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Stephen Hawking and Mark Zuckerberg backing microscopic spaceships project

Physicist Stephen Hawking, Facebook founder Mark Zuckerberg and Russian internet billionaire Yuri Milner have announced a project that could send microscopic spaceships to hunt for extraterrestrial life in the far reaches of space.

The 100 million dollar (£70 million) project, known as Breakthrough Starshot, will examine the possibility of sending a host of tiny spacecraft far beyond the boundaries of our solar system to our nearest neighbour, the Alpha Centauri star system.

The project aims to find out whether the "nanocraft", each weighing far less than an ounce, could fly at a fifth of the speed of light and capture images of possible planets and other scientific data.

Current spacecraft would take around 30,000 years to get to Alpha Centauri, which is 25 trillion miles or 4.37 light years away.

But the nanocraft, powered by a sail pushed by a light beam, could potentially travel the distance over 1,000 times faster and make the journey in 20 years.

The project is the latest part of Mr Milner's years-long search for extraterrestrial life.

Speaking at the One World Observatory in New York, Prof Hawking, who is on the board of the Breakthrough Starshot initiative, said: "What makes human beings unique, there are many theories.

"Some say it's language or tools, others say it's logical reasoning. They obviously haven't met many humans. I believe what makes us unique is transcending our limits.

"Gravity pins us to the ground but I just flew to America. I lost my voice but I can still speak thanks to my voice synthesiser. 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.

"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."

Astronomers estimate there is a reasonable chance of an Earth-like planet existing in the "habitable zones" of Alpha Centauri's three-star system.

A number of scientific instruments, based on Earth and in space, are being developed and will soon identify and characterise planets around nearby stars.

The Starshot concept involves creating a tiny robotic spacecraft, no larger than a mobile phone chip, which would carry cameras, thrusters, a power supply and navigation and communication equipment.

Each would also have a "lightsail" no more than a few hundred atoms thick

A "mothership" carrying thousands of the nanocraft would be sent into a high-altitude orbit before a laser beam sent from Earth would focus on each lightsail and give the tiny craft a strong push, launching them to 20% of the speed of light within minutes.

Once at their destination they would send back images of any planets and scientific data via an on-board laser communications system, and the same beam of light that launched them would receive the information - taking over four years to get home.

Years of research and development stand in the way before the first spacecraft could be launched to other stars.

But Mr Milner believes the nanocraft could eventually be mass-produced at the cost of an iPhone and launched for less than a million US dollars (£700,000).

He said: "Today I want to ask a question. The question is, can we reach the stars? Can we literally reach the stars? And can we do it in our lifetimes?"

Speaking 55 years to the day after his namesake, Yuri Gagarin, became the first man in space, he added: "The moon still marks the furthest point that human beings have come.

"Since then we have delegated the job to our robots, some roving across planets, some roaming in space. One of them, Voyager 1, has now reached interstellar space, flying at 40,000 mph.

"Is that as far as we can go, and what will be our next great leap?"

Talking about his project, he said: "If this comes to fruition it will tell us as much about ourselves as about Alpha Centauri. For the first time in human history we can do more than just gaze at the stars. We can actually reach them."

The project follows on from another announced last year, Breakthrough Listen, which will use the world's finest telescopes and millions of personal computers to scan the skies for signs of life, including searching the entire Milky Way and 100 nearby galaxies, in the "most comprehensive search programme ever".

The Breakthrough Initiatives projects are being financed by Mr Milner, who was named after Gagarin and made his fortune through investments in technology companies such as Facebook.

He has harnessed the innovation of Silicon Valley and brought together top scientists for his Breakthrough Foundation, funding projects that governments decide may be too ambitious.