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Lunar site selection

How NASA chooses where on the Moon to land and operate. The only source the wiki holds that is devoted to it is the 2023 white paper "Lunar Site Selection" (data-gap pages such as DN-014 L mention site selection in passing): an "iterative process" that balances "where we want to go" (the objectives) with "where we can go" (safe landings), shifting as the architecture moves from Human Lunar Return to the later segments. Rev C's own word on location is the legacy decision LD-03-L, the South Pole region, made before the architecture existed. This page holds the 2023 paper's content; it is context, older than Rev B and Rev C, and doesn't mention the Moon Base.

Where Rev C stands

Rev C settles the region, not the sites. LD-03-L: "The lunar South Pole region will be the initial landing area for crewed missions, enabling NASA to take advantage of its unique lighting conditions and the possible presence of volatiles (e.g., ice)" (ADD Rev C, p. 74). Its reasoning (p. 190) and the related decisions on NRHO (LD-04-L) and surface stays (LD-07-L) are on Key definition tasks. The South Pole's conditions as the Moon Base sources describe them are on Moon Base environment.

In the 2023 Lunar Site Selection white paper

From the paper, a 2023 Architecture Concept Review paper filed with the February 2024 workshop briefings.

Where we want to go, where we can go

"Selecting sites for lunar operations requires identifying locations that would enable stakeholders to address one or more of NASA's Moon to Mars Objectives: in essence, 'where we want to go,' balanced with locations where safe lunar landings can be conducted, or 'where we can go'" (p. 1). Human Lunar Return missions "will need to find safe landing locations close to the intended destination of surface operations as new systems are tested for the first time"; later, more accurate landings, longer traverses and longer missions "will relax the need for proximity", and "'where we want to go' will influence requirements for new systems" (p. 1).

Objectives decide "where we want to go". Some need no particular place ("observations of the human response to the lunar environment or gravity transitions"); others need "access to lunar volatiles in persistently or permanently shadowed regions or locations near multiple diverse terrain types". Longer stays favor "sustained access to greater-than-average amounts of sunlight", and "NASA must achieve a balance between visiting previously unexplored terrain and developing routine and repeatable presence at select locations" (p. 1).

Lunar conditions

  • Terrain and data. Initial landings need "relatively flat terrain, with only small blocks and impact craters that are within the lander's hazard tolerance", which also suits early EVAs. "The highest resolution image data for the Moon has a resolution of roughly a single meter, but this resolution is not universally available across the polar regions" (p. 2).
  • Lighting. Early landings will be at times when the site "is largely sunlit throughout the entire mission", so "the initial Human Lunar Return landing site should be sunlit for approximately 6–6.5 days". "There is no known location in the South Pole region that is continuously sunlit"; higher terrain, and hardware raised off the surface, get more sunlight; the sunlight ratio "can vary significantly over short distances", so "the concept of a lunar day/night cycle at the poles is not consistent across the region". A desired site may be dark when a mission launches, "brief or extensive, lasting weeks or months" (p. 2).
  • Seasons. "The Moon will experience roughly 11 seasonal cycles per 10 Earth years", so the best months for lighting at the South Pole shift against the Earth calendar. "Increasing capability to land in all lighting conditions will enable additional site opportunities" (p. 3).
  • Earth visibility. Before relays, a Human Lunar Return site "would likely need to depend on direct-to-Earth communications", so Earth must be in the sky. At the poles, on the limb, visibility varies; past the pole toward the far side Earth "may only be visible from high-elevation terrain", and low terrain on the near side may lose it for periods. Relays "will enable more site selection options", and traverses need the same planning as landings (p. 3).

End-to-end mission availability

Site conditions combine with when the vehicles can fly (pp. 3–4):

  • SLS Block 1 goes first to an elliptical low-Earth orbit, so EGS, SLS and Orion "can only achieve lunar orbit for roughly half of the Moon's orbit around Earth, nearly centered around the Moon's minimum lunar declination".
  • Orion's arrival in NRHO must leave time for crew preparation, and the HLS "is expected to be viable to conduct a lunar landing for about 90 days" there, so "the crew must arrive within that window". "Carrying multiple landing site options maximizes the likelihood of a successful landing across a calendar year."
  • SLS Block 1B inserts into a circular orbit, removing Block 1's constraint, but Orion must ferry the co-manifested payload to NRHO, and "the mass of that payload can significantly affect mission availability".
  • Later missions "benefit from the presence of Gateway and a lunar relay", which ease direct-to-Earth limits, "ultimately opening additional lunar site availability". Contingency protections "further restrict overall mission availability", a "risk-informed decision".

Figure 3 puts "Mission Availability" at the center of six overlapping considerations: launch performance, transit eclipsing, site accessibility, communication, rendezvous and transit performance (p. 4, image checked).

How site selection evolves

In Foundational Exploration and Sustained Lunar Evolution, "reusable surface assets are likely to be consolidated at one or more locations, which will have an impact on where we land". Communications will reduce the need for Earth visibility; terrain knowledge "might lead to landing options in regions that are partially or entirely dark". Repeat landings at one place mean "that site selection drives the mission". New concerns arrive with infrastructure: hardware left on the surface as an obstacle, "plume surface interactions that landers create during descent and ascent", and deployed hardware blocking sunlight for other elements. "All partners operating on and around the Moon will need to consider these factors" (pp. 4–5).

The paper's three take-aways (p. 5): physical and environmental conditions ("surface roughness and slope, lighting, and Earth visibility") strongly influence site selection; "the early Human Lunar Return mission sites will largely depend on when the crew launches"; and as infrastructure is emplaced, "accessibility to sites of interest should increase and establish a stronger link to NASA's Moon to Mars Objectives".

What changed by Rev C

The wiki's comparison:

  • The region is unchanged. Rev C keeps the South Pole (LD-03-L) and NRHO (LD-04-L). The paper's 6–6.5 sunlit days sit beside LD-07-L's first sorties of "approximately 6 Earth days" and its NRHO "of around 6.5 days" (ADD Rev C, pp. 74–75, 191).
  • Rev C has no section on how sites are chosen (search of its text for "site selection" and "landing site"). It says NRHO "links Gateway with other aspects of the Artemis architecture, such as landing site selection" (p. 190), and its Non-polar Lunar Sortie reference mission notes that "several objectives — particularly those related to science and utilization — could require landing sites beyond the South Pole", flown "opportunistically" (p. 30). The paper's objectives-first logic is the ADD's general "architecting from the right" (Objective decomposition).
  • Data needs. Rev C's data gaps include sub-meter imaging (DN-001 L), elevation maps (DN-002 L), thermal mapping (DN-003 L) and rock sizes (DN-014 L) for South Pole sites, and plume surface interaction (DN-017 L). They sit beside the paper's "roughly a single meter" image data and its plume concern; the paper names no data gap, and the pairing is the wiki's.
  • Vehicle plans have moved. The paper's Artemis III (Orion meeting Starship in NRHO, the last Block 1 flight) and its reliance on Gateway are superseded by the 2026 sources; see the source page.

Key definition tasks · Human Lunar Return · Foundational Exploration · Moon Base environment · #0301 Systems to Survive Extended Lunar Shadow · C&PNT Systems

Sources