Light from our sun takes more than four years to reach our nearest neighbor, the Alpha Centauri star system. Tiny spacecraft, each integrated on a chip and accelerated by laser light, might be able to close the distance in a respectable 20—assuming at least some of them survive the radiation and dust collisions en route. This interstellar scheme, developed with NASA funding, got a big boost last year with the launch of the US $100 million, privately funded Breakthrough Starshot program. As you might expect, there are myriad technical challenges to overcome. Here are five that top the list for Philip Lubin, a physics professor at the University of California, Santa Barbara, who road-mapped the idea and continues to research it with NASA support.
Getting in Phase
A kilometer-scale array of lasers could be used to create a spacecraft-pushing beam with many gigawatts of power, but the light has to come together very precisely in order to deliver that energy to the spacecraft. Designing an Earth-based array is an added complication because it must quickly adapt to atmospheric perturbations.
Reflecting the Laser Back
Laser light needs to bounce off the spacecraft’s sail in order to push it, delivering 10,000 gs or more of acceleration. This sail, which may need to change shape during the mission, will be a tall order. It must be tough but extraordinarily thin, to cut down on mass, and have very low absorption so it won’t be blown to bits by the laser.
Riding the Beam
With a mass of a gram or so, a “spacechip” will need to ride a laser beam for only minutes to get up to speed. But the beam needs to stay pointed at the sail during that critical time. Shaping the sail so the spacecraft can self-stabilize, shifting position or orientation to compensate for pointing errors, could help.
Sending Word Back
As the spacecraft zips by Alpha Centauri, it could use a small onboard laser, perhaps focused by its sail, to send data back. But there will be limited energy for transmission, and an Earth-based receiver will have to discern the dispatch over background noise from solar system dust and the universe at large.
Finding the Juice
Miniaturization will be a challenge for onboard energy generation and storage. A small thermoelectric generator, fueled with plutonium-238, could be a long-lasting source. Solar cells may generate significant power as the spacecraft closes in on its stellar quarry. A thin supercapacitor integrated on-chip could be used for storage.