Asteroids' close encounters with Mars
For nearly as long as astronomers have been able to observe asteroids, a question has gone unanswered: Why do the surfaces of most asteroids appear redder than meteorites — the remnants of asteroids that have crashed to Earth? In 2010, Richard Binzel, a professor of planetary sciences at MIT, identified a likely explanation: Asteroids orbiting in our solar system’s main asteroid belt, situated between Mars and Jupiter, are exposed to cosmic radiation, changing the chemical nature of their surfaces and reddening them over time. By contrast, Binzel found that asteroids that venture out of the main belt and pass close to Earth feel the effects of Earth’s gravity, causing “asteroid quakes” that shift surface grains, exposing fresh grains underneath. When these “refreshed” asteroids get too close to Earth, they break apart and fall to its surface as meteorites. Since then, scientists have thought that close encounters with Earth play a...