Keeping Earth’s germs from journeying to the moon proved to be a tall order, however. At least one bacterial species, Streptococcus mitis, found its way inside the Surveyor 3 camera that had spent some 2.5 years on the moon before the astronauts of Apollo 12 retrieved and returned it to Earth. Experts now believe Surveyor 3’s S. mitis came from post-return contamination by human investigators, rather than surviving lunar conditions. Even so, subsequent research has conclusively shown certain terrestrial organisms—Deinococcus radiodurans and Bacillus subtilis bacteria as well as tiny invertebrates called tardigrades—can indeed survive extended exposure to the harsh conditions of space. Both then and now forward contamination—the transfer of Earthly life-forms to other worlds—is the most vexing challenge of planetary protection.

Forward contamination is a familiar concern for mission planners seeking to preserve the environments of Mars and ocean-bearing icy moons of the outer solar system (such as Saturn’s Enceladus and Jupiter’s Europa) so astrobiologists can identify native life there—should it exist. But how should planetary protection’s prohibitions and restrictions apply to the moon, and what lessons from the Apollo era might be applicable in the coming years as we aim to go back?