Article IX of the Outer Space Treaty obligates states to avoid the harmful contamination of extraterrestrial environments. “Forward contamination” from biological material brought from earth could jeopardize scientific integrity in the search for extraterrestrial life. States also are to prevent “back contamination,” that is adverse changes to the earth’s environment caused by the introduction of extraterrestrial matter.

State practice to prevent forward and back contamination has been to adhere to the Planetary Protection Policy (PPP) developed by the Committee on Space Research (COSPAR). The PPP applies to all space objects that travel beyond earth orbit. Specific requirements are established for the return to earth of samples of extraterrestrial matter. In addition, guidelines have been articulated for crewed missions to Mars. The substance of the PPP is recognized by all launching authorities conducting interplanetary missions, and has been incorporated into the governing regulations of NASA and ESA.

The PPP began as a planetary quarantine policy which required near-sterility for interplanetary spacecraft such as Viking. The PPP is subject to continuous review and modification, and the current policy establishes standards of conduct based on a combination of target body and mission objectives. Five categories are delineated, with increasing levels of planetary protection based on their relative importance in the search for life. Missions to Category I bodies, such as the sun, have no specific protections. Category II bodies, including the moon, require only minimal documentation. Category III is for fly-by and orbiting missions to Mars, and requires that spacecraft comply with either specified decontamination levels or particular orbital parameters.

Category IV applies to missions intended to land on Mars, or Titan or Enceladus. These spacecraft must meet specified requirements, and missions which will search for evidence of life are subject to more stringent decontamination obligations. Moreover, similarly strict decontamination requirements must be met for all spacecraft which will land in “special regions” of Mars, where the temperature and presence of H2O may be hospitable to sustaining life.

Significant gaps exist in the policies to protect interplanetary environments, including that states have not made the COSPAR PPP applicable to private missions in their regulatory regimes. Nevertheless, the PPP has been crucial to prevent forward contamination and protect scientific integrity in the search for evidence of extraterrestrial life.