In chapter 4, Grace asks Stratt why he is the only scientist currently investigating the astrophage. He quickly realizes that he is a guinea pig; Stratt wishes to avoid harm to other scientists working with this previously unknown and potentially dangerous life-form.

Our own strategies for mitigating the risk of interplanetary contamination have evolved throughout humanity’s history of space exploration, eventually coming to be known as planetary protection (not to be confused with planetary defense, which involves protecting the Earth from impacts).

Scientists since the 1950s have raised concerns about the possibility of contaminating the Moon and other celestial bodies with Earth microorganisms. The Committee on Space Research (COSPAR), founded in 1958, included biology among its priorities— specifically the need for sterilization and quarantine of spacecraft.

The risk of contaminating other worlds with Earth microorganisms is known as forward contamination and has remained a primary focus in modern space science, especially with the robotic exploration of Mars and missions like Europa Clipper and Juice that plan to explore our solar system’s potentially habitable icy moons.

Back contamination, on the other hand, is Stratt’s concern with the astrophage. Collecting samples from space and returning them to Earth requires special care and caution to avoid contamination of the terrestrial biosphere and its inhabitants with unknown materials or even yet-to-be-discovered organisms.

For some interesting real world examples, we can contrast the unrestricted return of OSIRIS-REx’s sample from asteroid Bennu with the planned future Mars Sample Return, which is classified as a restricted return due to the risk of potential contamination of Earth with Martian matter. The terrestrial investigation of Martian samples will involve high-containment biosafety procedures and equipment.