When you're flinging a tiny circuit board 25 trillion miles through the universe, it's bound to hit some stuff. And when it's moving at one fifth the speed of light, even the tiniest speck of space dust can hurt.
Breakthrough Starshot, billionaire Yuri Milner's project to send tiny, laser-powered spacecraft to our nearest star system, Alpha Centauri, faces this problem. A Harvard University study led by Milner's scientific team examined the effects of interstellar gas and dust would have on the team's intrepid nanocraft. They found that a grain of dust, only a hundredth of a millimeter big, could completely destroy the craft. At the rate and distance it'd be traveling, 30 percent of the craft would be eaten away by dust before it got there.
The team proposed a graphite layer as a shield, and mechanisms to tuck the craft's laser-powered sails away safely for most of the trip, but these things add weight — the enemy of speedy spaceflight.
As Proxima Centauri's planets get more and more intriguing, the motivation to get out there grows stronger. Dusty space won't ground Starshot's plans yet, but staying realistic about its ambitions from the start will help keep the mission moving forward.