Europe's Hera spacecraft is racing through deep space at 12 kilometers per second. This November, it will finally reach its destination: a curious pair of asteroids. The smaller one, Dimorphos, measures 151 meters, about the size of the Great Pyramid of Giza. It orbits Didymos, a mountain-sized rock 780 meters across. In 2022, NASA's DART spacecraft deliberately crashed into Dimorphos to change its orbit. Hera's mission is to examine the results of that historic impact.
What would a visiting astronaut actually see there? Both asteroids are made of dark, silicon-rich rock, roughly as bright as the Moon's highlands. Didymos spins once every 2.2 hours, so fast that it is close to coming apart. Scientists suspect Dimorphos formed from material thrown off this rapidly spinning parent. The smaller asteroid resembles a pile of rubble, covered in boulders the size of cars and houses. The DART collision has probably reshaped its surface significantly.