Earth encounters as the origin of fresh surfaces on near-Earth asteroids

A shake-up for asteroids The 'ordinary chondrite problem' has been a factor in Solar System astronomy for three decades. It refers to the apparent anomaly that whereas about 80% of the meteorites falling to Earth are 'ordinary chondrites', they are rare among asteroids. The usual explanation is that 'space weathering' processes alter ordinary chondrite surfaces, producing reddened 'S-type' asteroids. A mystery remains, though, in the shape of a rare class of asteroids, the Q-types. These are found only near the Earth, and they do display 'fresh' spectral matches to ordinary chondrites. Now the combination of a new data set of 95 asteroid spectra with their detailed orbital histories shows that all Q-type asteroids have recently passed close to Earth at least within the lunar distance. Thus tidal stresses or seismic shaking during these encounters may have exposed new unweathered material on the surface. Intriguingly a test of this hypothesis may be at hand: 99942 Apophis, a potentially Earth-threatening asteroid currently displaying 'weathered' spectral colours, is due to pass within six orbital radii of Earth in 2029. It is predicted that it will experience a seismic 'fresh shake', which should expose new unreddened material on the surface.