For Yuri Milner, the Russian Internet entrepreneur and billionaire philanthropist who funds the world’s richest science prizes and searches for extraterrestrial intelligence, the sky is not the limit—and neither is the solar system. Flanked by physicist Stephen Hawking and other high-profile supporters today in New York, Milner announced his most ambitious investment yet: $100 million toward a research program to send robotic probes to nearby stars within a generation.

“The human story is one of great leaps,” Milner said in a statement released shortly before the announcement. “55 years ago today, Yuri Gagarin became the first human in space. Today, we are preparing for the next great leap—to the stars.”

“Breakthrough Starshot,” the program Milner is backing, intends to squeeze all the key components of a robotic probe—cameras, sensors, maneuvering thrusters and communications equipment—into tiny gram-scale “nanocraft.” These would be small enough to boost to enormous speeds using other technology the program plans to help develop, including a ground-based kilometer-scale laser array capable of beaming 100-gigawatt laser pulses through the atmosphere for a few minutes at a time, and atoms-thin, meter-wide “light sails” to ride those beams to other stars. Each pinging photon of light would impart a slight momentum to the sail and its cargo; in the microgravity vacuum of space, the torrent of photons unleashed by a gigawatt-class laser would rapidly push a nanocraft to relativistic speeds.

"Without new methods of propulsion we simply cannot get very far," Hawking said at the announcement. "Light is the most pragmatic technology available."

Deployed by the thousands from a mothership launched into Earth orbit, each nanocraft would unfurl a sail and catch a laser pulse to accelerate to 20 percent the speed of light—some 60,000 kilometers per second. Using a sophisticated adaptive-optics system of deformable mirrors to keep each pulse coherent and sharp against the blurring effects of the atmosphere, the laser array would boost perhaps one orbiting nanocraft per day. Each laser pulse would contain as much power as that produced by a space shuttle rocketing into orbit.

The array would need to be built at a dry, high-altitude location in the Southern Hemisphere, perhaps on a peak in Chile, South Africa or even Antarctica—somewhere within sight of Breakthrough Starshot’s primary targets: the twin stars of Alpha Centauri, which at 4.37 light-years away make up the nearest neighboring star system to our own. NASA has already sent five spacecraft on trajectories taking them beyond our solar system, although even the fastest of these would require more than 30,000 years to reach Alpha Centauri. The nanocraft would make that same interstellar crossing in just 20 years. With no onboard ability to decelerate, they would briefly gather data about any planets in the Alpha Centauri system and beam it back toward Earth before plunging deeper into interstellar darkness and out of communication range.

"If this mission comes to fruition it will tell us as much about ourselves as about Alpha Centauri," Milner said at the press conference.

Breakthrough Starshot is the latest of Milner’s Breakthrough Initiatives, a multidisciplinary collection of projects marshaling private funds to address existential questions about life in the universe. Last year, also with Hawking, he announced the $100-million, 10-year Breakthrough Listen initiative to search more than a million stars and a hundred galaxies for signals from alien civilizations as well as an accompanying $1-million Breakthrough Message initiative to compose potential cosmic communiqués to broadcast to any attentive extraterrestrials. Like Breakthrough Starshot, which involves the most sizeable lump sum ever dedicated purely to achieving interstellar flight, these other initiatives offered similar financial sea changes for their respective fields—which due to their extremely speculative nature have long languished in the hinterlands of federal science funding.

Big Plans, Small Spacecraft

Serious planning for the project began about a year ago, when Milner consulted experts to consider options for practical interstellar travel. One was Avi Loeb, an astrophysicist at Harvard University and new chairman of Breakthrough Starshot’s advisory board who has a reputation for performing groundbreaking work on unconventional research topics.

Loeb and his fellow consultants noted that we already routinely accelerate subatomic particles to near light-speed in modern particle physics experiments, and that the smaller a spacecraft is, the more likely it can be made to travel at an extreme velocity. “Strip an iPhone from its case and interface, and the electronics—including the camera and the communications device—weigh on the order a gram,” Loeb says. “That’s almost everything you need for a nanocraft, and we practically have it right now, thanks to the ongoing miniaturization of electronics.”

After evaluating and dismissing propulsion options as exotic as rockets fueled by antimatter annihilation or nuclear fusion reactions, the consultants narrowed their considerations to laser-powered light sails, a concept dating back to the 1960s. They focused on the recent work of Philip Lubin, a physicist at the University of California, Santa Barbara, who was just completing a “roadmap” for developing minuscule, laser-powered interstellar spacecraft as part of a modest NASA-funded study. With minor tweaks, that roadmap offered a notional template for Breakthrough Starshot, and Lubin is now one of the project’s key scientists.

“There are two axes to the problem of interstellar flight,” Lubin says. “Things like antimatter or fusion rockets are all on the ‘real’ axis. The known laws of physics tell us they are realistic solutions, even if we don’t know how to realize them. Things like warp drives and wormholes are on the ‘imaginary’ axis—these are what I would call fictional solutions, because no one knows how to do them.” The laser propulsion concept Lubin detailed in his roadmap rates high on his "real" axis, he says, because “it is both realistic and realizable.”

Lubin’s roadmap laid out myriad obstacles that any laser-propelled interstellar mission would have to overcome, such as linking many smaller lasers into a kilometer-scale array and engineering lightweight, gossamer-thin sails strong enough to endure the array’s gigawatt-scale pulses as well as persuading policy makers to allow the construction of a laser system that could in principle be used as a weapon. The probes will also need to transmit observations back to Earth using onboard lasers with just a few watts of power—a problem potentially solvable by using the giant Earthbound laser array as a receiver. But the biggest obstacle of all was simply a matter of cost: At an estimated present-day price of approximately $10 per watt of laser power, building and operating Breakthrough Starshot’s 100-gigawatt array today could cost as much as $1 trillion.

But as steep as that sounds now, the market prices of just 10 years ago would have rendered it a hundred times more expensive. Driven by demand in consumer high-speed telecommunications systems as well as defense-related projects, the cost of critical technologies for a gigantic laser array are now decreasing by approximately a factor of 2 every 18 months, Lubin says. Those exponential rates of change suggest that in 10 years a giant laser array’s per-watt cost would drop from $10 to 10 cents. “Breakthrough Starshot is really about choosing core technologies to scale up massively, and looking at what prevents or enables that scaling,” Lubin says. “If things are static for the next 30 years in terms of cost per watt, we will be in big trouble.”

According to Loeb, however, another obstacle is a more subtle and social phenomenon: the “giggle factor,” or the tendency for far-out concepts to be laughed off. “Any revolution in science or technology has an initial phase where people laugh at it,” Loeb says. “Sometimes the laughter is inspired by valid criticisms of an argument, but can also be because something appears very different and strange…. What is certain is that the mainstream scientific community that works on research you are not supposed to laugh about—research that has a giggle factor of zero, let’s say—keeps making major mistakes in giggling about the wrong things.”

“We are serious people,” Loeb continues. “We will find whether this project is doable or not, and if it is not, we will admit that and move on.”

Centauri Dreams

Following Lubin’s tweaked roadmap, most of Milner’s $100 million is meant to fund research grants to develop solutions to about 20 major technical obstacles identified by the project. Those solutions would then feed into a prototype system that could perhaps be built for a few hundred million dollars more. Provided that photonics and electronics technologies continue their trends of plummeting costs and soaring performance, assembling a fully functional system would require funding on the same scale as the world’s current multibillion-dollar science projects such as the Large Hadron Collider and the James Webb Space Telescope—funding that Milner alone could not provide. Governments would be one possible sponsor; collectives of billionaire philanthropists would be another. Along with Milner and Hawking, the third member of Breakthrough Starshot’s board of directors is Mark Zuckerberg, the wealthy founder and CEO of Facebook.

“This must be viewed as a collective effort, because that is the only way this can be done,” Milner says. “If we can do this within our lifetimes as we hope, that is pretty exciting, but if not, we will pass it to the next generation—not as an idea, but as a developed roadmap and technology. We are not hundreds of years away from this—only dozens…. This $100 million is meant to last for the next few years, to focus on every single one of the potential deal breakers we have found, to see how far we can push and if we hit any roadblocks.”

Besides the present lack of hardware and full-scale funding, one more key item is missing from Breakthrough Starshot’s plans: There are as yet no confirmed planetary targets in Alpha Centauri. In 2012 a team of European astronomers announced their discovery of an Earth-size planet in a three-day orbit around one of the system’s two stars, but further investigation cast serious doubts about those claims. According to Breakthrough Initiatives’ chairman Pete Worden, the former director of NASA Ames Research Center and current director of Breakthrough Starshot, the organization is also planning a second, related initiative to build new ground-based instruments and maybe even a small space telescope to search for and study Alpha Centauri’s possible planets. Such instruments and telescopes could be turned to other nearby stars, too, possibly revealing additional targets for interstellar voyages. Ultimately, nanocraft in their wispy millions could fan out on photons to explore many more stars, transforming what we do and know on galactic scales.

In addition to making practical interstellar flight a reality, Worden says, the Starshot project could also be transformative for other applications closer to home. Starshot’s final laser array could prove useful for detecting and characterizing potentially threatening near-Earth asteroids, and its many deformable mirrors could be repurposed to make the array a massive telescope for gathering starlight rather than creating laser beams. The lasers could also power nanocraft on rapid flybys of all the solar system’s planets, sending probes to Mars in a few hours or Pluto in days for a few hundred thousand dollars per shot.

“Once we launch these things and deliver close-up images of a planet around another star, that begins to define humanity as a whole and humanity’s future,” Worden says. “This is something that, if it works, changes how we think about ourselves as a species and as a planet.”

Already, the planning for Breakthrough Starshot is changing how members of the Breakthrough Initiatives organization view its other related projects, such as Breakthrough Listen, the effort to tune in to deliberate or inadvertent transmissions from cosmic civilizations. If we can seek out potentially habitable planets in other star systems and use lasers to send flotillas of miniaturized spacecraft to investigate them, there is no reason to assume we are the only ones in the Milky Way doing so. The spacecraft themselves would be essentially invisible: A gram-size interstellar probe striking the Earth’s upper atmosphere at 20 percent the speed of light would release roughly a kiloton of energy, indistinguishable from airbursts produced by meter-scale space rocks that regularly pepper our planet at a rate of about once per year. Were it to stream into one of Earth’s more capable ground-based observatories, however, the light from the lasers propelling such probes could conceivably be detected. Most of Breakthrough Listen’s efforts revolve around seeking signs of chatty aliens conversing via radio waves, but the project also funds searches for any glints of laser light from interstellar space.

In remarks prepared ahead of Tuesday’s news conference, Stephen Hawking explained his support for the project as less about science and more about survival. “Earth is a wonderful place, but it might not last forever,” Hawking says. “Sooner or later, we must look to the stars. Breakthrough Starshot is a very exciting first step on that journey.”

Loeb agrees. “At some point, we must find alternatives to living on Earth, and as with any big journey, this has to start with a first small step—you can’t just give up,” he says. “Remember what Oscar Wilde said—‘We are all in the gutter, but some of us are looking at the stars.’ When I look up at the stars at night, they seem like lights in a giant ship sailing through space. And each time I ask myself the same question: Are there other passengers riding in this giant ship, near those lights? If this project comes to fruition, we could visit them and find out.”