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Implications of Planetary Protection Category I Versus Category II for Small Body Missions

Both National Aeronautics and Space Administration (NASA) and Committee on Space Research (COSPAR) policies categorize all missions to small solar system bodies as either Category I or Category II. Current NASA Policy (NASA Procedural Requirement [NPR] 8715.24) includes the following description of planetary target priorities to differentiate between Category I and Category II missions:

Using these broad definitions, both NASA and COSPAR currently consider missions to undifferentiated, metamorphosed asteroids as Category I and missions to comets and all other types of asteroids as Category II. No cleanliness requirements or organics inventories¹ are imposed on either Category I or Category II missions. Category I missions carry no planetary protection requirements at all. The planetary protection requirements for Category II missions are limited to relatively routine information about the mission. This includes documenting the intended target body, the mission intent (flyby, landing, or impact), the planned trajectory, the post launch trajectory status, and the final disposition of the spacecraft.

For some small-body missions, if the trajectory is intended to fly by, or has a significantly high risk of approaching a Category III/IV target body (such as Mars, Europa, or Enceladus), then the mission will receive a Category III categorization and additional planetary protection requirements will apply, such as those for spacecraft cleanliness. NASA missions to small bodies that received Category III categorization include Dawn and Psyche, both due to the intent to fly past Mars.

The Juno mission to Jupiter provides an example of mission receiving a revised categorization as the result of its extended mission plan. The initial Juno mission was Category II. After launch, the mission was extended to include flybys of Europa, Ganymede, and Io, and received a revised Category III categorization.

PLANETARY PROTECTION DOCUMENTATION FOR CATEGORY II MISSIONS

NPR 8715.24 lists six possible document requirements for Category II missions:²

• Planetary Protection Requirements Document
• Planetary Protection Implementation Plan

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¹ The requirement for an organics inventory may be required for certain missions to the Earth’s Moon; such missions carry a Category IIb categorization. Category IIb is not associated with any missions to small bodies.

² NASA, 2021, “NASA Procedural Requirements: Biological Planetary Protection for Human Missions to Mars,” NPR 8715.24, Washington, DC: NASA, September 24.

• Pre-Launch Planetary Protection Report
• Post-Launch Planetary Protection Report
• Extended Mission Planetary Protection Report
• End-of-Mission Planetary Protection Report

The document further notes that “Some missions may be able to demonstrate compliance with planetary protection requirements with a reduced document set.”³

The planetary protection categorization of NASA missions is guided by COSPAR policy and guidelines. COSPAR Policy provides additional detail (see Appendix B) on four specific documentation requirements for Category II missions:

1. A brief Planetary Protection Plan outlining intended or potential impact targets,
2. A brief Pre-Launch Planetary Protection Report detailing impact avoidance strategies, if required,
3. A brief Post-Launch Planetary Protection Report detailing actual trajectory and any updates of previous documentation, and
4. An End-of-Mission Report providing the final actual disposition of the launched hardware and impact location.

Documents 2, 3, and 4 can be viewed as a progression. Prior to launch, Document 2 captures all the essential information required of the mission for planetary protection purposes. Document 3 can either supersede or supplement that information, including post-launch trajectory data. Likewise, Document 4 either supersedes or supplements Documents 2 and 3 with summary updates at the conclusion of the mission. If the intent of a mission is to fly by or orbit the target body, but not land on the target body, then the information requested will include a discussion of the approach to avoiding unintentional impact with the target or any other Category II object. For some Category II missions, the NASA Office of Planetary Protection has required additional trajectory analysis to demonstrate a sufficiently low risk of impact (and potential contamination) of other solar system bodies, particularly Mars and Europa.

Past practice within NASA has been that the specifics of the documentation required for a Category II mission is detailed in a Categorization Letter issued by the Office of Planetary Protection.

ACCESS TO PLANETARY PROTECTION DOCUMENTATION

Planners for future small-body missions intending to study the processes of chemical evolution and the origin of life need to know what target bodies have been visited and whether any target body, as the result of a spacecraft landing or impact, may have been contaminated by material brought from Earth that would compromise their planned observations. There may be an interest in avoiding any previously visited target or landing site to ensure pristine scientific measurements. Alternatively, there may be benefits to returning to a previously visited target with the objective of continuing studies and/or operations, or minimizing mission costs by returning to a known environment using heritage spacecraft systems. In either case, knowledge of previous mission impact targets and locations, intentional or inadvertent, is important. The previous section of this chapter describes the relevant documentation to provide this information.

COSPAR recommends and NASA currently requires this information for Category II missions. Yet there does not appear to be a central repository to archive and retain the data for either current or future

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³ NASA, 2021, “NASA Procedural Requirements: Biological Planetary Protection for Human Missions to Mars,” NPR 8715.24.3.1.1.6, Washington, DC: NASA, September 24.

needs. Given the increasing number of small-body missions (Table 2-1), such an archive is quickly becoming important. Moreover, past missions have been sponsored by large space-experienced government agencies that have their own archival requirements as well as collaborative agreements to facilitate information sharing. The future, however, is likely to bring many more small-body missions, including those initiated by new entrants to space exploration, such as private-sector entities with unknown archival standards. These changes argue for a cooperative archival effort, which could be accomplished by the establishment of a common repository.

Ideally, such a planetary protection document repository would include:

• Planetary protection documents from all space missions (international and domestic, government-sponsored, and private-sector), including the location of organic inventories if such inventories were required.
• Established standards for data storage.
• Established processes for archival, retrieval, and maintenance.
• International access with provisions to ensure the protection of proprietary and sensitive data.
• Appropriate authority to establish, maintain, and operate the archive.
• Agreement for sustained funding to maintain and upgrade the archive as needed in perpetuity.

NASA’s Planetary Data System (PDS) sets an example of how such a repository might be designed and executed. Adding a new node to the PDS to accommodate planetary protection documents, might provide an efficient and cost-effective option to fulfil this archival need.

Such a planetary protection repository would benefit all missions interested in understanding the process of chemical evolution or the origin of life, regardless of their planetary protection category.