Two spacecraft lost at Mars in ten weeks
Climate Orbiter flew to 57 km altitude instead of 226 because two teams used different units. Ten weeks later Polar Lander probably shut its engines off 40 metres up.
Mars Climate Orbiter lit its main engine at 09:00:46 UTC on 23 September 1999 and went behind the planet 49 seconds earlier than predicted. It never came out the other side.
The cause was identified six days later, and it is now the most famous unit error in engineering. Lockheed Martin's ground software wrote thruster impulse values into the angular momentum desaturation file in pound-force-seconds. The JPL navigation software's interface specification required newton-seconds. Nobody converted, so every one of hundreds of small thruster firings across nine and a half months was modelled as 4.45 times weaker than it really was. Navigators had noticed that their Doppler-only solutions kept putting the spacecraft closer to Mars than the combined solutions did, and said so; no process existed that forced the discrepancy to be resolved. The final trajectory correction aimed at a first periapsis of 226 kilometres. An hour before the burn, better tracking put it at 110. The board's reconstruction afterwards put it at 57. The minimum the orbiter could survive was 80.
Ten weeks later, on 3 December 1999, Mars Polar Lander entered the atmosphere above 76 degrees south and was never heard from. Here there is no evidence at all, because to save money the mission carried no telemetry during entry, descent and landing. The JPL review board under John Casani concluded, from ground tests rather than flight data, that the most probable cause lay in the Hall-effect touchdown sensors in the three landing legs. They emit a spurious pulse when the legs snap into place at about 1,500 metres. The flight software latched that pulse, and when it started reading the sensors at 40 metres it saw "landed" and commanded the descent engines off. The lander was already coming down at about 13 metres per second; with the thrust gone it would have hit the ground at about 22, against a design touchdown speed of 2.4. The board was careful to call this probable, not proven; it never will be.
Two small penetrators rode down with it, Deep Space 2's Scott and Amundsen, and were never heard from either. Nobody has ever established why.
The programme did not survive the pair. The Mars Surveyor 2001 lander was cancelled and put into storage. Thomas Young's independent assessment in March 2000 found NASA's Mars programme underfunded by roughly 30 percent against what it had been asked to deliver, and the phrase "faster, better, cheaper" quietly stopped being said out loud.