Russian internet billionaire Yuri Milner today proposed making a genuine journey to the stars: a program to develop a tiny spacecraft, weighing less than a gram, and propel it across nearly 40 trillion kilometers of space to the nearest star to study any planets there. Instead of a futuristic propulsion system, the “nanocraft” will “leave their fuel behind,” Milner told a press conference in New York City this afternoon. The craft will have a solar sail, a few meters across and weighing a few grams. Powerful Earth-based lasers would boost the craft, which would be waiting in Earth's orbit, with an acceleration of 60,000 g for a few minutes to reach 20% of the speed of light—fast enough to reach Alpha Centauri in 20 years. “It’s the first time in human history that we can do more than just stare at the stars,” Milner said.

Milner, who made a fortune investing in fast-growing internet companies such as Facebook, Twitter, Spotify, and Groupon, has a track record of funding far-out science. He created the Breakthrough Prizes and last year launched the Breakthrough Initiatives, including a $100 million, 10-year effort to search for extraterrestrial intelligence using optical and radio telescopes.

Milner has now launched another $100 million effort, dubbed Breakthrough Starshot, to prove the principle of sending multiple tiny craft to a star. “With light beams and a light sail and the smallest spacecraft ever created, we can launch a mission to Alpha Centauri in a generation,” U.K. physicist Stephen Hawking told the press conference. Hawking was joined on the podium by other high profile scientists backing the project, including Princeton University physicist Freeman Dyson, Massachusetts Institute of Technology in Cambridge astrophysicist Avi Loeb, and Pete Worden, former director of NASA Ames Research Center.

The effort faces considerable technical challenges but Milner said it was “based on technology that is available or available in the near future.” The first challenge is in microfabrication, which he says is being driven by the cellphone industry. The project will be developing a Starchip, essentially a spacecraft on a chip, which will have cameras, photon thrusters, power supply (a radionuclide source), navigation, and communication equipment. The team estimates that today such a chip can be built weighing just 370 milligrams; by 2030 it will be down to 220 milligrams. Starchips could be mass produced for about the cost of an iPhone, Milner claims.

The second challenge will be in nanotechnology: to develop a light sail material that is no more than a few hundred atoms thick so that they can build a sail a few meters across that weighs no more than a few grams.

The third challenge comes in laser technology. Powerful enough lasers don’t yet exist, but if developments continue to follow Moore’s law the team calculates technology will be ready in a few decades’ time. The team envisages a “light beamer,” a kilometer-sized array of possibly hundreds or thousands of powerful lasers producing a combined beam of 100 gigawatts. It would only be used for a few minutes a day to boost one nanocraft, so could draw its power from the grid, but for each shot it will have to store up hundreds of gigawatt hours, either in batteries or other storage technology such as flywheels. Particle accelerators and fusion reactors use similar techniques.

The light beamer will have to use advanced adaptive optics to compensate for the distorting effects of Earth’s atmosphere and ensure the beam stays on target. “It’s the first new form of propulsion in a century,” Worden told the press conference. He added that there will obviously be policy issues involved in beaming that much power through the sky but, he said, “regimes exist to get approval” for such things.

Nanocraft will be sent out toward the target star in large numbers so that the loss of some will not be a disaster. Different craft will also be equipped with a range of sensors to study any planets around the star. Data will be returned by laser beam from the Starchip, with the light sail doubling as a reflector to boost the signal. Traveling at 20% of light speed, however, will mean that they zoom past Alpha Centauri in less than an hour. Loeb said that at such a speed, the craft’s cameras will see misshapen stars and planets, their vision distorted by the effects of special relativity.

“There is a big difference between physically exploring regions of space and observing them from a distance,” Loeb told the press conference. “I don’t see any deal-breakers in terms of fundamental physics. I think we can do this.”

Milner said that if they can prove the concept, “considerably more funding will be required” to actually achieve the mission. “It will take a generation, but this is the first time we can say with conviction that it can be done on this timeframe.”