This post was contributed by Thomas Ormston, a spacecraft operations engineer here at ESOC, and highlights the science, the engineering, the technology and the incredible teamwork involved in getting ExoMars/TGO captured into orbit around another planet.
The most critical moment so far in the ExoMars Trace Gas Orbiter’s journey will be Wednesday’s Mars Orbit Insertion burn – the long (ca. 134 mins) 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 2.2m-diameter high gain antenna on the Trace Gas Orbiter will be locked into a safe ‘boost position’ [the opposite of what’s shown here – Ed.] 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!