khoji parkSpace · Cape Canaveral Air Station, Florida · 1999
The Mars Mission Failed Where Two Sets of Units Met
A measurement discrepancy between metric and US customary units caused the loss of the Mars probe.
Read in slides1. Launch of Probe
The Mars Climate Orbiter was launched by NASA from Cape Canaveral on December 11, 1998, using a Delta II launch vehicle. Carrying scientific instruments to study the Martian atmosphere and surface, the probe began a nine-month interplanetary journey. The spacecraft design incorporated dual redundancy on major systems and used an IBM RAD6000 processor for computing tasks. Mission planners intended for the orbiter to act as a communications relay for the subsequent Mars Polar Lander.
The spacecraft proceeded on a Hohmann transfer orbit toward Mars.
2. Trajectory Correction
On September 15, 1999, the navigation team executed Trajectory Correction Maneuver-4 to position the spacecraft for an optimal orbital insertion. The planned maneuver aimed to bring the probe around Mars at an altitude of 226 kilometers on September 23. During the week preceding the insertion, however, calculations indicated that the insertion altitude would be significantly lower than planned. Navigators noted the discrepancy but did not formally document their concerns according to procedural rules.
Preparations continued for the impending orbital insertion maneuver.
3. Orbital Insertion
Mars Climate Orbiter commenced the planned orbital insertion maneuver on September 23, 1999, at 09:00:46 UTC. The spacecraft went out of radio contact when it passed behind Mars four minutes later, which occurred 49 seconds earlier than expected. Communication with the probe was never reestablished following the occultation. Post-failure calculations showed that the actual trajectory would have taken the spacecraft within 57 kilometers of the Martian surface.
Radio contact with the space probe was permanently lost.
4. Destruction in Atmosphere
At an altitude of 57 kilometers, the spacecraft encountered atmospheric drag that was much denser than anticipated, causing the orbiter to be destroyed in the atmosphere or re-entered heliocentric space after leaving Mars's atmosphere. The Mars Climate Orbiter Mishap Investigation Board released a Phase I report on November 10, 1999, detailing the suspected issues. Investigators concluded that the spacecraft encountered Mars at a lower-than-anticipated altitude due to human error.
The mishap board began analyzing telemetry and navigation records.
5. Unit Mismatch Discovery
The investigation attributed the failure to a measurement mismatch between two software systems. Ground software supplied by Lockheed Martin produced results in United States customary units of pound-force seconds. A second system supplied by NASA expected those results to be in SI units of newton-seconds. The trajectory calculation software used these results to update the predicted position of the spacecraft with an error factor of 4.45.
The discrepancy caused significant errors in the calculated spacecraft position.
6. Missed Warnings
Prior to the encounter, at least two navigators noticed the discrepancy between calculated and measured positions. Their concerns were dismissed because they did not follow formal administrative rules for documenting such issues. A meeting was convened to consider executing Trajectory Correction Maneuver-5, but the maneuver was ultimately not performed. Officials acknowledged that appropriate checks and tests by NASA could have caught the unit discrepancy.
Project management faced intense scrutiny over communication breakdowns.
7. Program Assessment
The loss of the Mars Climate Orbiter occurred two and a half months before the loss of the Mars Polar Lander. An independent assessment team reported that the Mars Surveyor 98 program suffered from inadequate funding and poor management. The review indicated that the program was underfunded by at least 30 percent. Total project costs for the orbiter and lander combined reached $327.6 million.
Review panels issued recommendations to improve software interface verification.
8. Legacy of the Probe
NASA’s investigation traced the navigation error to an interface where impulse values were supplied in pound-force seconds while the receiving system expected newton-seconds. The board also examined testing, staffing and communication failures that allowed the mismatch to remain undetected. The spacecraft was lost at Mars. The investigation’s findings described how that mission failed, without establishing that every later spacecraft interface had become immune to unit errors.
The unit mismatch survived the checks until the Mars encounter.