The 8 Most Expensive Unit Conversion Mistakes in Engineering History — and the Software Bugs That Caused Them

TL;DR Eight engineering disasters. Zero arithmetic errors. Every single one was caused by two numbers — both correct, both carefully computed — meaning different things on opposite sides of a software interface. One cost $65 billion. Another killed 28 soldiers because 0.1 can't be represented in binary. The fix is never the math. The fix is the label. There is a particular kind of silence in a control room when someone realizes the number on the screen is in the wrong unit. It lasts about two seconds. Then it's replaced by the kind of noise nobody wants to hear. On September 23, 1999, that silence happened at the Jet Propulsion Laboratory in Pasadena, California. The Mars Climate Orbiter had just disappeared behind the planet. Telemetry showed the spacecraft at 57 kilometers above the surface. It was supposed to be at 140. The silence was four seconds long. Then someone said "oh no" — the official NASA transcript uses a stronger word — and $327 million of aluminum, titanium, and human effort disintegrated into the Martian atmosphere. What follows are eight stories about the same bug, wearing different uniforms. Some are famous. Some you've never heard of. Two of them are pure software failures that every developer who's ever written for (let i = 0; i < 10; i += 0.1) has come within a rounding error of replicating. 1. The Patriot Missile — When 0.1 Is Not 0.1 (1991) Let's start with the one that belongs in every CS curriculum. Because this isn't a "unit conversion" error in the traditional sense — nobody confused meters and feet. The error was in the way a computer counted time. And it killed 28 American soldiers in a warehouse in Dhahran, Saudi Arabia. The MIM-104 Patriot missile system tracks incoming targets using a phased-array radar. The radar scans the sky, and the fire-control computer predicts where the target will be when the interceptor arrives. That prediction depends on knowing exactly when the radar echo returned. Time is measured by the system's interna