The Post was one of just a few select media agencies to be briefed on the details of the ambitious project before the ­announcement.

Today’s fastest spacecraft would take 30,000 years to get to Alpha Centauri. To cut the time to just 20 years, the Breakthrough Starshot team plans to build small, unmanned probes that can carry cameras, power supplies and navigation and communications equipment while still weighing less than a smartphone.

The so-called “nanocraft”, powered by laser, will travel at ­almost 20 per cent the speed of light at 160 million km/h.

Advances in nanotechnology – that can produce super metamaterials just a few hundred atoms thick – had made it possible to create such tiny space probes, the Post was told.

Thousands of these nanocraft will be fired towards Alpha Centauri. After a 20-year journey, it would take at least another four years for the probes to transmit data and images gathered in the star system back to earth.

US space agency Nasa’s first probe, the Voyager 1, launched in 1977, holds the record for travelling farther into space than any other man-made object. Even so, it took the Voyager 1 more than three decades just to exit the solar system into interstellar space, the area that exists between star systems within a ­galaxy.

If successful, the Breakthrough Starshot project – possibly the largest and most ambitious scientific exploration ever undertaken – will extend mankind’s presence to another star system, within a lifetime.

The announcement of the project marks the 55th anniversary of Russian astronaut Yuri ­Gagarin’s maiden space flight on April 12, 1961.

“The human story is one of great leaps,” said Milner, who last year funded another US$100 million project to pick up extraterrestrial radio signals.

“55 years ago today, Yuri ­Gagarin became the first human in space. Today, we are preparing for the next great leap – to the stars,” the Russian tech ­tycoon added.

Milner is one of the earliest investors in many of today’s most successful tech firms, including Facebook, Twitter, Xiaomi and ­Alibaba.

Hawking, who has previously warned that hostile alien life might destroy humanity, said of his participation in the project: “Earth is a wonderful place, but it might not last forever.

“Sooner or later, we must look to the stars. Breakthrough Starshot is a very exciting first step on that journey.”

Hawking launched his own Weibo account on Chinese social media yesterday, gaining some 800,000 followers in just the first four hours.

The Breakthrough Starshot project will receive an initial investment of US$100 million for scientific and engineering research to ensure the feasibility of the technology.

The research is expected to take “a number of years” to complete, after which the actual journey to Alpha Centauri will need “the largest scientific-experiment budget” to take off.

The ambitious project calls for global collaboration as the costs and technical challenges involved exceed any country’s independent ability to fund and manage.

Launching the nanocraft will require the construction of a ground-based, kilometre-long laser beamer at an area of high ­altitude and dry conditions – such as in Tibet.

Scientists will then launch a mothership to carry to space thousands of nanocraft, each of which will in turn release a light sail that can be extended to a few square metres in size.

The laser beamer will focus its beams on the light sails to propel them towards their target, accelerating their speed to 160 million km/h in a matter of minutes.

Data captured will be transmitted back to earth via a compact on-board laser communication system.

Not all the nanocraft are expected to reach their destination, as some may be destroyed in interstellar dust ­collisions.

Beijing Planetarium’s senior engineer, Kou Wen, said the project should consider housing the giant laser beamer in Tibet.

Tibet, being the world’s highest plateau and with its cold, dry climate, would be an ideal location to reduce atmospheric absorption and disturbance to the laser-beaming process, according to Kou, who has participated in the building of large telescopes in the region.

“Tough Tibet is a politically sensitive region [but] I believe the Chinese government would be very interested in this project because the nation is gearing up for space exploration with the completion of its first space station just a few years away,” Kou said.

Professor Wang Hao, researcher at Zhejiang University’s Micro-Satellite Research Centre, said the project would attract many Chinese scientists from various fields and disciplines.

Ultra-light nanosatellites today were able to carry out simple missions in near-earth orbit, but tremendous challenges had to be overcome for an interstellar journey, Wang said.

Using a very large laser array to pick up faint light signals sent back by the nanocraft in outer space was also possible, at least in theory, according to Wang.

He said the nanocraft could be mass produced at costs similar to that of an iPhone, and China was a potential production centre for them.

China’s mainstream scientific community, however, met the Breakthrough Starshot project with scepticism.

Peking University astronomy professor Xu Renxin said he did not believe the experiment would work.

“Life will go on as usual as if it never happened,” Xu said, adding that he expected the tiny spacecraft to encounter myriad obstacles on their long journey.

A single grain of dust, for instance, would significantly slow down a nanocraft’s initial speed upon collision, he said.

“Even if some of them do reach Alpha Centauri, they would not be able to send any signal back to earth due to the small size of their antennae,” Xu said.