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Humans to Mars (H2M)

Summary. The fourth segment: "Initial capabilities necessary to establish a human presence on Mars" (ADD Rev C, p. 23). It covers the first crewed missions to Mars: getting there, landing, and returning safely. Its use cases and functions are decomposed (Appendix B), but reference missions and element mapping are "forward work" (p. 38). 44 of the 57 technology gaps list it, 11 of them only this segment, as do all six Mars data gaps. Lunar segments do Mars-forward work in parallel (p. 22). Four 2024 white papers cover crew numbers, surface power, landing and ascent propellant (below).

All page numbers below are ADD Rev C unless marked otherwise. For what segments are, see Segments. The code H2M is from the ADD acronym list (pp. 292–293), and the gaps index uses it too.

Description

  • "The Humans to Mars segment captures the capabilities, systems, and operations necessary to enable the initial human exploration of the Red Planet. These systems will represent transportation, logistics, utilization, and more. This segment is an enabling capability of continued deep space exploration, with additional efforts to be identified as the architecture matures" (p. 23).
  • "Building on previous segments, this segment will include the initial capabilities and systems necessary to safely travel to Mars, land on its surface, and return safely to Earth. After landing humans on the Red Planet, NASA will prepare for progressively longer and more complex missions there" (p. 37).
  • Mars-forward work happens earlier. Segments "are not exclusively serialized", and lessons from one destination inform another, "i.e., Mars-forward activities performed during lunar segments" (p. 22). Foundational Exploration is where the ADD places Mars analogs and risk reduction (FE page).

Objectives

"These objectives include cross-cutting science and operations goals, as well as Mars-specific infrastructure and transportation and habitation goals" (p. 37). Codes are quoted as printed; prefixes are the goals on Objectives: the ten goals.

  • Science (p. 37). Many science objectives apply at both destinations. Mars adds "investigating the origins of life and the existence of past or present life elsewhere in the solar system (LPS-4), which cannot be addressed on the Moon".
  • Infrastructure (p. 38). "developing power (MI-1); communications (MI-2); and position, navigation, and timing (MI-3) capabilities for human Mars exploration, as well as demonstrating ISRU capabilities (MI-4)".
  • Transportation and habitation (p. 38). "transportation systems that can operate between Earth and the Mars vicinity and surface (TH-5, TH-6), systems enabling crew to explore and conduct science on the Martian surface (TH-7), systems that pair human and robotic explorers (TH-10), and systems that return cargo from Mars to Earth (TH-12)".
  • Operations (p. 38). "While there are no operations objectives unique to Mars, Mars and lunar operations are closely intertwined, with lunar operations providing extensibility for Mars missions. Mars missions also have unique operational considerations, especially due to increased distance from Earth."

These are the ten objectives the 2022 Moon to Mars Objectives marks Mars-only (M). Their full wording is on Objectives (Moon to Mars Objectives, pp. 6, 9–10). The objectives marked LM apply to Mars as well as the Moon.

Use cases and functions

"The objective decomposition for the Humans to Mars segment follows the same philosophy as the lunar exploration segments" (p. 38). The ADD's worked example: objective TH-04-M, "Develop in-space and surface habitation systems for crew to live in deep space for extended durations, enabling future missions to Mars", drives characteristics and needs to "allow crew to live in deep space, manage crew health and performance, and conduct missions on the Martian surface". The use cases cover crew living "in deep space and on the Martian surface for varying durations"; the functions include "managing crew health and performance, enabling crew exercise, managing waste, and maintaining a pressurized environment" (p. 38). The 2022 document marks this objective TH-4 LM, both bodies, and prints "habitation system(s)" (Objectives). The method is on Objective decomposition.

The full Mars decomposition is in Appendix B.2.3–B.2.4 (pp. 127–147) and the Mars decomposition spreadsheet (December 2025): 46 objectives, 148 Mars use cases, 263 Mars functions, with no segment label and no element allocation. In it, TH-04 LM's use cases are crew staging and assembly in Earth vicinity, habitation and crew living in deep space and on the surface, uncrewed operation, and crew transfer between habitat and outside (nine in all). The wiki's pages:

Reference missions and elements

  • Reference missions: "Future work for this segment will include defining reference missions and detailed concepts of operations. Many historic Mars architecture studies have developed reference missions and examined the Mars trade space" (p. 38). The ADD points to the Mars architecture studies on NASA's architecture website (footnote 11). That page lists seven documents from 1989 to 2014, among them Design Reference Architecture 5.0 (2009) with two addenda, the 1997 Reference Mission and the 1989 "90-Day Study" (Mars Architecture Studies page). The list is held; the documents themselves are not.
  • Elements: "Mapping of elements to use cases and functions for the Humans to Mars segment is forward work. Mars elements fall into the similar sub-architectures as existing lunar exploration elements, which helps NASA assess the extensibility of lunar systems for Mars exploration, or vice versa" (p. 38).
  • Roadmapping. "Decisions about the design of certain elements and sub-architectures will significantly influence other elements and sub-architectures and could impact agency-level values such as cost, risk, and schedule. NASA has developed an architecture-level roadmapping process that considers the end-to-end architecture when evaluating options across the trade space" (p. 39). Section 3.1 describes it: see Architecture definition process. Four completed key definition tasks are Mars ones (MD-07 surface power, MD-02 target state, MD-04 loss-of-crew risk methodology, MD-05 "no fewer than four (4) crew to the surface"; pp. 75–76; Key definition tasks). The "Architecture Definition" white paper (Dec 2025) prints a snapshot of twelve Mars priority key definition tasks, MD-01 to MD-12, with these four shaded as closed. It says NASA's review of heritage Mars studies and its objective decomposition "identified nearly 100 candidate definition tasks for the Mars architecture" (white paper, pp. 4–5; Key definition tasks). ADD Appendix C's table of open key definition tasks holds 71 (67 open, the four above done), all Mars or destination-neutral, from science priorities to ascent propellant and "Inspiration Priorities" (pp. 194–197; Key definition tasks).
  • The target state, narrowed (MD-02). Appendix C says what the 2025 down-select left: the options combined "number of crewed surface missions, surface mission duration, and number of sites visited", and the down-select "removed all single-mission scenarios and the most expensive scenario, leaving three remaining target state options: multiple short-duration missions that return to the same site, multiple long-duration missions that return to the same site, and multiple short-duration missions to different sites" (p. 193). All three keep more than one crewed surface mission in the segment, and NASA said so at the January 2026 workshop: "The Initial Human to Mars Segment will be scoped to consist of more than one crew landing", with "Additional assessment needed" on the same three options (2026 industry and academia workshop, slide 36).
  • Crew to the surface. The companion 2025 Architecture Update explains MD-05: the floor of four rests on the EVA "buddy" rule, the "one-way time delay of up to 22 minutes" that forces Earth-independent operations, and the vehicle mass, consumables and cost that grow with every extra crew member. (The "Why Moon and Mars?" white paper, from the same month, gives "between 4 and 24 minutes" (white paper, p. 2); open question 31.) Whether more crew stay "in Mars orbit" is still open (Update, p. 13). The Update says six is considered as an upper figure ("up to six"); the ADD says "no fewer than six" (pp. 76, 193); a January 2026 workshop slide says "the minimum to be up to six (6)" (slide 31). All three versions, and the reasoning briefed at that workshop, are on Key definition tasks.
  • 2026. One of the Update's three focus areas for 2026 is "examining the communications and navigation needs of crewed Mars missions". NASA also plans to conduct "architecture definition tasks that bring the first human missions to Mars into focus" (Update, p. 18).

Areas of future work

"The functional mapping for this segment remains forward work" (p. 39). The areas, as printed:

  • "Assessing the Mars trade space, including transportation; return propellant strategies; surface infrastructure needs; entry, descent, landing, and ascent options; and more"
  • "Establishing science objective priorities, including inputs from an ongoing study by the National Academies of Science, Engineering, and Medicine" (footnote 12 names it "A Science Strategy for the Human Exploration of Mars")
  • "Researching human health and performance in the context of Mars missions"
  • "Defining data and knowledge gaps for the Mars architecture and evaluating pathways to close those gaps"
  • "Defining an initial human Mars campaign driven by NASA's three pillars of exploration (science, inspiration, and national posture)"

NASA's Mars trade space web page

NASA's "Mars Architecture Trade Space" page, which the 2025 Update links (p. 13), sets out the open questions in plain words. Undated; fetched 2026-10-01 (source page).

  • What it is. "the trade space is the range of possibilities for a mission or campaign. The Mars trade space represents the many approaches that NASA could take to land the first humans on the Red Planet. Currently, this space is relatively open." The fission decision is its example of a choice that narrows it ("Introduction").
  • Five areas: "Getting to Mars" (transportation: "chemical, nuclear electric, nuclear thermal, solar electric, or hybrid systems", perhaps "one type for human transportation and another for cargo missions"); "Landing on Mars" (EDL: lander size, how to "slow down and land softly", "the order in which NASA conducts individual landings"); "Living on Mars" (crew systems: suits, communications, food, water, clothing); "Working on Mars" (surface systems, defined by "the number of crew members, the length of their stay on Mars, and their exploration goals"); "Getting Back from Mars" (ascent: NASA "has never attempted an orbital ascent from the Martian surface"; a lander that doubles as the ascent vehicle, or one "delivered before the crew arrives"; propellant from Earth or ISRU).
  • Forward work. NASA "will update the Architecture Definition Document to reflect new elements, the target state, and expected operations", and "Future white papers will provide deep dives" ("Forward Work").

The five areas line up with the ADD's areas of future work above and with the four 2024 papers below (the wiki's comparison). The page names no MD task, gap or element.

The ACR24 Mars white papers (2024)

Four papers from the 2024 Architecture Concept Review take up four Mars subjects, most of them on the list above: crew numbers, surface power, entry, descent and landing, and ascent propellant. They predate Rev C and are context. Each has a source page, and its content sits on the page for its subject:

Paper In brief Wiki home
Mars Crew Complement Considerations The number of crew to Mars vicinity and to the surface "may be different values" (p. 1); five groups of considerations; three concepts of operations (crew to vicinity only, split crew, all to the surface); no number Key definition tasks, MD-05
Mars Surface Power Technology Decision Fission chosen at the 2024 review as "the primary surface power generation technology for crewed missions to Mars" (p. 1); needs from "at least 10 kilowatts" to "megawatt (MW)-class" (p. 2); six technologies weighed; the Moon as a testbed Key definition tasks, MD-07
Mars Entry, Descent, and Landing Challenges Heritage systems, all scaled from Viking and landing about 1 t or less, "do not scale" to human-class landers, projected at 26–36 t landed (pp. 4–6); atmosphere, hazards, plume-surface interaction, no full test on Earth Transportation Systems
Mars Ascent Propellant Considerations Ascent propellant is "the single largest category of landed mass" for most proposed architectures (p. 4); no, limited and comprehensive ISRU; mass, site selection, production rate; no decision ISRU Systems

Briefings of three of the four, from the February 2025 workshops (before Rev C), are among the sources. What the slides add is on each briefing's source page; no briefing of the ascent-propellant paper is among the sources.

What the wiki reads from the four (its comparison, not the papers'):

  • One decided then, one since, three still open. Only the power paper records a decision (MD-07, ADD p. 75). Crew to the surface was decided a year later (MD-05, ADD p. 76). Crew to Mars vicinity (MD-06), the ascent propellant strategy and the two Mars EDL technology tasks are still open in Appendix C's table (ADD pp. 194–197; Key definition tasks). None of the papers prints a task name or number.
  • Two-crew cases that Rev C has since moved past. The 2024 ISRU study's "single mission of four crew, two of whom would descend to the Martian surface" (ascent paper, p. 2) and the power paper's minimum case of "two crew members" for "no more than 30 days" (p. 2) both put two crew on the surface. MD-05 now sets a floor of four, and MD-02 "removed all single-mission scenarios" (ADD pp. 76, 193).
  • The subjects depend on each other, by the papers' own account. Crew numbers drive "ascent vehicles" and "power generation" (crew paper, p. 4); making or conditioning ascent propellant needs "abundant surface power" (power paper, p. 2); ISRU "naturally constrains site selection" (ascent paper, p. 3). The crew paper points to the power decision as its example of the process (p. 3).
  • No ADD element named. The papers name concepts: a Mars "pressurized rover" and "a small crew ascent vehicle" (power paper, p. 2), a "Mars Ascent & Landing Vehicle" and "Fission Surface Power" (ascent paper, Figure 2), a projected "Human-Scale Lander" (EDL paper, Figure 5). None is an ADD element, and Rev C maps no element to this segment (p. 38). The Mars "pressurized rover" is not the lunar Pressurized Rover.

Gaps that list this segment

Derived from the spreadsheets' segment columns.

  • Technology gaps: 44 of 57 (tech gaps spreadsheet, Segments column; gaps index). Eleven list only Humans to Mars (short labels; each page has the full title): #0102 Deep Space Communications, #0104 Earth-Independent Surface PNT, #0606 Mars ISRU, #0802 Mars Dust, #0803 Mars Suits, #0902 Mars Surface Power, #1102 Mars Precision Landing, #1103 Mars EDL, #1104 Mars Transportation Propulsion, #1105 Mars Ascent Propulsion and #1202 Planetary Protection. The other 33 also list one or both lunar segments.
  • Data gaps: all six Mars gaps, DN-001 M to DN-006 M (data gaps spreadsheet, Segment column). Defining Mars "data and knowledge gaps" is also one of this segment's areas of future work (p. 39).

Moon Base relevance

Not established by the sources so far. No source maps this segment to a Moon Base phase. The Users Guide describes seven Mars-forward areas the Moon Base will advance, without using the term "segment"; see Mars-forward.

Segments · Foundational Exploration · Sustained Lunar Evolution · Mars-forward · Objective decomposition · Mars use cases · Mars functions · Mars objectives · Gaps index · Data gaps index

Sources

ADD Rev C, pp. 22–23, 37–39, 45, 75–76, 127–147, 193–197, 292–293 · Mars objective decomposition spreadsheet, "Use Cases to Objectives" (TH-04 LM) · 2025 Architecture Update, pp. 13, 18 · "Architecture Definition" white paper, pp. 4–5 · "Why Moon and Mars?" white paper, p. 2 · Tech gaps spreadsheet · Data gaps spreadsheet · Users Guide (via the Mars-forward page) · Moon to Mars Objectives (2022), pp. 6, 9–10 · Mars Crew Complement Considerations (2024), pp. 1, 3–4 · Mars Surface Power Technology Decision (2024), pp. 1–2 · Mars EDL Challenges (2024), pp. 4–6 · Mars Ascent Propellant Considerations (2024), pp. 2–4 · Mars Architecture Trade Space page · Mars Architecture Studies page · February 2025 briefings: Mars surface power (copy), Mars crew complement, slides 3, 7 (copy), Mars EDL