NEW YORK CITY—The top of the new World Trade Center building was buried inside the clouds, but everyone's focus was on the stars. Yuri Milner, the man whose investments have helped fund the Breakthrough Prizes and Breakthrough Initiatives, was here to announce his newest venture: Breakthrough Starshot, an effort to send hardware to the nearest stars quickly enough for many of us to live to see their arrival.

Present to back the project was physicist Stephen Hawking. "I believe what makes us unique is transcending our limits," Hawking told the audience. "Gravity pins us to the ground, but I just flew to America."

He went on to ask, "How do we transcend these limits? With our minds and our machines. The limit that confronts us now is the great void between us and the stars. But now we can transcend it."

On hand for the announcement was a group that any geeks would consider an all-star cast: physicist Freeman Dyson, Ann Druyan, the woman behind COSMOS, Harvard astronomer Avi Loeb, astronaut Mae Jemison, and former NASA researcher Pete Worden, who will lead the project.

Milner announced the program on the anniversary of Yuri Gargarin's trip to space, noting he was named after him. He chose the location based on the fact that the top of the World Trade Center was "closer to the stars than any other rooftop in America." Milner said that the effort was driven by a single question: "Can we literally reach the stars, and can we do it in our lifetime?"

Milner is backing the $100 million R&D program necessary to get this to work. Existing technology won't do; New Horizons is the fastest spacecraft we've ever launched, and it would take 78,000 years to get to any of the stars in Alpha Centauri, a nearby three-star system. The plutonium in its power systems alone weighs 11kg and would require staggering amounts of energy to accelerate to the necessary speeds.

Instead, Breakthrough Starshot plans to build what's essentially a spacecraft on a chip, which Milner called a nanocraft. A gram-scale wafer will include "cameras, photon thrusters, power supply, navigation and communication equipment." The technology behind the power supply wasn't mentioned; communications at these distances will require something with pretty considerable power, even when using the optical communication that Breakthrough Starshot plans to rely on.

Each device would cost roughly the same as a high-end smartphone to make, allowing a massive number to be sent on the journey, providing some significant redundancy. Milner held up an early prototype during the announcement.

Propulsion will be outsourced to a facility on Earth. The small spacecraft will be equipped with a light sail, and a phased array of lasers in the 100GW range will provide the sail with enough push to get the craft moving at roughly 20 percent the speed of light in just a matter of minutes.

All of this, Milner said, comes from the revolutions in electronics, nanotechnology, and photonics—electronics to make the starship, nanotechnology to build the light sails, and photonics to build the drive system. The technology isn't quite ready yet, but Milner felt it was close enough that a focused effort could have it ready in a reasonable amount of time. He said that his website lists 18 distinct problems that his team will have to solve, along with some ideas on how to overcome these hurdles.

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

Each of the panelists took some time discussing their views on the project. Freeman Dyson, for his part, emphasized that the space between here and Alpha Centauri isn't empty, and we'll find interesting things on the trip. He hoped that Breakthrough Starshot will lead to a sustainable exploration program, one that wouldn't wind down once its main goal was achieved.

Avi Loeb took up this theme, saying, "Starshot is hoping to image planets in the neighborhood of Alpha Centauri—we may find other things." He noted that we can sacrifice ships to hit closer targets, mentioning that it would take only three days for these craft to reach Pluto, and we could drive one right into Saturn's rings to sample the material there. Worden later agreed that the basic technology could help move things around within the Solar System, possibly including moving heavier hardware at a somewhat slower pace.

For Mae Jemison, the new project is in keeping with one of her previous ones, the 100 Year Starship. Contrary to its name, the program simply hopes to have the technology developed to enable interstellar travel within a century. Breakthrough Starshot or something like it, in her view, is a necessary component of this. For the why, Jemison argued that space exploration is something that society's always done. "We started space exploration a thousand years ago, when we started to track the stars," Jemison said, later adding "We are as much a part of this Universe as any speck of stardust."

Before the questions started, Worden was given the last word, and he used it to say, "I've just gotta conclude by saying 'this is really cool.' From these hard challenges, on to the stars."

We're going to ask a few questions and look into the group's existing literature, and be back with a more detailed analysis in the next few days.