🌙 Moon Design Document

Document type: System Design Specification
Product: Earth's Moon (Luna-1)
Manufacturer: Solar System Assembly Line
Design lead: Gravity (uncredited)
Status: Approved — in production for 4.51 billion years
Classification: Public

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Table of Contents

1. Executive Summary
2. Requirements
3. Architecture
4. Architecture Decision Records (ADRs)
5. Trade-offs
6. Version History
7. Known Issues
8. Future Roadmap

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Executive Summary

The Moon is a 7.342 × 10²² kg natural satellite deployed in a stable orbit around Earth at an average altitude of 384,400 km. It was formed approximately 4.51 billion years ago via a Giant Impact between the proto-Earth and a Mars-sized body designated Theia. The Moon has been in continuous production since deployment with zero unplanned downtime.

Primary functions include:

• Nighttime illumination (reflective, albedo 0.12)
• Tidal regulation of Earth's oceans
• Stabilization of Earth's axial tilt
• Eclipse generation (solar and lunar)
• Impact shielding (crater absorption)

The Moon has no atmosphere, no magnetic field (since ~1 Ga), and no tectonic activity (since ~3 Ga). It is a low-maintenance, high-reliability system.

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Requirements

Functional Requirements

ID	Requirement	Priority	Status
FR-001	Provide nighttime illumination via reflected sunlight	High	✅ Delivered
FR-002	Generate ocean tides through gravitational interaction	High	✅ Delivered
FR-003	Stabilize Earth's axial tilt (reduces climate chaos)	Critical	✅ Delivered
FR-004	Absorb meteor impacts (shield Earth)	Medium	✅ Delivered
FR-005	Exhibit visible phase cycle (~29.53 days)	Medium	✅ Delivered
FR-006	Produce solar eclipses when aligned with Sun	Low	✅ Delivered
FR-007	Produce lunar eclipses when in Earth's shadow	Low	✅ Delivered
FR-008	Store water ice at polar regions (for future refueling)	Low	✅ Delivered
FR-009	Support human visitation (surface landing, EVA)	Low	✅ Delivered (12 users, 1969–1972)
FR-010	Ring like a bell when struck (seismic resonance)	Nice-to-have	✅ Delivered (discovered 1969)

Non-Functional Requirements

ID	Requirement	Target	Actual
NFR-001	Uptime	>99.999%	100% (4.51 Ga, zero downtime)
NFR-002	Orbital stability	Stable for >1 Ga	✅ Stable for 4.51 Ga
NFR-003	Mass budget	≤ 1.5% of Earth mass	1.2% ✅
NFR-004	Surface temperature range	Operate in vacuum	−173°C to +127°C ✅
NFR-005	Maintenance overhead	None	✅ Zero maintenance
NFR-006	Drift rate	Minimal	3.8 cm/year (acceptable)
NFR-007	Response time (light travel)	<2s from Earth	1.28s ✅
NFR-008	Mean time between failures	>1 Ga	✅ No failures recorded

Constraints

ID	Constraint	Reason
C-001	Must remain tidally locked to Earth	Stable same-face orientation required for observation
C-002	Must not exceed escape velocity threshold of 2.38 km/s	Prevents accidental atmospheric acquisition
C-003	Orbit must be prograde (same direction as Earth's rotation)	Retrograde orbit would destabilize system
C-004	Eccentricity must remain <0.1	Prevents excessive tidal stress on Earth
C-005	Inclination must remain <10° to ecliptic	Ensures eclipse alignment occurs periodically
C-006	No liquid water on surface	Would violate vacuum environment constraint
C-007	Must not generate its own light	Illumination is reflective only (budget restriction)

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Architecture

System Architecture Diagram

    ┌─────────────────────────────────────────────────────────┐
    │                    SOLAR SYSTEM                         │
    │                                                         │
    │      ☀️ Sun (power source, 150 million km)              │
    │       │                                                 │
    │       │  sunlight (photon stream, ~1361 W/m² at Moon)   │
    │       ▼                                                 │
    │  ┌───────────────────────────────────────────────────┐  │
    │  │              EARTH-MOON SYSTEM                    │  │
    │  │                                                   │  │
    │  │    ┌─────────┐        gravity        ┌────────┐   │  │
    │  │    │  Earth  │◄────── 1.98 × 10²⁰ ───│  Moon  │   │  │
    │  │    │ (host)  │        N (force)      │ (sat)  │   │  │
    │  │    └─────────┘                       └────────┘   │  │
    │  │         ▲                                │        │  │
    │  │         │         384,400 km             │        │  │
    │  │    tidal force                    reflected light │  │
    │  │    (ocean tides)                  (12% albedo)    │  │
    │  │         │                                │        │  │
    │  │    ┌────┴────┐                    ┌───────┴────┐  │  │
    │  │    │ Oceans  │                    │  Regolith  │  │  │
    │  │    │ (tides) │                    │ (surface)  │  │  │
    │  │    └─────────┘                    └────────────┘  │  │
    │  └───────────────────────────────────────────────────┘  │
    │                                                         │
    └─────────────────────────────────────────────────────────┘

    Data flow: Sun → Moon (photons) → Earth (reflected light + gravity)
    Control: Gravity (autonomous, no manual override)

Internal Architecture

    ┌──────────────────────────────────────────┐
    │             LAYER 0: SURFACE             │
    │  Regolith (0–10 m) | Craters | Maria     │
    ├──────────────────────────────────────────┤
    │             LAYER 1: CRUST               │
    │  Anorthosite (highlands) | Basalt (maria)│
    │  Thickness: 30–60 km                     │
    ├──────────────────────────────────────────┤
    │             LAYER 2: MANTLE              │
    │  Olivine, pyroxene                       │
    │  Thickness: ~1,400 km                    │
    │  State: Solid (rigid — high Q factor)    │
    ├──────────────────────────────────────────┤
    │             LAYER 3: OUTER CORE          │
    │  Liquid iron-nickel                      │
    │  Thickness: ~200 km                      │
    ├──────────────────────────────────────────┤
    │             LAYER 4: INNER CORE          │
    │  Solid iron (~240 km radius)             │
    └──────────────────────────────────────────┘

Component Inventory

Component	Material	Mass	Status	Notes
Crust	Anorthosite / basalt	~7 × 10²¹ kg	Intact	Heavily cratered, dual-layered
Mantle	Olivine, pyroxene	~6.5 × 10²² kg	Solid	Rigid — high seismic Q
Outer core	Iron-nickel alloy	~4 × 10²⁰ kg	Liquid	No dynamo (dead since ~1 Ga)
Inner core	Solid iron	~1 × 10¹⁹ kg	Solid	~240 km radius
Regolith	Impact breccia, dust	~10¹⁵ kg	Growing	Append-only (craters accumulate)
Water ice (polar)	H₂O ice	~10⁸–10⁹ tonnes	Present	Stored in shadowed craters
Atmosphere	—	~10⁴ kg	Trace only	Effectively vacuum
Magnetic field	—	—	Off	Dynamo ceased ~1 Ga

Subsystem: Orbital Mechanics

Parameter	Value	Mechanism
Semi-major axis	384,400 km	Gravitational equilibrium
Eccentricity	0.0549	Primordial + tidal evolution
Inclination	5.14°	Primordial
Sidereal period	27.32 days	Kepler's Third Law
Synodic period	29.53 days	Phase cycle (apparent)
Tidal lock	Yes (rotation = orbit)	Gravitational braking over ~100 Ma
Drift rate	+3.8 cm/year	Tidal angular momentum transfer
Orbital velocity	1.022 km/s	vis-viva equation

Subsystem: Thermal Management

    Lunar Day (14 Earth days):         Lunar Night (14 Earth days):
    ┌──────────────┐                   ┌──────────────┐
    │  Sun → Moon  │                   │  No sunlight │
    │  +127°C max  │                   │  −173°C min  │
    │              │                   │              │
    │ No atmosphere│                   │ No atmosphere│
    │ = no thermal │                   │ = no thermal │
    │   buffer     │                   │   retention  │
    └──────────────┘                   └──────────────┘

    Temperature range: 300°C swing
    Thermal cycling: ~13,000 per billion years
    No thermal runaway possible (no atmosphere to trap heat)

Note

The Moon's thermal system is passive — no active cooling, no heating, no thermostats. It simply radiates to space. During the day, it absorbs sunlight and heats up. During the night, it radiates to deep space and cools down. This is the simplest possible thermal design: a blackbody in vacuum. It cannot fail because it has no moving parts.

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Architecture Decision Records (ADRs)

ADR-0001: Moon Formation Method — Giant Impact

Field	Value
Status	Accepted
Date	~4.51 Ga
Decider	Gravity + thermodynamics

Context: Proto-Earth needed a large satellite to stabilize axial tilt and generate tides. Available formation methods:

1. Co-accretion (form together from disk)
2. Capture (capture a passing body)
3. Fission (spin off from Earth)
4. Giant Impact (collide with Theia)

Decision: Giant Impact.

Rationale:

• Co-accretion: would produce a Moon with Earth-like isotopic ratios and composition → doesn't match the iron-poor crust observed
• Capture: energetically improbable for such a large body; captured moons have different compositions → doesn't match isotopic similarity
• Fission: requires Earth to spin impossibly fast; doesn't explain angular momentum
• Giant Impact: explains the Moon's low iron content (Theia's core merged with Earth's), the isotopic similarity (material from Earth's mantle), the angular momentum, and the initial molten state

Consequences:

• ✅ Moon formed with Earth-like isotopic ratios (confirmed by Apollo samples)
• ✅ Moon has a small iron core (Theia's core merged with Earth)
• ✅ System angular momentum matches impact models
• ⚠️ Initial surface was fully molten (magma ocean) — required ~100 Ma to crystallize
• ⚠️ Giant Impact also tilted Earth's axis to ~23.5° (this was a feature, not a bug)

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ADR-0002: Tidal Locking — Same Face Always Points to Earth

Field	Value
Status	Accepted (gradually)
Date	Completed ~3–4 Ga
Decider	Tidal forces (autonomous)

Context: The Moon initially rotated faster than its orbital period. Tidal bulges raised by Earth's gravity created internal friction, dissipating rotational energy as heat.

Decision: Synchronize rotation period with orbital period (27.32 days).

Rationale:

• Minimizes tidal dissipation (lowest energy state)
• Stabilizes one hemisphere for continuous Earth observation
• No active control required — passive gravitational braking
• Once locked, system is self-maintaining (no maintenance overhead)

Consequences:

• ✅ Near side always faces Earth → enables Earth-based observation
• ✅ Far side shielded from Earth's radio noise → ideal for radio astronomy (future)
• ⚠️ Far side inaccessible from Earth → requires relay satellites for communication (discovered 1959)
• ⚠️ No "back side" view from Earth → caused "dark side" misconception (ongoing PR issue)

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ADR-0003: No Atmosphere

Field	Value
Status	Accepted (by default)
Date	~4.4 Ga (immediately after magma ocean solidified)
Decider	Gravity + thermal escape

Context: The Moon has insufficient gravity (1.62 m/s², escape velocity 2.38 km/s) to retain a significant atmosphere. Any gases released by volcanic outgassing during the mare volcanism period (4–3 Ga) exceeded escape velocity and were lost to space.

Decision: No atmosphere. Vacuum surface environment.

Rationale:

• Reduces maintenance (no weathering, no erosion, no cloud cover)
• Preserves craters indefinitely (4-billion-year archival storage)
• Simplifies thermal model (pure radiative, no convective complexity)
• Eliminates wind loading on surface installations
• No atmospheric drag on orbit → orbit is stable and predictable

Consequences:

• ✅ Craters preserved for billions of years (geological archive)
• ✅ No weather → no weather delays, no storms, no erosion
• ✅ No atmospheric distortion → ideal for astronomy
• ❌ No breathable air → requires spacesuits for all surface operations
• ❌ No thermal buffer → 300°C temperature swings
• ❌ No radiation shielding → solar wind and cosmic rays hit surface directly
• ❌ No sound propagation → radio-only communication
• ❌ Micrometeorites not burned up → all impactors reach surface

Tip

The "no atmosphere" decision was not a choice — it was a physical inevitability given the Moon's mass. If the Moon had an atmosphere, it would have lost it billions of years ago. You can't argue with escape velocity.

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ADR-0004: No Magnetic Field (Decommissioned)

Field	Value
Status	Decommissioned
Date	Dynamo ceased ~1 Ga
Decider	Core cooling (autonomous)

Context: The Moon once had a magnetic dynamo in its liquid outer core, generating a surface field of ~5–10 μT (stronger than today's ~0 μT). As the core cooled, convection weakened and the dynamo shut down.

Decision: Magnetic field decommissioned. Surface is now exposed to solar wind and cosmic radiation.

Rationale:

• Core cooling reduced below the threshold for sustained convection
• Small core size (~240 km inner core) cannot maintain a self-sustaining dynamo
• No plate tectonics to drive core heat flow
• Decommission was not a decision — it was a natural consequence of cooling

Consequences:

• ❌ Surface exposed to solar wind (implants helium-3 into regolith — future mining opportunity)
• ❌ Cosmic radiation reaches surface (~30 rem/year baseline)
• ❌ Solar flares can deliver lethal doses to surface astronauts
• ✅ Solar wind implantation created helium-3 deposits (potential fusion fuel)
• ✅ No magnetosphere → simpler space weather model (it's all bad, all the time)

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ADR-0005: Append-Only Surface (No Tectonic Recycling)

Field	Value
Status	Accepted (by default)
Date	~3 Ga (end of mare volcanism)
Decider	Thermal evolution

Context: Earth uses plate tectonics to recycle its crust — old crust subducts, new crust forms. The Moon, being much smaller, cooled faster and its lithosphere thickened. No slab pull, no ridge push, no convection strong enough to drive plates.

Decision: No plate tectonics. Surface is append-only. Craters accumulate permanently.

Rationale:

• Insufficient internal heat to drive mantle convection at plate-driving force levels
• Thick rigid lithosphere (~1,000 km) prevents fracturing into plates
• No liquid water to lubricate subduction (water lowers rock melting point)

Consequences:

• ✅ Surface is a 4-billion-year archive of Solar System impact history
• ✅ Apollo landing sites preserved indefinitely (no erosion, no subduction)
• ❌ No volcanic activity → no CO₂ cycling, no atmosphere regeneration
• ❌ Crater count increases monotonically → surface gets more cluttered over time
• ⚠️ Minor global contraction (~100 m shrinkage over 3 Ga) → occasional moonquakes

Note

The append-only design is a feature for geologists and a bug for anyone who wants a "clean" surface. The Moon does not support DELETE operations. See the Moon API Reference for details.

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ADR-0006: Regolith as Surface Layer

Field	Value
Status	Accepted (accumulated naturally)
Date	Ongoing since ~4 Ga
Decider	Micrometeorite bombardment

Context: Without an atmosphere to burn up micrometeorites, every impactor — no matter how small — hits the surface and shatters rock into fine particles. Over 4 billion years, this created a global layer of regolith.

Decision: Regolith is the default surface layer. Depth: 2–10+ meters (varies by location).

Rationale:

• Passive accumulation — no installation required
• Provides radiation shielding (2 m of regolith ≈ Earth's atmosphere for radiation blocking)
• Source of helium-3 (implanted by solar wind)
• Contains oxygen (43% by mass — extractable for life support)
• Thermal insulator (stabilizes subsurface temperature)

Consequences:

• ✅ Free radiation shielding for future habitats (just pile it on the roof)
• ✅ Resource: oxygen, helium-3, trace metals
• ❌ Abrasive — damages spacesuit seals, mechanisms, and lungs
• ❌ Electrostatically charged — clings to everything, impossible to fully clean
• ❌ Sharp particles (no erosion to round them) — microscopic glass shards

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Trade-offs

Trade-off Matrix

Decision	Benefit	Cost	Verdict
No atmosphere	Preserves craters, no weather, simple thermal model	No air, no radiation shielding, no sound, extreme temps	✅ Worth it — the archival value outweighs the inconvenience
Tidal locking	Stable Earth-facing view, simplified observation	Far side inaccessible without relay, "dark side" misconception	✅ Worth it — continuous near-side observation is valuable
No tectonics	4-billion-year geological archive	No resource recycling, surface clutter increases	✅ Worth it — the archive is irreplaceable
Low gravity (1/6g)	Easy to land on, easy to launch from	Dust doesn't settle well, human muscle atrophy on long stays	✅ Worth it — low gravity is an advantage for launch economics
Small core	Lighter total mass, more crust material	No magnetic field, no dynamo, radiation exposure	⚠️ Acceptable — the Giant Impact was a one-time event
Append-only surface	Perfect crater preservation	No garbage collection, no DELETE operation	✅ Worth it — the Moon is an archive, not a database
3.8 cm/year drift	Gravitational equilibrium (stable system)	Total eclipses will end in ~600 My	⚠️ Acceptable — eclipses are a bonus feature, not core functionality
No liquid water (surface)	Maintains vacuum environment	No erosion, no water cycle, requires ice mining at poles	✅ Worth it — ice is available at poles for future use

Trade-off: Atmosphere vs No Atmosphere

    WITH ATMOSPHERE                 WITHOUT ATMOSPHERE
    ┌───────────────────┐            ┌──────────────────┐
    │ [x] Breathable    │            │ [ ] No air       │
    │ [x] Temperature   │            │ [ ] 300°C swing  │
    │     buffer        │            │ [ ] No radiation │
    │ [x] Radiation     │            │     shielding    │
    │     shielding     │            │ [x] Craters      │
    │ [ ] Erodes craters│            │     preserved    │
    │ [ ] Weather       │            │ [x] No weather   │
    │ [ ] Atmosph. blur │            │ [x] Sharp optics │
    └───────────────────┘            └──────────────────┘    
    The Moon chose: NO ATMOSPHERE.
    Reason: Escape velocity (2.38 km/s) is too low to retain one.
    It wasn't a design choice, just physics.

Trade-off: Tidal Locking vs Free Rotation

    TIDALLY LOCKED                     FREE ROTATION
    ┌──────────────────┐                ┌─────────────────┐
    │ [x] Same face    │                │ [x] Full surface│
    │    always toward │                │     visible over│
    │    Earth         │                │     time        │
    │ [x] Far side is  │                │ [ ] No stable   │
    │     radio-quiet  │                │     observation │
    │ [ ] Far side     │                │ [ ] No radio-   │
    │     needs relay  │                │     quiet zone  │
    └──────────────────┘                └─────────────────┘
    
    The Moon chose: TIDALLY LOCKED.
    Reason: Gravity did it. Over ~100 Ma of tidal braking, the system
    reached its lowest energy state. There was no alternative.

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Version History

Version	Date	Event	Changes	Impact
0.1.0-alpha	4.51 Ga	Giant Impact (Theia collision)	Moon formed from debris disk	Initial release. Fully molten. No surface features.
0.2.0	~4.44 Ga	Magma ocean crystallization	Anorthosite crust forms (flotation)	First solid surface. Crust 0–60 km.
0.3.0	~4.3–4.0 Ga	Late Heavy Bombardment	Large impact basins created (Imbrium, Serenitatis, etc.)	Major surface sculpting. POST /impact endpoint heavily used.
0.4.0	~4.0–3.0 Ga	Mare volcanism	Basaltic lava fills impact basins → maria	Dark patches visible from Earth. Surface composition diversified.
0.5.0	~3.8 Ga	Tidal locking complete	Rotation synchronized with orbit (27.32 days)	Same face always toward Earth. Far side permanently hidden.
0.6.0	~3.0 Ga	Volcanism ceases	No new maria. Surface becomes geologically quiet.	System enters low-power mode. No new features.
0.7.0	~1.0 Ga	Magnetic dynamo stops	No global magnetic field. Surface exposed to solar wind.	Radiation environment changes. Helium-3 accumulation begins.
0.8.0	~500 Ma	Complex life on Earth (Cambrian)	Moon visible to evolving organisms	New user base: multicellular life. Moon becomes culturally significant.
0.9.0	~2.5 Ma	Humans evolve	Moon visible to early hominids	Future API consumers discovered.
1.0.0	1609	Galileo observes Moon with telescope	First detailed surface mapping	Surface features (craters, mountains) documented.
1.1.0	1969-07-20	Apollo 11 landing	First human visitors. 21.5 kg of samples returned.	POST /landing endpoint first used. Footprints installed (permanent).
1.1.1	1969–1972	Apollo 12–17	6 crewed landings, 382 kg samples, 3 rovers	Extensive surface testing. Seismometers deployed.
1.2.0	1959–2024	Robotic exploration	Luna, Surveyor, Chang'e, Chandrayaan, Kaguya, LRO	Full surface mapped. Polar ice confirmed. Far side explored.
1.3.0-dev	2027 (planned)	Artemis III	First crewed landing since 1972. South Pole target.	POST /landing endpoint reactivated. New user: Artemis program.
2.0.0	~600 My future	Moon too far for total eclipses	Drift exceeds apparent Sun size ratio	Breaking change: GET /eclipses returns annular only.
3.0.0	~50 By future	Tidal locking of Earth	Earth rotation = Moon orbital period	No more tides. Both bodies locked. System reaches final equilibrium. (Sun will have consumed both by then.)

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Known Issues

ID	Issue	Severity	Status	Workaround
BUG-001	Regolith is abrasive and electrostatically charged	High	Open (unfixable)	Use dust mitigation protocols, seal protection
BUG-002	Temperature swings 300°C (no atmospheric buffer)	High	Open (unfixable)	Active thermal management for all hardware
BUG-003	No radiation shielding (magnetic field decommissioned)	High	Open (unfixable)	2+ m regolith burial for habitats
BUG-004	"Dark side" misconception persists despite documentation	Low	Open (PR issue)	Education. Read The Moon Manual
BUG-005	Moon is drifting away (3.8 cm/year)	Low	Open (unfixable)	None. Total eclipses will end in ~600 My. Enjoy them now.
BUG-006	Moon rings like a bell (seismic vibrations last >1 hour)	Low	Open (feature)	Accept it. The Moon is a percussion instrument.
BUG-007	No CORS headers (Moon predates browsers)	Low	Open (won't fix)	Use a proxy
BUG-008	DELETE /crater always returns 405	Medium	Won't fix (by design)	Craters are append-only. Accept the architecture.
BUG-009	Far side requires relay satellite for communication	Medium	Open (infrastructure)	Deploy relay satellites (e.g., Queqiao)
BUG-010	Moon illusion — Moon appears larger near horizon	Low	Open (client-side bug)	This is a human perception bug, not a Moon bug. Hold a ruler at arm's length.

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Future Roadmap

Milestone	Target	Status	Notes
Artemis III crewed landing	2027	Planned	South Pole, Shackleton Crater region
Lunar Gateway (orbital station)	2028	In development	Near-rectilinear halo orbit
Permanent lunar base	2030s	Proposed	South Pole, near ice deposits
ISRU (in-situ resource utilization)	2030s	Research	Extract O₂, H₂O from regolith/ice
Helium-3 mining	2040s+	Speculative	Fusion fuel from regolith
Far-side radio telescope	2040s+	Proposed	Shielded from Earth's radio noise
Lunar mass driver	2050s+	Concept	Launch payloads without rockets
Earth-Moon space elevator	2100s+	Speculative	Requires materials stronger than current carbon nanotubes
End of total solar eclipses	~600 My	Scheduled	Moon too far to cover Sun. Annular only after this date.
Tidal locking of Earth	~50 By	Won't happen	Sun consumes Earth-Moon system in ~5 By first.

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Related Documents

Document	Description	URL
The Moon Manual	Product documentation (user guide)	https://github.com/lunar-me/the-moon-manual
Moon API Reference	Developer documentation (REST API)	https://github.com/lunar-me/moon-api-reference
Moon Design Document	System design (this document)	https://github.com/lunar-me/moon-design-document

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License

MIT — the Moon is open source and belongs to everyone. See LICENSE.

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Moon Design Document is an educational project. The Moon was not actually engineered — it formed from a Giant Impact 4.51 billion years ago and has been running unattended ever since. The requirements, architecture, and trade-offs are presented as if the Moon were a designed system. The astronomy is real; the design process is not.