Problems in sterilization of unmanned space vehicles.

The probability of achieving and maintaining sterility of an unmanned spacecraft with varied suggested procedures is examined in detail, as are alternative techniques for avoiding biological contamination of the planets. The required degree of assurance against contamination of Mars, Venus and the Moon with Earth organisms is also considered. For Mars landers and orbiters, sterilization of the spacecraft or capsule by dry heat, with no subsequent access, is found to be desirable. For the present, sterilization of Venus landers still seems desirable. For Mars flybys, and for Venus orbiters and flybys, control of the trajectory to minimize the chance of unintentionally entering the planetary atmosphere appears the method of choice. For the Moon, sterilization seems unnecessary, but microbial counts should be kept low. Sterilization lowers spacecraft and system reliability. It reduces the chance of launching within periods fixed by astronomical constraints and increases costs. The gain which should be achieved through spacecraft sterilization, in return of significant biological information about the planets, must be balanced against these losses. In particular, one should keep in mind the loss in return of biological data occasioned by failure of a spacecraft to fulfill its mission; the probability of such failure is increased by sterilization.