I don’t know if you’ve been reading Photo Synthesis, but if you haven’t you’re missing a real treat. Currently there’s some incredible photography and video of amateur rocketry at fairly large scales. Amazing stuff. A hundred years ago the idea that an amateur of comparatively middle-class means (well, after a lot of saving anyway) could send rockets to the edge of space would have been laughable.
But we’ve got a long way to go as well. Not long ago Gene Expression commented on a Seed Magazine piece on planetary astronomy (do you subscribe?) and noted that the problem of interstellar exploration is pretty far out of reach. Of course the goal of exploring the galaxy has been around as long as we’ve known that there were galaxies. Now however we’re on the verge of astronomical instruments that might be able to tell us something meaningful about potential earthlike planets elsewhere in the nearby interstellar universe:
“If planets are found around Alpha Centauri, it’s very clear to me what will happen,” Marcy said. “NASA will immediately convene a committee of its most thoughtful space propulsion experts, and they’ll attempt to ascertain whether they can get a probe there, something scarcely more than a digital camera, at let’s say a tenth the speed of light. They’ll plan the first-ever mission to the stars.”
So let’s say it happened. Alpha Centauri is about 4 light years away, so at a tenth the speed of light you could reasonably expect to get a signal back in your lifetime if your life expectancy has still got around 50 years on it. That’s still something of a sketchy proposition for a lot of scientists, but there’s plenty of scientists working diligently on proposals for all kinds of projects they’ll certainly never live to see. Especially those poor high-energy guys looking to build the post-LHC collider.
But could it be done at all? The top speed of a rocket in a vacuum is directly proportional to the velocity at which its propellant is being blasted out of the engine. A good rule of thumb is that your rocket can reach twice the speed of its propellant*. A liquid fuel rocket spits out its blast of fire at around 4-5 km/s, which leads to a pathetic 0.003% of the speed of light. So much for NASA’s normal methods. Ion engines have been tested with real success and generally end up around 30-60 km/s. This is maybe 0.04% of the speed of light. Still way too slow. VASMIR is another potential kind of ion engine, though not one that has yet seen deployment. It claims theoretical jet maximums of around 300 km/s. And that’s actually a solid 0.2% of the speed of light for the speed of the spacecraft. But incredible though that is, it’s still not 10%. A travel time of 40 years is long; a travel time of 2000 years is just depressing.
Can we do better? Probably, but it’s not likely to be popular. Get a spacecraft, fill it up with nukes, throw the nukes out the back one at a time, and let the blasts propel you forward. Crazy as it sounds, it’s been extensively studied and is not at all beyond the reach of today’s technology. It probably wasn’t beyond the reach of 1970s technology – overall it’s a very simple system. Theoretically you could probably squeeze close to 1000 km/s out of a rocket thus propelled. Still just 0.6% of the speed of light.
In terms of current feasibility that’s probably about it. There are some undeveloped technologies that can in theory do better. The fission-fragment rocket uses nuclear energy by directing the actual fissioning nuclei out as the rocket exhaust. Since no explosions are involved it wouldn’t be as controversial and theoretically it can give exhaust speeds of up to 5% of the speed of light. And that’s exactly where we need it to be to get 10% for our spacecraft. Fusion engines could do even better, but at that point we’re straining the bounds of what NASA may plausibly be able to do in the real world.
Further thoughts? My personal opinion is that it would certainly be worth spending a large fraction of the space exploration budget on such a project if a potentially life-supporting planet were found around Alpha Centauri, which I’m certainly not holding my breath over. Even if it were, we can only do what we have the technology to do. But it would certainly be worth a shot.
*Formally it can reach the speed of its propellant times the natural logarithm of the ratio of fuel mass to non-fuel mass. The 2x number assumes a mass ratio of a little over 7, which is reasonable.