* Astronomy

Spoiler

On December 3, 1999, Mars Polar Lander encountered Mars while mission operators began preparing for landing operations.
However, after the descent phase was expected to be complete, the lander failed to reestablish communication with Earth.
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Undiscovered code bug is likely cause of Mars Polar Lander crash: The conclusions reached by an investigation team from post-crash testing and analysis; since there was no telemetry from the MPL during atmospheric entry, descent, or landing, is that there can be no way to know for sure exactly what happened.

While a number of failure scenarios were examined, a specific candidate failure scenario emerged - one borne out by repeated ground-based, post-failure testing. Four tests in early 2000 by Lockheed Martin in Denver showed that sensors in the MPL's landing gear sent out spurious signals. Software that should have been designed to ignore these signals was not designed to do so after all. These signals were interpreted by the spacecraft as being indicative of the forces landing legs would experience upon landing and the descent engines were shut off. This design flaw was not caught before launch since a series of tests used sensors that were wired incorrectly. Ergo, the problem was not detected, and Mars Polar Lander was sent on a doomed flight to Mars. Again, while there is no way to be certain that this flaw was the actual cause of the MPL failure, but the investigation team was more or less certain that the engines were shut off at an altitude of 40 metres and that the MPL crashed into the Martian surface at 22 metres per second.