After a seven-months investigation, ESA has released the results of its inquiry into the crash of its unmanned Schiaparelli lander on Mars last October. From telemetry data and orbital images, the investigators have confirmed that the accident was due to a sudden, violent rotation of the module that fooled the onboard computer into thinking that it had already landed and caused it to shut down its landing thrusters while still high above the planet's surface.
On October 16, 2016, Schiaparelli attempted to land on the Red Planet several days after it separated from its Trace Gas Orbiter (TGO) mothership. The six-minute hypersonic entry into the Martian atmosphere went as expected as the spacecraft collected data and beamed it back to the TGO.
Then Schiaparelli jettisoned its parachute 3.7 km (2.3 mi) above the surface and fired its thrusters for just three seconds instead of the planned 30. The end result was that the probe dropped 3,700 m (12,000 ft) before hitting the ground.
Analysis of the computer telemetry showed that the lander's onboard computer mistakenly believed that it was on the ground when it was still thousands of feet in the air. In fact, it had even initiated its ground program and was sending housekeeping data. Later images by NASA's Mars Reconnaissance Orbiter (MRO) confirmed that the spacecraft had crashed and was destroyed by the explosion of its fuel tanks on impact.
ESA set up an independent external inquiry chaired by ESA's Inspector General to determine the cause of the accident and to make recommendations as to how to avoid a repetition.
The newly released report identifies the circumstances and the root causes, stating that about three minutes after entering the atmosphere, Schiaparelli deployed its parachute. As it did so, the spacecraft oscillated on the tether and was subjected for about a second to a violent rotation that was outside of the design specifications of the Inertial Measurement Unit (IMU), which tracks the rotation rate of the lander as it descends.
Despite the fact that Schiaparelli had another system for measuring altitude, the saturation of the IMU caused a large attitude estimation error by the guidance, navigation and control system software. This, in turn, caused the computer to miscalculate data from the Doppler radar and it believed that it was not only landed, but was underground. As a result, the landing thrusters were shut down 27 seconds too early and ended up striking the Martian surface at a speed of 540 km/h (335 mph).
Ironically, ESA contends that the landing was successful – aside from the crash, explosion, and total loss of the spacecraft. However, the inquiry says that the data from the event will be valuable in planning the upcoming ExoMars 2020 landing.
"Interestingly, had the saturation not occurred and the final stages of landing had been successful, we probably would not have identified the other weak spots that contributed to the mishap," says Jan Woerner, ESA's Director General. "As a direct result of this inquiry we have discovered the areas that require particular attention that will benefit the 2020 mission."
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