The idea of journeying to the stars is nothing new. As early as 1929, the British scientist J. D. Bernal wrote about the possibility of generation ships—enormous spacecraft, like miniature worlds, which would take hundreds or thousands of years to reach their goal and aboard which many generations of travellers would live out their lives. In both science and science fiction, concepts such as suspended animation have also been used to allow people to cross the light-years to other stars, even at relatively low speeds.
But, almost certainly, practical interstellar travel will demand that vehicles reach far higher speeds than any that have been achieved in spaceflight to date. This in turn will mean that new forms of propulsion have to be developed that go beyond the capabilities of chemical rockets or even ion engines.
One of the first practical designs for a robotic interstellar probe was that of the British Interplanetary Society in the mid-1970s. Known as Project Daedalus, it called for a ship to make the voyage to Barnard’s Star, a red dwarf 5.9 light-years away, in a travel time of 50 years, powered by a nuclear-pulse rocket that could propel the craft to about 12 percent of the speed of light (36,000 kilometers per second). This type of engine, which would use a series of nuclear fusion explosions—effectively, hydrogen bombs—had already been studied by Freeman Dyson and his colleagues as part of Project Orion in the 1960s.
Daedalus would be constructed in Earth orbit and have an initial mass of 54,000 tons, including 50,000 tons of fuel and 500 tons of scientific payload. The first stage would be fired for two years, taking the spacecraft to 7.1 percent of light speed, before being shut down and jettisoned. Then the second stage would fire for 1.8 years before being shut down to begin the 46-year cruise to Barnard’s Star. Since the design made no provision for deceleration upon arrival, Daedalus would carry 18 autonomous probes, equipped with artificial intelligence, to investigate the star and its environs.
In 1988, researchers from NASA and the US Naval Academy came up with a design for an interstellar probe to Alpha Centauri. Project Longshot, as it was called, would have been assembled at the International Space Station, and propelled on a 100-year flight to the nearest star system by a pulsed fusion engine of the type proposed in the Daedalus study. The 6.4-ton spacecraft would have carried a 300-kilowatt fusion reactor to power instruments and engine startup and used a 250-kilowatt laser to transmit data to Earth.
Of course, no one expected these early proposals to be put into action straight away. However, they showed that practical interstellar travel is possible, at least in terms of the propulsion systems. Other problems remain for any spacecraft, crewed or robotic, that must travel for many years at a significant fraction of the speed of light. These include surviving collisions with even small particles, such as dust grains, at speeds of tens of thousands of kilometers per second, and the durability of sensitive components, especially electronic ones, over such long time periods.