There is also a pragmatic reason to visit Ryugu. NASA researchers have identified it as a potentially hazardous asteroid, which means its orbit comes close enough to Earth to create a non-negligible chance of collision. While the risk is small, the complex forces acting on asteroids as they loop around the sun make it difficult to accurately predict their trajectory more than a few decades into the future.

EARLY ON SUNDAY morning, the skies above a secluded military complex in central Australia will be brightened by a fireball plummeting to Earth. It will be a flamboyant homecoming for the sample return capsule from Hayabusa2, a Japanese spacecraft launched almost exactly six years ago on a mission to shoot an ancient asteroid and steal some of its dirt. If the capsule survives its fiery descent, its payload of pristine space rock will help scientists understand the earliest days of our solar system, shed light on the mysterious origins of meteorites, and may even provide clues about the emergence of life on Earth.