U.S. planetary protection program: Implementation highlights

The implementation of planetary protection in the United States space program has reflected the trend in policy from an absolute to a probabilistic prohibition of the contamination of the celestial bodies of the solar system. The early emphasis on spacecraft sterilization (e.g. Ranger) was replaced by the imposition of contamination control procedures on later missions such as Pioneer, Viking, and Voyager. Similarly, analytical and laboratory techniques were developed to demonstrate compliance with probabilistic requirements. Microbial burden reduction methods that are not hazardous for spacecraft reliability supplanted the abstract concept of sterilization. The United States implementation of planetary protection has been completely successful. In an exploration program that has included Mercury, Venus, Mars, the Jovian system, and the Saturnian system, there have been no accidental impacts or detection of false positives (terrestrial microbes). Further, the contamination control and microbial burden procedures have proved beneficial to spacecraft systems and on-board science instruments. We review in this paper the implementation of planetary protection procedures by the Pioneer (10 and 11), Viking and Voyager projects.