In the Southern Hemisphere’s sky, there is a constellation, a centaur holding a spear, its legs raised in mid-gallop. The creature’s front hoof is marked by a star that has long hypnotized humanity, with its brightness, and more recently, its proximity.

Since the dawn of written culture, at least, humans have dreamt of star travel. As the nearest star system to Earth, Alpha Centauri is the most natural subject of these dreams. To a certain cast of mind, the star seems destined to figure prominently in our future.  

In the four centuries since the Scientific Revolution, a series of increasingly powerful instruments has slowly brought Alpha Centauri into focus. In 1689, the Jesuit priest Jean Richaud fixed his telescope on a comet, as it was streaking through the stick-figure centaur. He was startled to find not one, but two stars twinkling in its hoof. In 1915, a third star was spotted, this one a small, red satellite of the system’s two central, sunlike stars.

Astronomers are now looking for planets around all three.

To say that Alpha Centauri is the nearest star system to Earth is not to say that it’s near. A 25 trillion mile abyss separates us. Alpha Centauri’s light travels to Earth at the absurd rate of 186,000 miles per second, and still takes more than four years to arrive.

Aerospace engineers have spent decades trying to push spacecraft to extreme speeds, and with great success. The New Horizons probe that just whistled by Pluto is our fastest flying spaceship. It can cover more than one million miles per day. But if we aimed New Horizons at Alpha Centauri, it would reach the star tens of thousands of years from now. Interstellar travel with existing propulsion technology is all but unimaginable.

But, new technology might be on the way.

At noon today, Yuri Milner, the Russian tech billionaire, will join Stephen Hawking atop Manhattan’s Freedom Tower, where the pair will announce Starshot, a $100 million dollar research program, the latest of Milner’s “Breakthrough Initiatives.” (Mark Zuckerberg will serve on Starshot’s board, alongside Milner and Hawking.) With the money, Milner hopes to prove that a probe could make the journey to Alpha Centauri in only 20 years.

“Everybody, including myself, thought that this wouldn’t be possible in our lifetime,” Milner told me in an interview.

Silicon Valley’s billionaires are famously pro-science, but even among this set, Milner has distinguished himself. He amassed most of his fortune by making gutsy investments in social-media companies, including an 8 percent pre-IPO stake in Facebook. But as a younger man, Milner read Isaac Asimov and Carl Sagan. And at Moscow State University, he studied physics. In 2012, after his net worth spiked into the billions, Milner helped found the most lucrative prize in science. Its three annual awardees each take home $3 million dollars, more than twice the award granted to Nobel Prize Winners.

Milner calls himself a child of the space race. Last year, he bankrolled history’s most ambitious, well-funded SETI program—a close search of the nearest million stars for signs of intelligent life, alongside less granular scans of the Milky Way’s center, and the nearest hundred galaxies.  

“I even carry the space race in my name,” Milner told me. Like many Russian boys born in the early 1960s, he was named for Yuri Gagarin, the first human being to reach orbit.

Today is the 55th anniversary of that historic first spaceflight. Milner will mark it by announcing his Alpha Centauri project, which will be headquartered on Sand Hill Road, in Menlo Park, the financial heart of Silicon Valley. Pete Worden, the former head of one of NASA’s largest research centers, will run day-to-day operations.

Milner wants his $100 million to fund research that will culminate in a prototype of a probe that can beam images back to Earth. He told me the images would arrive less than 5 years after the probe reached the star.

There are no official specs yet, but Milner said the probe would have a two-megapixel camera, along with star-finders to help it get its bearings, after it boots up on the approach to Alpha Centauri. The probe will target one of the system’s two sunlike stars. It will be aimed at a planet (or planets) in the star’s habitable zone, the temperate region where oceans don’t boil or freeze, but instead flow, nurturing the kind of complex chemistry that is thought to give rise to life.

I asked Milner what the images would look like.

“The engineers tell me that we might be able to make out continents,” he said.

* * *

Milner’s Starshot initiative dwarfs all previous investments in interstellar travel. In 2012, DARPA announced its own “100 Year Starship” project, to much fanfare—but its budget was only $1 million, and perhaps for good reason. At the time, most interstellar mission designs were theoretical to the point of fantasy, or they were too expensive to pursue. Some had price tags in the trillions.

Milner’s Breakthrough Initiatives are meant to stake scientific research that is high risk, but high impact. He told me he vetted a number of speculative projects, before picking this one. Interstellar flight was always on his list, but he figured it was impractical. When he started reviewing mission concepts, he was surprised to find a steady drumbeat of support for a “light-sail,” a large reflective sheet that could reach cosmic speeds, when pushed with a powerful laser beam.

The “light-sail” idea has a distinguished pedigree. In a famous 1610 letter to Galileo, Johannes Kepler mused about “sails adapted to the heavenly breezes,” and in the 19th century, James Maxwell showed that light could be one of these breezes, because light can push—it carries momentum. Jules Verne went as far as to say that light would “probably be the mechanical agent” that propelled a mission to the stars. During the 20th century, several scientists warmed to the idea, in part because it eliminated the need for heavy, onboard fuels. A few even tried to work out how it could be done, in detail.

“The scientist that articulated this idea with a lot of scientific rigor was Robert Forward,” Milner told me. Forward proposed a large mesh light-sail, measuring a full kilometer wide. He called it a “starwisp,” and he imagined a microwave beam could accelerate it to speeds that would put the local stars within reach.

“Forward’s idea was elegant, but it was unrealistic,” Milner said. “It would take a lot of energy, comparable to all the energy we have on our planet, to propel an enormous spacecraft like that. What we needed was a dramatic reduction in the size of a spacecraft. And by dramatic, I mean many, many orders of magnitude. We need to build a spacecraft that would weigh a few grams, together with the sail. If we can’t go down to a few grams, there is no conversation.”

Milner envisions a sail that’s only a few meters wide. Picture a thin disc about the size of a round picnic tabletop. It would have miniaturized electronics onboard, including a power source, cameras, photon thrusters for navigation, and a laser for communication. Some of this kit would be bundled into the disc’s center, and some would be distributed through the rest of the sail. But it would all be a single unit: If you saw it streaking by, it would look like a flat, round sheet of reflective material.

Milner wants to launch a small “mothership,” filled with hundreds of these thin, disc-like probes. (He thinks each probe can eventually be manufactured at roughly the cost of an iPhone.) Once the mothership reaches orbit, it would release one probe per day. The probe would exit the larger spacecraft, and use its photon thrusters to position itself in the path of a ground-based laser beam.

The laser would be located somewhere in the Southern Hemisphere. “You need to put it high in the mountains,” Milner told me. Too much air or moisture, and the laser will be distorted on its way out of the atmosphere. “An interesting place would be the Atacama desert in Chile,” he said. Apart from the poles, Atacama is the driest place on Earth. Its arid peaks tower more than 16,000 feet, and already, it’s a pilgrimage site for those seeking cosmic communion—Atacama’s more remote perches host some of the world’s most far-seeing observatories.

* * *

Interstellar mission concepts are difficult to vet. Very little research money has been spent on star travel, meaning there isn’t much institutional expertise. But there is an interstellar subculture of sorts, a small community of engineers and scientists who write speculative papers about far-fetched missions in their spare time.

Paul Gilster’s popular Centauri Dreams blog (and its comment section) functions as a kind of salon for people who like to think about star travel. When I asked Gilster about the light-sail mission concept, he said he was surprised that Milner was considering a ground-based laser. Most light-sail research assumed a space-based laser, positioned near Mercury, where huge gobs of solar power are easy to come by. In space, you don’t have to worry about atmospheric turbulence throwing off your laser. And you don’t have to worry about it burning a hole in the ozone layer.

Milner dismissed the idea of a space-based laser. He hinted that the turbulence problem would be solved with adaptive optics, a cutting edge technology that allows observatories to adjust for atmospheric distortion in real time.

“People who talk about lasers in space don’t think about policy issues, and they don’t think about cost,” he said. “Nobody will allow you to build something that you can point in all different directions, as you would be able to in space. This is a very big laser. It can do quite a bit of damage.” On Earth, it could only point in certain directions, and would be much easier for other governments to inspect.

(Later, Milner said, darkly, that one of his big questions is, “whether we are a mature enough civilization to be doing this.” And by “this,” he meant building a starship powered by a superweapon.)

Milner told me that a ground-based laser could run off a giant power plant devoted solely to the mission. It could be a solar array in the Atacama desert, given how much sunlight pours onto its stark landscape. To make it work, the array would have to stretch for tens of miles, and it would need a battery large enough to store fodder for the daily firing of the world’s most powerful laser cannon.

Chile’s Atacama Desert is one possible site for the laser (Danielle Pereira)

The laser team would need to time its daily blast carefully, to avoid destroying the satellites and planes that pass overhead. When fired, the beam would shoot up through the atmosphere, and slam into the disc-like probe, sending it hurtling toward the edge of the solar system. After only a few minutes, the probe would be traveling at a significant fraction of the speed of light. It would pass Mars in less than an hour. The next day, it would streak by Pluto. (New Horizons took 9 years to achieve this feat.) As the probe headed deeper into the Kuiper belt’s recesses, another one would pop out from the mothership, and float into the laser’s line of sight.

“If you have a reasonable sized battery, and a reasonable sized array, and a reasonable sized power station, you probably can do one shot a day,” Milner told me. “And then you recharge and shoot again. You can launch one per day for a year and then you have hundreds on the way.”

By sending a whole stream of probes, you get more data, and also redundancy. Any encounter with interstellar dust would be fatal for a thin, flimsy disc traveling at cosmic speeds. A few hundred probes would probably be enough to guarantee that one slipped through—although it’s not a certainty. When I reached out to Freeman Dyson, who has signed on as an advisor to Milner’s project, he noted that we still don’t know what all lurks in the dark expanse between stars. The interstellar medium might be filled with rocky debris, or ice particles, or rogue planets, or other unknown objects that would make the path to Alpha Centauri more perilous than expected.

That wasn’t Dyson’s biggest worry, though. “The most difficult thing is the integrity of the spacecraft,” he told me. “You are zapping it with a hugely powerful laser beam, and the question is: Will it survive?”

I asked Milner how an ethereal, nearly weightless spacecraft could withstand a blast from a mammoth laser. “You need to reflect 99.9 percent of the light or else it will evaporate,” he said. There are ways to protect the spacecraft from the beam, but it’s tough to do it without adding too much mass.

“It’s the mass that gets you every time,” said Andreas Tziolas, the director of Icarus Interstellar, a group that researches star travel. “Every time I’ve seen a beam-propulsion study, they quote the size of the sail without any physical or mechanical support. They work so hard to build something the size of a table that weighs one gram, but then they add a support, like a wire or a piece of steel, and it goes to 10 kilograms.”

Tziolas did say there was interesting work being done in Japan, involving flexible sail-like materials that stiffen when charged, eliminating the need for a heavy support system. But he wasn’t sure they could be used in space.

* * *

Milner acknowledged the difficulty of these problems, but seemed confident they would be solved. “My time in business made me cynical,” he said. “It’s difficult to fool me. I asked these engineers and scientists a lot of questions. I tried to prove this wouldn’t work, and I couldn’t do it.”

Milner said he’s imagining a spacecraft that weighs a mere few grams. He said the sail would be exceedingly thin, perhaps only a few hundred atoms. He said its electronics would be miniaturized by the “magic of Moore’s law,” before adding that the law might need to be “sped up just a bit.” (It appears to be slowing down.)

Even if the probe can survive the laser’s impact, and the journey to Alpha Centauri, its challenges won’t end there. For one, it’s not clear that there will be something to see. There might not be any planets in Alpha Centauri’s habitable zones, or any planets at Alpha Centauri at all. Thanks to the ongoing exoplanet revolution, many astronomers now assume that most stars have planets—but there are exceptions. Some models of the Alpha Centauri system suggest that its two sunlike stars create an unstable gravitational environment, making it impossible for planets to form.

In 2012, that view was dealt a mighty blow, when astronomers thought they spotted a rocky, Earth-mass planet circling one of Alpha Centauri’s sunlike stars. But just last year, new data made that discovery look dodgy. And the follow-up search has slowed, because the two central stars are too close together in their orbits, as seen from Earth. When they blur together, it’s difficult to perform the fine observations required to spot planets. The good news is their separation, which is already underway, will coincide with the deployment of several planet-finding missions, on the ground and in space. Before the decade is out, we should know whether Alpha Centauri is home to worlds like our own.  

In the best-case scenario, we’d find another blue marble around one of its stars, and we’d send Milner’s probes to take crisp color images of it. But beaming the images back will be tough. Human beings have never sent data across interstellar distances. Milner told me the probes will use a small laser to communicate with Earth, but a signal like that will be faint by the time it arrives. Perhaps faint enough to be drowned out by the soft, electromagnetic afterglow of the Big Bang.

Some researchers have contemplated mission concepts that leave a trail of breadcrumb-like relay stations in their wake, to ease the interstellar data transmission challenge. But it’s hard to see how that would be possible with a spacecraft of this size.

* * *

When I spoke to scientists in the interstellar community, they all said Milner was wise to spend money developing a light-sail mission concept. The only alternative anyone mentioned was fusion propulsion, but for that to work, we’d need to invent fusion. “Beam-driven propulsion is the interstellar technology with the least unknowns,” Tziolas told me.

“The beauty of this is you can start small, and if it works you can go further,” Dyson said.

Milner told me his team had looked into “20 challenges” that engineers will face, as they try to design this mission. “Each one could have been a deal breaker,” he said. “But it looks like we found a reasonable path forward for each.” He wants the final, flight-ready mission to be priced in the range of other flagship scientific missions, like CERN’s Large Hadron Collider or the James Webb Space Telescope. That’s possible, in principle, but much will depend on the length and shape of his “paths forward.”

“That’s what the $100 million is for,” Milner said. “It’s to do extensive research into all of these challenges, and try to convince ourselves that this is possible in the lifetime of a single generation.”

The Starshot team has its work cut out. But it’s hard not to root for them.

Before we hung up, I told Milner I could see the Alpha Centauri mission being the first in a series. You could imagine giant laser arrays at several high-altitude sites across the globe. You’d have one atop California’s White Mountains, one on the edge of a Hawaiian volcano, one in Australia’s outback, one at the South Pole.

Every year, the lasers would blast streams of probes towards new star systems. Before long, there would be probes en route to every star within ten light years. As the search radius expanded outward, data return times would lengthen to decades. But the delay might produce a lovely effect. As the centuries wore on, an expanding sphere of the universe would slowly reveal itself, in vivid detail, as waves of images returned from the stars.  

I asked Milner how quickly a mission of that scope would be possible, and he laughed.

“It will probably be our children who do something like that,” he said.