Space 4 November 2026 4 MIN READ
Mars Climate Orbiter: Pounds and Newtons
One unit mix-up put a Mars orbiter about 170 km too low. What the NASA board found about the people and process failures behind it.
THE SHORT ANSWER
Mars Climate Orbiter was lost on 23 September 1999 because ground software gave thruster impulse in pound-force seconds instead of newton-seconds, a factor of 4.45. NASA's board blamed the process: unshared decisions, too few people, informal warnings and no end-to-end test.
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A probe that came in too low
NASA’s Mars Climate Orbiter was launched on 11 December 1998 on a Delta II rocket, and its twin lander, Mars Polar Lander, followed on 3 January 1999. On 23 September 1999 the orbiter went behind Mars to fire its engine and enter orbit. Radio contact was lost at 09:04:52 UTC, 49 seconds earlier than predicted, and never came back.
NASA’s Mishap Investigation Board found the reason in a ground-software file. It says the single root cause was that a file used in trajectory models had not been coded in metric units.
Pounds against newtons
The interface specification said that the impulse from the small thruster firings used to shed built-up angular momentum had to be delivered in newton-seconds. The ground software, which reported them in pound-force seconds, did not do that. The navigation software therefore underestimated the effect of those firings by a factor of 4.45, the number of newtons in a pound of force. The spacecraft’s own onboard software used metric units correctly; only the ground-side computation did not.
The effect was cumulative. Those thruster firings occurred 10 to 14 times more often than the navigation team had expected, which magnified the error. The reason was physical: the orbiter had a single solar array, asymmetric relative to its body, so sunlight pushed on it and built up angular momentum faster than on a spacecraft with symmetric arrays.
How far off was it?
The Board’s figures show how the error grew.
- Planned: a first closest approach, or periapsis, of 226 km above Mars after the final planned correction.
- Final week: the operations team’s navigation estimate dropped to 150–170 km.
- About an hour before the burn: as low as 110 km. The minimum survivable altitude was 80 km.
- Reconstructed afterwards: 57 km, with the corrected values.
The Board’s summary is that at Mars insertion the trajectory was approximately 170 km lower than planned, which agrees with 226 minus 57. The Board could not say definitively what happened to the spacecraft. Either it was destroyed in the atmosphere, or it left it and re-entered orbit around the Sun. NASA’s mission page today simply says it burned up, which simplifies the Board’s two possibilities.
It was not just the units
The Board stressed that the unit mix-up was a process failure. Mistakes are normal, it said, but the project’s processes should have caught this one and did not. It found several gaps.
- A decision nobody passed on. A systems-engineering decision to delete a planned daily flip of the spacecraft, intended to cancel momentum build-up, was never communicated to the operations navigation team.
- Newcomers without the context. That team joined shortly before launch and skipped design reviews and ground-software testing, so it lacked knowledge of the spacecraft.
- Too few people. One operations project was running three missions at once. Navigation of the orbiter was handled by only a team lead and one navigator.
- Warnings by email. When the data did not agree, the team used email instead of the formal reporting process for incidents, surprises and anomalies, and the problem slipped through the cracks. Doppler residuals from the thruster firings were noticed, but only informally reported.
- A correction never made. A last emergency trajectory correction that could have raised the altitude was discussed verbally shortly before insertion but never executed.
- No end-to-end test. Testing of the ground software against the interface specification did not appear to have been done.
A second NASA review, published on 13 March 2000, found that the programme had accepted significant cuts in money and people for this mission without adding process discipline, and that there was virtually no oversight of the contractor’s subsystem work.
What it cost
Costs depend on what is counted. NASA and JPL’s fact sheet gives $327.6 million in total for the orbiter and the lander together, excluding the Deep Space 2 probes: $193.1 million for development, $91.7 million for launch and $42.8 million for operations. Science News, citing the panel that reviewed the Mars programme, reported the orbiter alone at $125 million and the lander at $165 million. Some sources attribute the whole $327.6 million to the orbiter, which merges the two.
Why it matters
A unit mix-up sounds trivial, and it is. What the report shows is how many checks had to fail for a trivial mistake to cost a spacecraft.
Our take: the bug was small; the silence around it was not. Several people noticed that the numbers looked wrong. The failure was that there was no process that obliged anyone to act on what they saw.
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Sources
- Mars Climate Orbiter Mishap Investigation Board Phase I Report · NASA (Mishap Investigation Board) · Accessed
- Mars Climate Orbiter Team Finds Likely Cause of Loss · NASA Jet Propulsion Laboratory · Accessed
- Mars Climate Orbiter Failure Board Releases Report, Numerous NASA Actions Underway in Response (Release 99-134) · NASA Headquarters · Accessed
- Report on Project Management in NASA by the Mars Climate Orbiter Mishap Investigation Board · NASA (Mishap Investigation Board) · Accessed
- Mars Climate Orbiter Fact Sheet · NASA Jet Propulsion Laboratory (Wayback Machine capture, 3 Oct 2012) · Accessed
- Reviewers see red over recent Mars programs · Science News · Accessed
- Mars Climate Orbiter - NASA Science · NASA Science · Accessed
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