Bad IMU Data Doomed ESA's Schiaparelli Lander

The European Space Agency (ESA) said today that erroneous data from an Inertial Measurement Unit (IMU) doomed its Schiaparelli Mars lander last month.  The bad data convinced onboard systems that the spacecraft had already landed when it actually was still 3.7 kilometers (km) above the surface.  The spacecraft made a free fall the rest of way, hitting the surface at a high velocity.  Schiaparelli is part of ESA’s ExoMars program and traveled to Mars with the Trace Gas Orbiter (TGO) spacecraft, which successfully entered orbit.

TGO and Schiaparelli were launched together in March. TGO will study trace gases, like methane, in the Martian atmosphere that may reveal whether life ever existed there. Schiaparelli and TGO are the first two of four spacecraft that comprise ESA’s ExoMars program, which it is conducting cooperatively with Russia’s Roscosmos state space corporation.  Initially ExoMars was an ESA-NASA program, but the Obama Administration declined to fund NASA’s portion and ESA turned to Russia instead. 

The other two spacecraft — a Russian lander and an ESA rover — are scheduled for launch in 2020 (delayed from 2018). Schiaparelli was designed to test entry, descent and landing (EDL) technologies in preparation for the lander/rover mission.

TGO and Schiaparelli separated from each other on October 16, three days before Mars arrival. TGO went on to successfully achieve orbit on October 19 while Schiaparelli aimed for the surface.  Contact was lost during descent.  Using imagery from NASA’s Mars Reconnaissance Orbiter (MRO) spacecraft, the crash site was located just two days later and high resolution images were available a week after that.  The imagery shows the parachute, front and back sections of the heatshield, and scattered debris from the lander itself.

Using data Schiaparelli transmitted to TGO as well as from an Earth-based Indian radio telescope that was tracking it, ESA said today that atmospheric entry and braking occurred as expected.  The parachute deployed as planned at an altitude of 12 km and the heatshield was jettisoned at 7.8 km. 

As Schiaparelli descended under the parachute, something went wrong. The “radar Doppler altimeter functioned correctly and the measurements were included in the guidance, navigation and control system.  However, saturation — maximum measurement — of the Inertial Measurement Unit (IMU) had occurred shortly after the parachute deployment. The IMU measures the rotation rate of the vehicle. Its output was generally as predicted except for this event, which persisted for about one second — longer than would be expected.  When merged into the navigation system, the erroneous information generated an altitude estimate that was negative — that is, below ground level.”

Consequently, the parachute released. the landing thrusters fired briefly and on-ground systems were activated “as if Schiaparelli had already landed.  In reality, the vehicle was still at an altitude of 3.7 km.”   ESA earlier estimated that Schiaparelli was traveling at more than 300 km/hour when it hit the surface and probably exploded since its fuel tanks were still fairly full.

David Parker, ESA’s Director of Human Spaceflight and Robotic Exploration, emphasized that this is a preliminary conclusion.  An external independent review board is currently  being established “as requested by ESA’s Director General, under the chairmanship of ESA’s Inspector General,”‘ Parker continued.  Its report is expected in early 2017.

ESA officials stress that the whole point of launching Schiaparelli was to test EDL technologies and they are pleased that the early phases went as planned even if the ending did not.

Only the United States has successfully landed spacecraft on Mars.  Seven of eight attempts since 1976 have succeeded:  Viking 1, Viking 2, Mars Pathfinder + Sojourner, Spirit, Opportunity, Phoenix and Curiosity.  Only the 1999 Mars Polar Lander (MPL) failed.  One
of the four landers sent to Mars by the Soviet Union in the 1970s sent
back data after landing, but for less than 20 seconds so is not
considered a success.  The United Kingdom sent the Beagle 2
lander to Mars along with ESA’s Mars Express in 2003, but it landed in a
semi-deployed manner and was unable to communicate. 

Interestingly, MPL failed for somewhat similar reasons as Schiaparelli. The MPL failure review board concluded that vibrations in MPL’s landing legs when they were deployed as it approached the surface were incorrectly interpreted by onboard software as an indication that the spacecraft had landed.  In fact, it was still about 40 meters above the surface and could not survive the impact.