On September 23, 1999, Mars Climate Orbiter approached the red planet at roughly 57 kilometers altitude. NASA had targeted between 140 and 150 kilometers, and the probe could not withstand being any lower than about 80 kilometers.
The root cause lay in a unit conversion that was never performed.
Lockheed Martin’s ground software expressed thruster firings’ impulse in pound‑force seconds. Navigation from the Jet Propulsion Laboratory (JPL), by contrast, read those same values as newton seconds. One pound‑force is about 4.45 newtons, and the discrepancy accumulated over the nine months of the journey.
- Lockheed Martin’s software expressed impulses in pound‑force seconds.
- JPL’s navigation read them in newton seconds, equating to a 4.45-to-1 discrepancy.
- Desaturations of the reaction wheels occurred 10 to 14 times more often than anticipated.
- The probe descended to roughly 57 km, about 23 km below the survival threshold.
Exact figures read in the wrong unit
Those small thruster firings are routine maintenance. The probe’s solar panel, asymmetric, bears the pressure of sunlight, causing its reaction wheels to spin up to saturation. Short bursts of thrusters are then used to desaturate them. According to a recent analysis of the report, these desaturations occurred 10 to 14 times more often than the navigators had anticipated, and each was logged in a file produced by the SM_FORCES application.
The specification required newton seconds. The code delivered pound‑force seconds, so every thrust modeled by the navigators weighed less than a quarter of its true value.
Neither calculation was wrong in itself. Lockheed produced values accurate in its own unit, and the JPL used correct formulas for its own unit. Even the onboard calculation, expressed in metric units, was correct: only the copy kept on Earth went off course, leaving the probe better informed about its path than those who steered it.
Nine months of drift that no one read
Launched on December 11, 1998 from Cape Canaveral aboard a Delta II rocket, the probe was to study the Martian climate. During the cruise, each misinterpreted ignition nudged the actual trajectory a little further away from the one calculated by the computers. Anomalies were found in the ground software files, then discussed informally, without ever being settled.
No one connected them to the trajectory.
On September 15, eight days before arrival, a final maneuver was supposed to place the probe at 226 kilometers above the surface for orbital insertion. Documents also cite a target of 140 to 150 kilometers depending on mission phase, but in every case well above the 80‑kilometer survival minimum. The investigative committee later concluded that an additional bailout maneuver might have saved the probe. The team had not understood its vital importance, nor had they analyzed or tested it.
On September 23, 1999, a signal lost 49 seconds early
The main engine fired at 09:00:46 UTC to slow the spacecraft. Teams expected to lose contact when it disappeared behind Mars, but the signal vanished at 09:04:52, 49 seconds earlier than planned. It was never recovered, even after the 21 minutes of occultation had elapsed.
Recalculations using the correct values place the lowest point of the trajectory at around 57 kilometers, about 23 kilometers below the survival threshold. The orbiter broke up in the Martian atmosphere.
Over a voyage spanning more than 600 million kilometers, the deviation of roughly 90 kilometers from the lower target represents less than one in a million of the journey. Mars Climate Orbiter did not miss its target by luck, but by a minuscule error, repeated with every ignition, which had nine months to grow. By comparison, 57 kilometers is about six and a half times the height of Mount Everest.
Mission cost: about $327.6 million.
The figures cited vary by scope—from around $125 million for the probe alone to $327.6 million including launch and operations. The loss also complicated NASA’s subsequent Mars program, since the orbiter was to serve as a communications relay for the Mars Polar Lander lander.
What the Committee Criticized
Published on November 10, 1999, the Mars Climate Orbiter Mishap Investigation Board report identifies the direct cause: the failure to adhere to the metric system in encoding SM_FORCES. It does not stop there. It points to insufficient testing, a breakdown in communication between the JPL and Lockheed teams, and incomplete training of operators. All of these gaps should have caught the error.
At Lockheed Martin, propulsion engineers had grown accustomed to expressing force in pounds, while space mission practice demanded metric units, a conversion that the JPL teams assumed had been performed. The committee extended its work with a second report, focusing on NASA project management, dated March 13, 2000.
Sources: spacedaily.com | simscale.com
