Small Forces
The Mars Climate Orbiter (MCO) spacecraft was intended to measure atmospheric properties and serve as a relay station for data from the subsequent Mars Polar Lander mission. The MCO Mishap Investigation Board (MIB, no relation to the movie of the same acronym) reported that, after traveling for nine months between planets, the MCO mission was lost when it entered the Martian atmosphere on a lower than expected trajectory.
MCO's calculated position began diverging from expectations four months after the December 1998 launch, but [t]hese discrepancies were not resolved. The final Trajectory Correction Maneuver (TCM-4) should have put the periapse (closest approach) at 226 km, but tracking data obtained just before the Mars Orbital Insertion (MOI) engine firing showed a 110-km periapse. The minimum periapse altitude considered survivable by MCO is 80 km.
The MOI burn started and, as planned, the spacecraft disappeared behind Mars. Signal was not reacquired following the 21-minute predicted occultation interval.
The Mars Surveyor Operations Project (MSOP), which was already handling data from the Mars Global Surveyor (MGS) spacecraft orbiting Mars, had assumed control of MGO's operations after launch. Their Software Interface Specification (SIS) defines both the format and units of data produced from spacecraft measurements that will be used by existing programs.
Although we think of spacecraft as following ballistic trajectories, forces ranging from solar pressure to thruster firings perturb the Newtonian ideal. A program called SM_FORCES (Small Forces) kept track of thruster firings and produced a file showing their effects.
The output from the SM_FORCES application code as required by a MSOP [SIS] was to be in metric units of Newton-seconds (N-s). Instead, the data was reported in English units of pound-seconds (lbf-s).
The spacecraft also computed the result of the forces. The...software installed on the spacecraft used metric units for the computation and was correct. As for the ground software, [s]ubsequent processing...by the navigation software underestimated the effect of the thruster firings on the spacecraft trajectory by a factor of 4.45 (1 pound force=4.45 Newtons). It seems the spacecraft's calculations were ignored.
During the first four months after launch the team wasn't using either program because of a file format mismatch involving other spacecraft parameters in the ground software. Instead, they'd get an e-mail with the thruster data values, hand-calculate the results, and update the trajectory. All was well until they switched over to the ground software. ...(within a week) it became apparent that the files contained anomalous data that was indicating underestimation of the trajectory perturbations...
Measurements made from ground tracking stations and from the spacecraft itself could not unambiguously identify the problem. The spacecraft had an asymmetric solar panel that required more frequent thruster firings that caused an unexpectedly large accumulation of "small errors" and further confused the situation.
The final trajectory computed just before MOI was too late to save the mission. A fifth, contingency TCM could have raised the periapse, but other events took priority and TCM-5 never happened.
...after the fact navigation estimates ...indicated an initial periapsis...of 57 km which was judged too low for spacecraft survival.
