Although a much longer time scale than Starshot’s idea, their proposal would not require a large laser to be built on Earth. Using sails designed around graphene, the photon pressure from our Sun alone would be enough to complete the journey, according to the authors.
“The laser has a few serious issues,” said Hippke. “When you start the laser, water vapor evaporates in our atmosphere and it becomes intransparent. Some argue it is not possible to build it on Earth because it is reflected from our atmosphere. You could build it in space, but the biggest issue then would be political, having a laser in space that could destroy every city in the world.”
Heller and Hippke’s proposal, though, is only possible when a rare alignment of the three stars occurs, to maximize the photogravitational effect. The next time this will happen is in 2035, and the next will not be until 2115. If the spacecraft launched in the former window, it would thus enter orbit well into the 22nd century; in the latter, the 23rd century.
This is clearly an obstacle to getting interest in the mission. When he announced his proposal last year, Milner said he wanted to see Starshot happen in his lifetime. Switching to this other idea would mean that no one alive on Earth today would likely see its completion.
Starshot's idea requires a massive laser to be built on Earth. Breakthrough Starshot
“The main constraint in defining the Starshot concept was to visit Alpha Centauri within our lifetime,” Avi Loeb from Harvard University, chair of the Advisory Committee for Starshot, told IFLScience. “Extending the travel time beyond the lifetime of a human, as advocated in this paper, would make it less appealing to the people involved.”
Loeb also noted that the design of Heller and Hippke’s spacecraft might pose problems. Their sail would weigh less than 100 grams (3.5 ounces), and would be only a few atoms thick, meaning it might be unlikely to survive passing close to our Sun or one of the other stars. “[It’s similar] to the mythological story of Icarus, who dared to fly too close to the Sun,” said Loeb.
(It should be noted that the authors have addressed this issue, saying that graphene can survive relatively high temperatures, and with a coating the sail would also reflect most of the incoming radiation, so it could theoretically survive.)