In the 1960s, the physicist and space futurist Robert Forward proposed a radical method of sending a spacecraft to the stars. Roughly speaking, the idea was to attach the spacecraft to a large light sail, and then push it by illuminating the sail with an enormous laser. Forward suggested that a powerful laser could accelerate a spacecraft to a large fraction of the speed of light, allowing it to reach some of our nearest stellar neighbors within a few decades.

Until very recently, this idea remained solidly within the realm of science fiction. But today the outlook is beginning to change. In April, the physicist-turned-internet-billionaire Yuri Milner, together with Stephen Hawking and other notable scientists and engineers, announced that the Breakthrough Foundation would begin funding work on the concept of a laser-propelled starship, with the long-term goal of reaching the closest neighboring star system to our own, Alpha Centauri. Their initiative, dubbed Breakthrough Starshot, is a Silicon Valley take on Forward’s vision that imagines shrinking the spacecraft down to a mass of a few grams. A fleet of such “nanocraft”, each tethered to a light sail a few meters wide, could be economically launched into space and then accelerated toward Alpha Centauri with a terrestrial laser system. Illuminated by tens of gigawatts of laser light, the miniature spacecraft would reach 20 percent of the speed of light in about 4 minutes.  After a 20-year cruise, they would zip through the Alpha Centauri system in a few hours and send data and images back to Earth from the brief encounter.

This might still sound like science fiction, but Starshot is by far the most realistic proposal for reaching another solar system. It would take, at best, tens of thousands of years to travel the 4.3 light-years to Alpha Centauri with chemical rockets. The other alternative propulsion strategies that have been proposed over the past several decades, including warp drives, nuclear fusion, and antimatter,…[Read more]