, Venus and Mercury, eventually achieving a weak capture by Mercury in December 2025 (Fig. 1). During cruise, electric propulsion (EP) is provided by the MTM module. EP is used continuously during 22 “thrust arcs” to adjust the trajectory (highlighted in red in Fig. 1), lasting from several days up to 2 months, with a total deltaV of 2731 m/s required. Shortly before Mercury arrival, the MTM module
Fig. 2. BepiColombo spacecraft in exploded view will be jettisoned. A series of manoeuvres will deliver the MMO to its operational orbit, and finally the MPO will reach its 1500x480 km polar orbit (2.2h period), with its scientific mission planned to last for one Earth year (1 year extension possible). 1.2 The Spacecraft See Fig. 2 and 3 for an artist’s view of the spacecraft. The combined stack can have the following configurations throughout the mission: • Mercury Composite S/C Cruise (MCSC): MTM, MPO, MMO sunshield (MOSIF) and MMO • Mercury Composite S/C Approach (MCSA): MPO, MOSIF and MMO following separation of the MTM • Mercury Composite S/C Orbit (MCSO): MPO and MOSIF following release of the MMO The JAXA-provided MMO includes 5 scientific instruments (see Table 1) and is a passive passenger during cruise, not involved in the control of the composite, which is done within the MPO, while the MTM provides propulsion means.
Fig. 3. BepiColombo in cruise configuration (left), MPO at Mercury (right)
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Table 1. Scientific instruments on MPO and MMO
T6 thrusters and pointing mechanisms High pressure regulator
Flow control units
Tanks
Power processing units
Fig. 4. BepiColombo electric propulsion system The MPO has a box-like shape with a size of 3.9×2.2×1.7 m, and a dry mass of about 1080 kg. 11 scientific instruments (see Table 1) are accommodated on the MPO. Nadir-pointing instruments at Mercury are facing the MTM in the combined stack. The tremendous heat load at Mercury imposes strong requirements on the spacecraft design, requiring high-temperature multilayer-insulation and solar array technology. A radiator to dump excess heat into space is mounted on one side of the MPO (ref. Fig. 3), which may not be exposed to Sun or Mercury. For deep space communications, the MPO uses a X/Ka-band deep space transponder with moveable high gain and medium gain antennae (HGA and MGA). Two low gain antennae (LGA) are used for mission phases at low Earth distance, and for backup uplink throughout the full mission. The AOCS performs 3-axis stabilised attitude and orbit control employing star trackers, inertial measurement units, fine Sun sensors, reaction wheels and chemical propulsion. AOCS design is impacted by the challenging environment, requiring special guidance profiles for the MPO solar array (to avoid overheating) and rapid S/C attitude stabilisation in case of contingencies. For these cases, the on-board computer contains a separate processing unit, the Failure Control Electronics (FCE), taking over S/C attitude control in case of transient unavailability of the main on-board computer. S/C modularity leads to high complexity of the AOCS [2], which must handle a range of S/C configurations throughout mission life that vary considerably in terms of their dynamic properties, as well as a high number of sensors and actuators, some of which duplicated on MPO and MTM. The MTM provides propulsion means for cruise. Apart from dual mode bipropellant chemical propulsion, it features electric propulsion with 4 moveable thrusters based on the Kaufman-type electric bombardment ion motor (125 mN thrust), ref. Fig. 4. It is possible to use either a single thruster or 2 thrusters at the same time.
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The high power demand by the MTM electric propulsion (up to 10 kW) is satisfied with large solar arrays (area of over 40 m2 in total), using the same hightemperature technology as for the MPO. The Monitoring Cameras System (MCAM) consisting of 3 optical micro-cameras mounted on the MTM is primarily used for outreach purposes, with the central camera processing unit located on the MPO. See [3] for a more detailed general description of the spacecraft. 1.3 The Mission Operations Centre at ESA/ESOC Operations of the composite spacecraft and the MPO are conducted at ESA/ESOC, using the typical setup for ESA deep space missions, including a SCOS-2000 based mission control system, a standalone mission planning system, and a SIMSAT-based S/C simulator. The simulator is running the platform on-board software
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on a processor emulator, and is a key tool for operations preparation, allowing testing with high fidelity. The Engineering Test Bed (ETB) shown in Fig. 5 was installed at ESOC in early 2018, with the final handover from the S/C manufacturer to ESOC virtually complete. The ETB contains flight-like hardware for all electrical equipment and is used for operations validation and special testing in case the S/C simulator is not considered representative enough. Contact with the spacecraft is established making use of the ESA 35m network of ground stations, located in Madrid (Spain), New Norcia (Australia) and Malargüe (Argentina). A ground station contact typically has a duration of around 10 hours. Depending on the level of activities, the number of station passes varies greatly. For quiet phases in cruise, the baseline is to have one station pass per week. Combined with significant propagation delays of up to 13 min one way (maximum distance to Earth in cruise is at about 1.6 AU), an “offline” operations approach is required: there is only occasional monitoring of the spacecraft by ground, and virtually all routine operations are uplinked for time-tagged execution from the on-board Mission Timeline (MTL), typically executing out of ground coverage. The S/C has been designed with a significant degree of autonomy, allowing it to deal with failures when ground is not there to react. BepiColombo operations are performed by the Flight Control Team (FCT), consisting of about 10 engineers and controllers at launch. The FCT is interfacing with various multi-mission support groups at ESOC, including Flight Dynamics (FD), ground segment software and hardware support, and ground station operations. FD is in charge of orbit determination and prediction, command generation (e.g. for orbit control manoeuvres) and monitoring of S/C status for FD-related items (e.g. star tracker performance). For deep space missions like BepiColombo, there is a particularly close relation
between FCT and FD due to the complex navigation and AOCS operations activities. In addition to the typical difficulties of interplanetary mission operations, the particular challenges of BepiColombo operations are the high S/C complexity due to S/C modularity, constraints linked to the harsh environment the S/C will be travelling to, and the complexity of electric propulsion operations in cruise. See [4] for a general overview. 1.4 First year of flight: operations overview Key operations phases in the first year included the following: • 20th Oct to 22nd Oct 2018: Launch and Early Orbit Phase (LEOP), performing the most critical operations required to get the S/C ready for interplanetary cruise. • 23rd Oct to 16th Dec 2018: Near Earth commissioning part 1, performing a detailed checkout of S/C platform and payloads for both MTM/MPO and MMO, with the end date driven by the need to start the first thrust arc. • 17th Dec to mid Feb 2019: first electric propulsion (EP) thrust arc, with a short touch up manoeuvre in early March 2019. • March to September 2019: partially a quiet cruise phase, this period saw intense delta NECP operations, and the installation and activation of new platform on-board software, addressing findings of the first few months of flight. • 12th Sept 2019 onwards: second EP thrust arc, planned to be completed in mid Nov 2019. 2. LEOP and near-Earth commissioning 2.1 Launch and Early Orbit Phase (LEOP) BepiColombo was launched into an Earth escape orbit on 20th Oct 2018 with Ariane-5 from Kourou. The first 2.5 days of the mission were conducted with 24/7 ground station coverage and the ESA/Industry combined mission control team working in shifts around the clock. An overview of LEOP activities is shown in Fig. 6. The LEOP timeline consisted of the following key activities: • Autonomous Sun acquisition after separation, with AOCS stopping in SASM-SAPP3 • First switch ON and checkout of star trackers and reaction wheels by ground • AOCS mode transitions to Normal Mode (NM) under ground control • Deployment of MGA and HGA, followed by a move of communications from LGA to MGA
Fig. 5. MPO element of the BepiColombo Engineering Test Bed (ETB) at ESOC
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• Orbit control test manoeuvre on MTM axial thrusters (0.33 m/s deltaV) The first safe mode of the mission occurred 7h after launch due to an inadequate tuning of an FDIR threshold on a reaction wheel surveillance. The resulting delay in activities could be recovered by the end of LEOP. See [5] for more information on GNC operations during LEOP. 2.2 Near Earth Commissioning Phase (NECP) part 1 NECP operations following end of LEOP took place from 22nd Oct to 16th Dec 2018 and were supported by daily station contacts (10-12h duration), 7 days a week. The aim of this phase was to perform a full checkout of the spacecraft (platform, payload, MMO) with on-site support of the S/C manufacturer (only for specific platform commissioning operations early in NECP), the scientific instrument teams, and JAXA. Near-Earth commissioning of the MPO and MTM platform included the following key activities: • Orbit control test manoeuvres on MTM transverse thrusters • B-side unit checkouts, including commissioning firings of MTM CPS B-side thrusters out of the AOCS control loop to verify health of the redundant thrusters • IMU and STR calibrations activities (IMU scale factor, alignment and drift bias, STR inter head alignment and focal length calibration) • MTM solar array drive “run in”, including several full rotations of the array to recover full performance of the slip ring in the mechanism • Electric propulsion commissioning, including a stepwise commissioning of the various SEPS elements, culminating in firing each thruster in closed loop in Electric Propulsion Control Mode (EPCM), as well as one EPCM entry with dual thruster usage (see Fig. 7 for an overview) • Data handling subsystem commissioning, including acquisition of memory reference dumps of the most relevant on-board memories • TT&C commissioning, measuring the subsystem’s performance and executing all key configurations, including a move of communications to HGA • Thermal control system commissioning, including verification of correct functioning of control thermistors and heaters • MPO battery state of charge lowering from 100% to 40% (baseline state of charge to reduce degradation during cruise phase) Due to the S/C modularity, the following units could not be fully commissioned at this stage, and can only be checked out fully upon Mercury arrival:
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Fig. 6. LEOP overview
Fig. 7. Electric propulsion commissioning 20th Oct – 7th Dec 2018
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• MPO chemical propulsion system (tilted as well as axial thrusters). • MPO +X and –Z Fine Sun Sensors (FSS +X located out of Sun illumination domain in MCSC configuration, FSS -Z obstructed by MOSIF). • MPO solar array, which must not be rotated in cruise to avoid (i) MTM CPS plume impingement, (ii) MTM FSS disturbances, (iii) MPO solar array backside illumination. Only a very small movement (“Nudging”) of the MPO solar array was commanded to verify health of the array drive mechanism. Each MPO scientific payload (ref. Table 1) was switched ON for the first time with the PI team present at ESOC, performing a checkout as far as possible in MCSC configuration. The MPO-MAG boom (ref. Fig. 3) was deployed shortly after LEOP, constituting the last deployment of appendages required after launch. MMO platform was commissioned with the JAXA team co-located at ESOC. In a later slot, MMO scientific instruments were also commissioned, with JAXA and the MMO instrument teams present at ESOC. Pictures were taken using the monitoring cameras system (MCAM) starting from LEOP. This included picture taking during special, one-off events like MTM solar array drive run-in or MPO-MAG boom deployment, as well as regular pictures taking of MGA and HGA to show movement of the antennae over time. Fig. 9 shows some examples. 2.3 NECP part 2 and CSW/FCESW activation Owing to the need to start the first EP thrust arc in mid Dec 2018, it had already been planned before launch to perform some of the lower priority near-Earth commissioning operations both for platform and payload only in spring/summer 2019, after completion of the first thrust arc. This slot was also used to perform investigation activities for issues encountered in NECP
Fig. 8. HGA pattern calibration slew to determine antenna boresight, with actual Earth direction at the origin of the coordinate system part 1. Key delta commissioning activities included the following: • Pattern calibration for both MGA and HGA (Fig. 8), performing special attitude slews to determine the antenna boresight, confirming good accuracy of the pre-launch settings • Characterisation of MTM solar array flexible modes • Delta NECP commissioning for MPO payloads PHEBUS and SERENA, including high voltage commissioning • MMO delta commissioning, including MMO payload high voltage operations Another major activity in this phase was the installation and activation of new platform on-board software on the OBC (CSW 3.3.3.2) and the FCE (FCESW 2.3.3). These new software versions addressed a number of improvements on issues encountered either
Fig. 9. Pictures taken with the monitoring cameras of the MTM solar array (left), MGA and magnetometer boom (middle) and the HGA (right) IAC-19-A3.5.2
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in the last few months before launch, or in the first 6 months of flight. The new software versions were tested extensively on the simulator and the ETB at ESOC, and finally uplinked in July 2019. While FCESW activation was a rather straightforward activity requiring a reboot of the FCE out of the loop, CSW activation through OBC reboot naturally implied a ground-commanded safe mode entry for the spacecraft. This was triggered on 17th July 2019 and successfully recovered thereafter. While the baseline for cruise to Mercury was to have all payloads off (only to be switched ON for bi-yearly checkouts to confirm their good health), the successful completion of NECP also allowed letting some of the payloads perform background science operations. MGNS, MPO-MAG and ISA were left in science mode throughout this phase, only to be powered off for special activities requiring all payloads to be off.
Fig. 10. AOCS mode transitions for EPCM entry/exit
3. Electric Propulsion Operations 3.1 Overview BepiColombo electric propulsion (EP) operations are driven by the following characteristics: Complexity of AOCS mode transitions: Fig. 10 shows the mode transitions between AOCS Normal Mode and Electric Propulsion Control Mode (EPCM). Entering EPCM requires slewing to the custom EPCM attitude in phase ETP-A1, rotating the MTM solar array to the EPCM position to maximise power in phase ETPA2 (it is typically offpointed in NM to reduce degradation), and starting up the selected thrusters in phase ETP-A3. Likewise, for EPCM exit the thrusters are switched off in EPCM, the MTM solar arrays are rotated back in phase ETP-B2, and the S/C slews back to NM attitude in phase ETP-B1. Each thrust arc requires a custom spacecraft attitude, with the attitude and solar array guidance profiles commanded by ground for (i) the chosen thruster selection, (ii) a backup thruster selection (if desired), and (iii) the “fallback” guidance profile in NM. A transition to the backup thruster selection or a fallback to NM can be triggered autonomously by FDIR. Including highly configurable EP settings, entering EPCM requires several hundred telecommands and may take up to 5.5 hours (1.5h required for EP start up, up to 4h required for the attitude slews from NM to EPCM). Visibility constraints in EPCM: the attitude required for thruster firing is driven by trajectory needs and the set of thrusters used (out of the 4 thrusters available). Despite two moveable antennae (MGA and HGA), visibility with either of them it is not guaranteed in the firing attitude, due to antenna movement constraints or blockage by other parts of the S/C body. It may hence not be possible to get S/C telemetry when in EPCM, i.e. S/C status can only be checked after exiting EPCM, retrieving playback telemetry. For each EP arc,
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Fig. 11. MGA visibility constraints for EP1 arc: as from 10th Jan 2019, link via MGA in the thrusting attitude became possible expected antennae visibility is analysed to find the best strategy. As an example, Fig. 11 shows the MGA coverage situation for the first thrust arc, displaying the MGA movement range: only if using thruster pair [2,4], MGA coverage would be possible from 10th Jan 2019 up to end of the arc. Orbit Determination needs: slight variations in thrust direction and magnitude when using electric propulsion impact the accuracy for orbit determination in deep space based on radiometric data (Doppler and ranging). To work around this, regular acquisition of radiometric tracking data over periods with the thrusters off was introduced, requiring to exit and re-enter EPCM. Power constraints: despite approaching the Sun, the mission is highly power-constrained. The available power generally does not allow firing the thrusters at their maximum thrust level. Technological limitations on the solar arrays require to offpoint the MTM solar array at closer Sun distances to avoid overheating, see Fig. 12. As the baseline during EP arcs in cruise (see Fig. 1) is to use maximum possible thrust to minimise the duration of the arc (thereby reducing the risk of not achieving the desired deltaV in time for the next planetary swingby), careful management of MTM solar array pointing and EP thrust levels during EP arcs is needed to get the most out of the system.
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3.2 EP thrust arcs in 2018 and 2019 Following Electric Propulsion commissioning in NECP in late 2018, the first thrust arc of the mission started on 17th Dec 2018 and was completed in Feb 2019, with a short touch up manoeuvre in early March 2019. Refer to Fig. 13 for an overview, showing the periods of thruster usage as well as the thrust levels used. The first thrust arc allowed for dual thruster operations. While balanced thruster usage was also a consideration, thruster selection was mainly driven by MGA/HGA visibility, with thruster pair [1,3] allowing HGA coverage in EPCM up to 23/12/2018, and thruster pair [2,4] allowing MGA coverage from 10th Jan 2019 up to the end of the thrust arc. In the period from 24th Dec 2018 to 9th Jan 2019 with neither MGA nor HGA allowing coverage in EPCM attitude, a special TT&C configuration was established with the S/C transmitting a carrier-only signal via LGA when in EPCM, allowing ground to check presence of that signal to confirm the S/C was still in EPCM. This special approach was only possible for this thrust arc close to Earth, with the link budget on LGA insufficient for this approach at further distances. In terms of ground station contacts, a weekly socalled “navigation pass” was taken, when EPCM was interrupted to gather radiometric data for orbit determination (i.e. transition to NM before the pass, and re-entry to EPCM after the pass). On top of these passes, two ground station contacts were taken per week to monitor S/C status in EPCM (or, when neither MGA nor HGA were visible, to at least check presence of a carrier-only signal from LGA), allowing a regular check of S/C performance and faster reaction time to unexpected EPCM interruptions. Overall thruster performance in the arc was nominal [6]. Three unexpected interruptions of thrusting in
Fig. 12. MTM solar array allowed Sun aspect angle depending on Sun distance (SAA of 0 deg means MTM SA is facing the Sun) EPCM triggering a fallback to NM and a recovery by ground to re-enter EPCM occurred in this thrust arc. These were caused by thruster-related software FDIR triggering too early, with ground tuning the settings accordingly thereafter. The gradual change of thrust level over time visible in Fig. 13 was due to the change of distance to the Sun, limiting the maximum power available for firing the thrusters. The second thrust arc of the mission (SEP2) started on 12th Sept 2019 and is expected to end by mid Nov 2019. Due to larger sun distance, the power available only allows using a single thruster. Either thruster 1 or 3 are used, as they allow to have MGA coverage in the EPCM attitude. While the overall operations concept is very similar to the SEP1 arc, the planning process has been optimised to reduce the workload on the team. 4. Conclusion and Outlook After an extensive preparation phase, the ESA/JAXA BepiColombo mission to Mercury was
Fig. 13. Thrusters and thrust levels used during SEP1 arc from Dec 2018 to Feb 2019, with greyed out periods indicating when thrusters were OFF IAC-19-A3.5.2
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finally launched in October 2018. The first year of flight saw the successful execution of LEOP (20th to 22nd Oct 2018), near-Earth commissioning part 1 (22nd Oct 2018 to mid Dec 2018), the first electric propulsion thrust arc (mid Dec 2018 to early Mar 2019), near-Earth commissioning part 2 (Mar 2019 to Aug 2019), and the start of the second EP thrust arc on 12th Sep 2019. Following installation and activation of a new platform on-board software in July 2019 to address findings from commissioning, the S/C is in excellent shape for the cruise to Mercury, with a planned arrival in Dec 2025. Regarding key upcoming operations, the second solar electric propulsion arc of the mission is planned to end by mid Nov, bringing the S/C on a trajectory to the first planetary swingby, at the Earth in April 2020. As from the Earth swingby, the S/C will move into the inner solar system, with a first Venus swingby in October 2020. A key aspect of this phase is that the S/C performance will be observable for the first time in a hotter environment. References [1] J. Benkhoff, J. van Casteren, H. Hayakawa, M. Fujimoto, H. Laakso, N. Novara et al, BepiColombo - Comprehensive Exploration of Mercury: Mission Overview and Science Goals, Planetary and Space Science, Vol. 58, Issues 1-2, Elsevier, 2010, pp. 2-20.
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[2] P. Espeillac, T. Strandberg, M. Casasco, C. Steiger, A. Altay and E. Montagnon, GNC Design Challenges for Variable Flight Configuration on the BepiColombo Mission to Mercury, 10th International ESA Conference on Guidance Navigation and Control Systems (GNC 2017), Salzburg, Austria, 2017, 29 May – 2 Jun. [3] R. J. Wilson, M. Schelkle, The BepiColombo Spacecraft, its Mission to Mercury and its Thermal Verification, 46th Lunar and Planetary Science Conference, The Woodlands, USA, 2015, 16-20 Mar. [4] C. Steiger, E. Montagnon and A. Accomazzo, Flight Operations Preparation for the BepiColombo Mission to Mercury: Concepts and Challenges, SpaceOps 2016, Daejeon, Korea, 2016, 16-20 May. [5] F. Budnik, G. Bellei, F. Castellini and T. Morley, BepiColombo: Flight Dynamics Operations during Launch and Early Orbit Phase, 18th Australian International Aerospace Congress (AIAC18), Melbourne, Australia, 2018, 24-26 Feb. [6] P. Randall, R. Lewis, S. Clark, K. Chan, H. Gray, C. Steiger, F. Striedter, BepiColombo – MEPS commissioning activities and T6 ion thruster performance during early mission operations, 36th International Electric Propulsion Conference (IEPC 2019), Vienna, Austria, 2019, 16-20 Sep.
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S0094-5765(20)30065-5
DOI:
https://doi.org/10.1016/j.actaastro.2020.01.041
Reference:
AA 7869
To appear in:
Acta Astronautica
Received Date: 13 December 2019 Accepted Date: 31 January 2020
Please cite this article as: C. Steiger, E. Montagnon, A. Accomazzo, P. Ferri, BepiColombo Mission to Mercury: First Year of Flight, Acta Astronautica, https://doi.org/10.1016/j.actaastro.2020.01.041. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 IAA. Published by Elsevier Ltd. All rights reserved.
70th International Astronautical Congress (IAC), Washington D.C., United States, 21-25 October 2019. Copyright ©2019 by the International Astronautical Federation (IAF). All rights reserved.
IAC-19-A3.5.2 BepiColombo Mission to Mercury: First Year of Flight Christoph Steiger*, Elsa Montagnon, Andrea Accomazzo, Paolo Ferri Mission Operations Department, ESA/ESOC, Darmstadt, Germany * Corresponding Author ([email protected]) Abstract Launched in October 2018, the ESA/JAXA BepiColombo mission is set to arrive at Mercury in late 2025. The modular S/C consists of two scientific Mercury orbiters and a cruise module with electric propulsion to support the 7-years cruise phase with planetary flybys at Earth (1x), Venus (2x) and Mercury (6x). Flight operations of the composite spacecraft are performed by ESOC Darmstadt, Germany. In the first year of flight, extensive near-Earth commissioning operations as well as two electric propulsion thrust arcs have taken place, with the S/C in excellent shape and well on its way to Mercury. Starting with a mission overview, the paper will present operations in the first year of flight, the current status of BepiColombo, as well as providing an outlook on upcoming activities and events. Keywords: BepiColombo, Mercury, Deep Space Mission, Flight Operations, ESA Acronyms/Abbreviations AOCS CPS CSW EP EPCM ESOC ETB FCE FCESW FCT FD FDIR FSS HGA IMU ISA JAXA LEOP LGA MCAM MCS MCSA
Attitude and Orbit Control System Chemical Propulsion System Central Software (running on OBC) Electric Propulsion Electric Propulsion Control Mode European Space Operations Centre Engineering Test Bed Failure Control Electronics Failure Control Electronics Software Flight Control Team Flight Dynamics Failure Detection, Isolation and Recovery Fine Sun Sensor High Gain Antenna Inertial Measurement Unit Italian Spring Accelerometer Japan Aerospace Exploration Agency Launch and Early Orbit Phase Low Gain Antenna Monitoring Cameras System Mercury Composite Spacecraft Mercury Composite Spacecraft Approach (stack consisting of (MPO, MOSIF, MMO) MCSC Mercury Composite Spacecraft Cruise (stack consisting of MTM, MPO, MOSIF, MMO) MCSO Mercury Composite Spacecraft Orbit (stack consisting of MPO, MOSIF) MEPS Mercury Electric Propulsion System MGA Medium Gain Antenna MGNS Mercury Gamma-ray and Neutron Spectrometer MMO Mercury Magnetospheric Orbiter MOSIF MMO Sunshade and Interface Structure MPO Mercury Planetary Orbiter MPO-MAG Mercury Planetary Orbiter Magnetometer MTL Mission Timeline MTM Mercury Transfer Module
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NECP NM OBC OCM OD OSR PHEBUS RAM RM RIU RWU SA SAPP S/C SASM SBM SEP SEPS SERENA SGM SHM STR TPE TPM TT&C
Near Earth Commissioning Phase AOCS Normal Mode On-board Computer AOCS Orbit Control Mode Orbit Determination Optical Surface Reflector Probing of Hermean Exosphere by Ultraviolet Spectroscopy Random Access Memory Reconfiguration Module Remote Interface Unit Reaction Wheel Unit Solar Array Survival Attitude Pointing Phase (of SASM) Spacecraft AOCS Sun Acquisition and Survival Mode AOCS Standby Mode Solar Electric Propulsion Solar Electric Propulsion System Search for Exospheric Refilling and Emitted Natural Abundance Safeguard Memory AOCS Safe Hold Mode Star Tracker Thruster Pointing Electronics Thruster Pointing Mechanism Telemetry, Tracking & Command
1. Introduction ESA’s BepiColombo mission is a collaboration with the Japan Aerospace Exploration Agency (JAXA), with the objective to study the planet and its environment, in particular global characterization of Mercury through the investigation of its interior, surface, exosphere and magnetosphere [1]. BepiColombo was launched in Oct 2018 and is set to arrive at Mercury in late 2025.
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Fig. 1. Cruise trajectory, showing Sun distance, electric propulsion usage (“SEP”), and planetary flybys at Earth, Venus, Mercury 1.1 Mission Overview The BepiColombo mission consists of two scientific spacecraft, ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (MMO), launched together as a single composite including a dedicated propulsion module (MTM). MPO and MTM have been developed under ESA contract by an international consortium led by Airbus Defence and Space. Flight operations are performed by ESA/ESOC. BepiColombo was launched on 20th Oct 2018 with Ariane-5 from Kourou. Launch is followed by a 7 years cruise phase, including planetary swingbys at Earth, Venus and Mercury, eventually achieving a weak capture by Mercury in December 2025 (Fig. 1). During cruise, electric propulsion (EP) is provided by the MTM module. EP is used continuously during 22 “thrust arcs” to adjust the trajectory (highlighted in red in Fig. 1), lasting from several days up to 2 months, with a total deltaV of 2731 m/s required. Shortly before Mercury arrival, the MTM module
Fig. 2. BepiColombo spacecraft in exploded view will be jettisoned. A series of manoeuvres will deliver the MMO to its operational orbit, and finally the MPO will reach its 1500x480 km polar orbit (2.2h period), with its scientific mission planned to last for one Earth year (1 year extension possible). 1.2 The Spacecraft See Fig. 2 and 3 for an artist’s view of the spacecraft. The combined stack can have the following configurations throughout the mission: • Mercury Composite S/C Cruise (MCSC): MTM, MPO, MMO sunshield (MOSIF) and MMO • Mercury Composite S/C Approach (MCSA): MPO, MOSIF and MMO following separation of the MTM • Mercury Composite S/C Orbit (MCSO): MPO and MOSIF following release of the MMO The JAXA-provided MMO includes 5 scientific instruments (see Table 1) and is a passive passenger during cruise, not involved in the control of the composite, which is done within the MPO, while the MTM provides propulsion means.
Fig. 3. BepiColombo in cruise configuration (left), MPO at Mercury (right)
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Table 1. Scientific instruments on MPO and MMO
T6 thrusters and pointing mechanisms High pressure regulator
Flow control units
Tanks
Power processing units
Fig. 4. BepiColombo electric propulsion system The MPO has a box-like shape with a size of 3.9×2.2×1.7 m, and a dry mass of about 1080 kg. 11 scientific instruments (see Table 1) are accommodated on the MPO. Nadir-pointing instruments at Mercury are facing the MTM in the combined stack. The tremendous heat load at Mercury imposes strong requirements on the spacecraft design, requiring high-temperature multilayer-insulation and solar array technology. A radiator to dump excess heat into space is mounted on one side of the MPO (ref. Fig. 3), which may not be exposed to Sun or Mercury. For deep space communications, the MPO uses a X/Ka-band deep space transponder with moveable high gain and medium gain antennae (HGA and MGA). Two low gain antennae (LGA) are used for mission phases at low Earth distance, and for backup uplink throughout the full mission. The AOCS performs 3-axis stabilised attitude and orbit control employing star trackers, inertial measurement units, fine Sun sensors, reaction wheels and chemical propulsion. AOCS design is impacted by the challenging environment, requiring special guidance profiles for the MPO solar array (to avoid overheating) and rapid S/C attitude stabilisation in case of contingencies. For these cases, the on-board computer contains a separate processing unit, the Failure Control Electronics (FCE), taking over S/C attitude control in case of transient unavailability of the main on-board computer. S/C modularity leads to high complexity of the AOCS [2], which must handle a range of S/C configurations throughout mission life that vary considerably in terms of their dynamic properties, as well as a high number of sensors and actuators, some of which duplicated on MPO and MTM. The MTM provides propulsion means for cruise. Apart from dual mode bipropellant chemical propulsion, it features electric propulsion with 4 moveable thrusters based on the Kaufman-type electric bombardment ion motor (125 mN thrust), ref. Fig. 4. It is possible to use either a single thruster or 2 thrusters at the same time.
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The high power demand by the MTM electric propulsion (up to 10 kW) is satisfied with large solar arrays (area of over 40 m2 in total), using the same hightemperature technology as for the MPO. The Monitoring Cameras System (MCAM) consisting of 3 optical micro-cameras mounted on the MTM is primarily used for outreach purposes, with the central camera processing unit located on the MPO. See [3] for a more detailed general description of the spacecraft. 1.3 The Mission Operations Centre at ESA/ESOC Operations of the composite spacecraft and the MPO are conducted at ESA/ESOC, using the typical setup for ESA deep space missions, including a SCOS-2000 based mission control system, a standalone mission planning system, and a SIMSAT-based S/C simulator. The simulator is running the platform on-board software
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on a processor emulator, and is a key tool for operations preparation, allowing testing with high fidelity. The Engineering Test Bed (ETB) shown in Fig. 5 was installed at ESOC in early 2018, with the final handover from the S/C manufacturer to ESOC virtually complete. The ETB contains flight-like hardware for all electrical equipment and is used for operations validation and special testing in case the S/C simulator is not considered representative enough. Contact with the spacecraft is established making use of the ESA 35m network of ground stations, located in Madrid (Spain), New Norcia (Australia) and Malargüe (Argentina). A ground station contact typically has a duration of around 10 hours. Depending on the level of activities, the number of station passes varies greatly. For quiet phases in cruise, the baseline is to have one station pass per week. Combined with significant propagation delays of up to 13 min one way (maximum distance to Earth in cruise is at about 1.6 AU), an “offline” operations approach is required: there is only occasional monitoring of the spacecraft by ground, and virtually all routine operations are uplinked for time-tagged execution from the on-board Mission Timeline (MTL), typically executing out of ground coverage. The S/C has been designed with a significant degree of autonomy, allowing it to deal with failures when ground is not there to react. BepiColombo operations are performed by the Flight Control Team (FCT), consisting of about 10 engineers and controllers at launch. The FCT is interfacing with various multi-mission support groups at ESOC, including Flight Dynamics (FD), ground segment software and hardware support, and ground station operations. FD is in charge of orbit determination and prediction, command generation (e.g. for orbit control manoeuvres) and monitoring of S/C status for FD-related items (e.g. star tracker performance). For deep space missions like BepiColombo, there is a particularly close relation
between FCT and FD due to the complex navigation and AOCS operations activities. In addition to the typical difficulties of interplanetary mission operations, the particular challenges of BepiColombo operations are the high S/C complexity due to S/C modularity, constraints linked to the harsh environment the S/C will be travelling to, and the complexity of electric propulsion operations in cruise. See [4] for a general overview. 1.4 First year of flight: operations overview Key operations phases in the first year included the following: • 20th Oct to 22nd Oct 2018: Launch and Early Orbit Phase (LEOP), performing the most critical operations required to get the S/C ready for interplanetary cruise. • 23rd Oct to 16th Dec 2018: Near Earth commissioning part 1, performing a detailed checkout of S/C platform and payloads for both MTM/MPO and MMO, with the end date driven by the need to start the first thrust arc. • 17th Dec to mid Feb 2019: first electric propulsion (EP) thrust arc, with a short touch up manoeuvre in early March 2019. • March to September 2019: partially a quiet cruise phase, this period saw intense delta NECP operations, and the installation and activation of new platform on-board software, addressing findings of the first few months of flight. • 12th Sept 2019 onwards: second EP thrust arc, planned to be completed in mid Nov 2019. 2. LEOP and near-Earth commissioning 2.1 Launch and Early Orbit Phase (LEOP) BepiColombo was launched into an Earth escape orbit on 20th Oct 2018 with Ariane-5 from Kourou. The first 2.5 days of the mission were conducted with 24/7 ground station coverage and the ESA/Industry combined mission control team working in shifts around the clock. An overview of LEOP activities is shown in Fig. 6. The LEOP timeline consisted of the following key activities: • Autonomous Sun acquisition after separation, with AOCS stopping in SASM-SAPP3 • First switch ON and checkout of star trackers and reaction wheels by ground • AOCS mode transitions to Normal Mode (NM) under ground control • Deployment of MGA and HGA, followed by a move of communications from LGA to MGA
Fig. 5. MPO element of the BepiColombo Engineering Test Bed (ETB) at ESOC
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• Orbit control test manoeuvre on MTM axial thrusters (0.33 m/s deltaV) The first safe mode of the mission occurred 7h after launch due to an inadequate tuning of an FDIR threshold on a reaction wheel surveillance. The resulting delay in activities could be recovered by the end of LEOP. See [5] for more information on GNC operations during LEOP. 2.2 Near Earth Commissioning Phase (NECP) part 1 NECP operations following end of LEOP took place from 22nd Oct to 16th Dec 2018 and were supported by daily station contacts (10-12h duration), 7 days a week. The aim of this phase was to perform a full checkout of the spacecraft (platform, payload, MMO) with on-site support of the S/C manufacturer (only for specific platform commissioning operations early in NECP), the scientific instrument teams, and JAXA. Near-Earth commissioning of the MPO and MTM platform included the following key activities: • Orbit control test manoeuvres on MTM transverse thrusters • B-side unit checkouts, including commissioning firings of MTM CPS B-side thrusters out of the AOCS control loop to verify health of the redundant thrusters • IMU and STR calibrations activities (IMU scale factor, alignment and drift bias, STR inter head alignment and focal length calibration) • MTM solar array drive “run in”, including several full rotations of the array to recover full performance of the slip ring in the mechanism • Electric propulsion commissioning, including a stepwise commissioning of the various SEPS elements, culminating in firing each thruster in closed loop in Electric Propulsion Control Mode (EPCM), as well as one EPCM entry with dual thruster usage (see Fig. 7 for an overview) • Data handling subsystem commissioning, including acquisition of memory reference dumps of the most relevant on-board memories • TT&C commissioning, measuring the subsystem’s performance and executing all key configurations, including a move of communications to HGA • Thermal control system commissioning, including verification of correct functioning of control thermistors and heaters • MPO battery state of charge lowering from 100% to 40% (baseline state of charge to reduce degradation during cruise phase) Due to the S/C modularity, the following units could not be fully commissioned at this stage, and can only be checked out fully upon Mercury arrival:
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Fig. 6. LEOP overview
Fig. 7. Electric propulsion commissioning 20th Oct – 7th Dec 2018
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• MPO chemical propulsion system (tilted as well as axial thrusters). • MPO +X and –Z Fine Sun Sensors (FSS +X located out of Sun illumination domain in MCSC configuration, FSS -Z obstructed by MOSIF). • MPO solar array, which must not be rotated in cruise to avoid (i) MTM CPS plume impingement, (ii) MTM FSS disturbances, (iii) MPO solar array backside illumination. Only a very small movement (“Nudging”) of the MPO solar array was commanded to verify health of the array drive mechanism. Each MPO scientific payload (ref. Table 1) was switched ON for the first time with the PI team present at ESOC, performing a checkout as far as possible in MCSC configuration. The MPO-MAG boom (ref. Fig. 3) was deployed shortly after LEOP, constituting the last deployment of appendages required after launch. MMO platform was commissioned with the JAXA team co-located at ESOC. In a later slot, MMO scientific instruments were also commissioned, with JAXA and the MMO instrument teams present at ESOC. Pictures were taken using the monitoring cameras system (MCAM) starting from LEOP. This included picture taking during special, one-off events like MTM solar array drive run-in or MPO-MAG boom deployment, as well as regular pictures taking of MGA and HGA to show movement of the antennae over time. Fig. 9 shows some examples. 2.3 NECP part 2 and CSW/FCESW activation Owing to the need to start the first EP thrust arc in mid Dec 2018, it had already been planned before launch to perform some of the lower priority near-Earth commissioning operations both for platform and payload only in spring/summer 2019, after completion of the first thrust arc. This slot was also used to perform investigation activities for issues encountered in NECP
Fig. 8. HGA pattern calibration slew to determine antenna boresight, with actual Earth direction at the origin of the coordinate system part 1. Key delta commissioning activities included the following: • Pattern calibration for both MGA and HGA (Fig. 8), performing special attitude slews to determine the antenna boresight, confirming good accuracy of the pre-launch settings • Characterisation of MTM solar array flexible modes • Delta NECP commissioning for MPO payloads PHEBUS and SERENA, including high voltage commissioning • MMO delta commissioning, including MMO payload high voltage operations Another major activity in this phase was the installation and activation of new platform on-board software on the OBC (CSW 3.3.3.2) and the FCE (FCESW 2.3.3). These new software versions addressed a number of improvements on issues encountered either
Fig. 9. Pictures taken with the monitoring cameras of the MTM solar array (left), MGA and magnetometer boom (middle) and the HGA (right) IAC-19-A3.5.2
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in the last few months before launch, or in the first 6 months of flight. The new software versions were tested extensively on the simulator and the ETB at ESOC, and finally uplinked in July 2019. While FCESW activation was a rather straightforward activity requiring a reboot of the FCE out of the loop, CSW activation through OBC reboot naturally implied a ground-commanded safe mode entry for the spacecraft. This was triggered on 17th July 2019 and successfully recovered thereafter. While the baseline for cruise to Mercury was to have all payloads off (only to be switched ON for bi-yearly checkouts to confirm their good health), the successful completion of NECP also allowed letting some of the payloads perform background science operations. MGNS, MPO-MAG and ISA were left in science mode throughout this phase, only to be powered off for special activities requiring all payloads to be off.
Fig. 10. AOCS mode transitions for EPCM entry/exit
3. Electric Propulsion Operations 3.1 Overview BepiColombo electric propulsion (EP) operations are driven by the following characteristics: Complexity of AOCS mode transitions: Fig. 10 shows the mode transitions between AOCS Normal Mode and Electric Propulsion Control Mode (EPCM). Entering EPCM requires slewing to the custom EPCM attitude in phase ETP-A1, rotating the MTM solar array to the EPCM position to maximise power in phase ETPA2 (it is typically offpointed in NM to reduce degradation), and starting up the selected thrusters in phase ETP-A3. Likewise, for EPCM exit the thrusters are switched off in EPCM, the MTM solar arrays are rotated back in phase ETP-B2, and the S/C slews back to NM attitude in phase ETP-B1. Each thrust arc requires a custom spacecraft attitude, with the attitude and solar array guidance profiles commanded by ground for (i) the chosen thruster selection, (ii) a backup thruster selection (if desired), and (iii) the “fallback” guidance profile in NM. A transition to the backup thruster selection or a fallback to NM can be triggered autonomously by FDIR. Including highly configurable EP settings, entering EPCM requires several hundred telecommands and may take up to 5.5 hours (1.5h required for EP start up, up to 4h required for the attitude slews from NM to EPCM). Visibility constraints in EPCM: the attitude required for thruster firing is driven by trajectory needs and the set of thrusters used (out of the 4 thrusters available). Despite two moveable antennae (MGA and HGA), visibility with either of them it is not guaranteed in the firing attitude, due to antenna movement constraints or blockage by other parts of the S/C body. It may hence not be possible to get S/C telemetry when in EPCM, i.e. S/C status can only be checked after exiting EPCM, retrieving playback telemetry. For each EP arc,
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Fig. 11. MGA visibility constraints for EP1 arc: as from 10th Jan 2019, link via MGA in the thrusting attitude became possible expected antennae visibility is analysed to find the best strategy. As an example, Fig. 11 shows the MGA coverage situation for the first thrust arc, displaying the MGA movement range: only if using thruster pair [2,4], MGA coverage would be possible from 10th Jan 2019 up to end of the arc. Orbit Determination needs: slight variations in thrust direction and magnitude when using electric propulsion impact the accuracy for orbit determination in deep space based on radiometric data (Doppler and ranging). To work around this, regular acquisition of radiometric tracking data over periods with the thrusters off was introduced, requiring to exit and re-enter EPCM. Power constraints: despite approaching the Sun, the mission is highly power-constrained. The available power generally does not allow firing the thrusters at their maximum thrust level. Technological limitations on the solar arrays require to offpoint the MTM solar array at closer Sun distances to avoid overheating, see Fig. 12. As the baseline during EP arcs in cruise (see Fig. 1) is to use maximum possible thrust to minimise the duration of the arc (thereby reducing the risk of not achieving the desired deltaV in time for the next planetary swingby), careful management of MTM solar array pointing and EP thrust levels during EP arcs is needed to get the most out of the system.
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3.2 EP thrust arcs in 2018 and 2019 Following Electric Propulsion commissioning in NECP in late 2018, the first thrust arc of the mission started on 17th Dec 2018 and was completed in Feb 2019, with a short touch up manoeuvre in early March 2019. Refer to Fig. 13 for an overview, showing the periods of thruster usage as well as the thrust levels used. The first thrust arc allowed for dual thruster operations. While balanced thruster usage was also a consideration, thruster selection was mainly driven by MGA/HGA visibility, with thruster pair [1,3] allowing HGA coverage in EPCM up to 23/12/2018, and thruster pair [2,4] allowing MGA coverage from 10th Jan 2019 up to the end of the thrust arc. In the period from 24th Dec 2018 to 9th Jan 2019 with neither MGA nor HGA allowing coverage in EPCM attitude, a special TT&C configuration was established with the S/C transmitting a carrier-only signal via LGA when in EPCM, allowing ground to check presence of that signal to confirm the S/C was still in EPCM. This special approach was only possible for this thrust arc close to Earth, with the link budget on LGA insufficient for this approach at further distances. In terms of ground station contacts, a weekly socalled “navigation pass” was taken, when EPCM was interrupted to gather radiometric data for orbit determination (i.e. transition to NM before the pass, and re-entry to EPCM after the pass). On top of these passes, two ground station contacts were taken per week to monitor S/C status in EPCM (or, when neither MGA nor HGA were visible, to at least check presence of a carrier-only signal from LGA), allowing a regular check of S/C performance and faster reaction time to unexpected EPCM interruptions. Overall thruster performance in the arc was nominal [6]. Three unexpected interruptions of thrusting in
Fig. 12. MTM solar array allowed Sun aspect angle depending on Sun distance (SAA of 0 deg means MTM SA is facing the Sun) EPCM triggering a fallback to NM and a recovery by ground to re-enter EPCM occurred in this thrust arc. These were caused by thruster-related software FDIR triggering too early, with ground tuning the settings accordingly thereafter. The gradual change of thrust level over time visible in Fig. 13 was due to the change of distance to the Sun, limiting the maximum power available for firing the thrusters. The second thrust arc of the mission (SEP2) started on 12th Sept 2019 and is expected to end by mid Nov 2019. Due to larger sun distance, the power available only allows using a single thruster. Either thruster 1 or 3 are used, as they allow to have MGA coverage in the EPCM attitude. While the overall operations concept is very similar to the SEP1 arc, the planning process has been optimised to reduce the workload on the team. 4. Conclusion and Outlook After an extensive preparation phase, the ESA/JAXA BepiColombo mission to Mercury was
Fig. 13. Thrusters and thrust levels used during SEP1 arc from Dec 2018 to Feb 2019, with greyed out periods indicating when thrusters were OFF IAC-19-A3.5.2
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finally launched in October 2018. The first year of flight saw the successful execution of LEOP (20th to 22nd Oct 2018), near-Earth commissioning part 1 (22nd Oct 2018 to mid Dec 2018), the first electric propulsion thrust arc (mid Dec 2018 to early Mar 2019), near-Earth commissioning part 2 (Mar 2019 to Aug 2019), and the start of the second EP thrust arc on 12th Sep 2019. Following installation and activation of a new platform on-board software in July 2019 to address findings from commissioning, the S/C is in excellent shape for the cruise to Mercury, with a planned arrival in Dec 2025. Regarding key upcoming operations, the second solar electric propulsion arc of the mission is planned to end by mid Nov, bringing the S/C on a trajectory to the first planetary swingby, at the Earth in April 2020. As from the Earth swingby, the S/C will move into the inner solar system, with a first Venus swingby in October 2020. A key aspect of this phase is that the S/C performance will be observable for the first time in a hotter environment. References [1] J. Benkhoff, J. van Casteren, H. Hayakawa, M. Fujimoto, H. Laakso, N. Novara et al, BepiColombo - Comprehensive Exploration of Mercury: Mission Overview and Science Goals, Planetary and Space Science, Vol. 58, Issues 1-2, Elsevier, 2010, pp. 2-20.
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[2] P. Espeillac, T. Strandberg, M. Casasco, C. Steiger, A. Altay and E. Montagnon, GNC Design Challenges for Variable Flight Configuration on the BepiColombo Mission to Mercury, 10th International ESA Conference on Guidance Navigation and Control Systems (GNC 2017), Salzburg, Austria, 2017, 29 May – 2 Jun. [3] R. J. Wilson, M. Schelkle, The BepiColombo Spacecraft, its Mission to Mercury and its Thermal Verification, 46th Lunar and Planetary Science Conference, The Woodlands, USA, 2015, 16-20 Mar. [4] C. Steiger, E. Montagnon and A. Accomazzo, Flight Operations Preparation for the BepiColombo Mission to Mercury: Concepts and Challenges, SpaceOps 2016, Daejeon, Korea, 2016, 16-20 May. [5] F. Budnik, G. Bellei, F. Castellini and T. Morley, BepiColombo: Flight Dynamics Operations during Launch and Early Orbit Phase, 18th Australian International Aerospace Congress (AIAC18), Melbourne, Australia, 2018, 24-26 Feb. [6] P. Randall, R. Lewis, S. Clark, K. Chan, H. Gray, C. Steiger, F. Striedter, BepiColombo – MEPS commissioning activities and T6 ion thruster performance during early mission operations, 36th International Electric Propulsion Conference (IEPC 2019), Vienna, Austria, 2019, 16-20 Sep.
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