Here Are The Latest Ideas For How Interstellar Travel Would Work

SS031 star trailDonald Petit/NASAAstronauts at the International Space Station took this long exposure image.

SPACE, as Douglas Adams pointed out in “The Hitchhiker’s Guide to the Galaxy”, is big. Really big. It is so big, in fact, that even science fiction struggles to make sense of it. Most sci-fi waves away the problem of the colossal distances between stars by appealing to magic, in the form of some kind of faster-than-light hyperdrive, hoping readers will forgive the nonsense in favour of enjoying a good story.
But there are scientists, engineers and science-fiction writers out there who like a challenge. On October 22nd a small but dedicated audience gathered at the Royal Astronomical Society (RAS) in London to hear some of them discuss the latest ideas about how interstellar travel might be made to work in the real world. The symposium was a follow-up to a larger shindig held earlier this year in San Diego.

Starship research is enjoying something of a boom. “A few years ago, there was only one organisation in the world working on interstellar travel,” Jim Benford, a microwave physicist and former fusion researcher, told the conference. “Now there are five.” The following day many of the speakers at the event would visit the British Interplanetary Society (BIS, the venerable organisation of which Dr Benford spoke) to discuss design details for a starship named Icarus.

Starship research has always been a small field, full of iconoclasts and dreamers fitting the activity around their “proper” jobs. 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.

How final a frontier?

After Daedalus, interest flagged. Lately, though, several developments have given the field a shot in the arm.

The internet has made it easier for like-minded dreamers to get in touch. Astronomers have discovered thousands of alien planets (including, possibly, one around Alpha Centauri B, which at 4.4 light-years away is part of the star system that actually is closest to the sun), and this exoplanet boom has caught the public’s imagination, as well as giving starship researchers a list of destinations. The rise of the private space industry, which aims to slash the cost of getting into orbit, brings hope that the sort of orbital infrastructure which would be needed to build a starship might one day be developed. And the involvement of DARPA, an arm of the American defence department, which is sponsoring a long-term project to develop the sorts of technology a starship might require, has brought money and attention.

The chief problem, as Adams noted, is distance. 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 {+3}He, 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.

Without fuel, the ships could be small, and therefore easy to accelerate. They might even be able to stop at their destinations by employing the solar wind of the target star to slow themselves down, using a second, so-called magnetic sail. The basics of the technology already exist: microwave sails have flown in laboratories. And the transmitter could be reused, which would make such ships cheaper than one-shot nuclear rockets.

Because it’s there

“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. 50 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.

Ian Crawford, an astronomer at Birkbeck College, London, pointed out that sending a robotic probe to another star would be much better, scientifically, than studying it through telescopes. He even presented a checklist of the instruments such a mission should carry, and of the questions–in stellar physics, planetary science and general astronomy–it could be designed to answer. But for many of those attending such conferences, “because we can” would be reason enough to try.

Several speakers at the RAS agreed that a starship would not be feasible until such time as human beings had spread through most of Earth’s solar system, and possessed an economy able to command the resources of more than one planet. Whether that day will arrive is an open question. Gregory Benford said that Thomas Jefferson, America’s third president, guessed it might take a thousand years for the American frontier to advance to the Pacific Ocean. Humans are bad at prediction, Dr Benford argued, and often things thought on the edge of possibility happen faster than anyone would have believed.

Of course, the past is not necessarily any guide to the future, and the magnitude of the problems involved in space exploration dwarf any earthly analogy. Gregory Benford may be wrong. But he and his fellow starship designers are, by necessity, an optimistic bunch.

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