The Schiaparelli lander crashed on the surface of Mars because it thought it was underground – when it was actually 3.7 kilometers (2.3 miles) above the surface.
“Landing on Mars is an unforgiving challenge but one that we must meet to achieve our ultimate goals,” said David Parker, ESA's director of human spaceflight and robotic exploration, in a statement.
Schiaparelli was intended to be a demonstration of landing on Mars, with a rover aiming to land via a similar method in 2021.
The probe travelled to Mars along with its mothership, the Trace Gas Orbiter, having launched from Earth on March 14, 2016. On October 16, the TGO released Schiaparelli, as the former entered into orbit – where it remains today and will soon begin measuring the atmospheric composition of Mars.
On October 19, after three days of solo travel, Schiaparelli entered the atmosphere of Mars. The landing was intended to last six minutes. The probe began by using a heat shield to survive the high temperatures at supersonic speeds. It successfully aero-braked in the atmosphere and deployed its parachute.
It then jettisoned its heat shield after passing through the atmosphere, and began measuring the distance to the surface with its Radar Doppler Altimeter (RDA). However, at this point things started to go wrong.
Schiaparelli was equipped with an Inertial Measurement Unit (IMU), used to measure the spacecraft’s rotation. During the descent, however, the spacecraft rotated faster than expected. This led to the IMU being “saturated” with data, and for one second it was unable to take any more.
The result was that the probe’s guidance, navigation, and control (GNC) software thought it was several meters below the surface of Mars. It was actually 3.7 kilometers above the surface.
This caused Schiaparelli to think it had landed. It detached its parachute and fired its thrusters for three seconds, rather than the 30 seconds required at this point. Not long later, it smashed into the surface at 540 kilometers per hour (335 miles per hour).
ESA’s investigation does not point the finger squarely at the IMU, however. It says that more could have been done to prepare for such an anomaly.
In particular, it says that uncertainty in the modelling of the parachute dynamics was partly to blame. The small error in the IMU should also not have been such a problem, with the spacecraft not being robust enough to prepare for such an anomaly. Ultimately, a “mishap in management of subcontracters” may have led to less than adequate hardware being used on Schiaparelli.
All this is pretty important, as ESA gears up for an even more ambitious landing. In 2020, it will launch its expensive and delayed ExoMars rover to Mars, with a landing expected in early 2021. This will use many of the same technologies as Schiaparelli to touch down, so learning what went wrong will be imperative.
"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," Jan Woerner, ESA's director general, said in the statement. "As a direct result of this inquiry we have discovered the areas that require particular attention that will benefit the 2020 mission."
Schiaparelli might just turn out to be a very useful failure, then.