The accompanying article of Horo witz et al. concluded with the view that the COSPAR recommendations re garding Mars should be adjusted to re flect new environmental information. Specifically, it was concluded that viable terrestrial microorganisms which are transported to Mars inside solid components in sealed spaces have a low probability of being released to the sur face or atmosphere, and that, if any are released, they are not likely to in fect the planet. We suggest, in addition, that both the COSPAR recommenda tions and U.S. planetary quarantine policy should be altered to take into account past and continuing Soviet prac tice regarding the. exploration of Mars and Venus. No amount of analysis by COSPAR, or of costly, self-imposed restrictions by the U.S. on its own planetary exploration program, can reduce the probability of contamination of either Venus or Mars below what the Soviets have already made it, or will make it as they continue their large planetary effort. All that U.S. policy can accomplish is to insure that U.S. efforts do not significantly increase the probability above that level. Any rec ommended policy which would require the U.S. to apply significantly more stringent restrictions is illogical in that, in effect, the U.S. would be asked to increase greatly the cost and complexity of its planetary program without achieving any significant reduction in the probability of actual contamination. There exists some parallelism be tween the problem of planetary quaran tine and that of radioactive fallout from atmospheric nuclear testing, al though the desirable solution to the quarantine problem is not merely to stop all activity. Both are multilateral problems, and individual national policy necessarily must reflect the policy of other nations. Thus, the real questions that must be faced by COSPAR, and by the U.S., are, (i) What is the prob able number of viable terrestrial micro organisms alreadyr transported to Venus and to Mars? and (ii) What is the to tal number to be expected in the next decade or so from foreseeable Soviet efforts alone? Then COSPAR can rec ommend, and the U.S. can decide, that the total U.S. contribution should be equal to some specified fraction of the total present and future Soviet contribu tion.

This approach in turn suggests that every effort should be made to induce the Soviets to supply additional de tails on the Zond 2 and Venus 3 mis sion and trajectory and, particularly, on the procedure used for sterilizing the components and assembly of both space craft. With such information, the proba ble number of viable terrestrial microor ganisms deposited on Venus and Mars could be estimated well enough to per mit a. realistic quantitative analysis of what U.S. policy and practice should be. However, if more complete informa tion on Soviet practice cannot be ob tained, then, it seems to us, the U.S. has no logical alternative but to per mit greater engineering freedom in lander delivery technique and to ac cept gaseous and other nonthermal sterilization procedures, where neces sary, in its own program. By relying on the demonstrated U.S. spacecraft reliability to insure that the U.S. con tribution to planetary contamination will remain significantly less than the Soviet contribution, we could reduce significantly the cost and time required to carry out serious scientific investiga tions of the surfaces of Venus and Mars.