Which lagrange (earth-moon) point is better for a space settlement?

Question

Which among the Earth-Moon Lagrange points (EML1, EML2, EML3, EML4, EML5) is best for a permanent space settlement which requires materials from the moon for construction?

Some say it's EML4 for offering stability, while others propose EML1 because any object can reach the settlement from the Moon within hours or in a day at most.

Which Lagrange point supports good communication?
Which Lagrange point can offer better movement to the Moon?

Is there any other orbit that can be better than EML points for positioning the settlement, such as a lunar orbit? If so, can you provide specifications, please?

Edit: The settlement will not depend on Earth resources.. but it will depend on lunar resources for construction only. After construction, it is independent... i.e. without depending on the Earth or lunar resources. By better, I mean... which is a more potential orbit to make sure things from the Moon can reach early and safely to the settlement and a stable orbit. Thanks a lot!

Answer 1

L2 has the advantage of serving communication from the far side of the Moon. I think L2 will be an early starting point for cis-lunar infrastructure, and like cities grow around a village which started as a single farm, it might become the grain for a future space colony (if that stuff will work the same way). A spacecraft doesn't stay put in a lagrange point, it orbits it. While orbiting L2 you can always have line of sight to Earth and the Sun, the spacecraft is never strictly in line with Earth and the Moon.

You don't need a space station to directly improve transportation to the Moon. You simply launch from Earth surface to lunar orbit directly, like the Apollo missions did. Once over there you can supply yourself with fuel mined on the Moon, if that will become a reality.

Another option to consider, for low mass satellites at least, is using solar sails to create non-keplerian orbits, so called statites. It could of course also be done with other propulsion technologies which require refueling. Using continuous low-thrust propulsion (especially sunlight/wind which is tax free) you can modify the geometry of effective geocentrical and other orbits as well as the lagrange points. But a "settlement" sounds too heavy for doing that.

And then there is DRO, distant retrograde (lunar) orbits. A minor drawback with them is their availability. You can launch from Earth to the Moon or any lagrange point any day. But something in DRO might have much rarer launch windows.

A "space station" need not be stationary. When it needs materials from the Moon, it will enter some cis-lunar orbit. When it needs more energy it will go closer to the Sun. If an asteroid or the Jovian moons are of interest, it could go there. Massive structures change trajectory slowly and expensively, but they don't have any roots. Where ever you want a space settlement, it will likely be best to construct (most of) it in earth or lunar orbit, and then push it to its intended destination.

Answer 2

In 1975 NASA did a design study that selected L5.

There remain the orbits about the five libration points. Three of these, L1, L2, and L3, are known to be unstable, and to maintain orbits around any of these three points for long periods of time requires appreciable expenditures of mass and energy for station keeping.

There do exist, however, large orbits around both of the remaining [l]ibration points, L4 and L5. These have been shown to be stable (refs. 27, 28). A colony in either of these orbits would be reasonably accessible from both Earth and Moon. One of these libration points, L5, is chosen for the location of the first space colony. This choice is somewhat arbitrary for the differences between L4 and L5 are very slight.

The study describes how the whole station could be built using lunar materials and technologies that were known at the time, or could be reasonably expected to become available.

Both L4 and L5 allow direct communication lines to Earth-bound or Earth-orbit communication networks. Both points also allow direct communication with one hemisphere of the Moon.

The whole study was produced in the heady days before the NASA budget was cut. It is pretty amazing.

You may also be interested in the NASA Space Settlements page.

Answer 3

Without much more details on exactly what the settlement will be used for, what materials / supplies / communication will be required from the Earth and the Moon and even how many and how frequently settlers will be ferried to / from the settlement, it is impossible to give any proper answer. I did a bit of research and can at least give a discussion on each point below:

L1

This would be an excellent point for communications between Earth and Moon. You would also have quick access to the Moon. This point would have lower exposure to solar wind and cosmic rays than the other Lagrangian points.

This point is not fully stable, so you will need active stationkeeping.

L2 and L3

This is at the far side of the moon and the opposite side of Earth from the Moon, so neither provide any real benefits to the settlement proposed.

L4 and L5

These points are stable, so no need for active stationkeeping.

They are, however, less easy to access from the Moon and will have higher exposure to solar wind and cosmic rays than L1.

Lunar orbit

A settlement in lunar orbit should be fairly easy to access from the Moon. The Moon doesn't have enough of an atmosphere to cause any real orbital decay, however, due to the Moon's lumpy gravity, there are only a few orbital inclinations available for low lunar orbits.

You can design the orbit so that communications should not be a problem. And even if the materials you need to mine is on the far side of the Moon, it would be easy to set up the orbit so that the settlement can be accessed from the mine with moderate delta-v requirements.

Relevant wikipedia article