Near Earth Objects (NEOs) are asteroids or comets whose orbits sometimes bring them close to the Earth, thereby posing a potentially threat. The asteroid that struck Chelyabinsk last year was an NEO about 40 meters in diameter.
While it is relatively easy to detect an NEO in visible light by watching its movement across the sky from night to night, determining its size and its potential hazard is more difficult because its optical brightness results from both its size and its reflectivity. CfA astronomers have for several years been using the IRAC infrared camera on Spitzer to measure the infrared light emitted by NEOs and, combined with optical measurements, to deduce their probable dimensions.
NEOs are thought to originate from collisional fragments of objects in the asteroid belt beyond the orbit of Mars, with over 10,000 known today. Short–period comets can also be NEOs, but unlike asteroids they most likely originated in the Kuiper belt, a reservoir of icy bodies outside the orbit of Neptune.