Inside Starshot, the audacious plan to shoot tiny ships to Alpha Centauri

Starship conferences attract a hopeful crowd: researchers, inventors, and hobbyists enthused by the idea of building spacecraft that can fly between star systems. The excitement at these gatherings can make it feel as if anything is possible—but also as if nothing is. Many of the schemes put forward are too vague, and they almost always have too many technological gaps to fill.

In 2015, Philip Lubin, a cosmologist from the University of California, Santa Barbara, took the stage at the 100-Year Starship Symposium in Santa Clara. He outlined his plan to build a laser so powerful that it could accelerate tiny spacecraft to 20% of the speed of light, getting them to Alpha Centauri in just 20 years. We could become interstellar explorers within a single generation. It was quite the hook.

Because Lubin is an excellent public speaker, and because the underlying technologies already existed, and because the science was sound, he was mobbed after the talk. He also met Pete Worden, a former research director of NASA’s Ames Research Center, for the first time. Worden had recently taken over as head of the Breakthrough Initiatives, a nonprofit program funded by Russian technology billionaire Yuri Milner. Six months later, Lubin’s project had $100 million in funding from Breakthrough and the endorsement of Stephen Hawking, who called it the “next great leap into the cosmos.”

Starshot is straightforward, at least in theory. First, build an enormous array of moderately powerful lasers. Yoke them together—what’s called “phase lock”—to create a single beam with up to 100 gigawatts of power. Direct the beam onto highly reflective light sails attached to spacecraft weighing less than a gram and already in orbit. Turn the beam on for a few minutes, and the photon pressure blasts the spacecraft to relativistic speeds.

Not only could such a technology be used to send sensors to another star system; it could dispatch larger craft to Earth’s neighboring planets and moons. Imagine a package to Mars in a few days, or a crewed mission to Mars in a month. Starshot effectively shrinks the solar system, and ultimately the galaxy.

It’s fantastic. And also a dream. Or a sales pitch. Or a long-term, far-out project that can’t be sustained long enough for the nonexistent technologies it requires to be built.

Lubin is a young 66. He walks fast, and his thick hair and full beard are dark. When I went to meet him in Santa Barbara this April, he told me that he had been a serious kid, disturbed by the realities of the world. He sought solace in math and science because he found them beautiful. “I loved school,” he explains. “I used to study all the time. It was like a retreat for me: ride my bike to the library and devour books.”

Even so, he didn’t expect he’d follow an academic path—it didn’t seem possible. His family valued education, but his Lithuanian father, who worked as a mail carrier, never even graduated from high school. His Russian-born mother was a secretary. “I grew up with an internalization that college was for other people,” he says. After encouragement from a school counselor in Los Angeles, though, he attended community college; teachers there prodded him to transfer to UC Berkeley. And there, his professors nudged him to apply to graduate school. Eventually he landed at Harvard. “When I look back on it,” he says, “I was a total knucklehead.”

Today Lubin is a cosmologist. For much of his career, he’s built equipment to measure the background radiation of the universe, but his scientific and technical interests are varied. It was at a defense technologies conference, talking about using lasers to defend Earth against incoming asteroids and comets, that he first came up with the idea for Starshot.

He also tells me about another obsession: propulsion. Most rockets today run on liquid fuel, much as they did when Germany invented the V2 during the Second World War. The last 75 years in computing, by comparison, have produced a trillion-fold increase in speed. “Wouldn’t it be neat if propulsion could advance like that?” says Lubin. “The SLS”—NASA’s super-heavy rocket, which has already cost $12 billion and still isn’t ready—“could cost less than a penny.”

Lubin’s labs at UC Santa Barbara feature a cluttered warehouse that feels typical of experimental physics setups: giant spools of optical fiber, racks of oscilloscopes, tool boxes, circuit boards. One cabinet for solvents, another for snacks.

As we walk through the labs, he is quick to acknowledge that Starshot still faces a lot of challenges. There is, for example, no laser yet powerful enough to do this kind of blasting. There are no light sails that could take such a beam without being obliterated. There are no less-than-gram-size spacecraft to make the journey, and questions about laser supply and laser location remain. And then there are the ethical and geopolitical implications of building such a powerful directed energy source. After all, it could also be a weapon.

At the whiteboard, postdoctoral researcher Peter Krogan begins walking me through the solutions to these issues. First up: building the laser array.

The challenge here is figuring out how to fix the frequency of billions of lasers, each 10 centimeters in diameter, and stabilize them so they can be combined into a single large beam. Locking more beams together allows the strength of the laser to be scaled up to the levels proposed. The team’s current working plan is for an array located on the ground, which keeps costs lower than if it were placed in orbit but adds other complications—such as overcoming atmospheric interference. This requires a beacon attached to the spacecraft that sends a signal back through the atmosphere, letting the ground-based lasers fix on their target. To couple the array, Krogan is working on “nested phase locking,” where a smaller array synchronizes before seeding the next layer in the array, and so on. If this can work for two layers of lasers—their immediate research goal—then it might just be possible to do it for the five layers that simulations say is best for a 100-gigawatt beam.

The second big challenge is the solar sail. While the concept has been around for decades, it wasn’t successfully deployed until 2010, when Japan’s Ikaros spacecraft tested a sail 14 meters (46 feet) square during its mission around the sun. But a sail that can take the gentle pressure of solar photons is drastically different from one that can withstand the most powerful laser ever built—the difference between letting an April mist hit your face and getting pummeled by a firehose.

To manage this, the Starshot sail needs to be extremely robust, though it must also be extremely lightweight. The key, Krogan explains, is to let some of that power leak through: the sail’s material must be transparent and reflective simultaneously. Glass is one of the more promising candidates, though it would need to have its properties adjusted to achieve the perfect mix of reflectivity and transparency. The ideal material still needs to be invented, but there are some promising advances, Krogan says.

The third major challenge is building the tiny spacecraft. The smallest objects orbiting Earth right now are cubesats, which are 10 centimeters on each side and weigh about a kilogram. Lubin’s team wants to shrink the entire craft to the size of a microchip—what they call “wafer-scale.” They’ve miniaturized prototypes to the size of a matchbook and even a quarter. But their best working models currently weigh about 100 grams, still 100 times too heavy for the Alpha Centauri mission. Obstacles include integrating the electronics and photonics, making it able to withstand the radiation in deep space, shrinking the power supply, developing an ultra-small onboard thruster … the list goes on.

But while the technical challenges are real, the major difference between Starshot and many other interstellar projects is that it doesn’t require new physics or even fundamentally new technologies. When Lubin was developing the idea, he sent the details to colleagues for feedback. They were “people who would rip it to shreds,” he says. “The people who take no prisoners and have no mercy and are completely comfortable saying, ‘You idiot!’… I said, ‘Please destroy this, because I’m tired of working on it.’ In the end, everyone I spoke with said, ‘Well, it should work.’”

By the time Breakthrough’s technical experts vetted the concept, the outline was solid. Worden was excited. “We were all convinced that this was the first really plausible interstellar technology that we could do in our lifetime and would be affordable,” he says.

And even if not all the problems will be solved, it’s worthwhile to solve some of them, he says. For example, developing a fully capable spacecraft that weighs less than a gram would be a major revolution. Cubesats were dismissed by many until just a few years ago; now there are constellations of them. Chipsats, he says, will mature soon and revolutionize science and communications. Low-cost, efficient laser arrays could be useful for jobs like pushing space junk out of the way. And advances in light sails would allow microscale spacecraft within our own solar system to reach other planets in months, not years. “That’s going to change our understanding of objects in our solar system, and the search for life,” says Worden. “And commercially, it’s going to be hugely valuable when looking for space resources.”

There is one issue that cannot be solved by technology, though: geopolitics. Lasers would help propel solar sails, says Joan Johnson-Freese, a professor of national security affairs at the US Naval War College who also sits on the Breakthrough board. “But when you start talking about firing lasers, people get very nervous.”

She suggests that international agreements would likely ensure the broadest, most beneficial use of such a powerful laser. And the military potential of space is not new: today anything China does in space is considered dual-use. “The same is true for the US,” she says. “China could interpret anything we do as threatening.”

One way forward could be to democratize exploration. Historically, the US and other superpowers have dominated space, but Starshot could open it for countries that don’t have access. A nation that launched a fleet of chipsats could access communications, exploration, and commercial reconnaissance that were previously unaffordable. It’s a rare project that has such big technological, scientific, commercial, and geopolitical implications.

“It requires some careful thinking, and also transparency, and possibly international collaboration and conversations in the long run,” Lubin says. “Luckily, we have some time, because we’re not deploying anytime soon.” 

So when will Starshot be realized? One goal is to get probes to Alpha Centauri by 2061, the 100th anniversary of Yuri Gagarin’s pioneering orbital flight. That’s a long way off, almost certainly beyond Lubin’s lifetime. He says the project will have a chance only if people realize that it is “milestone based,” a road trip with many points along the way.

But that long horizon means it’s going to need money. NASA’s contributions expired this year. Other cash has come from an anonymous philanthropist. And so far, Breakthrough’s funding has yet to arrive.

“We’re a new organization, and we’re still in the startup phase,” says Worden, promising that the cash will come once negotiations between universities, contractors, and regulators have been completed.

It’s a puzzle, but Lubin isn’t afraid of a little complexity. That’s exactly what this whole endeavor is about. “This isn’t just a single-use technology,” he says. “It’s not just wafers to the stars. It’s cubesats to Europa, or humans to Mars quickly, or the ability to keep a spacecraft in orbit longer at low altitudes, or to protect the planet from external threats like asteroids. If you don’t understand the whole breadth of this technology, then you’re missing the beauty of the transformation it makes possible.”

Kate Greene is an essayist, poet, and former laser physicist.