Getting mass off a planet or moon and into orbit is very energetically expensive. In the future, we may use launch assist systems to deliver mass with factory-like economics to reduce costs (I call mass-lifting). But even if we do this on objects like the moon, the projectiles would be like tiny little morsels of matter painfully pushed through an incomprehensibly long and thin straw.
For building large space colonies in orbit (I mean really huge), mass-lifting from the moon is a tricky proposition. Asteroids are clearly a good alternative, but they are not a panacea for massive space industry because you must either choose between small masses, inaccessible locations, or outrageously long travel times.
If the goal is something like an O’Neil space colony, then a more extreme scheme will be needed to do build habitats at low cost. Here, I will begin to intellectually entertain the process of deconstructing an entire astronomical body, particular with Martian moons, large asteroid belt objects, or Jovian moons in mind as the source material.
The Mass Distribution Problem
Many people are surprised to find out that our moon is larger than all objects in the asteroid belt put together by a large margin. Evolution of our solar system by gravitational interactions and collisions underwent something like Ostwald ripening:
The big ones get bigger, and the small ones get eaten. That is the reason that the space around Earth is extraordinarily clean, and why mass values of astronomical objects are distributed in a hierarchical pattern. This is a big problem for people who envision a space society based around asteroids, because the asteroid materials are either relatively scarce (like Near Earth Asteroids) or have extremely inconvenient orbital parameters, such as high inclinations or being highly elliptic.
Lifting — Essentially Complete Deconstruction of an Astronomical Object
A launch gun on the moon is “mass-lifting”, but make the object itself the first word in that phrase, “moon-lifting”, “asteroid-lifting”, or “star-lifting” and you express a very different concept. While mass-lifting would bring a marginal fraction of the mass out of the gravity well, moon-lifting would completely destroy the object’s gravity well, dispersing the material en-mass into chunks which are all much smaller than the original object.
Why Would We Ever Do This?
To repeat the scenario mentioned about O’Neil habitats, someone would likely process the materials to make the steel hulls of massive space habitats.
Fleets of Island Three habitats would likely require radical mass-delivery methods like moon-lifting. Other, more creative, applications for bulk masses also exist.
Mining Trace Minerals
A growing number of asteroid bulls believe that we will conduct mining operations on asteroids. The point is not to transport the entire mass, but to set up refining operations on the asteroid and only deliver a small quantity of precious metals extracted from it. Digging into a body with significant gravity presents a sort of complication that’s not necessary for small objects. As such, moon lifting could eventually become something tantamount to strip mining in space where enormous quantities of matter are scoured by methods that have extreme economies of scale by traversing newly exposed surfaces from a moon or asteroid.
Micro-gravity opens up exotic possibilities for extraction and manufacturing industries. The lack of gravity allows for the very low effort in opening up seams in the rock. Arrangement of the rock can be manipulated in however one sees fit, which is important for mining extremely small quantities of rare trace elements. You can imagine slabs of rock with regular spacing that the mining robots pass in-between and use unique methods of extracting the desired minerals, like magnetic attraction. This also allows for highly effective imaging technologies to scan the constant-width slabs with instruments on both sides.
Spin-Up Disassociation
High gulfs of Delta V introduce massive inefficiencies, and the concept of moon-lifting benefits from the fact that only relatively low Delta V values are involved. Because of that, completely ordinary materials can be employed in the operations to disassociate the mass of the object. The most obvious first step to this is to simply spin the moon until parts of it begin to break apart. The angular momentum isn’t actually that hard to provide, since it comes from the produce of (mass) x (radius) x (velocity). In our scenario, a large radius is the trump card.
So the scheme goes like this: break off a part of the moon (it may not need to be a large part) and put it in orbit with a tether dangling own toward the surface. Now, tie this tether down to some kind of track (fancy electromagnetic couplings or conventional, it depends on the size of the moon you’re working with). Now just tug along the new moon of the moon.
That process will raise the orbit of the orbiting rock and spin-up the moon. It’s interesting to note that the vehicle on the surface that’s dragging the moon actually goes slower and slower as the moon gets further away, but the ground underneath it starts to move. Even as the ground itself approaches free-fall, the vehicle still experiences some gravity since it’s running opposite to the direction of movement. Maybe this isn’t an important detail to point out.
The orbital parking spaces for the resultant fragments would either need to stay within a cluster of their own gravitational interactions, or attain some distinct orbits around the planet or larger body that the entire system orbits. For most cases, the former sounds more reasonable to me.
Followup Calculation Work
There’s much too much to say about this topic to put in an introduction article. My arguments here inherently hinge on the crude numbers on the subject. As I’ve argued again and again, good futurism involves comparing quantities that we can know about competing future scenarios. Again, moon-lifting is like strip mining, so we’re balancing the benefit of getting those materials against the energy needed to lift the material. That energy is approximately equal to the total binding energy of the object.
Perspective on Gross Energy Needed
I like the units of TW-years, Terrawatt year units. Earth right now consumes around 15 TW, maybe 16, my memory could be a few years outdated. So right away, we can start talking about a role of thumb about which moons are viable to moon-lift without assuming a starkly different scale of industrial society in space than what we have on Earth.
Competitive Comparison to Asteroid Herding
Destruction of a moon is a competitive process with going out into deeper space, capturing an asteroid, and bringing it back near to the rest of your facilities. Both of these processes can have hard numbers put on them, and even better, time tables. Asteroids are in heliocentric orbits, so there’s a certain time measured in years that it would take to move one into the needed orbit. Since moons are clustered more toward planets, they are likely (not guaranteed) to already be in the general region where it’s needed, with only the gravity well (from binding energy) standing in the way.
Thus, we have a direct competition between binding energy and orbital changing maneuvers for asteroids.
Competitive Comparison to Space Steel Manufacturing
This one is more difficult to explain, and it involves a more fringe concept, described in my alternate blog on the subject.
We can avoid using structural materials in favor of passive pressurization from gravitation. The catch is that doing this requires a huge amount of material. Those materials are convenient to get if you start out at a cherry-picked asteroid, of which 100s or even 1,000s of suitable candidates may exist, but they don’t exist quite in the right places. For a more well-connected and larger society, moon-lifting as a source for the material would make a vast amount of sense.
I have a mathematical rule-of-thumb in mind for the gravitational binding energy versus the compression energy in the air contained therein. These two things are about equal, but the rock density is greater than the air by a factor of around 2,000. Also, very importantly, moon material is typically much higher density than what asteroids can offer. Asteroids might be more useful as a source of certain elements necessary for organic chemistry than as big dumb rocks, a role better served by moons.
So the challenge for moon-lifting is exactly how much extra energy you can tolerate in the construction of a gravity balloon. A comparative economic option lies both in the energy required to move an asteroid (almost certainly too high) as well as the energy needed to manufacture structural materials that present an alternative option to passive pressurization.
