Putting Heat on Voyager

During preparations for its successful trip to the moon, Surveyor was spared a severe test that future unmanned spacecraft on missions to Mars and Venus will have to endure: dry-heat sterilization to prevent the contamination of other planets by earthly microorganisms. The terrestrial bugs can do little harm on the lifeless moon, but experts agree that their premature arrival on other planets could obliterate or alter possible native life forms before they could be studied. There is a growing feeling, nonetheless, that the U.S. may have accepted international sterilization standards that are unnecessarily high.

The high criteria require that there be less than one chance in 10,000 that a single living microorganism be aboard an unmanned spacecraft designed to land on a planet. To comply, the U.S. plans to seal its Mars-bound Voyager landing capsule in a canister and bake it for as long as 53 hours at a temperature of 257 DEGF.--enough heat exposure to kill even the organisms within the solid metal structures of the spacecraft. Aware that sterilization of some early Ranger moonships damaged spacecraft systems and led to the failure of missions, scientists are spending time and money to design new Voyager systems that will withstand prolonged heating.

Russian Bugs. Not worth it, says a study group led by Biologist Norman Horowitz of the California Institute of Technology. In a report in Science, the scientists argue that Mars has too little oxygen or water and too much ultraviolet radiation to support the growth of earthly organisms, and that Venus apparently has surface temperatures high enough to kill any earthly bugs. In any event, the report says, there is little chance that organisms entrapped within solid structures in the spacecraft could work their way free. Thus it is important only to kill microorganisms on the exposed surfaces of the spacecraft either by brief heating or by poison gas, neither of which would be harmful to conventional spacecraft systems.

In the same issue of Science, a group headed by Caltech Geologist Bruce Murray contends that the Russians may well have already contaminated both Venus and Mars. In 1965, there was a failure aboard Russia's Venus 3, which was to parachute a sterilized instrument capsule to the surface of Venus. As a result, they believe, both the capsule and the unsterilized spacecraft hit the Venusian surface. A similar mishap that same year may have caused the unsterilized Russian probe called Zond 2 to impact on the surface of Mars.

Problems arising from the design of heat-resistant spacecraft systems have already contributed to the postponement from 1969 to 1971 of a U.S. mission to eject a sterilized Martian landing capsule from a flyby vehicle. They have also forced cutbacks on equipment to be carried aboard the Voyager capsule scheduled to land on Mars in 1973. And they have certainly increased the possibility that heat-weakened Voyager components may fail in flight.

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