Some folks that stop by out here in the Selenian Boondocks might be wondering just what exactly this whole L-1 thing is all about.
Simply put, L-1 is the point on the line connecting the center of mass of the Earth and the Moon where the pulls from either side are balanced out. It’s mostly a function of the Earth’s gravitational pull, the Moon’s gravitational pull, and centrifugal force. An object, like a space station, placed at this point will remain there, absent any outside displacement. Since both the Sun and Jupiter are outside displacements, objects put there aren’t going to stay there. This also means that debris will not accumulate there.
There are ways to solve the problem, like halo orbits and a modest (on the order of 10s of m/s) expenditure of station-keeping propellant. Other orbits, like a lissajous orbit, are more complex but offer different benefits. What is important, though, is that the L-1 point is the gravitational ‘high ground’ between here and the Moon. This makes it the lowest delta-V launch point to a number of other destinations of interest.
It’s exact distance from the Earth or Moon varies throughout the month proportional to the variance of the distance to the Moon in its orbit around the Earth, but generally the average is taken to be around 58,500km from the Moon, or 57,660km, or 66,000km, or somewhere in between depending on which source you use. And of course, most sources don’t specify whether that’s to center of mass or to the surface of mass (the more important consideration, since that’s where you’re landing).
Its orbit doesn’t follow the regular rules of orbital mechanics, since its period has to match that of the Moon. Even more bizarrely, objects that move ahead of the L-1 point in its plane of orbit will be pushed back towards the L-1 point. Same thing if it falls behind. This is because the gravipotential warp starts climbing ‘uphill’ towards the L-4 and L-5 points in front of and behind the Moon. It helps to look at a topographical type image to picture it.
The thing to remember is that it is closest at the Moon’s perigee and farthest at apogee. Because its location is defined by the gravitational fields, it lies in a network of gravity boundaries. Objects sent along these curves are in effect ‘surfing’ the gravity boundaries between all the planets. They use very little fuel, at the expense of time, to get where they’re going. Much more importantly, they can be made to return to Earth, which opens up a completely new realm of possibility in how we study our Solar system.
There are a few papers available on the Internet to explore this fascinating topic.
“The Lunar L1 Gateway: Portal to the Stars and Beyond” by Martin Lo and Shane Ross. These are the guys that helped to pioneer the concept of the ‘Inter-Planetary Superhighways’ or IPS that connects all of the planetary Lagrange points in our Solar system (though its use dates back to the early sixties). It includes lots of graphics to illustrate the points, and notes that the Genesis spacecraft return trajectory was very similar to the one taken by comet Shoemaker-Levy-9 in it’s date with fate on Jupiter, and quite possibly also the asteroid that wiped out the dinosaurs. This kind of knowledge only serves to highlight how important it is that we make a concerted effort to catalogue the other objects in orbit around the Sun, in part so that we can develop a gravitometric map of near-Sun space and look for those objects that might sneak in through the back door. Such a map would serve a lot of other useful purposes as well.
Kind of an interesting paper is “CEV Architectures – Cost Effective Transportation System to the Moon and Mars” by Leisman, Joslyn and Siegenthaler. These are folks that teach in the Department of Astronautics at the USAF Academy and work in flight research at the USAF Test Pilot School. They effectively advocate an EELV launch architecture and station at L-1. Some of the assertions are nothing short of astounding, like “A 70% reduction in launch costs could be realized if EELV has block buys over 30” (sourced from AIAA 2002-4314). From which follows the same wisdom I advocate (though from a vastly different analysis perspective):
“Not only would economies of scale be realized in the American launch industry for the first time in years, but the demand would enable a healthy launch market to maintain both EELV contractors ensuring NASA, the DoD, and commercial users assured access to space”
(Under the heading ‘NASA Should Not Develop an Ultra Heavy Lift Vehicle’)
“If NASA launches three Heavy EELV per year along with the military and commercial requirements for Medium and Heavy EELV, you have just built economies of a scale to make the EELV family a low cost system for all three customers (win/win/win)”.
Gotta love that. The paper also advocates use of LH and LOX, since both are also used in fuel cells and water needs.
So what happens when a group of Aerospace Engineering undergraduates at UMCP takes a look at the problem? You end up with a nice project paper like “Clarke Station: An Artificial Gravity Space Station at the Earth-Moon L1 Point”. This is a nice design project that ventures a little more into the station concept than the prior references.
“Strategic Considerations for Cislunar Space Infrastructure” by two well-respected Lunar scientists, Wendell Mendell and Steven Hoffman, also looks at EML-1 and strongly advocates its use based on meeting such criteria as:
-It should be near the edge of the Earth’s gravity well so that reusable interplanetary vehicles can come and go with a minimum of propellant.
-It should be accessible from the Earth and the Moon with a minimum of constraints on launch windows.
-There should be no hazard from artificial debris.
-The region of space in which it sits should support colocated unmanned scientific platforms which can be reached from the space station by small delta-V transfers.
-The fuel requirements for station-keeping should be small.
Sounds to me like they stacked the deck a bit, but those are all important considerations for a permanent space-based space exploration infrastructure. There are a couple of very useful tables, including mass surcharge comparisons.
These should be helpful in understanding why the L-point between here and the Moon is really the next logical destination beyond LEO. First to install instruments, later facilities. An investment now in doing so can have enormous value benefits not just in going back to the Moon, but also for operations in GEO, traveling to NEOs of interest, and launching for Mars so that half the planet can watch and cheer on those brave souls.
Since our space efforts are about human culture as well, below are the lyrics that I sing to ZZ Top’s Lagrange. I always envision a crew vehicle coming sweeping in towards the station, with the Moon further off in the distance, and then everyone disembarking to stretch their legs, breathe some new air, and hit the bar to socialize. And of course a shot of the lovely and competent young lady working comm during the ‘nice girls’ line, coquettishly winking at the camera.
Well you must’ve been down
in that station town,
in that shack outside Lagrange
You know what I’m talking about.
Just let me know
if you want to go
to that home out on the range
They got a lot of nice girls, heh. 😉
Well the air is fine
if you’ve got the time
and the grand to get your ship in.
A hmm, hmm.
And I hear it’s tight
most ev’ry night,
but now I might be mistaken.
hmm, hmm, hmm.
(with my sincere apologies to ZZ Top)