Occasionally, Mainzer says, asteroids assume comet-like orbits. “It’s possible these objects represent some kind of dead comet,” she says, that lost all their ice as they rattled inward from the Kuiper Belt.
Originally, we thought never the two should meet.” With NEOWISE, Mainzer says they sometimes spot unusual dark rocks-“like printer tone dark”-that approach the sun like comets but don’t reflect light. “Astronomers have seen asteroids that behave like comets and comets that sometimes behave more like asteroids. However, those distinctions are simplistic, says Mainzer. Astronomers used to think that comets were limited to the icy bodies that raced toward the sun from the remote hinterlands of the Kuiper Belt (which lies outside the orbit of Neptune) and beyond, and asteroids in the asteroid belt were rocks that more or less stayed in their orbital place. Mainzer says observations from NEOWISE support the idea of a blurry distinction between comets and asteroids (the team plans to release its latest round of data in March). scientists study the rugged surface of near-Earth asteroid Bennu. She directs the NASA-funded Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) project, an asteroid-hunting mission that measures infrared radiation from space rocks. Although scientists think it broke off a larger asteroid in the asteroid belt in a catastrophic collision between about 1 and 2 billion years ago, and hurtled through space until it got locked into an orbit near Earth's, they don’t expect that these. “There’s sort of a continuum of asteroid types out there,” says Amy Mainzer, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, California. Bennu, on the other hand, is a time capsule from the early solar system, having been preserved in the vacuum of space.
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