Executive Summary

The ultimate goal of planetary protection for outbound missions is to prevent harmful contamination that would inhibit future measurements designed to search for evidence of the existence or evolution of extraterrestrial life. This goal is codified by the Outer Space Treaty of 1967, consensus guidance from international scientific organizations such as the Committee on Space Research (COSPAR), and National Aeronautics and Space Administration (NASA) policy. Preventing harmful contamination is achieved by following specific guidelines based on existing scientific knowledge about the destination and the type of mission. Planetary protection policy categorizes missions according to the intended targets (e.g., Mars) and mission objectives (e.g., flyby, orbiter, or lander).

Small bodies discussed in this report include Main Belt asteroids (MBAs), near-Earth objects (NEOs), Trojan asteroids, comets, Centaurs, and Kuiper Belt objects (KBOs). Exploration of some of these objects has been performed in the past by government space agencies such as NASA to understand the nature of these objects and what they can reveal about the formation of the solar system. These studies, including both ground- and space-based astronomical observations, compose the bulk of knowledge about these objects to this date, and have been conducted from a purely scientific perspective.

With the advent of nongovernmental space missions, including by private-sector enterprises, that paradigm will change. Private entities are preparing to expand space operations to include mining small bodies, especially NEOs, to obtain resources for future space exploration. Therefore, at this critical juncture, it is timely to reassess planetary protection requirements for missions to these types of objects.

NASA and COSPAR currently consider missions to undifferentiated, metamorphosed asteroids as under planetary protection Category I and missions to comets and all other types of asteroids as under Category II. Missions to KBOs are considered Category II by both NASA and COSPAR. Neither COSPAR nor NASA cite a categorization for missions to Centaurs. No requirements for spacecraft cleanliness or organics inventories are imposed on either Category I or Category II missions to small bodies. Category I missions carry no planetary protection requirements at all. Category II only requires the provision of information about the mission important for planetary protection, such as the mission’s intended target, the spacecraft’s trajectory, and the final deposition of the spacecraft.

The committee learned that there are often misconceptions within the commercial space sector about these requirements. There is the incorrect perception that missions to small bodies are required to have material inventories or to use special cleanroom procedures to meet planetary protection requirements. That is not the case. Such decisions are only motived by either (1) the sensitivity or reliability requirements of the mission itself or (2) a mission trajectory that includes a flyby of a body more sensitive to biological contamination (e.g., Mars) and thus merit a more stringent categorization. Engineering missions have cleanliness requirements driven by spacecraft reliability.

The committee also found that confusion extends into the scientific community with regards to the planetary protection requirements for missions to small bodies. Members of the community were found to believe that planetary protection measures for missions to small bodies were in place to prevent contamination whereas Category II missions do not have any such requirements. Generally, science missions have cleanliness requirements that are not dictated by planetary protection but by the nature of the scientific measurements to be performed.

This report responds to NASA’s request for a study on planetary protection categorization of missions to small bodies, including whether there are particular populations of small bodies for which contamination of one object in the population would not be likely to have a tangible effect on the opportunities for scientific investigation using other objects in the population. NASA also asked that, if such populations exist, would it be suitable to categorize future missions to those bodies as Category I.

In addressing NASA’s request, the committee considered surface composition of target bodies and their importance for prebiotic chemistry, along with size of the small-body populations, the current state of knowledge on the types of objects, the likelihood of a future scientific mission returning to any specific object, active object surface processes, and the size. Each factor has varying degrees of importance to the consideration of planetary protection categorization.

The committee offers these specific findings related to the statement of task of this study:

In summary, some small solar system bodies are likely to contain prebiotic organic compounds that are relevant to the study of the origin of life in the solar system. However, based on current knowledge, it is highly improbable that small bodies harbor extinct or extant life, or that terrestrial microbes carried by a landing spacecraft can proliferate there. Therefore, the committee finds that, with the exception of large objects typified by the special case of Ceres, the forward contamination of small bodies is not expected to have a tangible effect on broader astrobiological investigations in the solar system. Nevertheless, given the importance of some classes of relatively primitive, volatile-rich, and organic-bearing small bodies to studies of prebiotic chemistry and the sparsity of current knowledge about them, the committee sees no reason to reduce the current categorizations (from Category II to Category I) for missions to such objects until such time as scientific knowledge changes. On the other hand, Category I is appropriate for missions to rocky, metamorphosed NEOs and MBAs. The committee strongly endorses periodic reassessments (on a timescale commensurate with the pace of new small-body science investigations) of the appropriate categorization of missions to all classes of objects as new scientific information becomes available. Such new or more detailed information could provide a basis for relaxing requirements for missions to some objects, or in the case of unique objects like Ceres, future mission requirements might need to be made more stringent. Finally, the community would benefit greatly from an archive of relevant mission planetary protection documentation and clearly defined lines of authority in terms of U.S. government planetary protection policy as it applies to nongovernmental space activities, and the committee strongly endorses the findings of prior reports on this topic.