Our neighbouring star Alpha Centauri ESO/Digitized Sky Survey 2

Today, billionaire Yuri Milner, along with physicist Stephen Hawking, announced the largest ever investment in interstellar travel: a $100 million fund to research and prototype a spacecraft capable of reaching the nearest star in just 20 years.

Forget starships, though. These “wafersats” would be small enough to fit in your hand, weighing just a few grams. Milner and his scientific advisory team believe recent developments in lasers and nanotechnology should make it possible to send thousands of these probes to Alpha Centauri, where they could beam back pictures and scientific data on any planets in orbit.

“We have done some recent research with some of the best minds in different areas, and to my surprise I have concluded it can be done within a generation,” says Milner. But the ambitious project, dubbed Breakthrough Starshot, is likely to raise eyebrows amongst the scientific community.


The plan involves launching spacecraft that are little more than a silicon wafer 10 centimetres across, comparable to the guts of a smartphone. These probes will use metre-wide lightsails of reflective material to capture the momentum from colliding photons and propel themselves along.

Sails powered by sunlight are in the works, but these only produce a small amount of thrust. That’s why, back on Earth, a 100 gigawatt laser will shoot into the sky and dump enormous amounts of energy into this sail, accelerating the craft to 20 per cent of light speed – enough to coast the 4 light years to Alpha Centauri in 20 years.

Fringe science

The technology required to do this doesn’t yet exist, but Milner is confident his team can develop it. “We have researched about 20 technical challenges, and we believe that none of those is insurmountable,” he says. “There is no physical law that contradicts this particular model.”

Yuri Milner is splashing the cash to send tiny spaceships to other stars Andy Rain/epa/Corbis

Interstellar travel is a fringe area of science, and while many plans exist on paper, space agencies like NASA have devoted little resources to fleshing out the details. Milner, who has funded science prizes and efforts to search for aliens, wants to change that.

So he created a scientific advisory board and settled on a scheme developed by Philip Lubin and his colleagues at the University of California, Santa Barbara. “They’ve enabled this to go from an idea to reality,” says Lubin.

Lubin’s road map to interstellar flight rests on two technological developments. The first is a technique for combining many smaller lasers into a single larger one, known as phased arrays, which was recently developed by the US Defense Advanced Research Project Agency (DARPA).

“Phased arrays have been made in small sizes, and there does not appear to be a fundamental limit of why we can’t go to larger sizes,” he says. But an array capable of beaming the required 100 gigawatts would stretch over a kilometre.

The other is the rise of smaller spacecraft, in part driven by miniaturisation for the smartphone industry. Cubesats, just 10 centimetres on each side, are a mainstay in orbit these days. Lubin thinks advances in semiconductor manufacture can crunch this down to a 2D-wafer, while still enabling a spacecraft bristling with sensors.

Cost of a phone

In the coming years, the team plans to develop and test such spacecraft, which would eventually be deployed from a “mothership” in orbit before being accelerated by the giant laser. The team aims to bring the cost of their probes down to that of an iPhone, allowing them to launch thousands of them. Such large numbers are needed because the tiny craft could easily be damaged by high-speed collisions with interstellar dust.

“We have a number of things we want to do in the first and second year,” says Avi Loeb of Harvard University, who heads Milner’s advisory board. “We want to demonstrate that we can send such craft, hundreds of them, with cameras to take pictures of the moon.” The team is already in talks to launch the craft on a conventional rocket to the moon in a year or two, says Loeb.

Some are sceptical that Lubin’s road map can be easily realised. “There is no physical limitation, but there are plenty of engineering challenges,” says Paulo Lozano of the Massachusetts Institute of Technology. These include building a laser far larger than any we have today, developing techniques to use it as both propulsion and communication for the spacecraft, and protecting a tiny probe from the hostile environment of interstellar space. “I honestly don’t think that we are ready with the technology that we have,” Lozano says.

“The technical challenges and economic costs are so high that it is not at all feasible and probably won’t be for a long time,” says Ian Crawford of Birkbeck, University of London. Such small spacecraft are likely to be limited in the data they can gather and beam back, but it may be worth trying the technology. “If we want to study exoplanetary systems in detail, we will need to be able to send spacecraft to these places,” Crawford says. “Exploring these beam-powered propulsion systems with low mass payloads would be a step on the road.”

Colossal scale

Even if the team can overcome the technical problems, there is no guarantee we’ll be seeing pictures from Alpha Centauri any time soon. The $100 million is only enough for research, not to fund the full mission, which Milner says will be a decades-long, multibillion dollar project on the scale of the world’s biggest scientific experiments, like the James Webb Space Telescope or the Large Hadron Collider.

Who will fund such a venture is unclear. “Yuri is optimistic he can convince many of his friends to contribute,” says Loeb.

A partnership with organisations like NASA is also a possibility, but the space agency may not be interested. In 2013, billionaire Dennis Tito announce funding for a similarly ambitious project to send a private crewed spacecraft to Mars by 2018, but soon went to NASA asking for assistance. It rejected the mission, which now seems unlikely to ever happen.

“We are not saying it’s easy. We are investing in something that is realistic in the next few years, and then we will take it step by step,” says Milner. “Flying to other stars is dramatically more complicated than within our solar system. We are only saying that we are technologically ready to do this.”