On December 11, 1998, NASA launched Mars Climate Orbiter.
The probe flies toward Mars, and from the outside everything looks fine.
On September 23, 1999, the time comes to slip into orbit.
Contact is lost.
Where did it go wrong?
One team on Earth works out the push from thruster firings in pound-force seconds. That is the English unit for impulse, the total shove of each burn.
NASA navigation software reads the same figures as newton-seconds. That is the metric unit for the same shove.
One pound of force is about 4.45 newtons.
Every course correction enters the math wrong. Over the whole flight the error stacks up.
The probe swings past about 37 miles (60 kilometers) above the surface of Mars.
Below 53 miles (85 kilometers)?
Simulations show that friction with the thin air there tears the fragile shell apart.
The planned orbit stays out of reach.
The craft burns up in the atmosphere of Mars.
The contract with Lockheed required data in newton-seconds. Nobody noticed the numbers were coming in as pound-force seconds.
The case turns into a textbook example.
Everyone wonders how the review missed the mistake.
You can reach Mars with careful math.
One confused unit is enough to burn.
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