'Data saturation' helped to crash the Schiaparelli Mars probe

Altitude reading of 'below ground level' turned probe into Magrathean Whale

 Schiaparelli separating from Trace Gas Orbiter. Pic: ESA–D. Ducros

The European Space Agency (ESA) has released results of its early investigations into the crash of the Schiaparelli Mars probe and it sounds like software may have been a part of the problem.

"A large volume of data recovered from the Mars lander shows that the atmospheric entry and associated braking occurred exactly as expected," says the agency's statement.

The probe's parachute popped out as planned and its heat shield flew off at just the right time.

"As Schiaparelli descended under its parachute, its radar Doppler altimeter functioned correctly and the measurements were included in the guidance, navigation and control system," the statement says. "However, saturation – maximum measurement – of the Inertial Measurement Unit (IMU) had occurred shortly after the parachute deployment. The IMU measures the rotation rates of the vehicle. Its output was generally as predicted except for this event, which persisted for about one second – longer than would be expected."

That saturation caused problems because it meant that when IMU data was "merged into the navigation system, the erroneous information generated an estimated altitude that was negative – that is, below ground level."

"This in turn successively triggered a premature release of the parachute and the backshell, a brief firing of the braking thrusters and finally activation of the on-ground systems as if Schiaparelli had already landed. In reality, the vehicle was still at an altitude of around 3.7 km."

The situation's been reproduced on terrestrial simulators.

Which meant that poor Schiaparelli found itself in a situation rather like that faced by the Magrathean Whale in The Hitchhiker's Guide To The Galaxy: certain its gravity-assisted plunge would result in its doom but with plenty of time to ponder its predicament on the way down.

The ESA says its statement represents "a very preliminary conclusion of our technical investigation" and that an external independent inquiry board will report in early 2017.

The agency still feels the mission will help the next ExoMars effort, scheduled for 2020. ®


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