As Mars Polar Lander started the landing sequence it hit the atmosphere at 6.8 km/s and had 4 minutes and 33 seconds to land. The heat shield slowed the craft. At an altitude of about 7.3 km and a speed of 500 m/s, the parachute deployed. The heat shield separated, and the parachute was jettisoned. The lander was 100 seconds away from safely touching down. The back shell separated at an altitude of 1.4 km and a speed of 80 m/s. Landing rockets fired, the final decent began. Landing legs deployed. That’s when trouble began.
Our investigation concluded the most likely cause of failure was a software error that identified vibrations caused by the deployment of the lander’s legs as touch-down. The resulting action was to shut-down the vehicle’s descent engines while still 40 meters up. It was known leg deployment could create the false indication, but the software did not account for that eventuality.
Moral of the story, don’t drop your lander on the ground from a height larger than a 13 story building.