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Click to enlarge. Credit: ESA
Artist rendering illustrating the TGO orbiter conducting its braking maneuver to enter orbit around Mars on Oct. 19, 2016. Click to enlarge. Credit: ESA
European mission controllers monitor separation between the TGO orbiter and the EDM lander. Click to enlarge. Credit: ESA
TGO in the orbit of Mars. Click to enlarge. Credit: ESA
ExoMars-2016 enters orbit of Mars
After a seven-month journey between the Earth and Mars, the TGO orbiter and the Schiaparelli lander comprising the ExoMars-2016 mission approached the Red Planet. On October 16, the orbiter released the lander to make a descent and soft landing on the planet's surface three days later. Almost simultaneously, the TGO spacecraft successfully entered orbit around Mars for a multi-year mission.
Final preparations for arrival
The activity at the European Space Agency's mission control center in Darmstadt, Germany, began picking up in August, as the spacecraft closed in on Mars. The communications sessions between the spacecraft and ground stations switched to a daily schedule. Some 10 days before the ultimate encounter with Mars ground stations in New Norcia and Malargue were expected to begin 24-hour radio contact with the spacecraft, while mission control conducted exhaustive testing of every system on the dual spacecraft and uploaded final commands on the Schiaparelli lander.
On October 7, ESA announced that the mission control center in Darmstadt had uploaded time-tagged stored commands to the spacecraft, ensuring that the lander could conduct its mission even when out of contact with any of the Mars orbiters that were to serve as data relays. The automated sequence also ensured that the lander would wake up from its power-saving sleep periods on the surface in time for communication links, ESA said. The timeline for the operation of the DREAMS instrument package onboard Schiaparelli was also contained in the code.
The commands were uploaded in two batches. The first set, containing the hibernation wake-up timers and the surface science instrument timeline, was uploaded on October 3 to the Trace Gas Orbiter, TGO. The next day, the software codes were transferred from the orbiter's computers to the lander's flight control system.
On October 5, mission controllers met with experts from ESA's ESTEC center and engineers from Thales Alenia Space, Italy, which built the spacecraft, for the final review of the mission timeline after the separation of Schiaparelli. By that time, mission control had completed simulations of expected actions in the orbit of Mars and on its surface in the dual mission. The meeting cleared the way to upload the second set of commands on October 7, containing the post-separation scenario of the mission.
From October 9, the TGO was put in constant communications with mission control without any planned interruptions.
As of October 11, mission control had already begun charging the batteries on the Schiaparelli in preparation for its landing on Mars.
According to the spacecraft operations manager Peter Schmitz, quoted by ESA web site, during the week of October 10, mission control "uploaded the commands to fully charge the lander’s batteries and prepare the orbiter’s data-handling system as well as power- and thruster system for separation and the subsequent trajectory tweak."
On October 14, at 08:45 GMT (4:45 a.m. EDT; 10:45 CEST), the joint TGO/EDM spacecraft initiated the final trajectory adjustment maneuver with its small thrusters before their separation. ESA was expected to confirm the results of the engine burn lasting around a minute at 10:06 GMT (6:45 a.m. EDT). According to the European Space Operations Center, ESOC, in Darmstadt, Germany, data indicated a minor over-peformance against the planned 1.4 centimeter per second velocity change that was planned during the maneuver. Still, mission control confirmed that the dual spacecraft was on track for separation.
On the morning of October 14, mission control also held a final pre-arrival team briefing for the ExoMars project team. The dual spacecraft and all support services, including US ground stations were declared fully ready for operations.
On October 15, at 02:00 CEST (at midnight GMT), the ExoMars flight operations switched to the main general-purpose control room at the European Flight Operations Center, ESOC, in Darmstadt, Germany. According to ESA, during the day, mission control teams planned to check the health of the spacecraft and to support another "delta DOR" navigation measurements conducted by ground stations. All files and configuration settings needed for the upcoming separation between TGO and Schiaparelli would have to be finalized by the end of October 15, ESA announced.
On October 16, at 02:42 GMT, mission control was expected to upload final commands for entry, descent and landing of the ExoMars-2016 mission. According to ESA, engineers from Thales Alenia Space (Italy) working at ESOC were to verify the final set of time-tagged commands, which have been uploaded via TGO and stored on board Schiaparelli so that it can function more or less autonomously throughout its mission.
In the final hours before separation, ESA reported that the lander had been activated around 10:10 CEST (4:10 a.m. EDT) and telemetry showed good temperature onboard. The separation timeline was activated at 13:25 CEST (7:25 a.m. EDT) and around 15:00 CEST (9 a.m. EDT), ESA confirmed that the TGO had attained the required attitude for separation.
The TGO spacecraft and the EDM Schiaparelli lander separated on October 16, 2016, at 14:42 GMT (10:42 a.m. EDT, 16:42 CEST, 17:42 Moscow Time) three days and six million kilometers before reaching Mars. As they parted at a gentle speed of just 0.3 meters per second, the TGO had to provide necessary attitude and slow stabilization spin to the disk-shaped Schiaparelli with a rate of about 2.5 rotations per minute.
Mission control anticipated that the separation of the 577-kilogram Schiaparelli would cause some wobbling on the 4.3-ton TGO "mothership." This could affect the very sensitive antenna pointing at Earth, which is needed to ensure a full data link. As a result, mission controllers planned in advance to monitor progress only via the basic ("unmodulated") radio carrier signal, with the signal acting like a beacon, ESA said. The full data link was cut off around 16:31 CEST (10:31 a.m. EDT).
The separation wobble was visible in the Doppler data associated with the carrier signal. With a one-way signal time of about nine minutes and 45 seconds, mission controllers were expected to see a first indication of progress around 14:52 GMT (10:52 a.m. EDT). Shortly before the actual event, ESA promised a confirmation from Flight Director Michel Denis after 14:55 GMT (10:55 a.m. EDT).
Mission controllers also hoped to see signals received via the GMRT radio telescope in Pune, India, however ESA warned that this was strictly an experiment and may not function as planned. A full confirmation was expected around 15:15 GMT (11:15 a.m. EDT) once controllers re-establish the full data link with the spacecraft, ESA announced.
Around 11:02 a.m. EDT, mission controllers publicly confirmed that the separation had indeed taken place based on the Doppler signal from the carrier. The GMRT telescope also recorded a very faint signal that indicated separation.
Telemetry interruption from TGO
At 11:28 a.m. EDT, Michel Denis confirmed the separation between the orbiter and the lander but said that the TGO had not re-established the normal telemetry link with mission control as planned, sending only the carrier signal. Experts were investigating the issue, Denis said. He then called for a meeting to review the situation.
At 12:30 p.m. EDT, ESA announced that "the anomaly that prevents TGO's telemetry from being sent is under investigation, and is expected to be resolved within the next few hours." However within minutes after the announcement, full telemetry link with the orbiter was restored via ESA's 35-meter ground station at Malargüe, Argentina.
Before the launch of ExoMars-2016, ESA enlisted NASA to train its 70-meter antennas in Canberra, Australia, and in Madrid, Spain, from the Deep Space Network, DSN, to listen to the departing lander. They will be able to confirm the crucial operation by measuring the changing Doppler shift of a carrier signal from the mission.
In case the first attempt to separate the two spacecraft did not take place, mission control had two more opportunities to repeat the operation and still ensure the nominal landing of Schiaparelli on the planet. The third backup attempt could also be made to simply get rid of the lander and still insert the TGO spacecraft into a nominal orbit around Mars. Finally, if all attempts to separate the lander failed, the joint spacecraft could make a flyby of Mars and continue its path around the Sun. It would have its next opportunity to enter into orbit around the Red Planet in about a year.
After separation, Schiaparelli was left on a direct collision course with Mars. During a big part of its three-day autonomous flight to Mars, the lander will be in hibernation mode to save battery power, the only source of electricity onboard.
On October 17, at 02:42 GMT (04:42 CEST), around 12 hours after dropping the lander (it was 10:42 p.m. EDT on October 16), and still 760,000 kilometers from the planet, the TGO spacecraft initiated a trajectory correction to avoid its own direct collision with Mars. The one-minute 46-second engine firing, known as the Orbiter Retargeting Maneuver, ORM, delivered 11.6 meters per second in velocity. The maneuver raised the pericenter (lowest point of the Martian orbit from the surface) to more than 500 kilometers, making it possible to enter its orbit around 60 hours later.
As usual, there was an interruption of communications with the spacecraft during the engine firing. Mission control was expecting to hear from the TGO again at 05:07 CEST (11:07 p.m. EDT on October 16). In reality, the signal was restored a few minutes earlier than expected.
Ground antennas in Canberra and Malargue monitored the maneuver.
Mutual positions of the Schiaparelli lander, the Mars Express orbiter and a Martian moon Phobos during the orbital insertion of the TGO spacecraft
Three hours after the Mars avoidance burn on October 17, the TGO should be configured for a fully autonomous maneuver to enter the Martian orbit. The spacecraft was to be placed into the so-called "failop" mode, which tells the flight control system to ignore minor glitches aboard the TGO during the critical engine firing.
On October 18, at 07:35 CEST (05:35 GMT, 01:35 a.m. EDT) mission control confirmed that orbit insertion commands, which will manage the critical engine burn, were successfully uploaded onboard the orbiter, apparently 4.5 hours ahead of schedule.
After around a 30-minute process of pointing its main engine against the direction of the flight, the TGO initiated the Mars Orbit Insertion, MOI, maneuver on October 19, 2016, at 13:04:47 GMT (9:04 a.m. EDT). In nine minutes 45 seconds, signals confirming the beginning of the engine burn should reach Canberra, Australia, the home of one of the ground stations controlling the ExoMars mission. The entire engine firing should last approximately 134 minutes (according to other sources 139 minutes). The exact duration of the burn will depend on actual performance of the engine, but it should not exceed 147 minutes. The full executed engine burn should reduce the speed of the spacecraft by 1,550 meters per second (1.55 kilometers per second) to ensure the capture of the TGO into Mars' orbit.
In the midst of the TGO's Mars Orbit Insertion maneuver, the Schiaparelli will hit the Martian atmosphere at 14:42 GMT (16:42 CEST, 10:42 a.m. EDT) and begin its descent. Its landing should then take place around 14:48 GMT (16:48 CEST, 10:48 a.m. EDT) or one hour 45 minutes after the TGO initiates its braking maneuver.
Minutes after the completion of the maneuver, the TGO was to begin taking its cruise attitude, deploying the antenna for communications with the Earth and pointing solar panels toward the Sun. However it would take the spacecraft nearly an hour to re-emerge from behind the planet and to establish contact with ground control.
When the spacecraft did call home, engineers quickly confirmed the spacecraft was in orbit and there were no reboots in the main computer which controlled the TGO.
TGO in the orbit of Mars
Following its braking maneuver, the TGO will initially enter a 298 by 95,856-kilometer orbit around Mars on October 19, 2016. It will take the TGO four Martian days (sols) to complete one revolution around the planet in its initial highly elliptical orbit.
Artist rendering of the TGO in the orbit of Mars.
The three initial orbits of the TGO spacecraft around Mars shown to scale with the planet.
In December 2016 (or by January 2017, according to other reports), the orbiter should reach the operational inclination of its orbit at 74 degrees toward the plane of the Martian equator. The spacecraft will also reduce its apocenter (the highest point of its orbit). As a result, the TGO will be make a single revolution around the planet in one sol (Martian day).
The TGO will then begin sweeping through the upper atmosphere of Mars in a process known as aerobraking. The resulting influence of the aerodynamic resistance from the atmosphere will make the TGO's orbit circular at an approximate altitude of 400 kilometers. The orbital period will be reduced to just two hours and the spacecraft will be fully ready to conduct its scientific mission.
According to the original plans, the a year-long aerobraking would commence at the end of 2016, however the start of the process was eventually postponed until March 2017.
In the midst of aerobraking process, from July 11 to August 11, 2017, the orbiter will cease all critical operations for a month, because the Sun will be exactly between the Earth and Mars, in the so-called superior solar conjunction, making communications difficult.
When the aerobaking is finally completed in March 2018, scientists will be able to point their instruments at the surface of Mars for the first time. The spacecraft will also be ready to begin communications relays from NASA's rovers on the surface of the planet.
According to original plans, by January 15, 2019, the TGO was to be ready to relay communications from the ExoMars rover and its landing platform. However since the the mission formerly known as ExoMars-2018 was postponed from 2018 to 2020, all interactions with the ExoMars rover had to be delayed until 2021. However from the outset, the TGO was designed to support the ExoMars rover until at least the end of 2022. It is possible that the TGO mission could be extended to continue the support for the ExoMars rover if it successfully operates on Mars. Currently, the official end of operations for TGO is set for December 2022.
Mission control during ExoMars-2016
Structure of scientific information processing in the ExoMars-2016 project. Credit: ESA
Mission control structure in the ExoMars-2016 project. Credit: ESA
As usual with spacecraft operations near Mars, it will take around 10 minutes for radio signals to reach the Earth during the mission.
The main nerve center of the ExoMars-2016 mission is located in Darmstadt, Germany, at ESA's European Space Operations Center, ESOC. The 15-member core flight control team of the ExoMars-2016 mission led by Spacecraft Operations Manager, SOM, Peter Schmitz is supported by many other specialists at ESOC and ESA's main technical center ESTEC in the Netherlands, to help with flight dynamics, ground stations, software and systems. A team at European Science Operations Center near Madrid, Spain and industrial experts from Thales Alenia Space (in Italy and France) are also involved in the mission.
To provide the most reliable communications during the ExoMars-2016 mission, the European Space Agency, ESA, engaged both American and Russian assets. NASA provided both ground antennas and spacecraft orbiting Mars, while Roskosmos agreed to point its 64-meter antenna onto the TGO to give an extra channel for downloading scientific information. Moscow-based Space Research Institute, IKI, was also expected to host a copy of all the scientific data coming from the TGO mission.
After separation of the lander, the TGO should monitor the UHF transmission from Schiaparelli, beginning from its coasting toward Mars and until its landing. A NASA orbiter will act as a data relay for Schiaparelli during its surface operations. Furthermore, ground-based communication arrays will also track the UHF signal during the entry, descent and landing phases.
ESA will be in full control of the TGO during all phases of its mission, including insertion into Mars orbit, orbit control, aerobraking, science operations and communications.
Signal delay during the ExoMars-2016 arrival at the Red Planet in October 2016:
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Page author: Anatoly Zak; Last update: October 19, 2016
Page editor: Alain Chabot; Last edit: October 13, 2016
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