One of the fun things about grad school in science or engineering is getting a bunch of highly technically educated people together to go see a movie. Like “Battle: Los Angeles.” If you want to see a movie with Marines being very Marine-y and some big gasoline explosions, go see this movie. If you want to see cool aliens, awesome technology, and innovative ideas, then, uh…don’t.

I’m not going to do a general review of “Battle: LA,” nor a general critique of the science. (I will leave the latter up to Ryan, and I’m sure if he does such a critique it will be a fantastic read.) I will say that I liked how the aliens basically use guns and jets/rockets instead of inexplicable hover-things and energy blasters, and I liked that the reason the aliens are unstoppable at first is not because of their tech but because our soldiers don’t understand how to fight them. (Of course, the usual video-game rules of technology apply: three bazillion M-16 rounds fired into an alien aren’t enough to kill it; but do one quick alien autopsy in the field and suddenly all our guns work with full effectiveness!)

It’s the premise of the movie I want to poke at. The whole reason the aliens are attacking Earth is to claim our resources. Sound familiar? In a brief glimpse of a TV news program, Professor Greybeard explains (scientists, get your cringes ready!):

The aliens must be attacking us for our resources. Specifically, our water. 70% of Earth’s surface is covered with water, and the chemical composition of our water is unique in the solar system: it is in liquid form.

(I paraphrased from what I could recall.)

This is both factually inaccurate and a ridiculous premise for an alien invasion, for three reasons:

1. The Earth’s water has exactly the same chemical composition as water anywhere else in the Solar System: two hydrogens stuck to an oxygen. And, in fact, water is one of the most common molecules in the Solar System – nay, universe!
2. The Earth is not the only place in the Solar System where liquid water exists: scientists are about as sure as scientists can be that there is liquid water under the crusts of Europa and Enceladus, and possibly Ganymede and Titan as well.
3. Water (liquid or ice) is available in many places throughout the Solar System, and as it turns out, the water on Earth’s surface is one of the hardest places to get at it, if your starting point is space.

Now, I will have to explain #3 a bit. My point relates to the depth of the Earth’s gravity well: in the words of xkcd, the reason “why it took a huge rocket to get to the Moon but only a small one to get back.” If aliens wanted to take our resources, presumably they want to do so because they need those resources for something. And since this alien civilization apparently makes a living moving from planet to planet (or star system to star system), they are going to have to move these resources or their products off of the planets they were harvested from. That means, for every kilogram of water the aliens pump out of Earth’s oceans, they need to produce spacecraft, rockets, and fuel to get the water up into space again. Think of how big the Space Shuttle is, and how much fuel we load it full of, just to get school-bus-sized Space Station modules into orbit. Contrast that with the tiny Lunar Module ascent rocket from the Apollo days.

Clearly, there must be a better way to get water off of planets. So, without further ado, the Quantum Rocketry Guide for Successful Star System Invasion and Resource Extraction for Nomadic Species:

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Step One

When to implement: as your invasion force approaches the heliopause

Tools required: telescopes, spectrometers

Survey the target system. Identify (if any) the home world of all radio-capable species. Identify all (if any) interplanetary spacecraft and immediately dispatch combat drones to shadow, assess, and possibly destroy them. Important note: if there are more than a several dozen such spacecraft, be prepared to complete only the first two to three steps in this guide, as further exploitation may result in retaliatory attacks.

Separate the remaining target bodies in the stellar system into three categories:

1. Resource-rich small bodies. This class includes comets with large mass fractions of water ice and asteroids or moonlets with useful metal or mineral compositions.
2. Resource-rich mid-sized bodies. This class includes moons and small planets with metal- and mineral-rich crusts.
3. Resource-rich planets. This class includes Earth-sized or larger planets and gas giants which contain useful minerals and gases. The cut-off between class 2 and class 3 will depend on the surface escape velocities of the bodies and the capabilities of your species’ ascent engines.

Disregard the home planet(s) of any technological civilizations located. Keep those planets under surveillance, and be prepared to deploy additional combat drones. (Your combat drones use radio, which lets you control them over interplanetary distances, right? And you have some diversified vehicles with independent AI intelligence or actual crews, right? Remember, you don’t want to keep all your klat’thra eggs in the same xikot!)

Step Two

When to implement: as your invasion force nears the first class 1 targets, in the stellar system’s Oort Cloud, Kuiper Belt, or orbiting outer gas giants.

Tools required: cutting tools, robotic manipulators, small thrusters, and foundry ships

Begin direct, in-space processing of class 1 bodies. Use heat to cut comets up into manageable-size ice fragments and then spray-coat them with epoxy to prevent sublimation of any of the precious resource. Attach small thrusters to these fragments to propel them off of their parent comets and towards your cargo and refinery vessels. Use other cutting and manipulator tools to break metal asteroids down into useful chunks, and harvest stony asteroids for useful minerals. Attach small thrusters to these chunks, as well.

Begin processing of resources with foundry ships to produce additional foundries, combat drones, or scout ships as necessary.

As a contingency, attach larger engines to some of the unusable asteroids. If necessary, these can be used to bombard pesky indigenous species – or at least give them something to worry about – from a safe interplanetary distance. As complete extraction of resources from a star system may take up to several centuries, depending on your species’ level of robotics and self-manufacturing capability as well as your long-term goals and objectives, such bombardment may be periodically necessary to ensure the uninterrupted operation and safety of your equipment.

Step Three

When to implement: when your invasion force reaches class 2 targets, most likely a rocky outer planet (if any) or the moons of an outer system giant planet (if any).

Tools required: descent engines, accelerators, mining equipment, planetary rovers, robotic manipulators, and foundry ships

Establish mining outposts on all icy or rocky class 2 targets and commence strip-mining the target bodies. On layered bodies like Europa, mine the ice (as a mineral) uniformly across the planet so that the thermal gradient freezes liquid water further down, preventing any of the water from boiling off into space and freeing your operation from having to build lots of tanks to hold liquid materials. On bodies with resource-rich atmospheres or surface liquids, such as Titan, your facilities will require the equipment to condense and package usable gases and liquids.

Construct linear accelerators, rail guns, or light gas guns on the surface. Use these facilities to catapult packages of mined resources into orbit around the target body, where they can be conveniently processed by refinery ships or stored in cargo vessels.

Step Four

When to implement: when your invasion force reaches class 3 targets, such as Mars-like inner planets.

Tools required: descent engines, ascent engines with in-situ fuel-production capability, surface factory components, mining equipment, planetary rovers, robotic manipulators, and foundry ships

Class 3 targets are those resource-rich bodies that are too large for catapult launches to reach orbit. These targets will require many ascents to deliver their resources, and will only present a net gain for your operation if you utilize fewer of your current in-space assets than the resources your equipment can deliver to orbit. Thus, mining and planetary ascent equipment should be manufactured in-situ, leaving your operation’s in-space assets free to process class 1 and 2 targets.

Begin by landing surface factories, exploratory rovers, and mining devices that can manufacture duplicates of themselves along with planetary ascent vehicles. On gas giants, a similar methodology can be applied but with buoyant balloon-based facilities. With a modest initial drop from orbit, your operation will soon see orbital delivery of ascent vehicles filled with metals, minerals, gases, and liquids mined from the planet below. From orbit, your craft can direct the resource packages to refinery or cargo ships.

Naturally, the operation will not be able to extract 100% of a target planet’s resources, as some resources will be tied up in the construction of mining apparatus, ascent vehicles, and fuel. Yield may be below 20-30% depending on the capabilities of your civilization. For that reason, this guide strongly suggests that the home planets of any technological civilizations be ignored unless the resources available on those planets are sufficiently desirable to justify the possibility of facing armed resistance and the military expenditures that such resistance will necessitate. In such cases, this guide recommends utilizing targeted asteroid impacts to limit the military-industrial base of such civilizations before commencing any extraction operation.

Step Five

When to implement: upon reaching the desired level of resource extraction

Tools required: N/A

Congratulations! If you have reached Step Five, you have successfully completed a star system invasion and resource extraction. This guide now recommends using your species’ foundry ships to deconstruct all recoverable vehicles and equipment from your completed operation. You may now proceed to the next target star system, and with vastly expanded resource capabilities!

A note: Advanced resource-gathering possibilities exist if your species possesses the technologies to extract plasmas from, enclose a Dyson sphere around, or otherwise manipulate the target system’s central star. See supplement.

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And now, I shall return to a more, um, human perspective. If some aliens tried to pull this one us, we would see – through our telescopes – robot refineries chewing up all the asteroids and comets in our Solar System; ice blocks being cut from Europa, Ganymede, Callisto, Enceladus, Titan, Rhea, Triton, Pluto, and other icy moons; and Mars and Mercury being strip-mined for metals. There’s very little about Earth’s resources that makes our planet really unique, and so the aliens wouldn’t bother with us. They wouldn’t need to. (Unless, of course, our governments got it together enough to recognize that our long-term future would be at stake and launched a crash military space program. Then the aliens would bother with us.) There would be very little that we could do, with our current capabilities; and when the aliens finish we would just see them leave.

I can, however, think of two reasons that might make the Earth worth invading to any species using the guidelines above. One is that the Earth is unique among all the bodies in the Solar System, so far as we know, in that it has plate tectonics. This constant churning and recycling of our crustal material might, conceivably, produce some kind of mineral resources that are only available on the Earth. That said, those resources would have to be pretty darned precious to get the aliens to try and launch an extraction campaign on the Earth – but if they went ahead with the operation, a bombardment from space would be the first order of business. I’m sorry to say it, but unless our near-Earth object detection gets way better, then not only will Earth be at risk from killer asteroids, but we simply won’t know what hit us in the unlikely event of alien invasion.

The second reason is more interesting: perhaps it is the Earth’s habitability (as defined by a species like us) that makes it desirable. Perhaps the aliens aren’t looking for resources – because they can have all the resources they want on worlds like asteroids and moons that are far easier to access – but maybe terraforming (extraterraforming?) presents some insurmountable barriers such that it is more efficient to seek out habitable planets in other star system and then fight for them. Incidentally, this is the premise behind the really excellent Old Man’s War books by John Scalzi. This scenario also might make the aliens less likely to pound us with rocks from orbit.

The guidelines for stellar system invasion I wrote above are based on two premises: first, a large technological disparity between the invasion force and the indigenous civilizations;  second, the goal of maximum resource extraction for minimal input effort. These premises lead me directly to an alien-invasion doctrine of quick victory by overwhelming force. However, since a lot of the challenge of moving things through space involves trying to reduce the mass of the things that are moving, it would make a lot of economic sense for the invasion force to build its vehicles and weapons on-site rather than carry them along everywhere. This is the flaw in the alien invasion plan: any non-trivial space defense force would represent a significant military investment by the marauders before they can begin extracting any benefit from the target system. Clearly, in order to defend the future of mankind against alien attacks, we must begin work on the Earth Defense Force as soon as feasible!