A handful of coins accelerates to one-fifth the speed of light. That's what Coracle interstellar probes will be like — tiny spacecraft weighing just a few grams, each with an ultrathin sail film. A powerful laser beam pushes them forward: light presses on the sail like wind fills a yacht's sails, except instead of air, it's a stream of photons. The swarm's target is Proxima b, a planet orbiting in the "habitable zone" (the region where liquid water can exist) around the red dwarf Proxima Centauri, the nearest star to us after the Sun.
A single probe would zoom by too fast to take a sharp picture. But when there are hundreds in a swarm, each sees the planet from a slightly different angle and at a different moment. Combining these disparate frames like a mosaic can yield a detailed portrait of a distant world. According to the authors' calculations, such an exoplanet would appear before us with gigapixel resolution — enough to distinguish the outlines of continents, oceans, and clouds. And if there are forests or even cities on the surface, their characteristic hue or geometry might reveal themselves in the spectrum of reflected light. The mission turns science fiction into an engineering challenge: not braking at the target, but using speed as an advantage, assembling a picture of the world piece by piece.
The light pressure that will push the probe's sail was first predicted by Maxwell back in the 19th century. Today, this phenomenon is the basis for real interstellar travel projects.
The idea of a swarm of reconnaissance probes assembling a single image recalls von Neumann's "locusts" or the observation networks in Alastair Reynolds's novels.