"The MCO had been on a trajectory toward Mars since its launch on December 11, 1998. All spacecraft systems had been performing nominally until an abrupt loss of mission shortly after the start of the Mars Orbit Insertion burn on September 23, 1999. Throughout spring and summer of 1999, concerns existed at the working level regarding discrepancies observed between navigation solutions. As MCO approached Mars, three orbit determination schemes were employed. Doppler and range solutions were compared to those computed using only Doppler or range data. The Doppler-only solutions consistently indicated a flight path insertion closer to the planet. These discrepancies were not resolved.
On September 8, 1999, the final planned interplanetary Trajectory Correction Maneuver-4 (TCM-4) was computed. This maneuver was expected to adjust the trajectory such that soon after the Mars orbital insertion (MOI) burn, the first periapse altitude (point of closest approach to the planet) would be at a distance of 226km. This would have also resulted in the second periapse altitude becoming 210km, which was desired for the subsequent MCO aerobraking phase. TCM-4 was executed as planned on September 15, 1999.
Mars orbit insertion was planned on September 23, 1999. During the weeklong timeframe between TCM-4 and MOI, orbit determination processing by the operations navigation team indicated that the first periapse distance had decreased to the range of 150-170km.
During the 24 hours preceding MOI, MCO began to feel the strong effects of Mar's gravitational field and tracking data were collected to measure this and incorporate it into the orbit determination process. Approximately one hour prior to MOI, processing of this more accurate tracking data was completed. Based on these data, the first periapse altitude was calculated to be as low as 110km. The minimum periapse altitude considered survivable by MCO is 80km.
The MOI engine start occurred at 09:00:46 (UTC) on September 23, 1999. All systems performed nominally until Mars's occultation loss of signal at 09:04:52 (UTC), which occurred 49 seconds earlier than predicted. Signal was not reacquired following the 21 minute predicted occultation interval. Exhaustive attempts to reacquire the signal continued through September 25, 1999, but were unsuccessful.
On September 27, 1999, the operations navigation team consulted with the spacecraft engineers to discuss navigation discrepancies regarding velocity change (DV) modeling issues. On September 29, 1999, it was discovered that the small forces DV's reported by the spacecraft engineers for use in orbit determination solutions was[sic] low by a factor of 4.45."
—Taken from: Mars Climate Orbiter Mishap Investigation Board Phase I Report, NASA, November 10,1999.
