The Moon

The Moon is three days away and has been visited more than any other body, yet landings still fail regularly. Proximity is not the same as ease, and the reasons are specific.

The environment

The absence of an atmosphere is the defining engineering fact. On Mars, an aeroshell and parachutes remove most of the arrival velocity for free. On the Moon, every metre per second must be removed propulsively, so a lunar lander carries propellant for the entire descent: roughly 1.9 km/s from low lunar orbit. See Delta-v budgets.

The 14-day night is the second driver. A solar-powered lander at a mid-latitude site faces two weeks without sunlight at −173 °C, which requires either radioisotope heating or acceptance that the mission ends at sunset. Most recent commercial landers have taken the second option, which is why their nominal missions are quoted in days.

Orbital instability

Low lunar orbits are unstable in a way that surprised early mission planners. In 1968 Muller and Sjogren discovered mascons, mass concentrations beneath the circular maria that make the lunar gravity field strongly non-uniform.

The consequence is that most low circular orbits develop eccentricity over weeks to months until periapsis intersects the surface. Several early orbiters were lost this way, and long-duration lunar orbiters must either station-keep continuously or occupy one of the few frozen inclinations, near 27°, 50°, 76° and 86°, where the perturbations average out.

This is why Gateway is not in low lunar orbit. A near-rectilinear halo orbit in the Earth-Moon three-body system is nearly stable, needs only a few metres per second per year, keeps continuous line of sight to Earth and avoids eclipses. See Lagrange points.

Regolith

The surface is covered by 4 to 5 m of regolith in the maria and 10 to 15 m in the highlands: fragmented rock produced by billions of years of micrometeorite impact, with no water or wind to round it.

That last point is the problem. Lunar dust particles are sharp, because nothing has ever eroded them, and they carry an electrostatic charge from solar ultraviolet. They adhere to everything, abrade seals and bearings, degrade radiator and solar cell surfaces, and were a persistent nuisance on Apollo, where suit joints wore visibly within three EVAs.

Dust mitigation is an unsolved problem at the scale a permanent base would require, and it is the reason airlock design and surface operations concepts receive as much attention as they do. See Lunar surface systems.

Polar volatiles

The Moon’s axial tilt is only 1.5°, so crater floors near the poles have never seen sunlight. These permanently shadowed regions are among the coldest places in the solar system, cold enough to trap water ice delivered by comets and asteroids over billions of years.

Water matters for three reasons: it is drinkable, it is breathable once electrolysed, and its components are hydrogen and oxygen, which is a rocket propellant. A lunar propellant supply would change cislunar economics substantially, because it removes the need to launch propellant out of Earth’s gravity well. See Refueling and propellant depots.

The qualification is that nobody knows the concentration, the physical form or the extraction cost. Ice mixed at a few per cent through cold, hard regolith at −240 °C in permanent darkness is not obviously economic to mine, and resolving that is the purpose of several current missions.

The Apollo record

Six landings between 1969 and 1972 returned 382 kg of rock and soil, deployed surface experiment packages, and established the fundamentals of lunar science: the Moon’s age, the giant impact origin hypothesis, the absence of water in the returned samples, and the impact chronology used to date surfaces across the entire solar system.

What Apollo left open is as significant as what it settled. All six sites were equatorial and near-side, chosen for operational safety rather than scientific diversity, so the poles and the far side were unsampled. Chang’e 6 returned the first far-side samples only in 2024. Volatiles were absent from Apollo samples, which supported a dry Moon consensus that polar observations have since overturned.

Some Apollo samples were deliberately kept sealed for future analysis. Sample 73001 was opened in 2022, fifty years after collection, using techniques that did not exist when it was sealed. That is a strong argument for the sample return approach generally.

The current wave

The CLPS model buys delivery as a service: NASA pays a fixed price for payload delivered to the surface and accepts a higher failure rate in exchange for lower cost and higher cadence. The early results have been mixed, with several failures and partial successes, which is the expected consequence of that trade rather than evidence against it.

Landing remains genuinely difficult. Chandrayaan-2 and several commercial attempts failed; Chandrayaan-3 and SLIM succeeded, the latter landing within about 100 m of its target and then coming to rest on its side. Terrain relative navigation, hazard detection and precision landing are the enabling technologies, and they are still maturing. See Computer vision in space.

Infrastructure

Sustained operations need communications and navigation that do not exist yet. A far-side mission has no direct line of sight to Earth, which China solved for Chang’e 4 by placing the Queqiao relay in a halo orbit about Earth-Moon $L_2$.

Both NASA’s LunaNet concept and ESA’s Moonlight propose a shared relay and navigation constellation, essentially a lunar equivalent of GNSS plus a communications backbone. Whether such infrastructure is built before or after sustained surface activity is one of the open strategic questions of the current programme.

References

• Heiken, G., Vaniman, D. and French, B. M. (eds.) Lunar Sourcebook: A User’s Guide to the Moon, Cambridge University Press, 1991.
• Muller, P. M. and Sjogren, W. L. “Mascons: Lunar Mass Concentrations”, Science, 161:680, 1968.
• Colaprete, A. et al. “Detection of Water in the LCROSS Ejecta Plume”, Science, 330:463, 2010.
• Folta, D. and Quinn, D. “Lunar Frozen Orbits”, AIAA 2006-6749.
• Crawford, I. A. “Lunar resources: A review”, Progress in Physical Geography, 39:137, 2015.