* Astronomy

Communication with the Mars Climate Orbiter lost, 1999

On November 10, 1999, the Mars Climate Orbiter Mishap Investigation Board released a Phase I report, detailing the suspected issues encountered with the loss of the spacecraft. Previously, on September 8, 1999, Trajectory Correction Maneuver-4 was computed and then executed on September 15, 1999. It was intended to place the spacecraft at an optimal position for an orbital insertion manoeuvre that would bring the spacecraft around Mars at an altitude of 226 kilometres on September 23, 1999. However, during the week between TCM-4 and the orbital insertion manoeuvre, the navigation team indicated the altitude may be much lower than intended at 150 to 170 kilometres. Twenty-four hours prior to orbital insertion, calculations placed the orbiter at an altitude of 110 kilometres; 80 kilometres is the minimum altitude that Mars Climate Orbiter was thought to be capable of surviving during this manoeuvre. Final calculations placed the spacecraft in a trajectory that would have taken the orbiter within 57 kilometres of the surface where the spacecraft likely disintegrated because of atmospheric stresses. The primary cause of this discrepancy was engineering error. Specifically, the flight system software on the Mars Climate Orbiter was written to take thrust instructions using the metric unit newtons (N), while the software on the ground that generated those instructions used the Imperial measure pound-force (lbf). This error has since been known as the metric mixup and has been carefully avoided in all missions since by NASA. 
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