Key definition tasks¶

Summary. A definition task is how NASA settles an open question in the architecture: once done, it lets NASA "set an architectural ground rule, make a decision, select a methodology, or otherwise shape the architecture" (ADD Rev C, p. 72). "Key" tasks are the ones that shape the whole architecture. ADD Rev C Section 3.1.3 summarizes seven legacy decisions (LD-01-L to LD-07-L), made before the first ADD, and seven completed key definition tasks (LD-101 to LD-103 for the Moon; MD-07, MD-02, MD-04 and MD-05 for Mars). Several of the lunar decisions, such as Gateway in lunar orbit and NRHO, are touched by changes since Rev C. ADD Appendix C (pp. 186–197) gives the reasoning behind each legacy decision and completed task. Its table of "Open Architecture Key Definition Tasks" holds 71: MD-01 to MD-12 (four of them done) and 59 without a number, in 13 categories. Every one is about Mars or names no destination; none names the Moon. Among the details: MD-02's down-select left three Mars target-state options, not one (below). The "Architecture Definition" white paper (Dec 2025) prints a snapshot of twelve Mars priority key definition tasks, MD-01 to MD-12, and shades the same four as closed (below). On MD-05, the number of crew to the Mars surface, the ADD and its companion 2025 Architecture Update disagree: a floor of four with "no fewer than six" under consideration, or with "up to six" (below). As the wiki reads a chart in the 2024 executive overview, it splits the same twelve Mars tasks, under older names, into seven identified at the 2023 review and five at the 2024 review. The match by title and the split are the wiki's, not printed (below; open question 54). Two 2024 white papers give the background to MD-05 and MD-07: how the crew number was to be weighed (below), and how and why fission was chosen for Mars, from "at least 10 kilowatts" for a small mission to "megawatt (MW)-class" (below).

All page numbers below are ADD Rev C unless marked otherwise. How definition tasks fit the annual cycle is on Architecture definition process.

What a definition task is¶

"Developing an exploration architecture requires stakeholders across NASA to examine numerous individual topics via architecture definition tasks. These tasks enable the agency to examine open, undefined parts of the architecture, answer questions about the architecture, and narrow down the trade space — the range of options for exploration missions. Once complete, they enable NASA to set an architectural ground rule, make a decision, select a methodology, or otherwise shape the architecture. Definition tasks bring future exploration into focus" (p. 72).
"Key" tasks. "Every definition task is important, but certain tasks (i.e., 'key' definition tasks) significantly influence the end-to-end architecture and flow down to impact many other areas of the trade space. These warrant much more scrutiny to balance a variety of priorities and demands" (p. 72).
What gets tracked. NASA's architecture roadmapping "only includes definition tasks that significantly influence the architecture and/or require collaboration between multiple, cross-agency authorities to answer an agency-level question. Identifying and sequencing these key definition tasks ensures that NASA works efficiently and remains a smart buyer of capabilities and services" (p. 72). How the tasks are sequenced is on Architecture definition process.
Key versus ordinary, by example. The "Architecture Definition" white paper repeats the ADD's wording and adds two examples: "deciding how many crew members an architecture must accommodate influences virtually every other aspect of the architecture. It requires high-level consideration and consensus between multiple programs and projects. At the other end of the spectrum, the number of windows on a specific habitat module — though it may affect many aspects of element design — is an engineering decision that does not require the same scrutiny" (white paper, p. 3).
What an outcome can be. "The outcome of a key definition task could include a decision, down-select, ground rule, or other determination that narrows the architecture trade space. Therefore, decision-makers must understand how the possible outcomes of key definition tasks relate to one another" (white paper, p. 1). Its three kinds, with examples, are below.
Terminology. Rev C renamed "architecture decisions" to "architecture definition" (p. 4). Older documents, and the "LD"/"MD" prefixes, come from the older term (Architecture definition process). The white paper supersedes one such older paper, "Key Mars Architecture Decisions (2023)" (white paper, p. 1). Neither the ADD (Appendices C and F included) nor the white paper expands LD or MD. The titles suggest lunar and Mars; that reading is the wiki's. The terms "definition task", "trade space" and the rest are defined three times, in Appendix C, Appendix F and the white paper, not always alike (Glossary).

Legacy decisions¶

"Lunar key definition tasks include both legacy decisions (decisions NASA made prior to the initial publication of the Architecture Definition Document) and priority key definition tasks. Legacy decisions shape many aspects of the Moon to Mars Architecture" (p. 74). The ADD's one-line summaries (pp. 74–75):

ID	Title	What NASA decided (pp. 74–75)
LD-01-L	Enable Human Exploration on the Surface of Planetary Bodies	"NASA will send human explorers — not just robotic missions — to explore the Moon and Mars, laying the foundation for the agency's exploration ambitions and the Moon to Mars Architecture."
LD-02-L	Deep Space Element(s) in Microgravity for Long-duration, Crewed Exploration	"To achieve long-duration crewed exploration in deep space, NASA will deploy Gateway in lunar orbit to host crew, support crewed missions to the lunar surface, conduct science, and develop deep space capabilities (habitation, propulsion, communications, operations)."
LD-03-L	Lunar Landing Region Selection	"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)."
LD-04-L	Crewed Lunar Orbit	"Crewed lunar orbital operations will use NRHO, which enables continuous communications with Earth and consistent access to the lunar South Pole."
LD-05-L	Integrated Crewed Lunar Mission Cadence	"NASA will conduct integrated (combined orbital and surface operations) crew missions on an annual cadence, maximizing exploration opportunities while meeting the production and processing needs of NASA and its partners."
LD-06-L	Number of Crew to Cislunar Space	"NASA will send up to four crew members during an integrated mission, including both orbital and surface operations."
LD-07-L	Crewed Lunar Surface Stay Duration Capability	"Crewed lunar surface missions will target surface durations of up to 33 consecutive Earth days for initial segments, with initial missions lasting for approximately 6 Earth days."

Related wiki pages: Gateway (LD-02-L, LD-04-L); Moon Base environment (the South Pole, LD-03-L); Lunar site selection (choosing sites within the region, from a 2023 white paper; LD-03-L, LD-04-L and LD-07-L); Initial Surface Habitat (its 7–33 days sit within LD-07-L's range; the wiki's pairing).

Completed key definition tasks¶

Section 3.1.3.2 (pp. 75–76), in the order printed:

ID	Title	Outcome, as the ADD summarizes it	When
LD-101	Lunar External Power Augmentation	"The agency will pursue power augmentation trades that balance element design, aggregate power demand, total surface landed mass, mission-to-mission flexibility, and architecture robustness."	not stated
LD-102	Lunar Logistics Strategy	"The agency will pursue a hybrid strategy for delivering required logistics to elements on the lunar surface using a variety of solutions ranging from small portable carriers to large mated carriers."	not stated
LD-103	Lunar Surface Communications Strategy	"In 2025, the agency adopted a combination of Space-to-Space Communications Systems, WiFi 6, and 3GPP (5G) as the foundational technologies for the agency's lunar surface communication network."	2025
MD-07	Mars Primary Surface Power Generation Technology	"the agency selected nuclear power technology (specifically, fission power) over non-nuclear power technology (in particular, photovoltaic arrays with energy storage) as the primary surface power generation technology for the initial human missions to Mars."	2024 ACR
MD-02	Initial Human Mars Segment Target State	"the agency made a down-select for the initial Humans to Mars segment target state that outlines a vision for the segment and will guide future architecture definition tasks."	2025 ACR
MD-04	Mars Architecture Loss of Crew Risk Methodology	"the agency selected a risk methodology that uses a combination of qualitative and quantitative risk assessment to for evaluating risk at the architecture level and enabling risk-informed decision making." ("to for" as printed)	2025 ACR
MD-05	Number of Crew to Mars Surface	"the initial Humans to Mars segment will target no fewer than four (4) crew to the surface, with consideration for no fewer than six (6) crew to enhance mission capability and/or provide risk reduction."	2025 ACR

Notes:

MD-02's target state is not described here. Section 3.1.3 says only that a down-select was made. Appendix C says what it left: three options, all with several missions (below). Humans to Mars has what Section 2.1 says about the segment.
MD-07 is the Mars fission decision that the Lunar Nuclear Fission System responds to (p. 58) and that the Users Guide repeats (gap #0902).
LD-101 and the lunar power white paper. The ACR25 paper "Integrated Lunar Power Strategy Considerations" weighs the same trades (delivered mass, centralized against distributed systems, manifest dependencies, flexibility) and says NASA "will implement external power augmentation" from the Foundational Exploration segment (white paper, p. 1). It never names LD-101 or any definition task; the pairing is the wiki's. See Power Systems.
LD-103's three technologies match the 3GPP and Wi-Fi networks in gap #0103 (Surface-to-Surface Communications), which cites ✓LD-103. The ACR25 C&PNT white paper says "NASA plans to use Third Generation Partnership Project (3GPP) cellular and 802.11 Wi-Fi standards" on the surface (C&PNT white paper, p. 2). It says "802.11 Wi-Fi" rather than "WiFi 6", doesn't mention Space-to-Space Communications Systems, and doesn't cite LD-103. Appendix C says what Space-to-Space Communications Systems is (below).
The Executive Summary's "three new results". It says NASA refined its definition tasks, "including capturing three new results that will shape the future of results" (p. 9). Three entries here are dated "At the 2025 Architecture Concept Review" (MD-02, MD-04, MD-05), but LD-103 also says "In 2025". The ADD doesn't say which are meant (open question 21). The companion 2025 Architecture Update doesn't settle it either. Its list of 2025 developments (two new elements, the data gaps, and the narrowed Mars crew range; p. 3) is not a list of definition-task results, and it doesn't use the phrase.

MD-05: Number of Crew to Mars Surface¶

The 2025 Architecture Update gives this decision a page, "Defining Number of Crew to Mars Surface" (p. 13). Page numbers in this subsection are the Update's unless marked ADD.

The decision, two ways. The two documents, both from December 2025, agree on the floor of four crew but differ on six:

	ADD Rev C	2025 Architecture Update
Floor	"no fewer than four (4) crew to the surface" (ADD pp. 76, 193)	"no less than four crew members to the Martian surface"; "no fewer than four crew members" (p. 13)
Six	"with consideration for no fewer than six (6) crew to enhance mission capability and/or provide risk reduction" (ADD pp. 76, 193)	"with consideration for up to six crew to enhance mission capability and/or provide risk reduction"; "with considerations for up to six crew members to the Martian surface" (p. 13)
More on six and above	Appendix C: "A minimum of six crew — while providing crew task redundancy and mission and utilization opportunity — will increase challenge to architecture feasibility" (ADD p. 193)	"While the agency has not formally constrained the maximum number, sending a larger number of crew (e.g., seven, eight, or more astronauts) introduces new challenges" (p. 13)

Each document words it the same way both times, so neither reading looks like a one-off typo. "No fewer than six" would make six the floor under consideration; "up to six" makes six a ceiling under consideration above a floor of four. Both are dated the same month, so the newer-wins rule doesn't apply. The wiki records both (open question 28). Appendix C adds only the drivers sentence quoted below; its Result column calls the outcome an "ACR25 Decision" (ADD p. 194). The 2024 white paper on Mars crew complement gives no number, so it doesn't settle the question either (below).

Briefed at the January 2026 workshop, in two wordings. The industry and academia workshop's deck states the outcome twice. Slide 15, "Architecture Definition Outcome", has the ADD's wording word for word: "no fewer than four (4) crew to the surface, with consideration for no fewer than six (6) crew to enhance mission capability and/or provide risk reduction". Slide 31 opens the briefing on the decision with a third wording: "The Moon to Mars Initial Human to Mars Segment will target no fewer than four (4) crew to the surface, with consideration for the minimum to be up to six (6) crew for enhanced mission capability and/or provide risk reduction", with the bullets "2025 Architecture Concept Review Decision", "Documented in Revision C of the Architecture Definition Document" and "Potential Future White Paper to Provide Detail" (2026 industry and academia workshop, slides 15, 31). A briefing ranks below both December documents, so neither slide settles the difference; slide 31's wording is a third one (open question 28).

The reasoning (p. 13, from the Update only; Appendix C adds that "crew health and performance and mission and operational risk were the main drivers", ADD p. 193):

Crew health and safety. "The safety and health of astronauts are NASA's most important concern." EVAs "follow a 'buddy' flight rule, where at least two astronauts work together … A solo EVA presents excessive risk, so a mission that sends a single crew member to the surface would be unacceptable."
Operations and Earth independence. Mars missions "will need to account for a one-way time delay of up to 22 minutes", so crews "must operate with significant autonomy and independence from Earth". "While no realistic crew complement could completely replace the services of a mission control center, this need to train for multiple specialized roles drives a larger crew to achieve acceptable risk levels." A larger crew "allows for more specialization, better proficiency retention, and redundancy of role coverage."
Engineering complexity. More crew means more "mass and size of descent and ascent vehicles within launch and landing capabilities, as well as mass of additional consumables … and overall mission cost". A larger crew "quickly becomes a case of diminishing returns and increased challenges."

The reasoning as briefed in January 2026 (the workshop deck's slides 32–35, by Patrick Chai, David Baumann and Eddie Terrell of NASA; source page):

Background (slide 32): "Assessment process involved cross-agency stakeholders, technical experts, and technical authorities"; "Decision primarily driven by crew operation and risks, and the need for new paradigm".
"Three Key Findings" (slide 33): "Mars Mission needs a Different Paradigm: Mars distance induced communication delay and/or blackout forces a paradigm shift away from the smaller crewed missions of LEO / Lunar heritage"; "Surface EVA is a primary driver for crew size (Buddy Rule, EVA Cadence, etc.)"; "Crew expertise, tasking, and training, are all key drivers for crew time, and crew time will be extremely precious for Mars surface missions". Its graphic gives a Mars one-way light-time delay of "4-24 Minutes" (the Update says "up to 22 minutes"; open question 31).
The crew-number chart (slide 34), as printed: "1 crewmember cannot meet operational needs"; "2 or 3 crew are technically feasible but not recommended due higher risk, especially for extravehicular activity operations"; "3 crew needed to offer minimum necessary expertise and coverage for contingencies"; "4 to 6 crew balances opportunity, risk, and complexity, while offering appropriate scalability and flexibility for Earth-independent operations with robotic/autonomous system support"; "7 + crew present significant architecture feasibility challenges for initial missions". "NASA will target no fewer than four crew to Mars surface for architecture development." "Each additional crewmember … increases available crew time, but growing architectural complexity could diminish available crew time" and "reduces risk by spreading workload and expertise coverage and protecting against contingencies". "Every crew will need adequate Earth-independent systems, robotic capabilities, and concepts of operations to account for communications delays preventing real time support from Earth."
"Crew Health and Operation Perspective" (slide 35): "Crew workload is a significant driver for mission success and crew safety"; "Orbiting crew around Mars will affect surface crew size"; "Even number of crew preferrable over odd number" (as printed); "Caution on over-reliance of the promise of automation/AI to make up for smaller crew sizes".

New against the December documents (the wiki's comparison): the 2–3 band, the three-crew minimum for coverage, the "4 to 6" balance, the even-number point and the caution on automation/AI. The 1-crew and "7 +" bands echo the Update's prose (a single crew member to the surface "would be unacceptable"; "seven, eight, or more astronauts" bring "new challenges") and the ADD's "will increase challenge to architecture feasibility" (p. 193). The chart doesn't say whether six is a floor or a ceiling under consideration, so it doesn't settle question 28 either.

What stays open. "This surface contingent does not necessarily limit the total number of crew in the future, but establishes a key lower bound for number to the surface that balances risk and achieving mission objectives. As the Mars architecture continues to develop, the agency could choose to send additional astronauts to remain in Mars orbit; that part of the trade space remains open" (p. 13).

The "Architecture Definition" white paper uses MD-05 as its example of an outcome that is "a reduction of the trade space to a viable or plausible range" (white paper, p. 5). That fits either wording, so it doesn't settle open question 28. Its Table One also lists a separate task, not shaded as closed: MD-06 "Number of Crew to Mars Vicinity Per Mission" (below).

Compare the lunar decision LD-06-L, "up to four crew members during an integrated mission" (above). See also Humans to Mars.

The 2024 crew-complement white paper¶

The ACR24 paper "Mars Crew Complement Considerations" (source page) prepared this decision a year before it was made. It prints no MD number and no number of crew. Its subject is "the number of crew to Mars vicinity and the Martian surface — which may be different values" (p. 1), which are the titles of MD-06 and MD-05; the pairing is the wiki's. Page numbers in this subsection are the paper's.

Why decide early. Crew complement has "flow-down impacts on most elements and sub-architectures", and "it was identified by NASA as a priority decision in the 2023 Moon to Mars Architecture white paper, 'Key Mars Architecture Decisions'" (p. 1). It drives habitable volume, life support, power, logistics and "human-rated ascent and descent vehicles". "In determining crew complement, it is important to look beyond just the first mission towards what the desired end state for the architecture is. For example, the first Space Shuttle flight only carried two astronauts, but the vehicle was designed to accommodate more" (p. 1).
Architecting from the right. "Historically, crew complement has been a secondary consideration defined by the capabilities of preselected exploration elements." The method of the 2023 strategy document instead lets NASA "evaluate the drivers and flow-down impacts of crew complement to identify the number of crew needed to achieve Moon to Mars objectives" (p. 1; see Objective decomposition).
Five groups of considerations (pp. 1–3), headings as printed:
- Crew Health and Performance. Human system risks "include crew behavioral health, team dynamics, probability of crew medical conditions (and duration of associated care), and integration of the human system with other exploration systems" (p. 1). "Human systems integration is perhaps the most complex of these risks" (p. 2).
- Crew Responsibility. "There are practical limitations on how many in-mission responsibilities a single crew member can support." Because of the delays, crew "will need to accomplish many responsibilities traditionally performed by terrestrial mission control" (p. 2).
- Crew Workload. A notional pie chart of crew time on a Humans to Mars surface mission, with ten labels from Science & Exploration to Sleep and no values ("All values notional"; p. 2). "Autonomous technologies and systems could reduce crew burden and, ergo, the minimum crew complement needed to support exploration systems and functions", if identified "early enough in the mission design process" (p. 3).
- Mission Concept of Operations. Three concepts: crew to Mars vicinity only, a split crew, and all crew to the surface (Figure 2, p. 3). "If the entire crew lands on the surface, any vehicles remaining in space would need to remain uncrewed and may increase vehicle autonomy needs." "Sending a crew member on an EVA alone or leaving a crew member alone in a habitation element while others are on EVA may result in unacceptable risks" (p. 3).
- Mission Complexity and Value. More crew may give "economy of scale"; a smaller crew drives "Earth-independent systems"; a decision "may reveal a need for precursor missions to demonstrate certain integrated systems"; more crew may need larger "crew training and Earth reconditioning facilities"; and "programmatic, administrative, budgetary, and schedule constraints" weigh in (p. 3).
What would come next. "NASA will develop an integrated decision package that includes analyses and recommendations for review and approval by agency leadership as part of the annual Architecture Concept Review process" (p. 3). The paper points to the Mars power decision as "an example of a key driving decision made using this process" (p. 3; below).

What changed by Rev C (the wiki's comparison):

The surface half is decided; the vicinity half isn't. MD-05 set a floor of four to the surface at the 2025 review (ADD pp. 76, 193). MD-06, crew to Mars vicinity, is still open in Appendix C's table and the white paper's Table One, and the Update says "that part of the trade space remains open" (Update, p. 13).
The 2025 reasoning echoes the paper's. The Update's EVA "buddy" rule ("A solo EVA presents excessive risk") matches the paper's warning about a lone EVA. Its need to "train for multiple specialized roles", because no crew can "replace the services of a mission control center", matches the paper's crew responsibilities. Its vehicle mass and consumables match the paper's flow-down to ascent and descent vehicles (Update, p. 13). No source says the Update drew on the paper. Appendix C's "crew health and performance and mission and operational risk were the main drivers" (ADD p. 193) names two of the paper's themes.
No number, so no help with six. The paper bounds the crew neither above nor below. It doesn't settle "no fewer than six" against "up to six" (open question 28).
A 2023 origin. Its statement that crew complement was named a priority decision in 2023 fits the wiki's reading of the 2024 overview's chart, which puts MD-05 and MD-06 on the ACR23 side (below; open question 54). It doesn't say which MD number goes with which 2023 decision.

ADD Appendix C: the full lists¶

Appendix C, "Architecture Definition" (pp. 186–197; p. 198 is the Appendix D divider), has two parts. C.1 describes architecture roadmapping: its terms, its value, and the process. That part is on Architecture definition process, and its terms are compared on the Glossary. C.2 holds the lists: "The lists below capture legacy decisions and completed and open key definition tasks. Subsequent revisions of this document will update these lists as NASA completes key definition tasks and identifies new ones" (p. 189). Page numbers in this section are ADD Rev C. The tables on pp. 194–197 were checked against the PDF.

Legacy decisions: the reasoning (C.2.1)¶

"Legacy lunar architecture decisions, introduced in Section 3, are decisions that were made prior to the establishment of the Moon to Mars Architecture and its architecture roadmapping approach. The list below presents these decisions with additional detail and rationale" (p. 189). What each entry adds to the one-line summaries above:

LD-01-L, human exploration of planetary surfaces (p. 189). It "lays the foundation for subsequent definition tasks spanning the entire 'blueprint' for space exploration". What follows from it: "Longer-duration missions, with increased exposure to radiation and varying gravity fields, will have unclear effects on the human body. Communications delays highlight the need for increased Earth independence."
LD-02-L, a deep space element (pp. 189–190). Gateway "will serve as an aggregation location for crewed vehicles and logistics payloads, and as a deep space laboratory". It constrains the surface: "The Gateway decision, alongside the decision to utilize NRHO, constrains surface operations and crew size. Lunar surface access is partially dictated by the orbit of Gateway, and the crew size capability of both Gateway and Orion constrain how many crew will operate on the lunar surface." For Mars it is "a testing platform for Mars-focused operations … while offering a rapid return capability to Earth".
LD-03-L, the South Pole (p. 190). "Key factors informing this decision include access to areas of both near-continuous sunlight and continuous darkness, access to ancient terrain, and access to completely unexplored areas." On power: "Systems developed for use in a near-continuous solar environment will not need excess capability designed to survive lunar night, decreasing complexity and power requirements." On science: volatiles preserved in permanently shadowed regions, and the South Pole-Aitken Basin, "the oldest impact basin in the solar system". On Mars: "The Martian poles contain ice, meaning work completed on the lunar surface will directly inform how Mars science objectives are completed." The wiki's pairing: the sunlight is near-continuous, not continuous, and the third-highest technology gap, #0301, is surviving 350 hours or more of continuous shadow at the same region.
LD-04-L, NRHO (p. 190). "NRHO's Earth-facing orientation allows for continuous communications, while also providing line of sight and coverage to the lunar South Pole". It is "relatively easy to sustain", suits "Mars-forward technologies, such as large-scale solar electric propulsion systems", and "drives downstream design requirements for Gateway, HLS, and lunar communication systems".
LD-05-L, annual cadence (pp. 190–191). It "originates from early requirements in the Constellation Program, specifically targeting a minimum mission rate of two crewed lunar missions per year". After Constellation's cancellation in 2011, "overall production capability was rescoped to provide for a yearly launch cadence, with the potential for surges of up to two missions each year". It "does not directly affect the Moon to Mars Architecture", but "an annual launch cadence will also serve as an analog for future Mars missions, which can only depart every 26 months".
LD-06-L, four crew (p. 191). Traced to "the 2005 Exploration Systems Architecture Study (ESAS)", which "baselined the CEV [Crew Exploration Vehicle] for up to four crew for lunar missions"; the CEV "eventually became what is now known as Orion". The benefits of "four crew split between orbit and surface include safety, risk management, expanded operational capabilities with surface monitoring, more efficient use of resources, support for longer surface missions, and refining techniques and lessons learned for future Mars missions."
LD-07-L, up to 33 days (p. 191). "For early lunar surface missions, crew will complete a sortie of around 6 days. This aligns with the NRHO orbit of around 6.5 days, ensuring reliable surface arrival and departure." And: "A 33-day lunar surface stay can also provide lessons learned for Mars exploration, because it closely matches the minimum Mars surface stay duration of 30 sols."

Completed tasks: what Appendix C adds¶

C.2.2, pp. 192–193. MD-05's entry is quoted above.

LD-101, lunar power augmentation. Why it was needed: "The Foundational Exploration segment is constrained by the amount of energy available to power crew life support systems, provide keep-alive support to surface elements, utilization payloads and equipment, and to make, move, or environmentally maintain critical infrastructure." What it did not decide: "This outcome does not down-select between technologies, sizing, or concepts; future studies will address these topics."
LD-102, lunar logistics. Logistics items are "food, water, air, spare parts, and other similar products", and their total "can be relatively large. A hybrid strategy that uses smaller crew portable carriers as well as larger mated logistics carriers enables the agency to maintain flexible and robust means to support these missions."
LD-103, lunar surface communications. "In 2025, the agency formally adopted a combination of three foundational technologies for the agency's lunar surface network. They are Space-to-Space Communications Systems, an Ultra High Frequency Time-Division-Multiple Access system designed to provide voice, commands, telemetry and data services in close proximity to the International Space Station; WiFi 6, an Institute of Electrical and Electronics Engineers networking standard from the nonprofit WiFi Alliance; and 3GPP — 3rd Generation Partnership Project — (5G), a technical specification for mobile networks." The aim is "high-bandwidth, high-availability communications to support long-term science, exploration, and industry needs."
MD-07, Mars surface power. Fission is "baselined as the primary surface power generation technology in the initial Humans to Mars architecture segment". "Mitigating loss-of-mission risk was the primary driver for this selection: although solar power may have a lower per unit cost, fission power is more robust to Martian environmental and atmospheric conditions, providing consistent power generation across a wide range of potential landing sites, around the clock, and during global dust storms, and a landed mass and volume advantage at the power levels needed for human Mars exploration." The entry's heading has "Mars Primary Surface Power Generation Technology" (p. 192); the table on p. 194 has "Primary Mars Surface Power Generation Technology".
MD-02, Mars target state. "The architecture options for the Mars target state at this stage included various combinations of three primary parameters: number of crewed surface missions, surface mission duration, and number of sites visited. Based on various factors of benefit, risk, and cost, a trade space 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." So the "down-select" narrowed the field; it did not pick one. Briefed at the January 2026 workshop under the label "INITIAL HUMAN MARS SEGMENT TARGET STATE": "The Initial Human to Mars Segment will be scoped to consist of more than one crew landing. Additional assessment needed to evaluate the following options: Multiple short-duration missions to a single site; Multiple short-duration missions to multiple sites; Multiple long-duration missions to a single site" (the same three options; 2026 industry and academia workshop, slide 36).
MD-04, loss-of-crew risk method. It "outlines a combination of both quantitative and qualitative assessment to evaluate loss-of-crew risk at the architecture level", to be "applied to future key definition tasks, architecture trade studies, and an annual architecture risk assessment update", and "continually improved over time, in particular with the subsequent MD-08: Mars Architecture Loss of Mission Risk Methodology definition task". The two risks are defined in the Glossary ("Loss of Crew", "Loss of Mission", p. 299). The January 2026 workshop deck shows "Established an architecture level methodology to evaluate crew risk across the vast architecture trade space" under the label "MARS LOSS OF CREW RISK POSTURE", the 2024 overview's name, which the wiki matches to MD-04 (below; slide 36).

Open key definition tasks (C.2.3)¶

The note above the table (p. 194):

"Key definition tasks captured here exist in various states of maturity. Through the roadmapping process, NASA identified certain key definition tasks to work in the near term and therefore prioritized the maturation of those tasks. As NASA has begun work on these priority key definition tasks in the near term, they have been assigned an identification number and appear at the top of the list in the table below. The remaining identified tasks that do not yet have a number are grouped by category and alphabetically. The order does not imply prioritization. NASA may add, remove, or modify key definition tasks in future updates to the Architecture Definition Document. Completed tasks are listed in Section C.2.2 and indicated in the table below."

The numbered tasks (p. 194, PDF checked). The four completed rows are shaded and carry a "Result". The last column is the wiki's search of the tech-gap sheet, from below.

ID	Title, as printed	Category	Result	Cited by a gap
MD-01	Initial Human Mars Segment Science Objectives Priorities	01 Science		#1201
MD-02	Initial Human Mars Segment Target State	02 Overall Strategy	ACR25 Down-Select	no
MD-03	Initial Human Mars Segment Mission Cadence	02 Overall Strategy		no
MD-04	Mars Architecture Loss of Crew Risk Methodology	03 Overall Risk Posture	ACR25 Methodology	no
MD-05	Number of Crew to Mars Surface	04 Human Systems & Habitation	ACR25 Decision	no
MD-06	Number of Crew to Mars Vicinity Per Mission	05 Human Systems & Habitation (as printed)		no
MD-07	Primary Mars Surface Power Generation Technology	05 Surface Systems & Infrastructure	ACR24 Decision	#0902
MD-08	Mars Architecture Loss of Mission Risk Methodology	03 Overall Risk Posture		no
MD-09	Maximum Mars Crew Surface Stay Duration	07 Surface Operations		#0401, #0402
MD-10	Mars Forward Contamination Planetary Protection Risk Posture	02 Overall Strategy		#0304, #0702, #0803, #1202
MD-11	Mars Backward Contamination Planetary Protection Risk Posture	02 Overall Strategy		#0304, #1202
MD-12	Maximum Allowable Crewed Communications Disruption	11 C&PNT		#0102, #0104, #1002, #1003, #1004

MD-06's category number, 05, goes with "Surface Systems & Infrastructure" everywhere else in the table; its category name, "Human Systems & Habitation", goes with 04. The wiki counts it under the name.

The 59 tasks without a number (pp. 194–197, PDF checked), by category, in the printed order. The "named by a gap" split is the wiki's search of the tech-gap sheet's Definition Tasks column, which shortens four titles to "Cargo Mars EDL Technology", "Crew Mars EDL Technology", "EDLA Systems Reuse Strategy" and "EDLA and In-Space Transportation Systems Functional Split".

Category	Named by at least one gap	Named by no gap
01 Science (4 with MD-01)	Mars Sample Analysis Strategy; Science Support Platform (Mars Surface)	Science Support Platform (In-Space)
02 Overall Strategy (6)	Maximum Total Crew Mission Duration; Minimum Mars Surface Element Design Lifetime
03 Overall Risk Posture (3)		Architecture Level Contingency Protection Posture
04 Human Systems & Habitation (9, counting MD-06)	ECLSS Strategy (In Space); ECLSS Strategy (Mars Surface); Food Refrigeration and Growth; Waste and Trash Disposal Strategy (In Space); Waste and Trash Disposal Strategy (Mars Surface)	Safe Haven Strategy (In Space); Safe Haven Strategy (Mars Surface)
05 Surface Systems & Infrastructure (7)	Crew Surface Mobility Strategy; Surface Ingress/Egress Strategy; Source of Mars Crew Consumables; Surface Construction Priorities; Surface Servicing Capability Strategy	Mars Landing Support Infrastructure
06 Surface Infrastructure Deployment Strategy (6)	Small Cargo Delivery, Stowage, and Return Strategy; Logistics Carrier and Interface Strategy	Surface Habitation Mobility Strategy; Logistics Deployment Timing (Mars Orbit); Logistics Deployment Timing (Mars Surface); Site Planning Schema
07 Surface Operations (5)	Exploration EVA Schema; Surface EVA Capability Strategy	Crew Landing Region; Number of Crew on Surface EVA
08 In-Space Systems & Infrastructure (3)	In-Space Refurbishment Capability Between Missions; In-Space Servicing Capability Strategy	Primary Mars In-Space Power Generation Technology
09 In-Space Transportation (10)	Cargo In-Space Propulsion Type; Crew In-Space Propulsion Type; Crew In-Space Return Propellant Strategy; Crew Mars Parking Orbit; Entry, Descent, Landing, Ascent and In-Space Transportation Systems Functional Split; In-Space Transportation Systems Reuse Strategy	Cargo Mars Orbit Capture Strategy; Crew Earth Capture Strategy; Crew Mars Orbit Capture Strategy; Element Delivery Strategy
10 Entry, Descent, Landing, Ascent (9)	Cargo Mars Entry, Descent, and Landing Technology; Crew Mars Ascent Availability; Crew Mars Ascent Propulsion Type; Crew Mars Ascent Propellant Strategy; Crew Mars Descent Availability; Crew Mars Entry, Descent, and Landing Technology; Entry, Descent, Landing, Ascent Systems Reuse Strategy	Crew Earth Ascent Vehicle Strategy; Crew Earth Descent Vehicle Strategy
11 C&PNT (6 with MD-12)	Surface PNT Strategy; Crew Communications Architecture; Minimum Crew Communications Capability; Minimum Sustained Communications Capability	Critical Events Communications Strategy
12 Robotics & Autonomy (2)	Priorities for Crew vs. Robotic Tasks Strategy	Priorities for Earth Independent Crew Tasks
13 Inspiration (1)		Inspiration Priorities

Category counts include the numbered tasks. In all: 71 tasks, 12 numbered and 59 not; 44 named by at least one gap (6 numbered, 38 not), 27 by none. "Crew Earth Ascent Vehicle Strategy" is as printed (p. 196).

What the wiki reads from the lists:

Three open subjects have a 2024 white paper. Crew to Mars vicinity (MD-06): crew complement. "Crew Mars Ascent Propellant Strategy": ascent propellant, on ISRU Systems. The two Mars EDL technology tasks: EDL, on Transportation Systems. None of the three papers names a task; the pairings are the wiki's. Each lays out options and makes no decision.
The three December 2025 sources agree on the twelve. Appendix C's MD titles match the white paper's Table One word for word (below), and both mark the same four as done, which are the four Mars tasks in Section 3.1.3 (pp. 75–76). The Result column's dates match Section 3.1.3: MD-07 at the 2024 review, MD-02, MD-04 and MD-05 at 2025.
Mars only, in practice. No open task names the Moon, and no LD task is open. The ADD doesn't say the open list is Mars-only. Several titles name no destination ("Crew Communications Architecture", "Logistics Carrier and Interface Strategy") and are cited by gaps that also list the lunar segments; where the wiki checked (#0103, #0701, #0806), the sheet lists them under "Mars:".
Fewer than "nearly 100". The white paper says the Mars analysis "identified nearly 100 candidate definition tasks" and that NASA "has since further refined this list" (WP p. 4). Appendix C lists 71. The ADD doesn't say how the two counts relate.
The Result labels and the white paper's three kinds of outcome. The labels are Decision (MD-05, MD-07), Down-Select (MD-02) and Methodology (MD-04). The white paper uses MD-05 as its example of "a reduction of the trade space to a viable or plausible range" (WP p. 5), where Appendix C calls MD-05 a Decision and MD-02 the Down-Select.
What the numbers mean. Numbers are given to tasks "As NASA has begun work on these priority key definition tasks", and "The order does not imply prioritization" (p. 194). Appendix C dates completions only. It doesn't say when each task was identified, so it neither confirms nor refutes the 2024 chart's seven-and-five split (below; open question 54).
The "three new results". Three Result entries say ACR25: MD-02, MD-04 and MD-05. That fits the Executive Summary's "three new results" (p. 9), though the ADD never links the two (open question 21).

The Mars priority key definition tasks (white paper Table One)¶

The "Architecture Definition" white paper (Dec 2025) prints twelve Mars priority tasks. Page numbers in this section are the white paper's. It is not the full catalog: the Mars analysis "identified nearly 100 candidate definition tasks", and "For a complete list of identified definition tasks, refer to the latest revision of the Architecture Definition Document" (p. 4). Table One is a "Snapshot of priority key definition tasks. Shaded rows indicate that NASA has closed a key definition task with a definition outcome" (p. 5, read from the PDF; the text layer scrambles the table).

Not a ranking. "Note that identification numbers and order in the table do not imply prioritization. NASA pursues multiple definition tasks in parallel" (p. 4).
A snapshot. "The list below represents a snapshot of the current state of architecture roadmapping. This list will evolve as NASA identifies new priority topics and completes existing definition tasks. Through these iterations, the agency shapes the Mars exploration architecture" (p. 4).

ID	Definition task, as printed (p. 5)	Shaded (closed)	In ADD Section 3.1.3	Gaps citing it in the tech-gap spreadsheet
MD-01	Initial Human Mars Segment Science Objectives Priorities	no	not listed	#1201, as "…Science Objective Priorities"
MD-02	Initial Human Mars Segment Target State	yes	completed, 2025 ACR	none
MD-03	Initial Human Mars Segment Mission Cadence	no	not listed	none
MD-04	Mars Architecture Loss of Crew Risk Methodology	yes	completed, 2025 ACR	none
MD-05	Number of Crew to Mars Surface	yes	completed, 2025 ACR	none
MD-06	Number of Crew to Mars Vicinity Per Mission	no	not listed	none
MD-07	Primary Mars Surface Power Generation Technology	yes	completed, 2024 ACR (as "Mars Primary Surface Power Generation Technology")	#0902, with ✓
MD-08	Mars Architecture Loss of Mission Risk Methodology	no	not listed	none
MD-09	Maximum Mars Crew Surface Stay Duration	no	not listed	#0401, #0402
MD-10	Mars Forward Contamination Planetary Protection Risk Posture	no	not listed	#0304, #0702, #0803, #1202
MD-11	Mars Backward Contamination Planetary Protection Risk Posture	no	not listed	#0304, #1202
MD-12	Maximum Allowable Crewed Communications Disruption	no	not listed	#0102, #0104, #1002, #1003, #1004

The last two columns, and this cross-check, are the wiki's:

The three sources agree on what is closed. The four shaded rows are exactly the four Mars tasks that ADD Section 3.1.3 lists as completed (pp. 75–76). The spreadsheet's ✓ is on MD-07 and on none of the five unshaded tasks it cites.
Three tasks are in neither ADD Section 3.1.3 nor the spreadsheet: MD-03 (mission cadence), MD-06 (crew to Mars vicinity per mission) and MD-08 (loss-of-mission risk methodology). ADD Appendix C lists all twelve under the same titles, with the same four done (above).
No lunar list. The paper names no LD task, though "Whether planning future lunar exploration or preparing for the first human missions to Mars, NASA uses the same roadmapping process" (p. 1).
MD-10 and MD-11, planetary protection. The ACR25 planetary protection white paper covers their subject, forward and backward contamination at Mars, but never names them; the pairing is the wiki's. See Planetary protection.
MD-12, communications disruption. The ACR25 C&PNT white paper is about the lunar Foundational Exploration segment and doesn't discuss it. For Mars it points to a 2023 white paper, "Mars Communications Disruption and Delay", which isn't among the wiki's sources (C&PNT white paper, p. 6).

Three kinds of outcome¶

"A completed definition task results in a definition outcome. These outcomes can take a variety of forms, though all reduce the trade space or have implications for architecture implementation" (p. 5). The paper gives one Mars example of each (p. 5):

Kind of outcome	Example
"tangible decisions about aspects of the architecture"	"the primary surface power generation technology for initial human missions to Mars" (MD-07)
"a reduction of the trade space to a viable or plausible range"	"the selection of the number of crew to Mars surface" (MD-05)
"the selection of a methodology or approach"	"the Mars architecture loss-of-crew risk methodology task" (MD-04)

The ADD's own list is to "set an architectural ground rule, make a decision, select a methodology, or otherwise shape the architecture" (ADD p. 72).

Case study: MD-07, Mars surface power¶

The paper's "Case Study: Mars Surface Power Generation Decision" (p. 5):

The task. "NASA's 2024 decision to select nuclear fission power as the primary surface power generation technology for initial human missions to Mars. This definition task examined the full range of power generation technologies that could enable human missions to Mars."
The trade. "NASA engineers examined a range of power sources, including solar power, nuclear fission power, and other sources (e.g., geothermal power, fuel cells). The agency weighed each technology's mass, power output, safety, suitability to the Martian environment, and other factors, and ultimately decided upon nuclear fission power, which offered the best balance across these factors."
What it unlocks. "With a technology selected, NASA can begin developing power generation and distribution systems, as well as surface infrastructure (e.g., habitation systems) that takes advantage of nuclear fission power's continuous output."
A demand signal. "This documented mission pull helps inform industry investments and future partnerships. By communicating its selection of nuclear power, NASA offers a demand signal for technology developers and positions itself as a future customer and partner. Industry and international partners can prioritize their technology development efforts, while NASA can focus its own resources on the areas with the most impact."
More detail is in the 2024 white paper "Mars Surface Power Technology Decision" (reference 4), summarized below.

See #0902 Scalable Mars Surface Power Generation, which still lists solar and fuel-cell child gaps (open question 12), and the Lunar Nuclear Fission System.

In the 2024 executive overview (its "Mars Surface Power Decision" feature, PDF p. 17; a year older than the paper above):

First of its kind. "At the 2024 Architecture Concept Review, NASA selected nuclear fission power as the primary surface power generation technology for initial human Mars missions. This was the first driving architecture decision made under the decision roadmapping process."
The constraints. "Any power generation source must be resilient to global dust storms, strong winds, and gravity that is about double that of the Moon. If a mission relies on in-situ resource utilization, power generation technology must also be deployed autonomously, before human explorers arrive."
The options and the result. NASA studied "solar power, nuclear power, fuel cells, geothermal energy, wind power, and biogeneration". "Trade space studies ultimately recommended that nuclear fission power offers the ideal combination of energy output, environmental resiliency, cost, and overall reduction of risk. The Artemis campaign offers the opportunity to test this technology on the Moon, reducing risk for later Mars missions."
A white paper excerpt, "Mars Surface Power Generation Trade Space", names nuclear and solar as "the most value" and gives one line on each of the others: geothermal ("limited data on local geothermal availability"), fuel cells ("require large amounts of landed reactant or large amounts of energy to make reactants in situ"), wind ("insufficient sustained winds") and biogeneration ("would greatly complicate planetary protection"). "Content has been abbreviated for this executive overview."

The wiki's comparison: the overview's criteria (energy output, environmental resiliency, cost, risk) and the 2025 paper's (mass, power output, safety, suitability to the Martian environment) overlap but aren't the same list. The lunar test it foresaw arrived as the Lunar Nuclear Fission System, added in 2025 (ADD p. 58).

The 2024 white paper¶

The ACR24 paper "Mars Surface Power Technology Decision" (source page) records the decision in the year it was made. The 2025 paper above cites it as its reference 4, so this pairing is the sources' own. Page numbers in this subsection are the 2024 paper's.

How it was decided. "As part of the 2023 Architecture Concept Review cycle, NASA began identifying driving decisions needed to define initial human missions to Mars. This effort identified the selection of the primary Mars surface power generation technology as a key decision because of its down-flow impacts on NASA's Mars architecture and Mars-forward considerations for NASA's lunar architecture." Then "ESDMD coordinated relevant data and technical expertise across NASA's mission directorates and technical authorities, collating these inputs into a decision package for consideration by agency leadership at the 2024 Architecture Concept Review and subsequent meetings of the executive council. These bodies reviewed the package and accepted the recommendation" (p. 1).
What it fixes, and what it doesn't. "Selecting nuclear fission establishes the primary power generation technology for the Humans to Mars architecture segment but does not dictate funding for technology development or restrict other power technologies that could operate on the Martian surface. Instead, it offers an initial assumption for narrowing the architectural trade space" (p. 1). "The potential for supplementary, backup, and redundant systems remains an open area of architectural analysis" (p. 1). The decision covers "primary and not supplementary power generation technologies specifically for initial crewed missions to Mars" (p. 3; italics as printed).
Why fission. The choice "was driven primarily by the need to mitigate the risk of loss of mission". NASA down-selected "to nuclear fission systems versus photovoltaic arrays with energy storage". "Although solar power may have a lower per unit cost, fission power is more robust and better suited to the Martian environment. Fission can provide consistent power generation for a wide range of potential landing sites, around the clock, and during global dust storms. It also offers advantages in landed mass and volume" (p. 1).
How much power. "Studies show that a modest mission of two crew members, conducting science and exploring the surface for no more than 30 days while living in a pressurized rover would require at least 10 kilowatts (kW) of surface power. (This includes propellant conditioning for a small crew ascent vehicle)." A larger crew, longer stays and propellant manufacturing "would require hundreds of kW. Some architectures could require megawatt (MW)-class power systems" (p. 2). The "pressurized rover" is a Mars concept here, not the ADD's lunar Pressurized Rover.
The challenges (p. 2), headings as printed:
- Environmental. Dust Storms (settled dust "proved fatal for the solar-powered Opportunity rover"); Reduced Solar Energy Availability (solar flux "at most 45 percent of typical Earth values", and a mid-latitude "25-hour cycle", "only illuminated for about 50 percent of that time"); Gravity and Wind Loads ("about twice the gravity of the Moon").
- Operational. Autonomous/Remote Power System Operation (power systems "may need to be deployed years in advance and support several human missions", to condition or make ascent propellant); Limited Repair Options; Plume-Surface Interactions; Planetary Protection Constraints (see Planetary protection).

The six options (p. 3), in the paper's words:

Option	Verdict
Nuclear	Radioisotope systems "only offer a few hundred watts"; "For higher power needs (e.g., crew life support or ascent propellant manufacturing), fission surface power is readily scalable."
Solar	"Solar power could be feasible if designed to address the challenges of dust accumulation and the day/night cycle", but dust removal "would not mitigate the problem of reduced solar availability due to suspended atmospheric dust during lengthy storms."
Fuel cells	"These systems do not trade well because they require large amounts of landed reactant mass and/or more energy to make reactants in-situ than the fuel cells could provide."
Geothermal	NASA has "limited data on local geothermal availability"; it is "less attractive for early missions."
Wind	Analysis shows "the Red Planet has insufficient sustained winds for reliable power production."
Biogeneration	It "would be greatly complicated by planetary protection constraints."

What was weighed (p. 4): "reliability and availability (i.e., their resilience to the environmental factors described above)", "ability to meet the power needs of a range of potential missions", "extensibility to future segments", and "key drivers of affordability".
The Moon as a testbed (p. 4). "The Moon's proximity to Earth offers opportunities to demonstrate candidate Mars surface power generation technologies with reduced consequences of failure." Lunar systems "would need to account for the environmental differences, including Mars' atmosphere, increased gravity, shorter day/night cycle, wind loads, dust storms, communications delay, etc. While implementing Mars-forward technologies at the Moon could add cost or complexity, surface power technology demonstrations during the Artemis campaign would significantly reduce risk for initial crewed missions to Mars."

What changed by Rev C (the wiki's comparison):

The decision stands. ADD Rev C lists MD-07 as completed at the 2024 review (p. 75), and the Users Guide repeats the selection (Users Guide, p. 14).
The lunar test has an element. The ADD ties the Lunar Nuclear Fission System, added in 2025, to "NASA's selection of nuclear fission" for Mars (ADD p. 58). The paper names no lunar element; setting the element beside its testbed paragraph is the wiki's pairing.
Three lists of criteria for one decision. The 2024 paper's four attributes, the 2024 overview's "energy output, environmental resiliency, cost, and overall reduction of risk", and the 2025 paper's "mass, power output, safety, suitability to the Martian environment" overlap without matching.
The gap still lists other technologies. #0902 keeps solar and fuel-cell child gaps beside the ticked MD-07. The paper's scope (fission doesn't "restrict other power technologies"; supplementary and backup systems are "an open area") is relevant, but no source says that is why the child gaps remain (open question 12).
The 10 kW case is below today's crew floor. It assumes two crew on the surface. MD-05 (2025) set a floor of four (above). No source gives a power figure for four.

In the February 2025 workshop briefing¶

The Strategy and Architecture Office briefed the 2024 paper at both February 2025 workshops, before Rev C (Mars surface power briefing; the partners' copy has the same slides). It calls the decision an "Exercise in process as much as decision outcome", "Documented in Appendix B" of the ADD (Rev B's letter; Rev C's Appendix C), and shows the working the paper leaves out:

Who scored it. Ten attributes under the paper's four headings, "developed, reviewed, and scored by STMD, SAO, M2MPO, OSMA", with "Consensus that 5 highlighted attributes are most impactful": robustness to solar flux variations, to nominal dust and to dust storms; scalability; affordability drivers (slide 4).
Masses. Three earlier reference missions (2009 DRA 5, 2016 EMC, 2020 POD) needing 9 to ~35 kW, with solar at 11.22 to 22.5 t against fission at 6 to 9 t (slide 6). Solar arrays lose "~ 0.2 % per sol without active dust mitigation"; studies before 2018 assumed storms "less than half as severe" as 2018's (slides 7–8).
A scoring. Fission and photovoltaics with storage against eight measures of effectiveness, fission ahead on access, power and mass and behind on cost; "This is the first decision to be added to the Mars architecture. A baseline Mars MOE assessment does not yet exist." (slide 11).
Affordability. "Space Policy Directive 6 encourages High Assay Low Enriched Uranium (HALEU)"; "NASA is leading a dozen federal agencies to identify/address space nuclear policy gaps"; "nothing on fission power systems requires EVA maintenance" (slide 10).

The criteria the slides list are the 2024 paper's four, broken down; the overview's and the 2025 paper's lists (above) still differ from them (the wiki's comparison).

In the executive overviews (2023 and 2024)¶

Both overviews are older than Rev C and use the older word, "decisions". Page numbers are their PDF pages.

Seven in 2023. The 2023 overview summarizes the white paper "Key Mars Architecture Decisions": "NASA has developed analysis tools to better understand the relationships between the many decisions it will need to make to begin planning initial crewed missions to Mars. Using these tools, seven key Mars architecture decisions have been identified. They are not the only questions to answer, but their answers will affect the many decisions that follow" (PDF p. 9). It doesn't list the seven. The paper isn't among the wiki's sources; the 2025 "Architecture Definition" paper supersedes it.

Decision roadmapping in 2024 (2024 overview, PDF p. 15):

"While every decision is important, some will have major flow-down impacts on other subsequent decisions. Mapping out these driving decisions and making them at the appropriate time is key to the success of an evolutionary architecture development effort. For example, when developing a Mars architecture, the decision to use a certain power technology, to send a certain number of astronauts to the surface, or to use a particular fuel for ascent will affect a huge number of later decisions."
"The agency developed a new decision methodology and built cutting-edge digital engineering tools to track the entire decision space (i.e., the network of decisions and the relationships between them). The latest revision of the Architecture Definition Document includes a new appendix documenting this effort."
"NASA provides a concise decision statement (e.g., how many crew to the Mars surface per mission?), expands on the context (e.g., how different numbers of crew to the surface changes the end-to-end architecture), and traces the flow-down relationships between this and other decisions. These are architecture decisions, not implementation decisions; they set a target while allowing for flexibility in mission planning."
"As part of the annual Architecture Concept Review cycle, NASA's architecture teams develop 'decision packages' that agency leaders can use to make those decision. As key driving decisions are made and documented in the Architecture Definition Document, NASA will add and track new or flow-down decisions" ("those decision" as printed).

The "Priority Decisions" chart (PDF p. 15, read from the PDF; the text layer drops it). Twelve hexagons, keyed "= Decision", each colored by a key "based on Systems Analysis of Architecture Drivers (2022)": "Why", "Where" and "When" ("We Will Go"), "What" ("We Will Do There"), "Who" ("Will Be Involved") and "How" ("We Will Get There and Back"). A dashed line, labeled "ACR23 | ACR24" at its foot, splits them. Which side each hexagon falls on and its color are the wiki's reading of the image. The last column is the wiki's match by title to Table One (above).

Hexagon, as printed	Color	Side	Table One task
Number of Crew to Mars Vicinity	Who	ACR23	MD-06
Initial Human Mars Segment Target State	What	ACR23	MD-02
Initial Human Mars Segment Cadence	When	ACR23	MD-03
Number of Crew to Mars Surface	Who	ACR23	MD-05
Mars Science Priorities	Why	ACR23	MD-01
Mars Loss of Crew Risk Posture	How	ACR23	MD-04, now "…Risk Methodology"
Mars Surface Power Generation Tech	How	ACR23	MD-07
Maximum Crew Communications Disruption	How	ACR24	MD-12
Mars Crew Surface Stay Duration Maximum	When	ACR24	MD-09
Backward Planetary Protection Risk Posture	How	ACR24	MD-11
Mars Loss of Mission Risk Posture	How	ACR24	MD-08, now "…Risk Methodology"
Forward Planetary Protection Risk Posture	How	ACR24	MD-10

NASA showed the same chart, with the same hexagons on the same sides, at the February 2025 international partners' workshop, under "Mars Decisions: New Priority Decisions" (the wiki's comparison of the images; Architecture Updates deck, slide 16). The slide calls the list Rev B's "Appendix B"; Rev C's Appendix C holds the tasks now.

What the chart adds, as the wiki reads it:

When each task was identified. Seven hexagons sit on the ACR23 side, matching the 2023 overview's "seven key Mars architecture decisions". By the title match they are MD-01 to MD-07, and the five ACR24 hexagons are MD-08 to MD-12. So the MD numbers may follow the order in which the tasks were identified. Neither overview prints any MD number but MD-01. ADD Appendix C doesn't settle it: it dates completions only, and says numbers are given as work begins (above).
No "Where" task. No hexagon has the "Where" color. Six of the twelve are "How".
Two renamings. Loss-of-crew and loss-of-mission "Risk Posture" in 2024 are "Risk Methodology" tasks in Table One. The outcome of MD-04 was indeed a methodology (above).
The same six questions. The color key's six are the six guiding questions of architecture roadmapping. The key credits them to "Systems Analysis of Architecture Drivers (2022)", one of the two papers the 2025 "Architecture Definition" paper supersedes.

The example, MD-01 (PDF p. 15): "MD-01 Initial Human Mars Segment Science Objective Priorities". "The agency's Moon to Mars strategy identifies science as one of three pillars on which the blueprint for sustained human presence and exploration throughout the solar system is built. The needed decision outcome is a formulation of more specific science objectives — traceable to NASA's high-level 'blueprint' science objectives — for missions carried out during the initial human Mars segment and prioritization of these objectives. Decision prerequisites will include inputs from and coordination between affected science communities and organizations such as academia, National Academies, affected NASA science advisory committees, and the Human Research Program. Priority science objectives have substantial flow-down impacts to most architecture and operations decisions. Therefore, the Mars science priorities key decision must be placed earlier in the Mars decision roadmapping process." The title has "Objective", as in the gap spreadsheet (#1201), where Table One has "Objectives". In December 2025 the task was still open (unshaded in Table One).

What changed by Rev C (the wiki's comparison):

"Decisions" became "definition tasks" (ADD Rev C, p. 4), and "decision roadmapping" became "architecture roadmapping" (p. 74). The outcomes now include methodologies and ground rules, not only decisions (p. 72).
Four of the twelve are closed: MD-07 at the 2024 review, MD-02, MD-04 and MD-05 at the 2025 review (above).
The appendix. The 2024 overview points to "a new appendix" in Revision B. Rev C's Appendix C, "Architecture Definition" (pp. 186–197), holds the lists (above). Its process description keeps the overview's "digital" model, now "a digital, SysML-based definition space model" (p. 187).

In the February 2024 workshop briefing¶

The first seven, named, a year before the 2024 chart: "NASA catalogued nearly 90 needed decisions for an initial crewed Mars mission and developed a decision roadmap. That process resulted in seven interrelated decisions needed to begin planning. In 2024, NASA has begun analyses needed to allow for informed decision-making by agency leadership." The seven hexagons are the 2024 chart's ACR23 seven, in the same colors, under a five-color key with no "Where" (Why, When, What, Who, How) (Overview and Updates, slide 12, read from the image). The same deck lists the paper as "Mars Priority Decisions", "White paper added after ACR23 Concurrence" (slide 11); the 2023 overview calls it "Key Mars Architecture Decisions", and so does the paper's own first page, as far as the briefing's thumbnail can be read (Mars decisions briefing, slide 15). Its forward work for 2024: "Developing decision packages for initial seven Mars exploration decisions" (slide 14).

Nearly 90 or nearly 100. The briefing of the paper itself, at the same workshop, says "Preliminary analysis identified nearly 100 key architecture decisions" and "Seven key decisions recommended for priority analysis in the 2024 analysis cycle" (Mars decisions briefing, slide 4). The opening deck's "catalogued nearly 90" and this deck's "Preliminary analysis identified nearly 100" (with the catalog still being refined) both date from one workshop, and neither deck says how the two figures relate. So the wiki can't read the 2025 paper's "nearly 100 candidate definition tasks" (above) as a change from 2024 in either direction (the wiki's comparison).

The seven, as briefed (slides 7–13 of the same deck; the MD numbers are the wiki's match by title):

MD	Slide title	Needed decision outcome (excerpt)
MD-01	"Science Priorities for Initial Human Mars Segment"	"Identify the highest science priorities for the initial human Mars segment", "both planetary and biological"; "Picking where before considering why may force us to revisit our how decisions"
MD-02	"Initial Human Mars Segment Target State"	"What is the target state ('vision') …? — Science missions to different sites, excursions from an established base at one site, or something else?"
MD-03	"Initial Human Mars Segment Mission Cadence"	"What is the cadence of missions …?"; the target state "will establish the segment scope"
MD-04	"Mars Architecture Loss of Crew (LOC) Risk Posture"	"Define probability LOC risk posture for a Mars mission."
MD-05	"Crew Complement to Mars Surface per Crewed Mission"	"How many crew will descend and land on the Mars surface per crewed mission? — Minimum number for first mission and upper limit for subsequent missions"
MD-06	"Crew Complement to Mars Vicinity per Mission"	"How many crew will travel to Mars vicinity per crewed mission?"
MD-07	"Mars Primary Surface Power Generation Technology"	"Select primary surface power generation technology."

Both crew slides add "Note that number of crew to surface is not necessarily the same as number of crew to Mars vicinity": surface and vicinity were two decisions from the first briefing. Against Rev C (the wiki's comparison): slide 8's open question became a down-select to three options, all with more than one mission (above); MD-04's "probability … risk posture" became a methodology; MD-05 was answered as a minimum ("no fewer than four (4)"), not a maximum; MD-07's slide title is the heading ADD Rev C gives it (p. 192). STMD told the same workshop it had a "Significant role in Mars Surface Power Technology Decision activity", MD-07 (Mission Directorate Panel, slide 13).

The tasks the gap spreadsheet cites¶

The tech gaps spreadsheet traces some gaps to definition tasks by ID, some marked ✓. The ADD's gap tables carry the legend "“✓” indicates completed definition task" (Appendix D, e.g. p. 204, checked against the PDF). The ticks agree with Section 3.1.3: every ticked task is a legacy decision or a completed task there, and no unticked one is. This cross-check is the wiki's.

Which tasks a gap lists. ADD Appendix D defines the column as a "List of architecture definition tasks with strong relevance to the gap; to be considered relevant, the definition task outcome affects the degree of need for the gap directly" (p. 199, checked against the PDF). For all 57 gaps, the ADD's gap tables print the same tasks as the sheet, and the same ✓ marks. Where the sheet writes "Moon:" or "Mars:", the ADD puts a Moon or a Mars icon beside each task (pp. 204–260, PDF checked; e.g. p. 206). The ticks on #0701, #0806, #0902 and #1101 in the table below are printed in the ADD too (pp. 234, 241, 245, 252).

Task, as the sheet prints it	✓	In Section 3.1.3	Gaps that cite it
LD-03-L Lunar Landing Region Selection	✓	legacy decision	#0301, #1101
LD-102 Lunar Logistics Strategy	✓	completed	#0701, #0806
LD-103 Surface Communications Strategy	✓	completed (as "Lunar Surface Communications Strategy")	#0103
MD-07 Primary Mars Surface Power Generation Technology	✓	completed (as "Mars Primary Surface Power Generation Technology")	#0902
MD-01 Initial Human Mars Segment Science Objective Priorities		not listed	#1201
MD-09 Maximum Mars Crew Surface Stay Duration		not listed	#0401, #0402
MD-10 Mars Forward Contamination Planetary Protection Risk Posture		not listed	#0304, #0702, #0803, #1202
MD-11 Mars Backward Contamination Planetary Protection Risk Posture		not listed	#0304, #1202
MD-12 Maximum Allowable Crewed Communications Disruption		not listed	#0102, #0104, #1002, #1003, #1004

The white paper's Table One agrees for the Mars tasks: MD-07 is shaded as closed, and MD-01 and MD-09 to MD-12 are not (above).

The sheet also names tasks without an ID (for example "Crew Communications Architecture" under

0103, "Logistics Carrier and Interface Strategy" under #0701). The ADD prints them in the same¶

column of its gap tables (e.g. #0103, p. 206), so by D.1's definition they are architecture definition tasks. They are key definition tasks too: the wiki's search of the sheet finds 38 distinct titles without an ID, and every one is in Appendix C's list of open key definition tasks, four of them under longer names ("EDL" and "EDLA" spelled out; pp. 194–197). The full list, with which titles a gap names, is above.

Why it matters for the gaps: depth, one of the four priority metrics, "measures the degree to which closing the gap is dependent on future architecture definition tasks". It is scored "based upon mapping to definition tasks, along with an assessment of how much those definition task outcomes affect the need for the gap's closure" (p. 78; see How NASA prioritizes the gaps). That is the same test D.1 uses to decide which tasks a gap lists (p. 199).

Changes since Rev C¶

Rev C describes the decisions as of December 2025. Newer sources report changes that touch some of them. They are quoted here; none of them says a decision has been revisited. The January 2026 workshop deck briefs the outcomes the wiki lists as MD-05, MD-02 and MD-04 (it prints no MD numbers) as the 2025 review left them (slides 15, 30–36). The international partners' deck of 24–25 February 2026 carries the same lines, both crew wordings included (slides 15 and 35–41, with other co-presenters; source page). See MD-05 and what Appendix C adds.

LD-02-L and LD-04-L (Gateway in lunar orbit, NRHO). The Users Guide says SR-1 Freedom repurposes "the Power and Propulsion Element originally planned for the Gateway space station" (Users Guide, p. 2). The August 2026 Moon Base update says Northrop Grumman is reusing HALO power and avionics hardware for surface demonstrations, "following NASA's shift from an orbital-focused lunar strategy to one centered on surface operations" (Moon Base update, Aug 2026, "Northrop Grumman"). The March 2026 fact sheet's updated Artemis plan mentions no Gateway visit (Going back to the Moon, pp. 1–2). See Gateway and open question 22.
LD-05-L (annual cadence). The fact sheet says the February 2026 plan "signals demand to industry that NASA is preparing for semi-annual crewed lunar missions after Artemis V" (Going back to the Moon, p. 1). Semi-annual is more often than LD-05-L's annual cadence, though Appendix C already allows "the potential for surges of up to two missions each year" (ADD p. 190).

Architecture definition process · Gaps index · Tech gaps spreadsheet · Segments · Humans to Mars · Gateway · ADD Rev C source page · "Architecture Definition" white paper

Sources¶

ADD Rev C, pp. 4, 9, 58, 72, 74–76, 78, 186–197, 199, 204–260 · "Architecture Definition" white paper, pp. 1, 3–6 · 2025 Architecture Update, pp. 3, 13 · Lunar power white paper, p. 1 · Planetary protection white paper · C&PNT white paper, pp. 2, 6 · Tech gaps spreadsheet, Definition Tasks column · Going back to the Moon, pp. 1–2 · Users Guide, pp. 2, 14 · Mars Crew Complement Considerations (2024), pp. 1–3 · Mars Surface Power Technology Decision (2024), pp. 1–4 · Mars Ascent Propellant Considerations (2024) · Mars EDL Challenges (2024) · Moon Base update, Aug 2026 · 2023 executive overview, PDF p. 9 · 2024 executive overview, PDF pp. 15, 17 · 2026 industry and academia workshop, slides 15, 30–36