conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized.”3 The findings and recommendations of the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System resulted from the deliberations conducted within a similar motivational framework.

COSPAR’s planetary protection policy categorizes spacecraft missions according to their type (i.e., flyby, orbiter, lander, or sample return) and the degree to which the spacecraft’s destination might inform the processes of chemical evolution and/or the origin of life (Table 1.1). The policy routinely changes in response to inputs from member organizations, including the NRC, which re-evaluate advances in scientific knowledge in both the planetary and the life sciences.

One such input came in 2000 when the NRC issued the report Preventing the Forward Contamination of Europa.4 The authors of that report were unable to agree on a methodology by which COSPAR’s existing categorization system could be extended to cover spacecraft missions to Europa.5 In place of categorization, the report recommended that spacecraft missions to Europa must reduce their bioload by an amount such that the probability of contaminating a putative Europan ocean with a single viable terrestrial organism at any time in the future should not exceed 10–4 per mission.

The 10–4 criterion proposed by the authors of the NRC’s 2000 Europa report is rooted in the history of COSPAR planetary protection policy statements and resolutions. Before its revision in 1982, COSPAR’s planetary protection policies were based on a quantitative assessment of the likelihood of contaminating planetary bodies of interest. The 10–4 contamination criterion can be traced back to a COSPAR resolution promulgated in 1964 concerning “any spacecraft intended for planetary landing or atmospheric penetration” and still earlier.6 Unfortunately, the historical literature does not record the rationale for COSPAR’s adoption of the 10–4 standard. Nor, in fact, has the committee been able to come up with its own quantitative rationale for this number. Even though COSPAR has all but eliminated quantitative approaches from its policy statements, the apparently arbitrary 10–4 standard continues to guide the implementation of planetary protection regulations, particularly with respect to those pertaining to missions to Mars.7 The adoption of a particular contamination criterion raises a number of questions. First, was it appropriate for the authors of the 2000 Europa report to apply a martian standard to Europa for any other than historical reasons? The current committee argues that since the advertised purpose of planetary protection is to preserve the integrity of scientific studies relevant to the origins of life and the processes of chemical evolution, the contamination standard for a particular object is directly related to the scientific priority given to studies of that object. Recent NRC reports such as A Science Strategy for the Exploration of Europa,8 New Frontiers in the Solar System: An Integrated Exploration Strategy,9 and Vision and Voyages for Planetary Science in the Decade 2013-202210 have ranked the scientific priority of studies of Mars and Europa as being, if not equal, then a very close one and two. Thus, a contamination standard applicable to one should, to first order, be applicable to the other.

A second question is determination of the standard itself. It should be possible, in principle, to come up with a standard that is simultaneously not arbitrary and still permits exploration. For example, it could be argued that the standard be such that the likelihood of contamination by spacecraft is less than the likelihood of contamination by meteoritic delivery of Earth microbes in impact-launched meteorites (integrated over some time period, say, the interval of anticipated spacecraft launches). But the adoption of such a standard may preclude the exploration of the icy bodies of the outer solar system.11

The committee’s decision to retain use of the historical 10–4 was predicated on two factors. First, planetary protection policies are deliberately conservative and strongly influenced by historical implementation practices. The 10–4 standard is conservative, but implementable, as evidenced by the extensive efforts undertaken to ensure that the Viking missions to Mars and the Juno mission to Jupiter were compliant. Second, the committee’s charge specifically focuses on the approach taken by the NRC’s 2000 Europa report committee and subsequent COSPAR actions related to planetary protection measures for the outer solar system. The introduction of a new contamination standard into the deliberations will, in the committee’s considered opinion, complicate the resolution of more serious issues arising from the methodology contained in the 2000 Europa report.

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