But the Sun’s light isn’t powerful enough to accelerate a ship to Alpha Centauri that would require an enormous, unwieldy sail, says Betts, who led a team that in 2015 deployed a 32-square-metre solar sail. Researchers at the Japan Aerospace Exploration Agency (JAXA) and the Planetary Society have demonstrated this in space by launching large sails propelled by sunlight. Since the turn of the twentieth century, scientists have known that light carries momentum and can give objects a push. So Starshot is focusing on harnessing light. “Chemistry will get you to Mars,” he says, “but it won’t get you to the stars.” Conventional rockets are out of the question because they can’t store enough chemical energy in the form of fuel, says Philip Lubin, an astrophysicist at the University of California, Santa Barbara, who is on the project’s advisory and management committee. The first truly challenging step in any mission such as Breakthrough Starshot is to accelerate the spacecraft to interstellar velocities. The effort would ultimately cost about $10 billion, leaders hope, and take another 20 years to reach Alpha Centauri. Leaders of the mission plan to start funding technology-development projects within months, with the aim of launching a fleet of tiny, laser-propelled probes in the next 20 years. “Starshot takes the best bits out of all of that, and puts them together into something new,” says Caleb Scharf, an astrophysicist at Columbia University in New York City who is not on the Starshot team. And even astrophysicists who are not involved with Starshot agree that it has the most realistic chance at an interstellar mission in the next few decades, thanks in part to scientists who have published many concept papers on interstellar travel. Other groups are also aiming for nearby stars, but none has the momentum-or money-of Breakthrough Starshot. Laser-propelled probes could make the interstellar journey to exoplanet Proxima b within fifty years. It all amounts to a monstrous engineering challenge, but project researchers say it is possible and are now moving towards that goal. Then it would need to collect useful data during a 60,000-kilometre-per-second fly-by of the Proxima system, and beam the information back across the 4 light years to Earth. To reach it within a scientist’s working lifetime, a probe would have to reach around one-fifth the speed of light and navigate a treacherous path through unseen debris in our own Solar System and interstellar space. Despite Proxima b’s name, it is still nearly 2,000 times farther from Earth than any human-made object has ever travelled. Anglada-Escudé et al. Nature 536, 437–440 2016), the project gained an even more tantalizing target. They announced Breakthrough Starshot, an effort backed by US$100 million from Russian investor Yuri Milner to vastly accelerate research and development of a space probe that could make the trip. In fact, a few months before the discovery of the exoplanet, a group of business leaders and scientists took the first steps towards visiting the Alpha Centauri star system, thought to be home to Proxima. The idea of reaching Proxima b is not just science fiction. The data beamed back could reveal whether the alien world offers the right conditions for life-and maybe even whether anything inhabits it. “Clearly it would be a huge step forward for humanity if we could reach out to the nearest star system,” says Bruce Betts, director of science and technology for the Planetary Society in Pasadena, California. Sending a spacecraft to the planet, dubbed Proxima b, would give humans their first view of a world outside the Solar System. It’s a tempting-some might say irresistible-destination. In August, researchers reported the discovery of a potentially habitable, Earth-sized planet orbiting the Sun’s closest stellar neighbour- Proxima Centauri, a mere 1.3 parsecs, or 4.22 light years, away. Anybody who longs to see an alien world up close got an exciting gift last year.
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