Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth, making them potential collision threats. NEOs also offer clues to the composition, dynamics and environmental conditions of the early solar system and its evolution, and because they are relatively close to the Earth they lend themselves to astronomical measurements. Most NEOs are discovered in optical searches, but one crucial NEO parameter, its size, usually cannot be determined from optical detections alone. This is because an NEO’s optical light is reflected sunlight, and an object can be bright either because it is large or because it has a high reflectivity. A CfA team has been using the IRAC infrared camera on Spitzer to measure NEO infrared emission signals which provide an independent measure of its size.
Source:
https://phys.org/news/2019-11-near-earth-asteroid-pairs-clues-composition.html