Mars Polar Lander - Loss of Communications

Loss of Communications

On 3 December 1999 at 14:39:00 UTC, the last telemetry from Mars Polar Lander was sent, just prior to cruise stage separation and the subsequent atmospheric entry. No further signals were received from the spacecraft. Attempts were made by Mars Global Surveyor, to photograph the area the lander was believed to be. An object was visible and believed to possibly be the lander; however, subsequent imaging performed by Mars Reconnaissance Orbiter resulted in the identified object being ruled out. Mars Polar Lander remains lost.

The cause of the communication loss is not known. However, the Failure Review Board concluded that the most likely cause of the mishap was a software error that incorrectly identified vibrations, caused by the deployment of the stowed legs, as surface touchdown. The resulting action by the spacecraft was the shutdown of the descent engines, while still likely 40 meters above the surface. Although it was known that leg deployment could create the false indication, the software's design instructions did not account for that eventuality.

In addition to the premature shutdown of the descent engines, the Failure Review Board also assessed other potential modes of failure. Lacking substantial evidence for the mode of failure, the following possibilies could not be excluded:

Surface conditions exceed landing design capabilities.
Loss of control due to dynamic effects.
Landing site not survivable.
Backshell/parachute contacts lander.
Loss of control due to center-of-mass offset.
Heatshield fails due to micrometeoroid impact.

The failure of the Mars Polar Lander took place two and a half months after the loss of the Mars Climate Orbiter. Inadequate funding and poor management have been cited as underlying causes of the failures. According to Thomas Young, chairman of the Mars Program Independent Assessment Team, the program "was under funded by at least 30%."

Quoted from the report
"A magnetic sensor is provided in each of the three landing legs to sense touchdown when the lander contacts the surface, initiating the shutdown of the descent engines. Data from MPL engineering development unit deployment tests, MPL flight unit deployment tests, and Mars 2001 deployment tests showed that a spurious touchdown indication occurs in the Hall Effect touchdown sensor during landing leg deployment (while the lander is connected to the parachute). The software logic accepts this transient signal as a valid touchdown event if it persists for two consecutive readings of the sensor. The tests showed that most of the transient signals at leg deployment are indeed long enough to be accepted as valid events, therefore, it is almost a certainty that at least one of the three would have generated a spurious touchdown indication that the software accepted as valid. The software—intended to ignore touchdown indications prior to the enabling of the touchdown sensing logic—was not properly implemented, and the spurious touchdown indication was retained. The touchdown sensing logic is enabled at 40 meters altitude, and the software would have issued a descent engine thrust termination at this time in response to a (spurious) touchdown indication. At 40 meters altitude, the lander has a velocity of approximately 13 meters per second, which, in the absence of thrust, is accelerated by Mars gravity to a surface impact velocity of approximately 22 meters per second (the nominal touchdown velocity is 2.4 meters per second). At this impact velocity, the lander could not have survived."

Quoted from the report

"A magnetic sensor is provided in each of the three landing legs to sense touchdown when the lander contacts the surface, initiating the shutdown of the descent engines. Data from MPL engineering development unit deployment tests, MPL flight unit deployment tests, and Mars 2001 deployment tests showed that a spurious touchdown indication occurs in the Hall Effect touchdown sensor during landing leg deployment (while the lander is connected to the parachute). The software logic accepts this transient signal as a valid touchdown event if it persists for two consecutive readings of the sensor. The tests showed that most of the transient signals at leg deployment are indeed long enough to be accepted as valid events, therefore, it is almost a certainty that at least one of the three would have generated a spurious touchdown indication that the software accepted as valid.

The software—intended to ignore touchdown indications prior to the enabling of the touchdown sensing logic—was not properly implemented, and the spurious touchdown indication was retained. The touchdown sensing logic is enabled at 40 meters altitude, and the software would have issued a descent engine thrust termination at this time in response to a (spurious) touchdown indication.

At 40 meters altitude, the lander has a velocity of approximately 13 meters per second, which, in the absence of thrust, is accelerated by Mars gravity to a surface impact velocity of approximately 22 meters per second (the nominal touchdown velocity is 2.4 meters per second). At this impact velocity, the lander could not have survived."

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