Is Earth’s Constant Companion a Stray Asteroid or a Chunk of the Moon?

Earth has a group of cosmic stalkers. Known as “co-orbitals”, these small bits of rock have a 1:1 mean motion resonance with Earth. Basically, they take the exact same amount of time to orbit the Sun as we do. Astronomers have long believed these objects wandered in from the main asteroid belt between Mars and Jupiter, but recent spectral analysis suggests they better match the space-weathered lunar silicates that make up the Moon’s surface. As such, there has been an ongoing debate about whether these cosmic stalkers are actually visitors from the belt or blasted pieces of the Moon. A new study, published in Icarus, from researchers Elisa Alessi and Robert Jedicke provides strong hints that the belt is the more likely source – but pretty soon we’ll get a definitive answer from a spacecraft.