A space station at the L1 Lagrange point of the Earth-Moon system
One might wonder why someone passionate about establishing a human presence on Mars would be interested in space stations near the Moon. I’ll tell you and describe what I think we could build.
First, the reasons why:
1. There are two major challenges for a spacecraft travelling from one planet to another. The first is the Entry, Descent, Landing (EDL) from the Earth’s atmosphere to its surface. The second is the EDL to the destination planet or other celestial body (currently only the Moon or Mars). Takeoffs from these surfaces and subsequent interplanetary injection are significant but secondary. However, there can still be issues arising from the rocket’s ascent (from the surface of the visited celestial body), which would need to be addressed before entering into the major EDL challenge. A ‘revision’ stopover in a zero-gravity space station would allow us to do this.
2. A stay on a rotating space station, with a gravity greater (I propose 0.5g) than that of the Moon (0.16g) or Mars (0.38g), would allow humans who have spent long periods on the Moon or Mars to readjust less abruptly to Earth’s gravity (1g). This 0.5g gravity would be the maximum given the station’s mass. Indeed, under the assumption—which I’ve adopted—of a torus, it’s difficult to imagine an even greater distance than 112m to travel from this torus to the center of gravity, or a number of rotations per minute of the torus higher than 2 revolutions per minute.
3. A stay in a humanized and medically equipped environment could serve as the last phase of a health quarantine (started with the travel) upon return from Mars and to conduct a final check of the risk of contamination an astronaut might present before returning to Earth.
4. An emergency departure from the Moon for health reasons resulting from insufficient gravity.
5. The touristic appeal of staying in this remote but not too remote space could be a source of revenue for the company operating the station, while simultaneously contributing to the financing of Mars missions by generating economies of scale in the unit cost of launches through the number of flights.
6. At the end of its life, the whole station could be left drifting into Space (good for ecology and the opportunity of some last observations).
Station location:
A station near Earth must be very close to it (less than 600 km). Otherwise, it would be too close to the Van Allen Belts (inner and outer) that surround Earth like a giant torus and can reach altitudes of up to 60,000 km, well beyond geostationary orbit (37,000 km). Of course, one could choose to locate the station just above this 60,000 km altitude or in an orbit above it up to the limit of our planet’s Hill volume (the point of equal gravity in a three-body system). Precisely, the L1 Lagrange point of the Earth-Moon system is at the limit of this volume. It corresponds to the point where the gravity generated by the Earth is exactly balanced by the gravity generated by the Moon. It is the closest of all our Lagrange points and is located 326,000 km from Earth and 58,000 km from the Moon. The advantage, besides its proximity to Earth, is that this point is not lost in space, that it avoids a very large and long orbit around the Earth (at L1, we know that the station is always close to the Moon), and that, if necessary, we can take refuge on the Moon or bring materials up from it (due to its low gravity). From a tourism perspective, one can imagine that being able to enjoy the view of the Moon and the Earth with a magnifying effect resulting from greater proximity would be quite appealing.
Minor drawbacks include eclipses and orbital stability:
Everyone will have noticed that along the Earth-Moon axis and in the vicinity of the Moon, there can be long occultations (‘eclipses’) of sunlight (the ‘new moon’ phase and, by extension, the phase between the last and first quarter moons). The Lagrange point L1, which lies on this axis, is obviously subject to these occultations. This is a ‘minor’ problem (to put it mildly) if we want to harness solar energy (the easiest source to use)! However, there is a solution: orbiting L1 in a ‘halo orbit’. We can estimate that such an orbit, with a radius of 20,000 km from L1, would protect us from most occultations: claude.ai estimates them at 2 or 3 per year, each lasting only a few hours. During these occultations, the lack of solar energy could be addressed with 10-15 tons of modern Li-ion batteries (capacity ~500-750 kWh). Note: in case of an accident, these batteries would allow us to operate for several days in reduced power mode.
Unlike points L4 and L5, points L1 and L2 are unstable. However, the latter two are quite far from the Earth-Moon axis, and their stability favors the concentration of matter (in the case of points L4 and L5, no asteroids but dust). The instability is manageable, though. It simply requires regular, slight trajectory corrections. Note: The necessary adjustments to Lissajous orbits are more frequent, which is why halo orbits, although more difficult to initiate, are preferable. Estimates from claude.ai indicate, for a halo orbit, approximately 10-50 m/s of Δv per year (depending on the orbit size) of adjustments (in our case, it would be ~30 m/s). For these adjustments (solar electrical power), the required expellable gas (xenon) would be very modest in mass, 30 kg per year, and the volumes very small (70 kg in a pressurized tank at 150 bars hold in a 50 cm x 50 cm cylinder). We could therefore have a 210 kg stock (for safety) and replenish it with 70 kg every year.
L1 is therefore the orbit we should choose.
Next week I will talk about the station’s structure and the advantages of orienting it towards the Sun while protecting it from its rays.
Title illustration: Earth – Moon system Lagrange points. Credit Wikipedia Commons.
Computations have been made with the help of claude.ai
