Some things are impossible because they violate fundamental laws of the universe, as far as we know. The theory of relativity says that neither matter nor information can travel faster than light. Matter because an object reaches infinite mass at the speed of light. (Though the recent measurement of neutrinos apparently traveling faster than light remains to be explained, most physicists suspect it reflects a subtle error, not an overthrow of the theory of relativity.) Information because that would reverse the order of cause and effect for some observers, effectively enabling time travel and violating everything we think we know about how nature operates. Other things are impossible, or at least extremely difficult, because of practical or engineering limitations rather than fundamental ones.

Travel to the stars has both kinds of constraints. The fundamental one is that no spacecraft can reach, let alone exceed, the speed of light. But that speed of 300,000 km/second is so far beyond what our spacecraft now typically achieve – 16 km/second or less at launch – that it is not currently relevant to a discussion of interstellar travel, though it may be in the future.

A speed of a few km/second has allowed us to send people to the Moon, and probes to Mars, Saturn, and more, and so it might seem that we can reach the stars, too. But travel to the stars is qualitatively different. This was convincingly expressed at the first conference devoted to the possibility of reaching the stars, the 100 Year Starship (100YSS) event held in Orlando, Florida, which I attended last September.

Sponsored by DARPA (Defense Advanced Research Projects Agency) of the U.S. Department of Defense, 100YSS asked whether, within the next century, we can build a spacecraft capable of reaching the stars. DARPA does not necessarily see direct military use for interstellar travel, but it has often invested in unconventional ideas that yielded benefits for the military and for civil society. A starship project would have an excellent chance of producing valuable new science and technology.

But as was emphasized at 100YSS, the problem is moving a starship fast enough to cope with the nearly ungraspable immensity of interstellar space. Our nearest neighbor star besides the Sun, Proxima Centauri, is four light years away (a light year or ly is about 6 trillion miles). That may not seem far when compared to the 100,000 ly diameter of our Milky Way galaxy but it is a lot when you consider the speed of spacecraft. NASA’s Voyager 1 craft has been steadily flying out of the solar system since its launch in 1977, but it has covered only 0.002 ly in those 34 years, and would need 80,000 years to reach Proxima Centauri. The travel time goes up proportionately for other interesting stars such as those with recently discovered exoplanets – like Gliese 876 with four planets, at 15 ly distance, or Kepler 11 with 6 planets, 2,000 ly away.

Voyager 1 and every other spacecraft use rocket engines that burn chemical fuels. Pack one of these craft with lots of fuel and its engine can keep firing for a long time to accelerate to high speeds – but there is one problem: the more fuel you put in, the greater the mass that must be moved, which reduces the acceleration. When the numbers are crunched, it becomes inescapable that chemical fuels do not hold enough energy to get spaceships to a really high speed.

That defines the basic problem, a central point of discussion at 100YSS – finding alternate means of propulsion to carry us to the stars in reasonable times. One possibility is to replace chemical fuels with nuclear ones, which in the processes of fission or fusion deliver millions of times more energy per kilogram. This method of propulsion surfaced early in the history of nuclear energy. In 1958, DARPA sponsored a project to use thousands of thermonuclear bombs, exploded one at a time, to push a spacecraft forward, but the idea had to be abandoned when the Nuclear Test Ban treaty came into effect in 1963.

Artist's rendition of the fusion-propelled starship proposed by the British Interplanetary Society in 1973, compared to the Empire State Building. More than 99% of its 53,00 metric tons is fuel.However, cleaner nuclear approaches have emerged, such as one put forth by the British Interplanetary Society, a private group that seeks to encourage space travel. In 1973, the group’s scientists and engineers designed a craft that would use controlled nuclear fusion to achieve a much higher speed than any chemical rocket, about 36,000 km/second. This is fast enough to reach their proposed target, Barnard’s Star, 5.9 ly distant, in 50 years, though with compromises: the ship would be unmanned, and would consist of more than 99% fusion fuel (deuterium and helium-3) and less than 1% payload. Other nuclear methods have also been proposed, for instance, a nuclear thermal rocket, where a fission reactor heats a fluid like liquid hydrogen until it expands as a gas through a rocket nozzle, creating thrust.

Artist's rendition of Japan's IKAROS spacecraft in flight with its 200 square meter solar sail deployed.A different propulsion method, space sailing, sidesteps the fuel problem by using light. Each of the photons making up a light beam exerts a tiny push when it encounters an object. This radiation pressure deflects a comet’s dust tail away from the Sun, as was first surmised in 1619. With a big “sail” to capture lots of sunlight, the result can be a meaningful force on a spacecraft. This was effectively demonstrated in 2010 when, after a conventional launch, Japan’s IKAROS spacecraft spread out a sail of area 200 square meters and began moving toward Venus, pushed by sunlight. The acceleration is small, but if sunlight is replaced by a powerful laser, it can push spacecraft to extremely high speeds, according to reports at 100YSS.

Nuclear propulsion and laser propelled sailing have good scientific bases and do not violate anything fundamental, but our technology is not yet up to making either a reality. Nuclear fusion has been studied for decades as a clean source of power, but has not yet yielded actual net energy production – though the technique of laser fusion shows promise at the Lawrence Livermore National Laboratory, and there is heavy investment in the new ITER fusion machine under construction in France. For space sailing, the ship would need to be accelerated by a laser with terawatts of power to reach high speed. Such a laser exists – it is the heart of the Lawrence Livermore laser fusion project – but it operates only in brief pulses, not continuously, and costs billions of dollars. Building a laser for space sailing would be a massive project.

Even if these propulsion methods could be made to work, the speed any of them would impart to a spacecraft is around 10% of the speed of light – certainly a huge improvement over our present rocket technology, but not a clear breakthrough for exploring the universe. At that speed, a trip to Proxima Centauri would take 40 years, and to the star Kepler 11 to examine its 6 planets, 20,000 years!

Science fiction-ish ways to deal with these long travel times were explored at 100YSS, such as sending people in suspended animation to survive the lengthy trips, or building huge spaceships where generations of humans could live out their lives as they voyage to the stars. And of course, there is the hope of achieving a true science-fiction dream, bypassing the fundamental limit on speed to allow faster than light travel. Space warps, wormholes, “quantized inertia,” and other speculative ideas to do so were all discussed at 100YSS. Though some had a reasonable scientific basis, no one presented even the beginnings of an engineering approach that would turn ideas into an engine that could move a spacecraft faster than light.

So at the moment, the impossible dream of reaching the stars remains impossible, but it is never smart to bet against rapid technological change or the chance of a major scientific breakthrough. DARPA does not think so, either. In early 2012, under the 100YSS project, it reportedly awarded $500,000 to a foundation operated by ex-astronaut Mae Jamison, the first black woman in space, to set up an organization that would create a starship. Astronauts are determined people who are used to overcoming obstacles, and we should not be surprised to find that this particular “impossibility” is being realized, and sooner than we could have believed.

Sidney Perkowitz is the Candler Professor of Physics Emeritus at Emory University. There’s more about interstellar travel in his latest book Slow Light, and his magazine article “Ad Astra! To the Stars!” in Physics World, January 2012, pp. 28-32. He can be reached for comments or questions at http://www.sidneyperkowitz.net/.

Image credits: Daedalus next to the Empire State Building – image by Adrian Mann. Japan’s IKAROS spacecraft – image by Japan Aerospace Exploration Agency.

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