Quantum Rocketry Guide: Star System Invasion!

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.

You will see a lot of guys hooah-ing and a lot of wreckage.

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: Continue reading Quantum Rocketry Guide: Star System Invasion!

Attack of the Space Junk!

There is a pretty cool gallery on Gizmodo of various pieces of spacecraft and rocket debris that have fallen to Earth, intact enough to have recognizable shapes.

A fallen rocket stage in Russia

This gallery puts in perspective what we mean when we say things “burn up” in the atmosphere. The friction of re-entry generates enormous heat, usually enough to break up rockets and satellites into tiny, unrecognizable pieces. But every now and then, conditions are just right for bits of spacecraft to make it all the way down to Earth. Some of the pieces are clearly spent rocket parts, which might not have been all the way up in orbit, but a few are most definitely chunks of previously orbiting spacecraft.

This sort of thing highlights one reason why launches all have range safety officers. If there is an unrecoverable problem during a launch, standard procedure is usually to blow up the rocket. The goal of such an action is to break the vehicle up into small pieces that will be much less likely to hit things on the ground.

As usual, the physics here is something that future spacecraft designers might be able to take advantage of: clearly, some spacecraft shapes other than the standard aeroshell capsules can make it to a planet surface. This idea has inspired certain spacecraft research groups to look at developing spacecraft that can get from orbit to the ground by fluttering down benignly.

Note: the problem of orbital debris, shown in the map in that gallery, is related but very different from the issues surrounding re-entering debris.

Jovian Electrodynamics

Jupiter is one of the most useful planets for planning spacecraft trajectories, and it’s home to some of the most interesting science targets in the Solar System. However, it also happens to be one of the most dangerous planets for spacecraft.

Jupiter is so dangerous because of two things: its magnetic field, and the moon Io. You see, Io is extraordinarily volcanically active – the only known extraterrestrial body with active volcanism, in fact – and constantly spurts all sorts of particles out from its interior. The energy of Io’s eruptions gives some of these particles escape velocity, and they end up orbiting Jupiter. Jupiter’s immense magnetic field or solar radiation can then strip electrons off these particles, creating a torus of raging ions around the planet. It’s the same phenomenon as Earth’s Van Allen radiation belts, but without the nuclear explosions and, well, Jovian in scale.

Jovian radiation belts. Sort of. My Jupiter model didn't work, so this picture looks terrible.

Jupiter’s radiation belt kills spacecraft. The Galileo probe, a Jupiter orbiter, eventually died when it crashed into the planet, a protective measure to prevent it from contaminating Europa with Earth life in the event its electronics became too fried to control the spacecraft. (Galileo contrasts quite a bit with the Cassini mission to Saturn, which has been considerably extended from its main science mission!) The next mission to orbit Jupiter, Juno, is encased in a titanium radiation shield. And you can pretty much forget human exploration of Europa.

But it’s not just tempting to send robotic probes to Jupiter – it’s practically a necessity! Any spacecraft headed to the outer Solar System, or trying to do wild orbit maneuvers, needs a lot of delta v (that is, the capability to change velocity – in magnitude, direction, or both). The Ulysses spacecraft needed a bunch of delta v to kick its orbit waaaaaaay up over the sun. And New Horizons needed a ton of delta v to get out to Pluto and beyond. One efficient way to get delta v is to perform a planetary swingby, or flyby, maneuver – and you can get more delta v from a bigger planet. Jupiter is king of the planets and gives space vehicles a huge boost. So, every spacecraft that has ever gone into the outer Solar System has visited Jupiter.

That magnetic field, though. Jupiter has a magnetic moment 18,000 times as large as the Earth’s. That sort of magnetic field is useful, if a spacecraft has the hardware to take advantage of it. For instance, an electrodynamic tether: a long conductive filament stretching from the spacecraft, along which the spacecraft runs a current. (Currents through the rarefied plasma filling the Jovian orbit environment complete the circuit.) The spacecraft will then experience a force proportional to the cross product of the current along the tether and the local magnetic field. If you don’t like vector math, don’t worry – just remember that the force is perpendicular to both the tether and the planet’s magnetic field near the spacecraft (which, near the planet’s equator, will run approximately parallel to the planet’s spin axis). In Earth orbit, there have been several missions testing tether physics, with applications including both electrodynamic propulsion and harvesting power from the Earth’s magnetic field. Around Jupiter, these methods will be even more powerful.

If a spacecraft is in circular orbit around Jupiter and orients a tether parallel to its direction of travel, then the force will be either directly toward or directly away from the planet’s center. This means that the spacecraft can, by running a current through a tether, generate a force that has the effect of either enhancing or weakening gravity. So a robotic probe performing a Jupiter flyby could get a much bigger gravity-assist boost with a little help from a current-carrying tether. Or, if the current is high enough, the net force could repel the spacecraft from Jupiter, changing the direction of the delta v it picks up in the flyby. The capability for spacecraft to perform that kind of maneuver could open up more launch windows for outer Solar System missions.

Another idea is to use electrodynamic tether propulsion to keep a Jupiter orbiter out of the worst parts of the radiation belt, so that it can get lots of data on the Galilean moons. If the probe has a slightly inclined orbit, then it could vary the current in its tether over the course of each orbit so that the spacecraft pushes its semimajor axis in and out each period. With the right parameters, this non-Keplerian spacecraft trajectory would skirt the ring of hard radiation around Io’s orbital radius.

A trajectory that skates around the radiation belt

Perhaps the spacecraft has some leeway into how far into the radiation belt it can venture – or its orbit is just bigger than the “danger zone.” Then, it could follow a Keplerian orbit (affected only by gravity) some of the time and use the passage of the tether through Jupiter’s immense magnetic field to generate electricity. If engineers can balance the numbers, then such an exotic orbit might come out power-neutral over the course of each orbital period, giving spacecraft a free, and safe, way to explore Jupiter’s Galilean moons for a very long time.

Speaking of power harvesting: Jupiter is far enough from the Sun that spacecraft around there can’t really get all the power they need from solar panels. Dragging a conductive tether around and letting the planet’s magnetosphere drive charges along the length of the filament would be one way to overcome that challenge.

These are kinds of technologies that we can develop in Earth orbit and deploy in the outer Solar System, to take advantages of the resources out there and allow us to learn more about the things in our backyard. After all, the more we understand about the different regimes of our own Solar System, the more we understand about our origins – and about the possibilities that exist in planetary systems around other stars.

Call Me Dr. Twit

After much pressure from my girlfriend, I’ve signed up for a Twitter account. I’m going to use the account to echo posts here on Quantum Rocketry.

It’s not a communication medium I really see a lot of value in; if I want to convey information I would much, much, much, much rather do so here, where I have more than 140 characters to develop an idea, in context, without conforming to the soundbite-based, ultra-distilled, headline-only view of things that seems to be the current trend on the Internet. (There goes my liberal arts background, shouting opinions at anyone who will listen!)  And if I feel like being silly and inane, I’d rather do that on a narrowcast medium like Facebook, where my silliness will go to my friends who know what to expect and how to interpret such activity, instead of a broadcast medium like Twitter.

But, though I shall continue my rebellion, I finally got an account anyway. (And I just have to continue my rebellion against all the people who instantly must check everything on their smartphones instead of interacting with me when I’m right in front of their faces!) It is jpshoer. This is 10% to allow further Twitter-based interaction with ‘netizens who read this blog, and 90% because a Twitter account is a requisite for NASA Tweet-ups.

Just remember, kids: one of the few things I think are more stupid than Twitter is the word “tweep,” so don’t call me that.