The five Lagrange Points in the Earth-Moon system
L1 Gateway Station
Tech Level: 12
L2 Farside Communication Relay
Tech Level: 12
L2 Astronomical Observatories
Tech Level: 12
L4, L5 Clusters
Tech Level: 15
This article concerns the Earth-Moon Lagrange Points. Structures at other solar system Lagrange Points will be discussed in a future article. Lagrange Points are also known as libration points.

Lagrange points are locations in space in a two-body system where the forces of gravity and orbital motion balance each other out, creating small regions of orbital stability. French mathematician Louis Lagrange originally worked out the orbital mechanics of the positions in 1772, thus the points bear his name.

An object placed at a Lagrange point will be in a state of gravitational equilibrium, and will orbit with the same period as the bodies in the system. In other words, in the Earth-Moon system, an object in a lagrange point will keep pace with the Moon in its orbit about Earth.

In any two body system where one body orbits the other, there are five lagrange points. The first lagrange point, usually abbreviated L1, is located directly between the primary and the satellite. In the Earth-Moon system, the L1 point is roughly 200,000 miles (323,110 kilometers) away, or roughly 84% of the way to the moon.

The L2 point lies in direct line with the L1 point, but at a distance of some 37,000 miles (60,000 kilometers) behind the moon. The L3 point also lies along the same imaginary line of L1 and L2, but on the opposite side of Earth from the moon, in the Moon’s orbit.

The L1, L2, and L3 lagrange points are all what’s called metastable. The forces of gravity and orbital motion are precisely balanced at these points, but even a slight nudge will send any object at them drifting off. Think of a ball balanced precisely on top of a hill; though the ball is stable and in equilibrium, even the slightest push will send it rolling off. Because satellites even in the vacuum of space aren’t completely devoid of forces acting on them (such as the solar wind, mircrometeors, and even light pressure), anything placed at the first three lagrange points will need periodic course corrections to keep them in place.

The L4 lagrange point lies 60 degrees trailing the Moon in its orbit, and the L5 lagrange point lies 60 degrees spinward of the Moon in its orbit, about 238,000 miles from both the moon and Earth, forming an equilateral triangle with those bodies. These are also called the Trojan Points, after the asteroids Agamemnon, Achilles and Hector that orbit in the L4 and L5 points of the Jupiter-Sun system.

Unlike the first three lagrange points, L4 and L5 offer true orbital stability. Whereas objects at lagrange points 1 through 3 can be held akin to a ball on a hill, objects at L4 and L5 can be thought of metaphorically as a ball in a large shallow pit on top of that hill. A gentle push at an object at these point will not send it drifting away, but will instead put it into “orbit” around the lagrange point.

Tech Level: 12
For large-scale exploration and colonization of the Moon, NASA has long since envisioned a “staging base” station at the Earth-Moon L1 point. It was part of their long-enduring, long-term architecture for manned exploration of the solar system, which envisioned a “step ladder” of different stations leading out into our interplanetary neighborhood. Gateway had been planned as the second rung on that ladder after the low-orbit International Space Station.

However, ever-tightening budgets forced NASA to abandon even considering an L1 station, and current plans to return to the moon generally do not mention it except in abstract. However, the presence of such a way station would be very advantageous to any effort to set-up a permanent manned presence on the Moon, and it may be constructed by future administrations just for the sheer practicality of it. The L1 structure is also mentioned often as a logical staging point for many missions further out into the solar system.

An L1 Gateway station would have a number of important functions. One, it is in a perfect location to monitor and coordinate communications among various expeditions and missions on the nearside of the Moon. A vessel launched from L1 could reach anywhere on the Moon within a few hours, which would make it ideal for coordinating and initiating crisis management. Also, it would function very much as a waystation, especially once built up, and would probably be used to handle tourists and casual visitors to our natural satellite.

Gateway can also serve an important function as a manufacturing and repair center. At first, one of its most vital functions would be to repair and service translunar spacecraft. As the station matured, it may also be handling a great deal of the raw materials lunar mining is expected to produce, and thus may become a major hub in Earth-Moon commerce and economics.

Given the conservatism (in general approach, not in the current political sense) of the world’s space programs, the initial L1 Gateway station would most likely resemble the current modular ISS with a number of upgrades and modifications. In fact, during the first few years of its existence, it is likely to remain unmanned for long periods at a time, and be used only when a major expedition or endeavor is currently at work on the lunar surface. This would probably change as a permanently manned moonbase is established, and Gateway would likely expand along with a manned presence on the Moon.

One major difference between Gateway and ISS would that its would have to be much more heavily insulated, as outside Earth’s magnetic field it would be constantly exposed to the full strength of both radiation from the sun and cosmic sources. Also, as the L1 point is only metastable, minor course corrections would have to be made at least once every two weeks or so to keep it in place.

Tech Level: 12
A farside halo satellite would be used on the L2 Lagrange point on the far side of the moon. It would be set up in a broad orbit perpendicular to the plane of the Moon’s orbit. This would give it the unusual property of always being visible from Earth at every point in its orbit, inscribing a circle in the sky slightly larger than that of the moon itself. In other words, to an observer on Earth, the satellite’s rounded path in the sky would always circle the disk of the moon but never slip behind it. The satellite would always therefore be facing the farside of the moon and Earth simultaneously, allowing for unbroken communication with and observation of any facilities, base, or phenomenon on the farside of the moon.

However, maintaining a halo orbit around a metastable Lagrange point can prove very tricky, and the satellite would have to be constantly monitored for necessary course corrections.

Tech Level: 12
The L2 point lies some 35,000 miles beyond the moon, and thus provides an unprecedented stable venue for astronomical instrument away from Earth’s glare and radio noise. Space-based radio telescopes and the like are already a proven technology, but one has never been placed so far out in space. Also, because L2 is metastable, periodic station-keeping would be required.

Tech Level: 15
Both the L4 and L5 points are functionally the same, only in different positions, and thus are grouped together here. One type of structure designed for these points, a space-based particle accelerator, is discussed in the Orbital Hot Lab article.

The L4 and L5 points are often the sites of gigantic space colonies in science fiction, such as Bernal Spheres or O’Neill Colonies or Hollowed Asteroids. This is because they offer true stability, and an object at or orbiting one of these points can stay in that position indefinitely. For large space structure whose course corrections in other orbits would require large expenditures of fuel and energy, this is a very attractive option.

Over time, the L4 and L5 points would likely become crowded as they would be prime real estate for large orbital structures, much like geosynchronous orbit is for communication satellites today. As an orbital infrastructure grows, numerous structures including colonies, stations, and automated satellites would likely be seen circling the latter lagrange points in tight, carefully choreographed halo orbits.

These clusters would also be logical locations for microgravity manufacturing efforts, as they would be in a very advantageous position to intercept raw materials launched from the Moon. In fact, large scale production of any lagrange cluster would likely require the establishment of an extensive lunar mining effort as a source of raw materials.

Solar Power Satellites, discussed in another article, would also be advantageously be positioned at L4 or L5. However, as colonies and stations are assumed to have some waste outgassing from time to time, this could create a tenuous “atmosphere” around the lagrange point that could degrade solar cell performance and operational lifetime. Thus large dedicated Solar Power Satellites and space colonies may not be a good mix in close proximity at L4 or L5 over any long period of time. The colonies’ own solar power arrays would have to deal with the phenomenon, but given the circumstances they would have little choice but to put up with the extra expense of more maintenance and lower energy conversion percentages.

It may be possible that one lagrange point may be reserved exclusively for solar power satellites, and the other exclusively for space colonies. The former may even be used to provide most of the power for the latter.


L1 Gateway

L2 Farside Communication Relay

In Print:

Encounter With Tiber by Buzz Aldrin and John Barnes

On The Web

L2 Astronomical Observatory

L4, L5 Clusters

In The Media:

The various Gundam anime series, et al.

On The Web: