What do NASA’s soon-to-be-launched Mars Exploration Rover (MER-1 and MER-2) spacecraft have in common with the Viking and Voyager spacecraft launched decades ago? Besides being interplanetary explorers, they will be among the most biologically clean spacecraft ever launched from Cape Canaveral. Making sure the spacecraft are as biologically clean and contamination-free as possible before they leave Earth is NASA’s planetary protection (PP) policy. It protects other solar system bodies from Earth life and protects Earth from extraterrestrial life that may be brought back by returning space missions.