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Just how dangerous is it to travel at 20% the speed of light?

Breakthrough Starshot is one of the more exciting scientific ideas that has popped up in the past decade, with its promise to deliver hardware to the nearest star in time for many people currently alive to see it. While the idea would work on paper as an extrapolation of existing technology, there are a lot of details that need to be thoroughly checked out, because it's possible that one of them could present a show-stopper.

There's a bit of good news there: Breakthrough Starshot is apparently funding the needed research to give its concept a thorough vetting. A recent posting to the arXiv describes a careful look at the odds of a spacecraft surviving an extended journey at the speeds planned for the trip. Overall, things look good, but a bit of shielding will be needed, and there's the potential for a catastrophic collision with a speck of dust.

The work, done by a team of four astronomers, focuses on one of the most basic issues: spacecraft survival. The goal of Breakthrough Starshot is to accelerate its craft to about 20 percent the speed of light. At that speed, even individual atoms can damage the vehicle, and a collision with a bit of dust could be catastrophic. So the team set out to quantify just how risky these collisions could be.

The space between here and the nearest star isn't completely empty. Earlier generations of stars have left a sparse collection of tiny dust particles, and various processes have left individual atoms scattered through the interstellar space. These particles pose very distinct risks, and the team worked through each separately.

The main problem with running into an atom isn't the physical collision but rather the energy deposited in the spacecraft by the impact, which can cause local heating that damages the craft in two ways. When high enough, the heating causes the material the craft is made of to evaporate into space. Below that threshold, it can cause local melting, leading to a change in material properties as the area resolidifies.

Using information on the bombardment of quartz (silicon dioxide) and measurements of the interstellar gas concentration, the authors performed some calculations to describe the damage that will occur during flight. They found that while hydrogen and helium are the most common atoms the spacecraft will encounter, heavier atoms—specifically oxygen, magnesium, and iron—will do the vast majority of the damage.

Dust presents a somewhat different problem. Small dust particles will essentially act like a simultaneous bombardment by a lot of gas atoms. That's because the energy binding things together in a dust particle is tiny compared to the energy of the collision itself, and the dust is largely composed of heavier atoms. But a sufficiently large dust particle will create a collision energetic enough to destroy a craft. And "sufficiently large" isn't very big; the authors estimate that it only has to be 15 micrometers across to kill off the craft. Fortunately, dust particles this size are rare, and the authors calculate the odds of running into one at 1050 to one against.

Overall, the authors find the effect of gas to be minor and only likely to cause damage down to a depth of 0.1 millimeters. Dust, however, is a different story. It will evaporate about 1.5 millimeters off the surface of the spacecraft, and melting will happen at depths of up to 10 millimeters. When every gram counts, this could be significant.

That said, the authors suggest a few potential ways to lower the impact of this problem. The simplest is to just limit the cross-section of the craft in the line of travel, which would involve folding the solar sail it relies on for its initial acceleration—and possibly placing it behind a shield. The main body of the craft could also be shaped a bit like a bullet in order to limit the area that presents a dust target. As the solar sail is used for communications with Earth in some potential designs, however, this may not be possible.

Since heating poses the biggest problem, the team also suggests adding a layer of graphite to the front of the craft, as this will diffuse the heat more efficiently. Comparison of graphite to quartz indicates that the damage caused by impacts is much smaller in this material.

The one problem that these calculations don't tackle, however, is the change in momentum caused by impacts. Each dust particle will create a small jet of evaporated material that pushes against the craft itself, redirecting it slightly. Over time, these jets are likely to roughly average out, but there will be changes to the flight path based on how rough that averaging is and how long it takes to happen.

So the collisions may make it hard to get the craft as close to any planets as we would like. And given that weight and space considerations will likely limit the quality of the sensors on board, getting the craft close to planets will probably be critical.

The arXiv. Abstract number: 1608.05284 (About the arXiv).