In comments below Is there any demonstrated or even proposed technology that can sterilize a spacecraft with 100% certainty and yet leave it electronically functional? there is a link to Europa Lander Study 2016 Report; Europa Lander Mission; JPL D-97667, Task order NNN16D011T; Europa Lander Mission Pre-Phase A. In the beginning of Section 7, Planetary Protection it says:

The mission design of Europa Lander (Chapter 10), as a spacecraft that will land and also potentially impact the europan surface prior to landing, should be compliant with the requirements of PP mission classification Category IV under current COSPAR and NASA planetary protection policy, i.e., less than 1×10−4 probability of contaminating the europan ocean by a viable Earth microorganism. The ultimate landing, a relatively low-velocity impact of Europa Lander onto Europa’s surface without the benefit of atmospheric heating, drives the derived requirement for bioburden reduction processing of the spacecraft hardware.

The importance of preventing forward contamination is two-fold, first as a legal and ethical consideration to protect potentially viable organisms and ecosystems beyond Earth, and second as a scientific imperative to ensure that false positives are minimized (NAS, 1958; COSPAR, 1964; Rummel and Billings, 2004; UNOOSA, 1966; and see Hand et al., 2009 for a more detailed discussion). The specific language to which the United States, and therefore NASA, committed to in 1967 is that of the Outer Space Treaty, Article IX: “States Parties to the Treaty shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination ... and, where necessary, shall adopt appropriate measures for this pur- pose.”

Question: Who decided that a 1 in 10,000 probability of contaminating the europan ocean by a viable Earth microorganism was legally and ethically sufficient to protect potentially viable organisms and ecosystems beyond Earth?

I'm looking for more than just an acronym of an organization. Was there a specific panel of experts or group of ethicists and legal scholars that was convened and then arrived at this <1×10−4 probability figure? Is the process by which this number was arrived explained and its ethical and legal soundness justified in some place where we can read it?

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    $\begingroup$ The NASA Planetary Protection History & other sources point to "COSPAR re-establishes first set of quantitative Planetary Protection objectives" in 1964. Successive COSPAR Planetary Protection Panel reports all refer to COSPAR. COSPAR RESOLUTION 26.5, COSPAR Information Bulletin, 20, 25-26, 1964 but I cannot find it online yet. $\endgroup$ Commented Jul 26, 2021 at 17:41
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    $\begingroup$ @BrendanLuke15 That 1964 directive almost certainly said nothing with regard to Europa and Enceladus. The focus back then was the Moon and possibly Mars. $\endgroup$ Commented Jul 26, 2021 at 17:59
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    $\begingroup$ @DavidHammen no but it may have introduced the 10e-4 $\endgroup$ Commented Jul 26, 2021 at 18:28
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    $\begingroup$ @BrendanLuke15 That apparently is correct. This isn't the original source, but it does reference it and quote it: sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/… . Note that the rules for Mars have been significantly changed from the $10^{-4}$ guideline. The odds are very high that Mars has already been contaminated. $\endgroup$ Commented Jul 26, 2021 at 18:47
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    $\begingroup$ I see ok, so in that case I think it's not surprising people are interpreting as if you want to find someone to blame (since I don't see any reason why knowing the process itself without the "who" is not satisfying, as well as the phrasing of the question seems a bit accusative, but that can be explained with just different writing style). But I can see from your comments that you are not trying to find someone to blame, so I guess different philosophy or curiosity perhaps. =D $\endgroup$
    – justhalf
    Commented Jul 28, 2021 at 22:58

1 Answer 1


I'll start with what COSPAR wrote in 1964, emphasis mine:

COSPAR "accepts, as tentatively recommended interim objectives, a sterilization level such that the probability of a single viable organism aboard any spacecraft intended for planetary landing or atmospheric penetration would be less than $1\times10^{-4}$, and a probability limit for accidental planetary impact by unsterilized fly- by or orbiting spacecraft of $3\times10^{-5}$ or less."

The OP appears to think this $1\times10^{-4}$ probability is terribly loose. It's the opposite. It is terribly tight. It has its origins in the very start of NASA. In 1959, Abe Silverstein, NASA's director of Space Flight Programs, made the U.S. space agency's first formal statements about planetary protection:

The National Aeronautics and Space Administration has been considering the problem of sterilization of payloads that might impact a celestial body. ... As a result of the deliberations, it has been established as a NASA policy that payloads which might impact a celestial body must be sterilized before launching.

This early sterilization policy required NASA to bake the first five Ranger spacecraft in the pursuit of 100% sterilization. This baking almost certainly resulted in some of the failures of the early Ranger missions:

Ranger bore the brunt of the difficult constraint imposed by sterilization. Any spacecraft likely to land on the surface of the Moon or a planet, by accident or by intent, had to be free of Earth's pervasive microbial population. This scientifically imposed international requirement placed a tremendous burden on Ranger engineering and development, and greatly multiplied costs. Of course, it also took its toll on the useful life and reliability of sterilized components and is believed to have had a seriously degrading effect on early spacecraft performance.

As an aside, the stringent requirements for heat sterilization may have, in the long run, resulted in the development of more reliable hardware, in much the same way that insecticides cause insects to develop an evolutionary resistance. But early Ranger spacecraft had to pay the price of meeting these stringent requirements. While there never was any positive proof that sterilization caused difficulties with electronic gear in space, there were many reasons to believe this was a factor in the high initial failure rates.

In addition to the problems with those first Ranger missions, there was a set of planned missions in the early 1960s that presented challenges with regard to planetary protection. The US wanted to land humans on the surface of by the end of the 1960s. That goal represented payloads that would never be sterilized. I can't find a reference that states how COSPAR backtracked on its initial 1964 statement with regard to the Apollo project, but this almost certainly happened.

The only way to make the probability of a probe releasing Earth life on some other body be zero is not to go there. NASA realized this very early on. Even with the most stringent processes, anything sent from Earth to land on another body will have a non-zero chance of infecting that body with Earth life. The $1\times10^{-4}$ probability was set as a very high bar. As an example, with increased interest on Mars, COSPAR lowered the bar in 1969:

COSPAR "accepts as the basic objective for planetary quarantine of Mars and other planets deemed important for the investigation of extraterrestrial life, or precursors or remnants thereof, a probability of no more than $1\times10^{-3}$ that a planet will be contaminated during the period of biological exploration. The period of biological exploration is assumed to be 20 years ending in 1988, and the number of missions to or near the planets is assumed to be 100."

COSPAR has repeatedly lowered the bar to something more achievable. Here are the planetary protection requirements for NASA's most recent Mars mission:

At launch, the entire payload going to Mars (the Perseverance rover, Ingenuity Mars Helicopter, cruise stage, aeroshell, and descent stage) carried fewer than 500,000 bacterial spores (dormant forms of bacteria). This is a tiny number as far as spores go and wouldn’t even cover a typical smartphone camera lens. Of this number, the parts of the Mars 2020 spacecraft intended to land on Mars – including the rover, the parachute, and the descent stage – had no more than 300,000 spores in total. The rover itself was allowed to have just 41,000 spores, spread out over the vehicle’s entire surface area.

The $1\times10^{-4}$ objective for missions to Europa or Enceladus appears to be an unmodified version of the 1964 "tentatively recommended interim objectives". My prediction: If NASA is funded to send a mission to Europa or Enceladus, that unrealistic goal of a $1\times10^{-4}$ chance of infecting the body with Earth life will be changed to something that is achievable.

  • $\begingroup$ "A doxing request is getting u̶p̶v̶o̶t̶e̶s̶ answers? Why is that?" The question is reasonable, this answer is good. It looks like there really was no actual determination that the number would be "legally and ethically sufficient" for this future mission, so I'm taking the answer to be somewhere along the lines of "Actually, nobody." $\endgroup$
    – uhoh
    Commented Jul 27, 2021 at 5:29
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    $\begingroup$ Hopefully, the don’t-interpret-me and the answer-the-question-as- written will be put aside, to allow the debate to rise up. Both question and answer are useful in highlighting that there is a defect in “space governance”. Hopefully, those who believe that “all is well in the state of Denmark” will discover that something is rotting. $\endgroup$
    – Ng Ph
    Commented Jul 27, 2021 at 8:13
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    $\begingroup$ There is a thorough and nicely-sourced summary published in Nature Feb. 13, 2020: Lessons in space regulations from the lunar tardigrades of the Beresheet hard landing $\endgroup$
    – uhoh
    Commented Jul 28, 2021 at 11:49
  • $\begingroup$ Yes, I think it is loose, and feel it is a reasonable view and "tight" isn't. The tight viewpoint assumes eminent domain; we have a right to do it and we're going to do it and so we need the limit to be consistent with what we can achieve so that it doesn't get in our way. The loose viewpoint assumes "at least do no harm" without regard to how easy that would be. Now I'm cognizant of how problematic the "no" is there and don't need to be re-re-re-reminded of it. But baking for month at 800 °C in some kind of multi-layered enclosure in deep space en route would come a lot closer to it. $\endgroup$
    – uhoh
    Commented Dec 30, 2021 at 21:19
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    $\begingroup$ @uhoh That's your opinion. Other people have different opinions. (That my opinion is close to yours is irrelevant.) Suppose a Mars rover finds definitive proof that Mars not only supported primitive life billions of years ago but also supports primitive life now. What should we do? Study it? Support it? No. There will be a massive clamor to NUKE IT, especially after having gone through two years of covid-19. $\endgroup$ Commented Dec 31, 2021 at 11:35

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