In order to retrieve samples from another place in the solar system that might harbor life, careful planning is required to ensure that mission designs incorporate measures to safeguard both the Earth and other solar system bodies from cross contamination. These measures, collectively known as planetary protection (PP) measures, are actually tied to international law. The Outer Space Treaty of 1967 specifically requires that all space exploration must be done in a way that avoids harmful contamination to celestial bodies or adverse changes in the environment of the Earth from the introduction of extraterrestrial materials.
Over the years, planetary protection policies for solar system
exploration missions have been developed by the members of the
international Committee on Space Research (COSPAR). PP policies
specifically address the prevention of two types of
cross-contamination: 1) forward contamination, the transport of
terrestrial microbes on outbound spacecraft; and 2) back contamination,
the introduction to Earth of organisms that could be present in
materials or samples returned from extraterrestrial locations.
The task of planning effective PP measures requires blending information about what we know about biological systems and extraterrestrial environments, with educated speculation about what conditions might exist in the extreme habitats where samples will be collected. Because of our ignorance about life beyond the Earth, PP measures are deliberately designed to deal with organisms at least as capable of surviving extreme conditions as the toughest organisms found on this planet. Admittedly, decisions about handling returned samples in the near term will be made in the face of scientific uncertainty and a lack of definitive information, but that doesn't make it the stuff of science fiction.
Because of public interest and concern about extraterrestrial life,
there will undoubtedly be plenty of questions about the safety of
returning samples that may contain alien life to Earth. Whether the
concerns are caused by overactive imaginations, repeated exposure to
Hollywood-style aliens, distrust of government bureaucrats, or basic
scientific questions, it will be important to address them responsibly.