Space exploration raises the risk of interplanetary contamination, both forward and backward, and questions about regulatory design and implementation of 'planetary protection' policies. In 1967, the Outer Space Treaty called on its parties to 'avoid… harmful contamination and… adverse changes' (Article IX). This issue is urgent again today, with ongoing missions to visit and return from asteroids, the Moon, Mars and beyond. The history of the Apollo missions to the Moon in the 1960s, NASA’s quarantine measures and contemporary activities all offer insights and challenges for risk regulation of potential interplanetary contamination. These include persistent inadequacies in our risk perceptions and planetary protection policies; lack of institutional coordination across regulatory agencies; lack of international cooperation regimes across governments; lack of regulation of private space missions; and disregard for the extraterrestrial other.