Starship research is enjoying something of a boom:
Serious work in the field dates back to 1968, when Freeman Dyson, an independent-minded physicist, investigated the possibilities offered by rockets powered by a series of nuclear explosions. Then, in the 1970s, the BIS designed Daedalus, an unmanned vessel that would use a fusion rocket to attain 12% of the speed of light, allowing it to reach Barnard’s Star, six light-years away, in 50 years. That target, though not the nearest star to the sun, was the nearest then suspected of having at least one planet.
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During the cold war America spent several years and much treasure (peaking in 1966 at 4.4% of government spending) to send two dozen astronauts to the Moon and back. But on astronomical scales, a trip to the Moon is nothing. If Earth — which is 12,742km, or 7,918 miles, across — were shrunk to the size of a sand grain and placed on the desk of The Economist’s science correspondent, the Moon would be a smaller sand grain about 3cm away. The sun would be a larger ball nearly 12 metres down the hall. And Alpha Centauri B would be around 3,200km distant, somewhere near Volgograd, in Russia.
Chemical rockets simply cannot generate enough energy to cross such distances in any sort of useful time. Voyager 1, a space probe launched in 1977 to study the outer solar system, has travelled farther from Earth than any other object ever built. A combination of chemical rocketry and gravitational kicks from the solar system’s planets have boosted its velocity to 17km a second. At that speed, it would (were it pointing in the right direction) take more than 75,000 years to reach Alpha Centauri.
Nuclear power can bring those numbers down. Dr Dyson’s bomb-propelled vessel would take about 130 years to make the trip, although with no ability to slow down at the other end (which more than doubles the energy needed) it would zip through the alien solar system in a matter of days. Daedalus, though quicker, would also zoom right past its target, collecting what data it could along the way. Icarus, its spiritual successor, would be able at least to slow down. Only Project Longshot, run by NASA and the American navy, envisages actually stopping on arrival and going into orbit around the star to be studied.
But nuclear rockets have problems of their own. For one thing, they tend to be big. Daedalus would weigh 54,000 tonnes, partly because it would have to carry all its fuel with it. That fuel itself has mass, and therefore requires yet more fuel to accelerate it, a problem which quickly spirals out of control. And the fuel in question, an isotope of helium called 3He, is not easy to get hold of. The Daedalus team assumed it could be mined from the atmosphere of Jupiter, by humans who had already spread through the solar system.
A different approach, pioneered by the late Robert Forward, was championed by Dr Benford and his brother Gregory, who, like Forward was, is both a physicist and a science-fiction author. The idea is to leave the troublesome fuel behind. Their ships would be equipped with sails. Instead of filling them with wind, an orbiting transmitter would fill them with energy in the form of lasers or microwave beams, giving them a ferocious push to a significant fraction of the speed of light which would be followed (with luck) by an uneventful cruise to wherever they were going.
“Cheaper”, though, is a relative term. Jim Benford reckons that even a small, slow probe designed to explore space just outside the solar system, rather than flying all the way to another star, would require as much electrical power as a small country — beamed, presumably, from satellites orbiting Earth. A true interstellar machine moving at a tenth of the speed of light would consume more juice than the entirety of present-day civilisation. The huge distances involved mean that everything about starships is big. Cost estimates, to the extent they mean anything at all, come in multiple trillions of dollars.
That illustrates another question about starships, beyond whether they are possible. Fifty years of engineering studies have yet to turn up an obvious technical reason why an unmanned starship could not be built (crewed ships might be doable too, although they throw up a host of extra problems). But they have not answered the question of why anyone would want to go to all the trouble of building one.
No Bussard love? Interstellar ramjet?
Ross, it turned out that the drag from the magnetic particle collection field is bigger than the thrust produced even if the system is perfectly efficient. There are some alternative designs that don’t slow the collected matter down so much or use antimatter to catalyze the fusion reaction, but it’s a thornier engineering problem (not that the basic ramjet wasn’t already!).
That said, with respect to “But they have not answered the question of why anyone would want to go to all the trouble of building one” the answer is that a planetary civilization probably wouldn’t. However, for a solar civilization utilizing a decent percentage of the solar-system’s industrial resources and having a population of trillions or tens of trillions spread across habs and colonies throughout the system, it’s much more reasonable.
Freeman Dyson once remarked that interstellar flight is a problem not of engineering but of biology. My sense is that he was thinking in terms of suspended animation or of radical life extension for the crews of very long space voyages, but another way that the matter could be approached is to use genetic engineering (which we seem to be getting better at) to boost IQ significantly. If we produce super-genius humans, then they might figure out a way for us to reach the stars.
But the moon is close enough, and positioned just right, so that we could build an Earth-Moon Tunnel!