On December 11, 1998, NASA launched the first of a pair of Mars explorer probes, the Mars Climate Orbiter (MCO). On January 3, 1999, the Mars Polar Lander (MPL) was successfully launched, as well. The plan was for MCO to enter orbit around Mars in October 1999 and begin transmitting signals back to Earth; MPL would then land in December, examine the surface of Mars for signs of liquid water, and other substances, and relay the information back to Earth via the MCO.

But on September 23, 1999, as MCO fired its main thrusters to enter into orbit around Mars, flight controllers lost contact with the spacecraft. After a frantic investigation, it was discovered that the wrong amount of rocket thrust had been applied (the navigation software had used the wrong mathematical units of force in its calculations). This simple, careless human error had caused MCO to crash onto the Martian surface. Scientists quickly sent the corrected information to the Mars Polar Lander, to be sure it would not suffer the same fate.

On December 3, 1999, as MPL came down for a soft landing, flight controllers lost contact with it less than twelve minutes before it was to touch down near the Martian south pole. Later investigations suggested that the engines had erroneously shut down while MPL was still more than one hundred feet in the air, causing a devastating crash landing.