Science in Science-Fiction: Interplanetary Spacecraft part 2

Okay in part one I discussed various ways that we might get up out of a planetary atmosphere and into space, and we can be hopeful that eventually we’ll have a space-elevator, orbital ring or launch loop, although I suspect they should be tried out first on the moon and later on mars where we will learn a little about their operation before one falls on Calcutta or Mexico City — but let’s take that as read and we can get to orbit, what kinds of craft can we sail out into the deep in?

Sunjammers

Count Dooku rode around in a Solar sailer in Star Wars, although it seemed a little nippier than the real thing. Well first and most poetic is a Sunjammer or solar sailer, not to be confused with the Solar Barge of Ra (which is the sun). Invented for a story by the same man who invented communication satellites, Arthur C. Clarke (although other authors do seem to have come up with the idea separately). He imagined a sheet of metallic foil bigger than football field tied to your capsule with thin strong cables. The solar wind, which consists of photons, electrons, neutrinos, alpha particles (and so on) hit the sail travelling at (or very close to) light-speed and push it, very very gently. This tiny push, across a huge enough sail can turn into enough thrust to allow us to sail out away from the sun, and with a bit of ingenuity and orbital mechanics it becomes possible to use the out flowing particles to decelerate your obit around the sun and descend inward. Sounds great, it’s very cheap and has the potential to shift huge masses around, but only at incredibly low accelerations. NASA is testing this, even giving it a good try with a technology demonstrator due to fly in 2014. That tiny level of thrust is useful for preserving an orbit or slowly sailing in or out, but for humans to ever really sail these vessels about it’s probably going to require enormous sails that can exert a few actual Newtons of force (at least). There is a possibility that we might be able to create some sort of magnetic sail hundreds of kilometres across, but that technology is hundreds of years (or one good genius jump) off yet. So what else have we got?

Ion drive

A working Ion Drive test firing from NASA JPL (via wikipedia)

The ion drive (or more properly thruster) sometimes called a Plasma jet, is another rocket. Rather than burning chemicals the ion drive instead throws electrons or protons really quickly out of the back end to provide thrust. In physics terms this elevates the Ion drive from chemical efficiency to electrical efficiency (which on paper is a very good thing). In real terms its a bit like like pointing a tiny machine gun backwards while wearing roller skates. Sounds great in theory, you can carry a lot more bullets if they are tiny, but each kick of the gun is weaker. At the moment our best try can produce about half a Newton of force, but we can sustain that for thousands of hours at a time. Which again is great for pushing satellites around, but we need to spend a lot (of time and money) perfecting these before we’ll be riding plasma tailed rockets through the firmament. In science-fiction the Ion drive is a regular staple and you’ll find it turns up all over the place, as the sub-light drive of choice, everywhere from Star Wars to the Impulse drive of Star Trek (although that is technically a magnetoplasmadynamic thruster which still produces an ionic exhaust. Most often the variation used is the ultra-cool sounding Plasma Drive, presumably powered by a Fusion reactor.

Pulse detonation thermonuclear drive

Back in the 1940s when mankind was thinking big after World War II, one Stanislaw Marcin Ulam, who worked at Los Alamos on the Manhattan Project and may have been the man behind the technological singularity as a concept, had a brilliant idea. What if a rocket rode a series of explosions rather than one continuous one, and then just to make it faster (and safer- weirdly) lets make the explosions atomic or even thermonuclear bombs. The idea was fleshed out during the 1950s and called Project Orion, which is why they are sometimes referred to as Orion drives. The idea of riding bomb explosions to the stars turns up quite a lot in sci-fi, any time the science gets hard and you are talking about generation ships, or ships to get out beyond Jupiter in fact, you’ll often find Orion drives are the go to drive. They even turn up in Star Trek (original series series 3 episode 8 : “For the World is Hollow and I Have Touched the Sky”) with a convenient asteroid ship.

The Messiah from ‘Deep Impact is described as an Orion drive vessel, but the geniuses at ILM just strapped some boosters together with a shuttle cab for the special effects shots.

Whilst it may sound like a crazy Sci-Fi idea, they actually did do a lot of the planning and everything looked good for the Orion design, until nuclear test treaties banned the use of nuclear weapons in space, so even if it is a good idea, it has been banned already. At least until we need to use one to get to that incoming Deep Impacter Asteroid like the one from this movie. Then it will probably turn out some Space-Agency or Air Force has secretly been keeping the technology up to date, at least it always does in the movies. Which brings us to another banned technology.

Torch ships

Torch ships were a favourite of early science-fiction authors, particularly Robert Heinlein. The early concept of a Torch ship (As discussed in Rocket Ship Galileo) is simply a nuclear reactor where you pour a reaction mass such as water or molten lithium or even lead into the reactor. This heats up quick and rather than trying to contain the resultant hot gas, you let it come out of nozzles and turn it into thrust. Of course, that hot exhaust gas has a significant problem, its highly radioactive. Later versions of the Torchship moved to a cleaner system of matter annihilation which may or may not have been based on Anti-matter, but could still take on almost anything as reaction mass, but water was the usual “fuel” the desperate heroes had to source. Actually doing the maths seems to prove that the original Torch-ship concept isn’t actually as efficient as the nuclear propulsion proposal of Project Orion, and the Ion drive is actually a more efficient use of the energy, so they’re pretty much off the cards, but there is another option.

Laser Drive

Technically the laser doesn’t actually propel the space ship it just vaporizes a reaction mass so that it expands explosively pushing out away from the ship exactly like a rocket. It’s just that you can use anything, ice, rock even metal as a propellant if your laser is powerful enough. The laser drive has many of the good points of an ion drive, it can be easily turned off and on, can be pulsed to conserve reaction mass, the laser can also ionise the explosion allowing for a hybrid engine that can act as an ion drive for months before finally applying higher accelerations by volatilising the propellant. In this mode it has all of the advantages of the chemical drive in that huge amounts of thrust can be generated from the reaction mass. It also has the problems of chemical rockets in that huge amounts of reaction mass can be consumed quickly. The laser ion drive has hope, but that powerful a laser also requires a power source and we’ll have to see what we can do about that… One possibility is to mount big lasers in place on the moon (or in orbit) and shoot at the ships hitting their reaction masses from a distance and potentially using one laser to propel all out-bound traffic and decelerate inbound. Of course, a laser powerful enough to vaporise the reaction mass could just as easily destroy the ship if it missed…

Project Daedalus

Orthographic image of Project Daedalus from www.icarusinterstellar.com

One variant of the laser drive is the idea behind the British Interplanetary Society’s Project Daedalus, which combines the efficiency of an Ion drive with the canned craziness of Project Orion. A high energy beam contains and detonates a pellet of deuterium that explodes as a small fusion bomb pushing the ship. The idea was so good that Projects Longshot and Project Icarus were also designed around this engine concept. This fusion bomb/laser combination has advantages over the Orion drive in that the fusion bomb is not quite as radioactive as a fissile device (or even a fissile triggered thermonuclear device), but is still a little hotter than you’d probably like. You certainly wouldn’t want to live near the launch site. Daedalus/Longshot is still probably our best hope for interstellar travel. The tiny looking living space at the top of that model would actually be the largest thing we’ve ever launched (in its own right) with that lower dome being about the height of a Saturn V. Daedalus as a project is rather like designing a flying Chrysler building, although since each of the spheres contains the thousands of Fusion “pellets” (or bombs as we’d call them), perhaps its more like building a huge flying armoury to the stars… In science-fiction laser fusion drives are fairly rare, or are considered a variation of the Orion Drive, and you might find giant generation ships cruising through deep space with Laser Fusion drives, but more often they will just be described as Orion drives. Daedalus is of course still banned by that pesky test treaty, so unless there is a pressing need for us to reach for deep space quickly, we’re never likely to build one. Of course the other small problem with the Daedalus is that the actual technology it is meant to work from has never actually been proven, although we are getting close now.

Gravity Drive

This is a purely hypothetical drive with none of the grounded physics of the above. It may be possible to contort space ahead of a ship (or above it if you are trying to hover) and effectively cause the ship to fall in that direction. Although to really get this to work we need a better understanding of space-time, mass and the Higgs’ field, but it does not seem impossible to suggest that space can be distorted in this way. Such vessels would appear to be capable of amazingly fast manoeuvres without placing undue stresses upon the frame or pilots as the gravity field would act uniformly across the craft. Sometimes the Gravity drive is called an Anti-gravity drive, because it can negate gravity. Although technically an Anti-Gravity drive would be a slightly different thing (a gravitational pushing force). Gravity drives are often thrown into the mix of science-fiction for alien spacecraft, particularly the flying saucers. As they do describe the sorts of behaviours that people observe UFOs engaging in (rapid high-G impossible turns and the like).

Into the deep

Well that’s about it for space drives. At the moment we are limited to simple chemical rockets and basic ion drives and soon tiny solar sails. If we are going to exploit the resources of our solar system, and as a species move beyond the next asteroid strike wiping us out then we are going to need to invest in some of these technologies. Technologically we are close to being able to assemble a Generation ship to begin the journey towards new worlds, but socially we are far from it. Mankind currently can’t really contemplate the sacrifices required to leave Earth for even the Moon, or Mars, let alone journey’s beyond the solar system. Although we might get to the point of building robot probes to explore another stellar system in the next hundred years or so, I don’t think we are within sight of even extra-planetary colonies, let alone extra-solar. Although the flurry of exo-planet findings of the last decade does look good for us eventually being able to. It certainly looks more possible now than it did thirty years ago. Being realistic though, I cannot see mankind moving to colonize other star systems without extreme pressures at home, like ecological collapse or impending disaster of some sort and strong investment by governments or very forward looking institutions. At least not unless someone creates a shortcut that will allow us to leap out to the stars. Next Month I’ll write about what that Star Drive might be…