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CHAPTER 28 DECONTAMINATION STANDARDS FOR MARTIAN EXPLORATION PROGRAMS CARL SAGAN and SIDNEY COLE MAN It has recently been established that typical samples of terrestrial soil contain populations of microorganisms capable of surviving the rigors of simulated average Martian environments [Scher, Packer, and Sagan, 1964; Packer, Scher, and Sagan, 1963]. Growth of microorganisms in simulated microenvironments having moisture and organic-matter contents higher than the anticipated mean values of Mars has also been demonstrated [Young, Deal, Bell, and Allen, 1964]. These results give experimental credibility to the concern, repeatedly expressed, that the landing of unsteri- lized space vehicles on Mars may obscure subsequent attempts to detect, in a pristine state, indigenous life on that planet. To avoid possible biological contamination of Mars, it is clear that entry vehicles should be carefully and conscientiously sterilized. This recommendation is in accord with others previously made by the COSPAR Committee on Contamination by Extra- terrestrial Exploration [CETEX, 1958; 1959], Lederberg [I960], the Space Science Board {Brown et al., 1962], and Imshenetsky [1963]. A policy governing the decontamination of spacecraft landing vehicles has been adopted by the National Aeronautics and Space Administration (see Brown, et al. [1962] ); this policy is being carried out. If contamina- tion is to be avoided, the Soviet Union must also implement a sterilization policy for Mars entry capsules. Methods for space vehicle sterilization are being actively developed in the United States, and there is a good indication 470

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Decontamination Standards for Martian Exploration Programs 471 that there are adequate sterilization procedures which do not compromise the engineering and scientific requirements of the mission [e.g., Phillips and Hoffman, 1960; Posner, 1961; Swift, 1961, 1963; Brown et al., 1962; Hall and Bruch, 1964]. At present, the most reliable procedure for insuring a high degree of sterility is to heat the entire landing capsule. In one scheme, the capsule is heated in three spaced cycles to temperatures above 135°C for 36 hr. Longer exposures to lower temperatures may possibly be equally effective. If heat sterilization is to be employed, the initial design of capsule com- ponents must, of course, be made with the necessity of subsequent heat sterilization in view. This seems to be a feasible objective; indeed, the reliability of some components—for example, certain transistors—can be enhanced by the heat-sterilization procedure. Nevertheless, sterilization introduces serious engineering problems, especially by decreasing compo- nent reliability. The type and duration of sterilization procedure must depend on some estimate of what constitutes an acceptable risk of planetary contamination. Our intention here is to provide a means of computing the level of spacecraft sterility as a function of this acceptable risk. One early study [Davies and Comuntzis, 1960] arbitrarily concluded that the probability of landing one viable microorganism on Mars should be kept below a = 10-° per mission. A recent reestimate by Jaffe [1963] places OCR for page 470
472 AVOIDING THE CONTAMINATION OF MARS v is the mean number of viable microorganisms within the landing cap- sule at the time of landing; a is the mean number of viable microorganisms per capsule which are distributed outside the capsule, on the Martian surface; N is the total number of experiments that must be performed success- fully before the experimental program can be considered completed; X is the mean number of experiments per capsule; /. is the probability that a capsule lands successfully, if it lands at all; E is the probability that an individual experiment will succeed, assuming that the capsule containing it has landed safely; M is the probability that a viable microorganism landed on Mars will subsequently multiply, and contaminate a significant area of the planet within the duration of the total experimental program and, finally, p is the probability that the program of N experiments will be completed without biological contamination of Mars. We will require p to be very close to unity. We will assume throughout that all parameters are constant during the experimental program. If our mean values of the above parameters are chosen properly, this assumption will introduce no significant errors into the final results. For all cases of interest, the product of a and M is much less than unity, and thus, the probability that a given capsule contaminates the planet is approximately aM. On the average, N/E experiments must be attempted for N to succeed, and m = N/^EL missions are required. To obtain an initial orientation, we will first assume that x = 1- The calculation of p may then be performed exactly by elementary methods. It is possible to write down the result immediately, using standard tech- niques (e.g., see Feller [1957]). However, for the benefit of the reader unfamiliar with probability theory, the result will be derived from first principles. We will then show that an excellent approximation to the exact expression may be obtained through an approximate method. We will then use this approximate method to derive an expression for the case x > 1. Since it is unlikely that every mission will be successful in its search for and characterization of life on Mars, there will be a number, Q > N, missions to Mars before the desired information is obtained. Consider the (N -f- /)th mission, and assume that it is the final in the series of N experi- ments. Thus, the (N + ;')th experiment must be successful, because, by hypothesis, we are content to risk a probability p that biological contami- nation of Mars occurs after N + / missions. The remaining missions may be arranged in any order whatever. Thus there are (N -f- / — 1)! permu- tations of the remaining missions. Of these, N — 1 will be experimentally successful, where again, the order, for which there are (N — 1)! possi-

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Decontamination Standards for Martian Exploration Programs 473 bilities, is immaterial. At the same time, / missions will be failures, and once again the ordering of the failures is unimportant. Further, there must be no contamination events in N -\- j missions. The total number of ways of obtaining such scenarios of successful experimentation before contami- nation will then be p- (N — I)!/! (1) j = 0 Thus, p is the probability that biological contamination does not occur until N experiments are performed. From the power series expansion for (1 — >>)-.*, we find T LE(\—aM) p = i Taking the Nth root of each side, we find LE(l — aM) i/y~i i n ww (3) since (In p)/N is a small (and negative) number. Since aM and aM/LE are also expected to be « 1, we may expand the lefthand side of (3), finding it to be approximately 1 — (aM/LE). Equation (3) then becomes LE Inp-. (4) We will always wish to arrange the over-all experimental program so that p is very close to unity. In this case, 7 p We may obtain the approximate equation (5) in a less rigorous, though more direct manner by making our approximations at the beginning of the calculation rather than at the end. We argue as follows: We wish to obtain the probability of scientific success in N experiments before biological contamination occurs. Because aM is very small, the probability of non- contamination is a very slowly varying function of the number of missions landed. Also, because N is fairly large, the probability distribution for the total number of experiments landed — successfully or otherwise — will clus-

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474 AVOIDING THE CONTAMINATION OF MARS ter sharply about its mean value, N/LE. Therefore, to a good approxima- tion, we may replace the average probability of noncontamination by its value at the mean—that is to say, pc^(\— oM)*'L». (6) Taking the logarithms of both sides, we obtain an expression which is equivalent to Equation (4). Our argument leading to Equation (6) can now be easily generalized to the case that x > 1- N is then replaced by N/x and the remainder of the argument is unchanged. Thus, we obtain '":- (1— P). (7) MN Up to this point, we have discussed only vehicles intended for planetary landings. There is a category of spacecraft, however, intended for planetary fly-by and orbit that may, nevertheless, accidentally impact the planet. If landers are sterilized but fly-bys and orbiters are not, the probability of planetary contamination from the latter spacecraft may be higher than from the former. The number of microorganisms on a typical unsterilized spacecraft may be v '—- 10-10- A significant fraction F of these escape from the vehicle upon impact. Therefore, the impact of one such spacecraft has a good chance of contaminating the planet, provided, as seems to be the case, that FM » 1010. The heat and deceleration of impact will not kill all the contained microorganisms. Thus, the probability of contamination from an intended fly-by or orbiter will be I, the probability of accidental planetary impact, a parameter ordinarily computed in the planning of interplanetary trajectories. We assume that, in the long run, the contribution to our knowledge of Martian biology from fly-bys and orbiters will be negligible, compared with the contribution from landers. Then, if n is the number of fly-bys and orbiters launched towards Mars in the same period as the landers, our previous arguments can be extended to give (8> The division of effort between sterilizing landers and controlling trajec- tories of unsterilized fly-bys and orbiters is a problem in operations re- search appropriate to national space agencies. However, to visualize the

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Decontamination Standards for Martian Exploration Programs 475 numerical constraints on a and /, let us arbitrarily require that the con- tamination risk from fly-bys and orbiters equal that from landers. We then have, finally, a requirement for landers: and for unsterilized fly-bys and orbiters: Some representative numerical values for a as a function of N and p appear in the accompanying graph. We have assumed that the mean num- ber of experiments per mission over the entire exploration program is X — 20, and that the mean probability, over the entire program, of spacecraft failure resulting in no biologically significant information from an entire mission is 1 — L ^ 0.1. We have also taken the mean probability of scientific success of a given experiment to be £ — 10-1. Estimating the value of E involves trajectory information, since presumably not all locales on Mars are equally likely to have detectable life-forms. It also involves an evaluation of the ability of biologists to design experimental packages to detect Martian life-forms in the face of almost complete ignorance of the biochemistry, morphology, or physiology of Martian organisms. While every tested terrestrial soil sample has microorganisms capable of surviving simulated mean Martian conditions \Scher, Packer, and Sagan, 1964], the exact fraction that survives is very difficult to determine. The problem is complicated by the wide variety of microbial survival conditions and the fact that no culture medium is optimal for all microorganisms. There are many microbial types which do survive freezing and dessication [Packer, Scher, and Sagan, 1963] . We adopt a preliminary value of the fraction of survivors ^ 10-1, a value which, after the initial freeze-thaw cycle, is essentially independent of time. But we emphasize that the uncer- tainty is at least one order of magnitude; intercomparison of soil samples shows a fractional survival range of at least this order. Survival and growth are by no means equally easily achieved, especially under average Martian environmental conditions. In the following discussion, we adopt M ^ 10-2. The graph shown in Figure 1 gives, for the choice of parameters just cited, minimum values of a. From Equation (9) and the graph, the reader can easily make his own choices of these parameters and redetermine OCR for page 470
476 AVOIDING THE CONTAMINATION OF MARS Figure 1. Risk of planetary contamination. See text. 0 -2 -4 -- -8 bS"° * -,2 -14 -16 -18 -10 -I* -24 Parameters assumed: X- 20; L =0.9; E = ' M • ICT8! n « 30. icriz icr10 icr" icr6 I-P 10-* KT2 0.3 biology. If there are Martian life-forms, biologists will need adequate time to investigate their anatomy, physiology, biochemistry, genetics, ecology, systematics, and behavior. The wealth of desired information per variety of Martian organism, coupled with the potential variety of Martian organ- isms, indicates that the value of N must be large. However, it is relevant that no existing plans for manned landings are responsive to the contamina- tion problem. There seems to be no immediate prospect of spacesuits with negligible contaminant leak. Indeed, the design difficulties involved may prove insuperable. The specification of a very large N in an experiment- centered strategy will be tantamount to requiring manned exploration of Mars in self-contained mobile vehicles entering and leaving the spacecraft through a sterilization lock. An alternative and, from the present vantage point, much less desirable procedure will be to accept a mission-centered strategy; to accept planetary contamination after manned landings on Mars; and to maximize the effi- ciency of the preceding unmanned exploration phase. In the spirit of this alternative, let us adopt 1984 as the date of the first manned landing on Mars. Assuming an average of three launches of landing vehicles at each op- portunity before 1984 by each of two space-faring nations, we have about 60 possible missions, of which 54 land successfully. With x — 20, this leaves 1080 possible experiments. With E = 10-1, we have N = 108 suc- cessfully completed experiments, a number small compared with the range of biological prospects, but hardly insignificant. During the same period, a number n — 30 launches of fly-bys and orbiters appears to be an upper limit. Suppose it were concluded that N = 108 is consistent with an experi- ment-centered strategy. If we desire 99.9% probability that 1080 biological

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Decontamination Standards for Martian Exploration Programs 477 experiments can be attempted before contamination, the mean number of viable organisms deposited on the planet by each spacecraft must be less than 9.3 X 10~4^ 10-3. With a mission-centered strategy the analysis fol- lows that preceding Eq. (6), and Eq. (9) is replaced by a <(l-p)/2Mm, where m is the total number of missions. But we have seen that m = 'N/x EL. The equation is therefore identical to Eq. (11), and whichever strategy is used, the numerical results are the same. However, in the ex- periment-centered strategy N will almost certainly be larger than 120, and OCR for page 470
478 AVOIDING THE CONTAMINATION OF MARS case, the parameters which were assumed for the computation of a and I should be periodically reevaluated, especially in the light of new informa- tion obtained about Mars from early fly-bys, orbiters, and landers. There are, however, other lines of work which, if properly pursued, might at some time in the future lead to an increase of the acceptable values of a and /. We recall that a is the probability that a single viable micro- organism be deposited on the Martian surface. In the case of an intact soft landing, only those organisms near the exterior of the spacecraft have a high probability of being deposited on the Martian surface, a point recently emphasized by Horowitz [1965]. It would appear at first sight that organ- isms contained within sealed components, such as batteries or transistors, have no chance of escaping from the spacecraft. If this were rigorously true, the entire sterilization requirement would be greatly ameliorated, because the microorganisms likely to contaminate the planet would then be precisely those which can be killed by chemical surface sterilants. The necessity for heat sterilization would then be diminished. However, microorganisms may diffuse through solid materials in time scales of the order of a decade [Sneath, 1964]. Also, there will always be a non-zero probability of accidental hard landing on Mars, in which case the fraction of the total load of contaminations which are distributed over the Martian surface will depend on the fragmentation size-distribution. If an impacting spacecraft fragments into spherical particles of characteris- tic radius r, the fraction of contained microorganisms of 5 /i dimensions which are sufficiently close to a surface to escape onto the planet will then be as follows: P(r) > l_(l_5/r)», (11) where r is in microns [Horowitz, 1964]; e.g., if r = 10 cm, P(r) > 1.5 X 10-4. This expression gives a lower limit to P(r), because it assumes a uniform fragmentation size. Small fragments make the greatest contribution to planetary contamination. In fact, P(r) should be multiplied by /(r), the frequency distribution of fragments of characteristic size r. Then, if we knew the probability that a given landing results in a particular fragmenta- tion distribution /(r), we could compute the probability of escape of a given organism due to an imperfect soft landing. If we also knew the specific probability that an organism migrates—by solid-state diffusion or through aeolian erosion—a distance r in time /, we could compute a/v, where v is the total number of viable organisms aboard the spacecraft. In general, a < v. At the present time it is difficult to make even a fair numerical estimate of the relevant parameters. It is clear that substantial work is required on

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Decontamination Standards for Martian Exploration Programs 479 the fragmentation distribution of model spacecraft, on the diffusion of microorganisms through solids, and on the rate of aeolian erosion on Mars. Moreover, a good estimate of the probability distribution of accidental hard landing at various impact velocities would be useful. Until such information is available, it is premature to assume that a/v « 1. For the present we must design space missions as if a/v — 1. It should also be emphasized that any spacecraft designed to sample periodically the Martian environment will also provide at the same time some opportunity of egress for microorganisms contained in the spacecraft. Sampling procedures imply a route for microorganisms from the inside to the outside of the lander. There are departures in spacecraft design which could greatly relax the standards on / and v. Precautionary terminal systems can be developed for assuming non-intercept trajectories in case of imminent accidental impact—for example, by extending the existing capabilities of midcourse- correction motors. Such systems must be failsafe. Possibly more prom- ising, both for intended fly-bys and orbiters and for intended soft landers, would be the development of impact-actuated terminal-destruct sterilization procedures. These procedures can be low in weight and cost, compared with adequate sterilization before launch. Prelaunch sterilization of fly-bys and orbiters is also desirable. The problems attending microorganism solid state diffusion and aeolian erosion of soft-landed vehicles can similarly be circumvented—here, by development of destructive heat-sterilization tech- niques to be activated after a lander completes its experimental program. The reliability of the timer and sterilization would have to be demonstrated before such techniques could supplant prelaunch sterilization. Several other decontamination requirements have been pointed out by Brown, et al. [1962]. These include provision for an independent author- ity to certify that the required values of a are achieved; an emphasis on biological and chemical experiments with the first Mars fly-by, orbiter, and landing vehicles; the acquisition and retention of large numbers of sterile samples by the earliest missions; and an active program to continue sterility precautions after beginning manned exploration of Mars. Future simulation studies can contribute to our evaluation of the proba- bility of biological contamination of Mars, and therefore to a better speci- fication of the vehicle sterilization parameter, a. It will be important to determine the fraction of survivors of simulated Martian environments which also survive spacecraft sterilization procedures. It is possible that organisms which are fortuitously pa-adapted to survive heat sterilization are extremely maladapted to Martian regimes of freeze-thaw cycling. Cer- tainly, there seems to be little selection pressure on Earth for survival of the same organisms both at 135°C and —80°C. Further characterization

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480 AVOIDING THE CONTAMINATION OF MARS of the Martian environment and of the nature and modes of surviving microorganisms should permit a better understanding of this problem. Summary. A significant fraction of terrestrial microorganisms survive the inclemency of simulated mean Martian conditions; a smaller fraction may be able to grow at favored times and places on Mars. To avoid bio- logical contamination of Mars by terrestrial microorganisms on unsterilized landing vehicles, the number of viable microorganisms, OCR for page 470
Decontamination Standards for Martian Exploration Programs 481 Feller, W. (1957), An Introduction to Probability Theory and Its Applications, John Wiley & Sons, New York, pp. 155-156. Hall, L. B., and C. Bruch (1965), Procedures Necessary for the Prevention of Planetary Contamination. In: Life Sciences and Space Research III, M. Florkin (Ed.), North-Holland Pub. Co., Amsterdam. Horowitz, N. H. (1965), "Spacecraft Sterilization." Chapter 27, this volume. Imshenetsky, A. A. (1963), Perspectives for the Development of Exobiology. In: Life Sciences and Space Research I, R. B. Livingston et al, (Eds.), North-Holland Pub. Co., Amsterdam, pp. 3-15. Jaffe, L. D. (1963), Sterilization of Unmanned Planetary and Lunar Space Vehicles—An Engineering Examination. Jet Propulsion Lab. Tech. Rept. 32-325 (Rev.); also published in Astronautics and Aerospace Engineering 1(1), 22. Lederberg, J. (1960), Exobiology: Approaches to Life Beyond the Earth. Science 132, 393-400. Packer, E., S. Scher, and C. Sagan (1963), Biological Contamination of Mars, 2: Cold and Aridity as Constraints on the Survival of Terrestrial Micro- organisms in Simulated Martian Environments. Icarus 2, 293-316. Phillips, C. R., and R. K. Hoffman (1960), Sterilization of Interplanetary Vehicles. Science 132, 991-995. Posner, J. (Ed.) (1961), Proceedings of Meeting on Problems and Techniques Associated with the Decontamination and Sterilization of Spacecraft, June 29, 1960, Washington, D. C. NASA Tech. Note D-771, Washington, D. C. Scher, S., E. Packer, and C. Sagan (1964), Biological Contamination of Mars, 1. Survival of Terrestrial Microorganisms in Simulated Martian Environ- ments. Life Sciences and Space Research II, M. Florkin and A. Dollfus (Eds.), North-Holland Pub. Co., Amsterdam, 352-356. Sneath, P. H. A. (1964), Private communication. Swift, J. (1961), Effects of Sterilizing Agents on Microorganisms. Jet Pro- pulsion Lab. Astronautics Information Literature Search No. 260; and Supplement (1963). Young, R. S., P. H. Deal, J. Bell, and J. L. Allen (1964), Bacteria Under Simulated Martian Conditions. Life Sciences and Space Research II, M. Florkin and A. Dollfus (Eds.), North-Holland Pub. Co., Amsterdam, 105-111.