Last month, Russian internet billionaire Yuri Milner announced plans to send thousands of tiny spacecraft to visit Alpha Centauri, the closest star system at 4.4 light-years from Earth. Dubbed Breakthrough Starshot, the mission aims to take close-up images and collect data from any potentially habitable planets there. In order to cover the vast distance—41 trillion kilometers—in a reasonable time, the proposed spacecraft will each weigh less than a gram. Once in space, they will unfurl lightweight sails to catch laser beams shot from Earth, accelerating to one-fifth the speed of light under light pressure. Launch could be 30 years off, and the trip to Alpha Centauri would take a further 2 decades.
Milner, who also supports the multimillion-dollar Breakthrough Prizes and Breakthrough Listen, a search for signs of extraterrestrial intelligence, has committed $100 million to this venture. But Breakthrough Starshot has polarized opinion: Some are enthused by its ambition, whereas others say it is costly and unnecessary, isn’t feasible, or is downright dangerous. Milner spoke with Science by phone about the challenges facing the project and how he answers his critics. His responses have been edited for clarity and brevity.
Q: How did your interest in space travel and in this mission to Alpha Centauri come about?
A: I was named Yuri after Yuri Gagarin because I was born the same year the Russian cosmonaut was launched on the first manned space flight. So I’ve carried this message about space travel in my name my whole life!
Breakthrough Starshot came from a small working group we put together to devise a practical space project to a neighboring star system that could achieve results within the lifetime of a generation. They considered various propulsion mechanisms for interstellar travel—including fusion engines and matter-antimatter propulsion—and concluded that the sail configuration is the most feasible in a reasonable time frame.
The idea of using spacecraft with solar-powered sails is actually very old, but until recently it was purely theoretical. Over the past 20 years there has been significant progress in microelectronics, nanomaterials, and laser technology that means we can now have a sensible conversation about making a gram-scale starship and accelerating it to 20% of the speed of light.
Q: You’ve contributed $100 million dollars, but the final project could cost $10 billion. Where will this extra money come from?
A: We’ve been open from day one that this is not something you can build in a garage and no one person can finance this machine. If this is going to happen, I envisage this as something that will need international collaboration on a financial scale comparable to CERN [the particle physics laboratory near Geneva, Switzerland].
The seed money covers the first 5 to 10 years’ research and development phase, and within that time we should know if the challenges the project faces can be overcome or if they are insurmountable. The second phase will be to build a prototype and I think that can be financed by private investors, too. The final machine will need international backing.
Q: Might the money be better spent on a new planet-hunting telescope?
A: We’re actually in negotiations to spend some of the money to increase the capability of some ground-based telescopes to take a direct image of possible planets around Alpha Centauri. That would use existing infrastructure and we hope to announce it soon. This is important because we don’t even know with any degree of certainty if there are potentially habitable planets in the Alpha Centauri system to target with Breakthrough Starshot.
But there is no substitute for a flyby and taking close-up images. This would be the equivalent of the New Horizons mission to Pluto. To get an equal quality image with a ground or near-Earth telescope, you would need a telescope on the scale of a few hundred kilometers and that’s not a small endeavor.
Q: Even if the project’s giant lasers can be built, what about the damage they could potentially cause to the environment or their misuse as a weapon?
A: Laser technology is following its own Moore’s law trajectory, so in a couple of decades’ time we think laser power will have increased sufficiently and such lasers will not be prohibitively expensive. But from the outset we identified that there must be some form of global consensus on its use. It may be that we have the technological capability but the project stalls because there is no agreement about the governance of such a machine.
Q: Won’t laser beams fired through the atmosphere lose power through dispersion? Wouldn’t a space-based array be better?
A: That would increase the cost 100 times and push the mission back a few hundred years. So for a space-based system, let’s just stop talking now. It’s not going to happen in our lifetime. A space-based laser also poses more serious policy issues because it could be pointed at Earth and is more difficult to control.
The power from Earth-based lasers will not be dramatically different. The basic principle would be to utilize the adaptive optics already used by ground-based telescopes to deal with the challenges of the distortion of light passing through the atmosphere.
Q: Critics have warned that the powerful laser beam could set the tiny craft spinning out of control or destroy the fragile sails. Have you considered such scenarios?
A: Our experts have been looking into this and now think that a spinning craft may actually be more stable than a nonspinning one. But we don’t know whether the sails will melt when the laser hits them or what the craft will meet in interstellar space. We’ve identified more than 20 technological challenges to the successful completion of this project. More work is needed and that’s what the research phase is for.
Q: Can you miniaturize the sensors, imaging, and signaling equipment to fit on such a small craft?
A: We have carried out pretty detailed calculations that show we can shrink down the imaging equipment and sensors, even today. And surprisingly, to send a signal over trillions of miles you only need a small laser on board, powered by a watt-scale battery, and that can be made gram-scale. The sail would then be used as a dish to help transmit the signal, while the laser array on Earth would act as a receiver. So miniaturization of nanocraft is probably the least of the problems—the sails and the lasers are bigger obstacles.
Q: Are you worried about sustaining a workforce for such a long-term project?
A: It took two or three hundred years to build some cathedrals, but people did not lose interest. We have proven that we can focus on long term scientific projects, too: 2016 will be remembered as the year we detected gravitational waves but the LIGO experiment took 50 years; CERN is another example of experiments stretching over decades. This is the exciting next stage of space exploration being ignited and the fire will keep burning.
Q: You have identified many ways this project might fail. Are you worried that your investment might ultimately go to waste?
A: Honestly, we are a very lucky generation because we are the first that could pull this off and, if we do, it will be incredible. But if not, we have promised to keep all the results of our research open to the public. One day our civilization will make use of it. It is human nature to explore the world around us and I don’t think that curiosity will ever go away.
Zeeya Merali is a writer in London.