The Mars Climate Orbiter was one-half of the Mars Surveyor ‘98 program, along with the Mars Polar Lander. The orbiter’s mission was to (duh) reach Mars orbit to study the weather and climate and to, ultimately, serve as a communications relay for the lander. The orbiter, launched on December 11, 1998, never made it into orbit, though, due to a software bug in a ground based system. When attempting enter Mars orbit on September 23, 1999, the orbiter approached at a lower than expected altitude, causing it to disintegrate. The cause was ultimately determined to be that the ground based software generated and sent thrust instructions using English measurements (pound-force), while the onboard software was expecting the measurements in metric (Newtons). Ooops.