Artemis and human lunar return

Artemis is the United States programme to return crew to the lunar surface and establish sustained cislunar operations. Its architecture is a compromise between existing hardware, new commercial capability and political constraints, and understanding which is which explains most of its structure.

The elements

SLS and Orion are the inherited elements. Both trace back to the Constellation programme cancelled in 2010, and SLS reuses Space Shuttle main engines and solid booster derivatives. That heritage is why SLS exists in its current form, and it explains both its capability and its cost structure: it is expendable, its engines are refurbished Shuttle hardware flown once and discarded, and its per-flight cost is high.

Why the profile is unusual

Apollo went directly to low lunar orbit. Artemis does not, and the difference is a consequence of Orion’s performance.

Orion with its European service module has less delta-v than the Apollo command and service module. It cannot enter and leave low lunar orbit and still return to Earth. So the architecture places the rendezvous point in a near-rectilinear halo orbit instead, which Orion can reach and leave.

The NRHO is genuinely well chosen for the constraints. From Lagrange points and Orbit types and regimes, it is nearly stable so station keeping costs only a few metres per second per year, it maintains continuous line of sight to Earth so communications never drop, and its 9:2 synodic resonance keeps it out of eclipse.

The cost is that it is far from the surface. Descent from NRHO requires more delta-v than from low lunar orbit, and that burden falls on the lander rather than on Orion. The architecture has moved the difficulty from the crew vehicle to the lander, which is a deliberate trade rather than an oversight.

The lander and its refuelling dependency

Starship HLS is the element with the most schedule risk, and the reason is specific rather than general.

A Starship in low Earth orbit does not have the propellant to reach NRHO, descend, ascend and return. It must be refuelled in orbit, which requires launching a series of tanker Starships and transferring cryogenic propellant between them in microgravity.

Number of flights

Estimates have ranged from a handful to more than a dozen tanker launches per lunar mission. The number depends on boil-off rate and transfer efficiency, neither of which is yet measured at scale.

Cryogenic transfer

Moving liquid methane and oxygen between vehicles in microgravity has never been demonstrated at this scale. There is no bottom to the tank, so propellant management devices or settling burns are required.

Boil-off

Propellant loaded early evaporates while waiting for later tankers. A long campaign loses propellant continuously, which sets a maximum time between the first and last flight.

Launch cadence

The whole campaign must complete within that window, which imposes a launch rate requirement no vehicle has yet sustained.

None of this is impossible, and it is the same capability that propellant depots would need. But it is a chain of unproven steps on the critical path of a crewed mission, and it is where the schedule most plausibly slips. See Onboard propulsion for the microgravity propellant management problem.

Blue Origin’s Blue Moon lander was selected as a second provider, which gives redundancy at the architecture level rather than the component level. That is the right kind of redundancy for a programme whose largest risk is a single provider’s development schedule.

Mission sequence

Artemis I validated SLS, Orion, the NRHO trajectory and the heat shield at lunar return velocity, which is about 11 km/s and considerably more demanding than return from low Earth orbit. The heat shield showed more charring and material loss than predicted, and understanding that behaviour became a significant item of work before crewing the vehicle.

Why the south pole

Apollo landed at equatorial near-side sites chosen for operational simplicity. Artemis targets the south polar region, and the reason is volatiles.

Permanently shadowed crater floors near the pole contain water ice. Adjacent crater rims receive nearly continuous sunlight because of the Moon’s small axial tilt, giving both a power source and a resource within a short traverse. That combination exists nowhere else on the Moon.

The operational cost is that polar lighting is difficult. The Sun stays near the horizon, shadows are long and deep, and the illumination geometry changes over a lunar day in ways that complicate both solar power and landing navigation. A lander approaching a site in near-permanent shadow cannot use conventional optical hazard detection.

The Artemis Accords

The Accords are a set of bilateral agreements between the United States and partner nations setting out principles for civil space activity: transparency, interoperability, emergency assistance, registration, release of scientific data, preservation of heritage sites, deconfliction through safety zones, and orbital debris mitigation.

They are politically significant because they attempt to establish operating norms outside the formal United Nations treaty process, which has produced no new binding space treaty since 1979. Supporters see a pragmatic route to norms that the treaty process cannot deliver. Critics note that a coalition led by one state, interpreting the Outer Space Treaty in ways favourable to resource extraction, is not the same as a multilateral agreement, and that the safety zone concept sits uneasily with the treaty’s prohibition on national appropriation. See Policy and regulation.

Where the risk sits

The last row is not a technical risk but it is the historically decisive one. Constellation was cancelled in 2010 after five years and substantial expenditure, and the elements that survived did so by being repurposed. A programme whose critical path is a decade long is exposed to changes in priority that no engineering margin covers.

References

• Smith, M. et al. “The Artemis Program: An Overview of NASA’s Activities to Return Humans to the Moon”, IEEE Aerospace Conference, 2020.
• Whitley, R. and Martinez, R. “Options for Staging Orbits in Cis-Lunar Space”, IEEE Aerospace Conference, 2016.
• Zimovan, E. M., Howell, K. C. and Davis, D. C. “Near Rectilinear Halo Orbits and Their Application in Cis-Lunar Space”, IAA-AAS-DyCoSS3, 2017.
• NASA Office of Inspector General, audit reports on SLS, Orion and HLS, published periodically.
• The Artemis Accords: Principles for Cooperation in the Civil Exploration and Use of the Moon, Mars, Comets and Asteroids for Peaceful Purposes, 2020.