Mars encounter -1 day

The most critical moment so far in the ExoMars Trace Gas Orbiter’s journey will be Wednesday’s Mars Orbit Insertion burn – the long (134 min) engine firing that will slow the spacecraft down sufficiently to be captured into Mars orbit. What actually happens during this critical burn, though? Here we thought we’d give you a more detailed rundown of that all-important moment.

The burn will be performed autonomously by the orbiter, based on commands uplinked beforehand by the control team at ESOC in Darmstadt. Around half an hour before the burn starts, currently set for 13:04:47 GMT (15:04:47 CEST) on 19 October, the spacecraft will begin turning around to point its big main engine toward the direction of travel.

As this is happening, the large 2.2 m (7.2 ft) diameter high gain antenna on the Trace Gas Orbiter will be locked into a safe ‘boost position’ for the burn. As this doesn’t point it at Earth, we will temporarily lose contact with the spacecraft. Also at almost the same time, the solar arrays will rotate and also lock into their safe boost position. Finally the orbiter will start reconfiguring its radios to send a beacon signal through its Low Gain Antenna.

When this radio reconfiguration is complete, the orbiter will start sending out this ‘carrier only’ signal. The Low Gain Antenna isn’t powerful enough to send data to Earth, hence why we use this simple beacon signal. The key advantage is that the Low Gain Antenna signal can be picked up almost no matter what orientation Trace Gas Orbiter is in. The signal will be acquired by NASA’s big 70m-diameter dish in Canberra, Australia and that will let the team on ground know that the orbiter is there. Critically it should also show a jump in frequency caused by Doppler shift as the orbiter fires its engine, allowing us to monitor the progress of the burn even without telemetry data.

As the spacecraft reconfigures itself, and points the engine nozzle in the right direction, the clock will tick down to the programmed ignition time. The exact ignition time that will be loaded is being refined all the time by our flight dynamics experts to take place at exactly the right second. At that time the valves above the main engine will open and the Monomethylhydrazine propellant and Mixed Nitrogen Oxides oxidiser will flood into the engine. These are hypergolic – meaning no ignition spark is necessary! The two liquids will ignite on contact and at this moment the big engine will roar into life, generating 424 Newtons [pushing with about the same force as that of a 45-kg weight on Earth’s surface – Ed.] of thrust in the direction of flight – effectively slamming on the brakes as the orbiter hurtles towards Mars.

With the engine firing with all its might, Trace Gas Orbiter will settle into the longest engine burn of its life. Just as our Flight Dynamics team are calculating the exact point of ignition, they are also calculating the predicted moment of shutdown. We currently expect that to be roughly 139 minutes after ignition, but this actually isn’t a time we programme on board the satellite. Trace Gas Orbiter has another trick up its sleeve – sensitive accelerometers will measure by how much the orbiter is decelerating and when exactly the right amount of braking force has been generated, it will shut the engine down. This approach allows the spacecraft to autonomously compensate for any over or under performance of the engine.

But suppose there’s a problem with the accelerometers, and they don’t signal the shut down – the engine won’t simply keep running forever. One hundred and forty seven minutes after the programmed ignition time, Trace Gas Orbiter will reach ‘MOI Timeout’. At this time, it will shut down the engine (if it’s still firing) no matter what and start reconfiguring back to a normal Earth-pointing communication mode.

It will safely isolate the main engine, allow the power-generating solar arrays to freely track the Sun once more and turn the big high-gain antenna toward Earth. Finally, it will turn on the main radio data signal and start telling us how the burn went.

Here lies the last bit of the puzzle though – we actually won’t be able to hear Trace Gas Orbiter at the end of burn! This is actually all planned – while the burn is happening, the spacecraft will pass behind Mars (occultation) and we’ll lose all radio contact with it. It will only emerge from behind the Red Planet after the burn is complete and it is reconfigured to talk to us. It will be a tense wait for everyone on ground but, all being well, it will come out of occultation on time and telling us all we need to know about the burn performance. Flight Dynamics will then measure and assess the orbit the spacecraft has actually achieved and provide us final confirmation that Europe has returned to Mars! 

Thomas Ormston, spacecraft operations engineer at ESOC

One day to arriving and landing on Mars!

18 October

08:20 CEST: The TGO’s Mars Orbit Insertion manoeuvre commands were confirmed uploaded this morning at 07:35 CEST, ready for executing tomorrow. The insertion manoeuvre is planned to start at 15:04 CEST on 19 October.

17 October

15:45 CEST: The ExoMars/TGO orbiter and the Schiaparelli demonstration lander module are in good health and continuing, since separation yesterday at 16:42 CEST, on separate paths toward Mars. Schiaparelli is already on a trajectory that targets a landing in an area close to the equator known as Meridiani Planum. Following this morning’s 11.6-m/sec orbit-raising manoeuvre, TGO has ‘lifted’ its trajectory so as to avoid the planet, while continuing to draw closer.

On Wednesday, TGO will conduct one of the most critical manoeuvres in its mission: a 139-min orbit entry manoeuvre planned to slow it by about 1.6 km/second. Earlier today, the mission control team at ESA’s ESOC operations centre configured TGO into a special mode for the orbit entry manoeuvre. The so-called ‘fail-op’ mode ensures that any routine problem that might arise − and that might trigger the craft to reset itself into ‘safe mode’ (which would shut down many ongoing activities, including propulsion) − will be ignored, so that the engine burn will in fact continue, more or less no matter what. The craft only gets one chance to enter orbit on the 19th.

Also today: mission controllers confirmed that TGO correctly received data transmitted by Schiaparelli during separation, and that the landing module had promptly gone to sleep shortly after being pushed away from the orbiter, as planned.

05:15 CEST: This morning at 04:42 CEST the TGO completed an orbit raising manoeuvre as planned. Without the manoeuvre, the spacecraft would, like Schiaparelli, remain on a collision course with Mars. Firing its engine for about 1m 46s raised the TGO’s orbit by several hundred km ‘above’ the planet, ahead of its planned orbit insertion on Wednesday. Signal with the TGO was reacquired after the burn, just after 05:00 CEST.

16 October

18:43 CEST: Full telemetry link with ExoMars/TGO has been restored via ESA’s 35m deep-space ground station at Malargüe, Argentina. 

18:30 CEST: The Schiaparelli module was released from the Trace Gas Orbiter (TGO) at 14:42 GMT (16:42 CEST) as planned. The Schiaparelli Entry, Descent & landing demonstrator Module separated from the TGO orbiter and is now en route on a ballistic trajectory to reach the Red Planet, enter its atmosphere and land softly in an area close to the equator known as Meridiani Planum.

However, TGO unexpectedly did not return telemetry (on-board status information), and sent only its carrier signal, indicating it is operational. The anomaly that prevents TGO’s telemetry from being sent is under investigation, and is expected to be resolved within the next few hours.

An update will be posted in the next few hours.

17:27 CEST: ExoMars Flight Director Michel Denis confirmed that separation of Schiaparelli has occurred and and signals from TGO have been reacquired. The signals do not contain the expected telemetry (information on the onboard status), and the teams are investigating the situation.

17:02 CEST: Flight dynamics team at ESOC confirms separation of Schiaparelli from TGO on the basis of Doppler signal from the carrier. The Giant Metrewave Radio Telescope (GMRT) near Pune, India, has also recorded a very faint signal that indicates separation. Official confirmation expected soon when telemetry from TGO is received.

16:42 CEST: According to the timeline, Schiaparelli should have separated from TGO. Confirmation is expected on Earth soon and will be announced by ExoMars Flight Director Michel Denis once the data is on ground.

15:50 CEST: Both Schiaparelli and ExoMars/TGO are in good shape! The A-team shift of the mission control team are now on console in Main Control Room at ESOC. ExoMars/TGO has completed its slew into separation attitude/orientation. Communication now provided by ESA’s 35m deep-space tracking station at New Norcia, Australia. Separation on track for 16:42 CEST.

13:25 CEST: The separation timeline has started! Teams at ESOC are extremely busy monitoring the sequence of events scheduled for today’s separation, expected at 16:42 CEST. Both the ExoMars/TGO orbiter and the Schiaparelli lander module are in great shape. The lander has been switched on and engineers monitoring telemetry – on-board status information – report that temperatures on Schiaparelli are nominal. ExoMars/TGO has already begun slewing – rotating in space – into the correct attitude for separation. Just prior to separation, at 16:31 CEST, mission controllers expect to lose the full data link with TGO, and then will follow progress by monitoring the basic unmodulated carrier signal only, as a sort of beacon. We may also see signals received via the GMRT radio telescope in Pune, India, although this is strictly an experiment and may not function as planned. One-way signal time today is 9 mins and 34 secs.

10:10 CEST: Today, Schiaparelli, still attached to ExoMars/TGO, is switched on, and its systems checked out one final time. Experts from Thales Alenia Space (Italy) working at ESOC will 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. TGO will eject Schiaparelli at 16:42 CEST, dispatching it on a three-day coast and a six-minute descent to the surface. Ground station coverage will be provided by NASA’s giant 70m Deep Space Network (DSN) ground stations at Canberra, Australia, and Madrid, Spain, which will listen for the spacecraft’s signals as the Schiaparelli module separates. It will be pushed away from TGO at just 30 cm/second, but this tiny push can be detected by the DSN stations.

15 October

At ESOC today: The ExoMars/TGO mission control team begins working from the large, general-purpose Main Control Room as of 02:00CEST. Activities include spacecraft health and status check-outs and ground station tracking passes to support the highly accurate “deltas DOR” navigation technique. By this evening, all files and configuration settings needed to support separation will be finalised. Separation is set for 14:42 GMT (16:42 CEST) spacecraft time tomorrow.

For separation, releasing a 577-kg lander will make TGO wobble. This could affect the very sensitive antenna pointing needed to ensure a full data link, so mission controllers will monitor progress only via the basic radio carrier signal, with the signal acting like a beacon. The separation wobble will be visible in the Doppler data associated with the carrier signal. With a one-way signal time of about 9 min and 45 secs, mission controllers will see a first indication of progress around 16:52 CEST. A full confirmation will come later (around 17:15 CEST) once controllers re-establish the full data link with the spacecraft.

14 October 

18:35 CEST: Our coverage of separation on Sunday, 16 October, set for 14:42 GMT (16:42 CEST) spacecraft time, will begin a bit earlier than previously announced. You can watch a live webstream from ESA’s mission control centre, Darmstadt, Germany, starting at 16:30 CEST.

14:05 CEST: The final pre-arrival orbit correction manoeuvre that took place at 10:45 CEST went very well. This burn was the last push needed to perfectly line TGO up on the right orbit to deliver Schiaparelli onto the surface three days after separation on Sunday. Flight Director Michel Denis reports a very tiny over performance from TGO’s thrusters, but the burn was overall very good. The next manoeuvre is scheduled for 12 hours after separation, at 04:42 CEST on 17 October, and will raise TGO’s orbit above the planet.

10:25 CEST: This morning, the final pre-arrival team briefing was held at ESOC for everyone involved in ExoMars. Teams at ESTEC and from industry listened in via audio conference. The briefing was held to review and confirm readiness for separation on 16 October and arrival at Mars on 19 October. ExoMars/TGO orbiter and Schiaparelli demonstration lander health/status? Both GO for arrival. Schiaparelli is GO for entry, descent, landing and surface operations. TGO is GO for Mars orbit insertion. At our ESOC mission control centre: Simulation training complete; teams from Flight Dynamics, Flight Control, the ESA ground stations, networks and systems are all GO. NASA ground stations have also confirmed readiness to support. ESA’s ExoMars Project Manager Don McCoy said: “People have put their hearts and souls into this. We’re ready to go. Thank you to everyone.”

08:50 CEST: This morning at 08:45 GMT (10:45 CEST) ExoMars/TGO will conduct the final orbit correction manoeuvre before separation on 16 October. The manoeuvre means that TGO’s thrusters will be fired for a minute or so to deliver a change in speed and direction of just 1.4 cm/second. This burn will refine the already highly accurate orbit, and line TGO up to deliver Schiaparelli on to its intended landing site on Mars.

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