April 5, 2001: NASA launches a spacecraft designed to last a few years. It's now 2026, and it's still the hardest-working machine in the solar system.

While everyone was obsessed with Nokia 3310s and *NSYNC's PopOdyssey tour, NASA was sweating bullets on the launchpad at Cape Canaveral. The Mars Odyssey was about to launch—and it had to work. Two previous Mars missions had just failed back-to-back. This was the rebound mission that couldn't afford to fail.

Named after 2001: A Space Odyssey, the spacecraft's mission was simple: Follow the Water. Using a gamma ray spectrometer (basically a super-powered "nose"), it would search for hydrogen buried in Mars's dirt—the telltale sign of water ice. If humans were ever going to Mars, we needed to know if it was actually habitable.

Here's the in ...