Since the early days of the space program, there has been concern about planetary protection: the prevention of human-caused biological cross-contamination between Earth and other bodies in the solar system. After the launch of Sputnik in 1957, scientists cautioned about the possibility of contaminating other places in the solar system with microbes from Earth: "Hitchhiker" bacteria and other organisms on spacecraft and equipment might cause irreversible changes in the environments of other planets. Moreover, these changes could also interfere with scientific exploration. In addition, it was felt that spacecraft or extraterrestrial samples returned from space might harm Earth's inhabitants and ecosystems.

The Outer Space Treaty of 1967 requires that exploration of outer space and other celestial bodies "avoid their harmful contamination" and "adverse changes in the environment of Earth" caused by the "introduction of extraterrestrial matter." In practical terms, the concerns are twofold: avoiding (1) forward contamination, the transport...