Nasa experts have begun to think the unthinkable and dream the impossible - a spaceship that can accelerate to a tenth of the speed of light, and carry colonies of human travellers across the void to the nearest stars.
Geoffrey Landis, an expert in space propulsion from the Nasa Glenn research centre in Ohio, told the American Association for the Advancement of Science meeting in Boston yesterday, that the star trekkers of tomorrow could be very different from the Star Trek of yesterday's television.
Human space voyagers would spend generations on their journey, even at 18,000 miles a second - and the ideal crew might be composed entirely of females, with a helpful bank of sperm to supply the future.
"In Nasa there has been a small and almost underground effort to look at how you would send a probe to the nearest star," he said. "Some very clever people have been chipping away at the problem, and now we think it could be possible without breaking the laws of physics."
The great problem is the distance. Voyager probes launched two decades ago are speeding out of the solar system at 10 miles a second.
"If at the end of the last ice age, the mammoth hunters had launched a star probe at the speed of Voyager - which is the fastest thing that humans have ever launched out of the solar system - it wouldn't even be 20% of the way to the nearest stars yet. It will take Voyager about 75,000 years."
One idea - by the space scientist Robert Forward - would involve building a sailing ship, with ultralight niobium or diamond sails only atoms thick, but stretching for tens of miles, driven by the "wind" from a powerful laser in orbit.
This laser beam could push the space clipper to 11% of the speed of light. In theory it would cover the distance to the nearest star in 40 years. But it would need to stop when it got there. So researchers are now working on a theoretical device - a huge magnetic parachute at Proxima Centauri - to slow the spacecraft.
And the process of braking could last 100 years. So anyone planning a voyage to the stars, even with technologies that Captain Kirk could never have imagined, would have to plan for a 200 year round trip, or more.
"Stopping is hard: it is just as hard to stop from 10% of the speed of light as to get to 10% of the speed of light in the first place. And getting back is just as hard as the original problem, if you have to send a laser to the other star system, to send you back."
But above all, it seemed a spacecraft bound for other stars would have to be big. "Such a ship would literally be an interstellar ark, carrying with it all forms of earthly life that colonists might need, at least in the form of embryos. It would be a self-sufficient, self-contained space colony, a city in space," he said. "Everything you don't bring, you must be able to make."
This raised a puzzle of human dynamics: sociologists, physicists and science fiction writers had gathered in Boston to consider who might survive such a voyage.
"What is the absolute minimum needed?" he asked. "The sociologists are going to say that you need a lot more than just a man and a woman."
The star trekkers would have to grow plants in space, and carry fusion reactors to run life support systems, because they would be too far from the sun for solar power.
"This is not something we are going to do next year," he said.
