Finding Near Earth Objects from Space with NEOWISE and NEOCam

Asteroids and comets periodically impact the Earth; the key questions are how often do they do so, and with what energy? Systematic telescopic searches to find, track, and characterize these objects are key to addressing these questions. The issue of identifying when a potential impact might occur is answered by discovering the objects and tracking them for sufficient time to enable a reliable prediction of close approaches to be made over the next century. The impact energy scales as the kinetic energy, which in turn depends roughly on the object’s diameter cubed. Thus, estimating sizes reliably is also important. Today, most surveys consist of ground-based telescopes operating at visible light wavelengths; for such telescopes, detectability depends on the amount of sunlight reflected by the asteroid or comet. With visible light alone, only approximate sizes can be determined, because asteroid and comet surface reflectivity in visible wavelengths varies significantly from objects as dark as coal to nearly white. A complementary approach to surveying for and characterizing asteroids and comets is to search using thermal infrared wavelengths that measure the heat emitted by the objects. Such telescopes are approximately equally sensitive to bright and dark objects, and can offer more robust constraints on effective size. When visible light data are also available, the reflectivity (albedo) can be computed, which offers insight into composition and density. The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) is an Earth-orbiting NASA mission that is currently searching for and characterizing asteroids and comets using two infrared channels, 3.4 and 4.6 microns. It began life as the Wide-field Infrared Survey Explorer (WISE), with its purpose to survey the entire sky in four channels (3.4, 4.6, 12 and 22 microns) in order to search for low mass stars and ultraluminous infrared galaxies. Although not originally designed to search for asteroids, it has successfully discovered many large, dark near-Earth objects, a complementary dataset to the ground-based surveys. Additionally, NEOWISE has provided the largest catalog of asteroid physical properties such as diameters and albedos to date, including more distant asteroids in the main belt, the Jovian Trojan population, Centaurs, and comets. Launched in late 2009 with a planned 7-month lifetime, the spacecraft is now nearly 10 years old; its two longest channels no longer function. However, its lifetime is limited by orbital decay. A successor mission, the Near-Earth Object Camera (NEOCam), has been proposed to make a comprehensive survey of Earth-approaching asteroids and comets using a design optimized for this objective.