Disaster 10 · Engineering disasters
The spacecraft lost to a mix-up of units
A Mars probe flew too close to the planet because one team's software reported thruster data in pounds while another team's software expected metric units.
- NASA Jet Propulsion Laboratory and Lockheed Martin
- Mars
- 1999
- 3 min read
On this page
Before you readWhat would you do?
A colleague sends you a spreadsheet of product weights for shipping quotes. Some figures look a little high compared with last month, but not by much. What should you do?
Pick an answer now. The file tells you whether you were right.
01The file in one minute
1Pounds and newtons
Contractor software reported thruster data in pound-force seconds. The navigators' software expected newton-seconds.
2Small errors added up
Each firing nudged the predicted path off a little more during the nine-month trip.
3Lost at arrival
On 23 September 1999 the orbiter came in far too low and was lost.
4The lesson
Label units, check data end to end where teams meet, and chase small surprises until they are explained.
02Timeline
11 December 1998
Mars Climate Orbiter launches from Cape Canaveral.
1999
Navigators notice the path differs from predictions. Concerns stay informal.
23 September 1999
The orbiter arrives far too low and is lost.
November 1999
The investigation board names the units mix-up and weak checks as causes.
03What happened
NASA's Mars Climate Orbiter launched in December 1998 to study the Martian weather. During the nine-month cruise, small thruster firings nudged the spacecraft. Software built by the contractor Lockheed Martin calculated the effect of those firings and sent the numbers to the navigation team at NASA's Jet Propulsion Laboratory.
The contract said the numbers should be in metric units, newton-seconds. Lockheed Martin's software produced them in pound-force seconds, which are about 4.45 times larger. The navigators' software treated them as metric, so each small error pushed the predicted path off course a little more.
On 23 September 1999 the orbiter fired its engine to go into orbit around Mars. It was far lower than planned, and it was lost, probably destroyed in the Martian atmosphere. The mission had cost about $125 million for the spacecraft alone.
The investigation board found that navigators had noticed the path did not match their predictions during the cruise. Their concerns were raised informally but were not pushed through the formal process, and a planned correction burn that might have helped was not carried out.
04Why it happened
The unit mix-up was the direct cause, but the board focused on why nobody caught it. Data passing between two teams was not checked end to end. Small surprises in the navigation results were discussed, not resolved. The project was also short on staff and running under a cost-cutting approach known at NASA as faster, better, cheaper.
A simple error survives when everyone assumes someone else has checked. The navigators assumed the numbers coming in were right, and the people making the numbers assumed they matched the agreed format.
The idea behind this caseLesson: Spreadsheets and data →
05Lessons for decisions
- When data passes between people or systems, write down the units and format and check a sample end to end.
- When a result does not match what you expected, find out why before moving on, even if the gap is small.
- Give everyone a formal way to raise a worry, and make sure each one gets a written answer.
- Label every number with its unit in spreadsheets, forms and messages, such as kg, km or VND.
07 — Sources
- Mars Climate Orbiter · NASA Science · 2026
- Mars Climate Orbiter · Wikipedia · 2026