At one point, the success rate fell below 50 percent, and as early as the 1970s, engineers at NASA’s Jet Propulsion Laboratory joked about the “great galactic ghoul” — a fictional monster that protected Mars against prying eyes by capturing and destroying space probes.

The Mars Climate Orbiter burned up in the Martian atmosphere in September 1999 because units were not converted from English to the metric system. Its computer was programmed to receive the spacecraft’s force data in the metric system (Newton-seconds) — but course correction instruction files from Lockheed Martin Astronautics were written in English (pounds-force/second). Consequently, the Mars Climate Orbiter underestimated the influence of the forces on the spacecraft as it entered Mars’ atmosphere and perished.

The Mars Polar Lander suffered a similar fate. It arrived on the planet in December 1999, only three months after the Climate Orbiter. Everything went well until contact with the probe was lost just as it was about to land. According to an independent report, several potential failures may have occurred; the Polar Lander may have thought it had landed even though it was still more than 130 feet above the ground. At that time, the probe deployed the three landing legs, triggering some vibrations, which the computer mistook for the landing bump. Consequently, the computer automatically shut off the engines, leaving the Polar Lander in free fall toward the surface.