Russian billionaire Yuri Milner plans to spend $100 million over the next few years to begin developing the technology needed to build a giant laser array to propel swarms of postage stamp-size spacecraft off on 20-year-long interstellar flights to Alpha Centauri, the nearest star to the sun, the internet investor announced Tuesday.

The tiny 1-gram nanocraft, or "StarChips," would be equipped with small, ultra-thin light sails and accelerated, one at a time, to 20 percent the speed of light by a powerful half-mile-wide array of ground-based lasers, boosting them to a cruise velocity of some 37,200 miles per second in a few minutes.

From that point on, the tiny spacecraft would sail on their own across the immense 4.3-light-year -- 25-trillion-mile -- gulf, flying through the Alpha Centauri system about 20 years after launch. Each surviving "spacecraft on a chip" would snap pictures and beam the data back to Earth using tiny on-board lasers, the faint signals arriving four years later.

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To put the immensity of the proposed trip into perspective, imagine shrinking the 92 million miles separating the Earth from the sun to 1 inch. At that scale, Alpha Centauri would be about 4.5 miles away. An Apollo moonship traveling at 25,000 mph -- the fasted piloted spacecraft ever flown -- would need 114,000 years to reach the Alpha Centauri system, made up of two and possibly three stars.

Milner -- named after cosmonaut Yuri Gagarin, who became the first man in space 55 years ago to the day -- and famed physicist Stephen Hawking unveiled the "Breakthrough Starshot" project during a briefing on the observation deck atop One World Trade Center in New York.

Facebook founder Mark Zuckerberg also is a board member and Pete Worden, the former director of NASA's AMES Research Center, will serve as program executive.

Milner, a self-described science philanthropist and founder of DST Global, an internet investment firm, announced plans last July to spend $100 million to expand and accelerate the on-going search for extra-terrestrial intelligence, or SETI, another "breakthrough initiative" supported by Hawking and directed by Worden.

Physicist Stephen Hawking endorsed plans to develop the technology needed to send small "nanocraft" to Alpha Centauri, saying humankind is poised to cross the "great void between us and the stars." Breakthrough Starshot

But the Breakthrough Starshot project is clearly his most ambitious undertaking, the first real effort to develop proof-of-concept technology aimed at interstellar flight.

Even so, Milner believes the technology to accomplish the first star voyage is nearly in hand and after initial research and development, funded with his $100 million, prototypes could be built and tested with additional funding.

The final cost of the project would rival the money spent on the largest science projects in the world, such as NASA's $8.8 billion James Webb Space Telescope or the $10 billion Large Hadron Collider in Europe. But Milner believes the goal can be accomplished within a single generation, and he hopes to be around to witness the initial launches.

"The limit that confronts us now is the great void between us and the stars," Hawking said through a speech synthesizer. "But now we can transcend it. With light beams, light sails and the lightest spacecraft ever built, we can launch a mission to Alpha Centauri within a generation. Today, we commit to this next great leap into the cosmos because we are human, and our nature is to fly."

The concept is relatively straight forward. In principle, at least.

An artist's view of a StarChip spacecraft's light sail getting hit by a powerful laser beam from the ground, propelling it to 20 percent light speed for a 20-year voyage to Alpha Centauri. Breakthrough Starshot

Milner said each nanocraft would be built around a 1-gram StarChip about the size of a large postage stamp.

"This is a StarChip," he said, holding a prototype between thumb and forefinger. "A gram-scale wafer containing cameras, photon thrusters, power supply, navigation and communications equipment. It's about the size of a large postage stamp, only a little bit thicker. This is the Silicon Valley approach to spaceflight: a fully functional space probe that can be held with two fingers and mass produced at the cost of an iPhone."

Each nanocraft would be equipped with an ultra-thin light sail just a few meters across and weighing a gram or less. A few hundred to a thousand or more StarChips would be packed into a cargo carrier atop a conventional rocket and launched to geosynchronous altitude 22,300 miles above the equator.

The nanocraft then would be released into space, once every day or so, to await their boost to Alpha Centauri.

That enormous kick would come from the combined light of synchronized lasers in a 1-kilometer-wide (0.62-mile-wide) array. The width of the beam at geosynchronous altitude would be about 13 feet across, packing an enormous amount of energy into a small area.

"If I pointed this laser at a nanocraft in space the light would actually push it," Milner said, holding a small laser pointer. "Not super fast. If I do this for a full day, it will push the nanocraft to about the speed of an ant. But advances in photonics have led to the development of phased light beams, many small lasers forming one powerful beam."

Entrepreneur Yuri Milner, holding up a prototype "StarChip" spacecraft he hopes to develop and, within a generation, launch to the nearest star using ground-based lasers and ultra-thin light sails. Breakthrough Starshot

Avi Loeb, chair of the astronomy department at Harvard University and chairman of the Breakthrough Starshot advisory committee, said the energy required to send a StarChip to Alpha Centauri at 20 percent the speed of light is roughly 100 gigawatts, "similar to the power needed to lift off the space shuttle."

The collimated beam hitting the sail of a nanocraft would accelerate it to cruise velocity in about two minutes, he said, briefly subjecting the craft to 60,000 times the force of Earth's gravity.

The spacecraft then would spend 20 years in transit before streaking through the Alpha Centauri system, snapping pictures of any planets they might find, perhaps recording environmental and other data and beaming it all back to Earth before flying out of the distant solar system and continuing on into the galaxy.

Swarms of nanocraft will be launched to protect against internal failures, guidance problems and high-speed collisions with unseen space dust and debris.

The project would be carried out in three stages: initial research and development, funded by Milner's $100 million; proof-of-concept demonstration flights, which would cost an additional few hundred million dollars; and finally, assembly of the actually nanocraft and laser array.

The phased-array laser system, or "light beamer," will pose major challenges. Loeb said engineers will have to figure out how to synchronize enormous numbers of powerful lasers, design and build adaptive optics systems to counteract turbulence in Earth's atmosphere and come up with precision beam guidance techniques.

"Breakthrough Starshot aims to bring economies of scale to the astronomical scale," the backgrounder said. "The StarChip can be mass-produced at the cost of an iPhone and be sent on missions in large numbers to provide redundancy and coverage. The light beamer is modular and scalable. Once it is assembled and the technology matures, the cost of each launch is expected to fall to a few hundred thousand dollars."

Milner doesn't believe government funding is required to see the project through to completion, although it would certainly help. Worden said NASA managers briefed on the project have expressed interest. Funding aside, international consensus will be required before launching spacecraft to a different solar system.

"The very nature of the project calls for global co-operation and support," Milner's release said. "Clearance for launches would be required from all the appropriate government and international organizations."