The Space Reviewin association with SpaceNews

lunar base illustration
Utilization of the Earth-Moon L-1 point could support human activities elsewhere, including a lunar base. (credit: NASA)

EML-1: the next logical destination

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The first Earth-Moon Lagrange point, or EML-1, offers a number of key advantages that make it an ideal destination for activities in cislunar space. Over the near-term, however, its utility is constrained by a lack of physical infrastructure. This can change if our approach to space moves away from destinations and towards a strategy of enabling capabilities.

Talk abounds of going beyond Earth orbit, although once beyond low Earth orbit (LEO) what happens next becomes a little fuzzy in most discussions. This need not be the case, as capabilities can be built from the very first test of a trans-LEO vehicle. Some sample test runs:

  1. Out to GEO: Given launch locations of likely US crewed vehicles, the mission could involve a plane change to geostationary (perhaps through a bi-elliptical transfer for extra-credit), close-approach to a “zombiesat”, and perhaps even retrieval of some old hardware for forensic analysis.
  2. Free-return trajectory: A loop around the Moon to give the heat shield a workout on the return. Perhaps some maneuvering out around the Moon. How close can they shave the rear-end of the Moon at perilune?
  3. EML-1 visit: The main purpose would be to establish a halo orbit. Once there, it would make sense to drop off a package of instruments that could serve a number of purposes.

These test runs highlight some of the future capabilities to be enabled by development of an EML-1 station in cislunar space.

The EML-1 neighborhood is good for looking not only outward, but also back in towards Earth.

Instrumental to the use of EML-1 is the concept of the halo orbit. It is a technique that allows an object to orbit an empty space, typically a Lagrange point, as with SOHO, WMAP, Genesis, the future JWST, and others. The best, though inapt, way to think about it is as a sort of gyroscopic effect, as with a bicycle wheel, where the hub is on the line connecting the centers of gravity of two objects (like the Earth and Moon), and the satellite is on the wheel. The action of the orbit helps to keep the object in place; some station keeping is required, but at a level orders of magnitude less than the ISS.

One space issue to which the public is highly sensitized is that of asteroids. We’ve gotten much better at find and tracking them, but we still have a huge blind spot that is highlighted by the YouTube video by szyzygy entitled “Asteroid Discovery From 1980 – 2010” (shown below). Watching the video, it quickly becomes obvious that the huge majority of objects are found looking away from the Sun; typically early in the morning at first, but across wider swaths of the night sky as the years roll on. Interestingly, in 2010, discoveries start being made ahead of and behind the Earth in its orbit as well as away from the Sun.

Still, we’re not looking inward, and the majority of the asteroids that we only get hours or days notice of their close flyby (or impact, as in Sudan) are coming out from the Sun, what I refer to as blindsiders. We are blind on the Sunward side, and we need space-based instruments to look that way (but not directly at the Sun). This, then, is an ideal “first mission” for instruments emplaced at EML-1. The instruments could revolve around the EML-1 point in a halo orbit, looking outward. The length of each observation period could be varied by adjusting the diameter of the halo orbit. Over the course of a month you can get a full sky survey, including out-of-the-ecliptic objects like comets. Instruments could start out simply identifying objects and basic characteristics, with additions over time as capital becomes available to resolve more wavelengths and better characterize each object. There is even a business case to be made that such data could be sold on a subscription basis to governments, NGOs, educational institutions, and corporations looking for a mining target.

The EML-1 neighborhood is good for looking not only outward, but also back in towards Earth. EML-1 is a naturally clutter-free environment, since space junk doesn’t normally have station-keeping capability, as well as a “high ground” that allows observation not only of the Earth but also everything that’s orbiting around the Earth out to GEO. Instruments could be placed there for the specific purpose of keeping an eye on the traffic in cis-GEO space. By the same token, it is also ideal for looking at the Moon, and serves as a natural gateway to the entirety of the Moon’s surface. Bigelow Aerospace has proposed using EML-1 as an aggregation point for modules to be emplaced on the Moon’s surface using Armadillo Aerospace rockets for the descent. As far back as 1986 a Lunar Spaceport was envisioned as a kind of motel/gas station/warehouse/restaurant/garage for space travelers.

In terms of propellant, it is cheaper to go from EML-1 to GEO and back to EML-1 than it is merely to go from LEO to GEO. Over the long term, it makes sense to stage GEO operations from EML-1.

The Moon is still a bit in the future, though, as there are still a lot of things to consider on the Earth side of EML-1. One of the key advantages of staging at EML-1 is, as Brad Blair notes, “its ability to fall into various inclinations without a major [delta-V] penalty, thus increasing the number of customers that could be reached by a small set of vehicles and systems elements.”. What this means is that the inclination you end up in in LEO is set by how you depart from EML-1.

If you picture the Earth-Moon system, and the line connecting their centers of gravity, EML-1 is about 85 percent of the way to the Moon. Trace an ellipse from the EML-1 point down around the Earth and back up to EML-1. Put the perigee close enough to Earth and you can even get some aerobraking. This ellipse can be assumed to be in the orbital plane of the Moon. Now, rotate that ellipse around the Earth-Moon axis. This means that every inclination of LEO orbit is accessible, from the 28.5° of Kennedy Space Center, to the 51.6° of ISS, to even polar orbits, although the latter suffer from more delta-V penalty by virtue of the Earth’s oblateness.

If all of the LEO inclinations are available from EML-1, then it is also true that any LEO inclination can get to EML-1. This means that the ISS can serve as a staging point for missions to EML-1 in the nearer term, and later stations in different inclinations can also reach EML-1 as they come online. There is no need to wait to get started. As soon as a crew vehicle comes online it can start staging from ISS to EML-1, first as test-runs, then as missions to emplace as well as service, upgrade, and refuel assets. By the time crewed facilities are emplaced there will already be regular traffic to the location.

One question often raised is “What would a crew do at EML-1?” There are a myriad of answers:

1) In terms of propellant, it is cheaper to go from EML-1 to GEO and back to EML-1 than it is merely to go from LEO to GEO. Over the long term, it makes sense to stage GEO operations from EML-1. What kind of operations? The easiest answer is salvage, given the hundreds of tonnes of scrap circulating in GEO. Crews could fall down to GEO, spend a few days retrieving defunct equipment like failed satellites, and then return to EML-1 to process it. Whatever could be reused in some way is unknown, but the real value is in the forensic analysis of how the satellite weathered in the GEO environment over a known period of time. That kind of information allows for better satellite design.

2) EML-1 is an on-ramp to what are known as the InterPlanetary Superhighways (IPS). These are a network of ridges and ripples in space created by the gravitational effects of the planets and Sun. A satellite pushed onto the IPS will travel very, very slowly along this network to its destination, where it can kick itself into a halo orbit around a Lagrange point and collect data. Locations of interest would include the Sun-Mars L-2 and Sun-Jupiter L-1, to observe the Asteroid Belt; the Sun-Venus equilaterals at L-4 and L-5 to provide communications relay when Mars is on the other side of the Sun from Earth; Sun-Saturn L-2 to look at the Kuiper Belt; Sun-Neptune L-2 to look at the Oort Cloud; Sun-Mars L-1 as a waypoint on the way to Mars and the Asteroid Belt; Sun-Earth L-1 to watch the Sun; Sun-Earth L-2 to watch the stars. The key is that all of these instruments would also be able to return via the IPS to EML-1 for regular maintenance and servicing. As more probes are added to the network, instead of thrown into the void, there will be an increasing stream of probes in need of work.

3) The time lag from EML-1 to the Moon and back is much less than that for Earth-Moon. As a result, it is a better location for safe teleoperation of robots on the lunar surface.

The next destination is EML-1. Because from there we can build for the Moon, Mars, asteroids, and more.

4) As lunar activities ramp up, there will be an increasing need for freight handling of goods destined for the Moon, as well as those from the Moon. Early lunar exports are likely to be low-value-added goods such as oxygen, water, raw regolith, and some metals, but as more capabilities are established the exports will start creeping up the value-added chain: foodstuffs from lunar greenhouses, crafts created by the locals from local materials, increasingly sophisticated entertainment like dance and music, and so forth.

5) EML-1 is an ideal location to aggregate mission components for a trip to an asteroid. Fuel can come from the Moon, while spacecraft come from Earth. Someone’s going to have to put all of that together.

6) A facility at EML-1 can serve as a communications node for lunar operations to overcome the line-of-sight issue. Additionally, with solar sails “pole-sitting” above the north and south lunar poles, communications with the far side can be established.

7) Port services. While probes returning on the IPS will end up in the neighborhood of EML-1, they will need to be picked up. Same thing with freeflyer platforms sent on low-energy trajectories around the Moon for production runs. A space tug would be a good tool to have, and someone has to fly it.

These are just a few ideas, which can easily be expanded. If a crew can fall down to GEO for a servicing run, it can also fall down to HEO, MEO, and LEO for servicing missions. If an asteroid mission can be assembled at EML-1, so too can a Mars mission. If crews are salvaging dead satellites and kick stages from GEO, they may be able to cobble together the parts for other missions, selling the result to whomever wants to buy a space probe.

The next destination beyond low Earth orbit is not Mars, or the Moon, or an asteroid. The next destination is EML-1. Because from there we can build for all of them and more.