Interstellar probes will be eroded on the way to Alpha Centauri

When you’re travelling at one-fifth the speed of light, even a small collision can hurt. Now we know exactly how much. A team working on a project to send tiny spacecraft to the stars have calculated the damage that hitting just a speck of dust could do.

Breakthrough Starshot is an ambitious plan to launch probes weighing little more than a few grams at interstellar speeds using lasers. The goal is to reach the Alpha Centauri star system in just 20 years, and hopefully send back pictures of any planets that might be lurking there. Unconfirmed reports from German newspaper Der Spiegel suggest the discovery of one such world around the star Proxima Centauri is due to be announced later this month.When billionaire Yuri Milner announced the Breakthrough Starshot project earlier this year, he said a team of scientific advisers had identified around 20 challenges that would need to be tackled for a successful mission, and stumped up $100 million to fund this research. The full mission will likely cost many billions.

Now Avi Loeb at Harvard University, who heads Milner’s scientific team, has completed the first of these studies, looking at the effects of collisions with the interstellar medium of dust and gas. “We did a thorough analysis, taking all the relevant physics into consideration,” he says. “We didn’t see any showstoppers.”

Little ping, massive energy
Normally, a speck of dust would bounce harmlessly off a spacecraft, although slightly larger micrometeoroids are known to cause trouble for telescopes and the International Space Station. But Breakthrough Starshot wants to send their probes travelling at a fifth of the speed of light, meaning the kinetic energy released by even a tiny ping will be massive.

The probes, dubbed wafersats as they are essentially just small circuit boards, will be mostly made from graphite and quartz, so the team studied the effects of impacts on these materials. They found that interstellar dust will hit the wafersats as a collection of heavy atoms, rather than a single particle, meaning they will bombard the surface, heating it up and forming craters.

Dust is far more damaging than gas, which consists of lighter elements that pack less punch. An encounter with a single grain slightly larger than a hundredth of a millimetre would be enough to completely destroy the spacecraft, though astronomical observations suggest most grains are much smaller than this.

Still, it’s going to be a bumpy ride. Depending on the exact configuration of the wafersat, up to 30 per cent of its volume could be eroded by the interstellar medium by the time it reaches Alpha Centauri.

Head-on collision
The wafersats are designed to be long and thin, reducing the chance of a head-on collision, and will weigh no more than a gram whilst hosting all the electronics needed to study Alpha Centauri and communicate with Earth.

Loeb’s team suggests adding a few millimetres of “dumb” graphite to the front of the craft to act as shielding, designed to be eroded away and keep the electronic systems safe.

The craft itself isn’t the only thing to consider. In the first phase of the mission, the wafersats will be equipped with a light sail, a mirrored surface designed to pick up powerful laser blasts from Earth and accelerate the probes to a fraction of light speed. This will need to be folded and retracted for the rest of the trip, says Loeb, but such protections will add weight to the spacecraft, meaning it requires more energy to launch. “There is a trade-off, always,” he says.“Erosion of solid surfaces will be a severe problem at these speeds,” says Ian Crawford at Birkbeck, University of London. It’s possible that the wafersats won’t be able to complete the journey, but their success or failure will teach us more about the interstellar medium, he says. “We will learn a tremendous amount that will help us build better starships.”

“I don’t think it’s going to be a showstopper, it’s just going to make things more complex,” says Paulo Lozano at the Massachusetts Institute of Technology. Studies like this one will help make Breakthrough Starshot a more realistic project, he says. “It is pretty clear they are paying a lot of attention to the details, and that’s good.”

Loeb agrees that frank, scientific assessment is the only way for the project to proceed. “Reality comes back to bite you if you try to ignore it,” he says. “It’s very important to recognise the difficulties and try to find the solution.”