Remember when NASA lost a spacecraft because it's simultaneously used Imperial and metric measurements on the same mission? The Mars Climate Orbiter disappeared 15 years ago this month and here's a very brief recap of exactly what went wrong.

The Mars Climate Orbiter launched on December 11, 1998 on a mission to orbit Mars. This first interplanetary weather satellite was designed to gather data on Mars' climate and also serve as a relay station for the Mars Polar Lander, a mission that launched a few weeks later. But you can't just launched a spacecraft towards Mars and trust that it's going to get where it's going. You to have to monitor its progress.

Many spacecraft have reaction wheels to keep them oriented properly and navigation teams behind interplanetary spacecraft that constantly monitor the angular momentum and adjust trajectory to make sure it gets exactly where it needs to go. In the case of the Mars Climate Orbiter, monitoring its trajectory and angular momentum involved a few steps.

First, data from the spacecraft was transferred to the ground by telemetry. There it was processed by a software program and stored in an angular momentum desaturation file that process data was what scientists used to adjust the trajectory. Adjustments that were made by firing the spacecraft's thrusters. Every time the thrusters were fired, the resulting change in velocity was measured twice once by software program on the spacecraft and once by software program off the ground. And here's where the problem comes in. It turned out that the two systems the processing software on the spacecraft and the software on the ground we're using two different units of measurements. The software on the spacecraft measured impulse, or the changes by thrusters in newton seconds a commonly accepted metric unit of measurement, while the processing software on the ground use the Imperial pound seconds. And it was unfortunately the ground computer's data that scientists used to update the spacecraft trajectory and because one pound of force is equal to 4.45 Newton's every adjustment was off by a factor of 4.45.

For a spacecraft traveling tens of millions of miles to destination a number of seemingly small errors really add up. During the Mars Climate Orbiters nine-month cruise to Mars seven errors were introduced into its trajectory that meant that when it reached the red planet it was 105 miles closer to the Martian surface than expected. This turned out to be an unsurvivably low altitude for its Mars encounter when the spacecraft fire its main engine for the orbit insertion burn that was designed to put it into an elliptical orbit nothing happened. NASA lost contact quite abruptly with the spacecraft.

So while we know the root cause of just what went wrong we'll never know exactly what happened to the Mars Climate Orbiter. The loss of the Mars Climate Orbiter very sadly happened in space.

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