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. Telescopic measurements of asteroids' colours rarely match laboratory reflectance spectra of meteorites owing to a 'space weathering' process that rapidly reddens asteroid surfaces. 'Unweathered' asteroids, however, with spectra matching ordinary chondrite meteorites, are seen only among small bodies with orbits that cross inside the orbits of Mars and Earth. Such unweathered asteroids are now shown to have experienced orbital intersections closer than the Earth–Moon distance within the past half-million years. Telescopic measurements of asteroids’ colours rarely match laboratory reflectance spectra of meteorites owing to a ‘space weathering’ 1 , 2 process that rapidly 3 reddens asteroid surfaces in less than 10 6 years. ‘Unweathered’ asteroids (those having spectra matching the most commonly falling ordinary chondrite meteorites), however, are seen among small bodies the orbits of which cross inside Mars and the Earth. Various explanations have been proposed for the origin of these fresh surface colours, ranging from collisions 4 to planetary encounters 5 . Less reddened asteroids seem to cross most deeply into the terrestrial planet region, strengthening 6 the evidence for the planetary-encounter theory 5 , but encou