WISE and Asteroids

WISE related Minor Planet Circulars

Asteroids are usually detected optically, but the typical asteroid is quite dark and usually absorbs ten times more Solar radiation than it reflects. The absorbed power goes into heating the asteroid, and this heat is then radiated away as infrared radiation primarily in the 12 and 23 micron WISE bands. As a result, WISE will be able to detect asteroids a million times fainter than Ceres, the largest asteroid. These faint small asteroids are not well catalogued, so WISE will provide a vast database of new astrometric and radiometric information on previously unknown asteroids. The image on the right is the 19 km long main belt asteroid Gaspra as seen by Galileo during a flyby.

The figure shows the flux of a 1 km diameter main belt asteroid (MBA) 2.7 astronomical units from the Sun compared to the WISE sensitivity. This asteroid would be a >5 standard deviation detection at 12 microns after the >8 frames that should contain the asteroid are appropriately stacked, so WISE data could be used to give radiometric diameters for almost all asteroids with known orbits. Bigger asteroids could be easily detected in single frames and thus WISE will discover a large number of asteroids with diameters > 3 km.

WISE will always be scanning about 90o away from the Sun and thus will be quite good at detecting near Earth objects. The dashed curve above is a 0.2 km diameter near Earth object (NEO) at a distance of 1 au from the Earth. It is easily detectable. The image at right shows three frames from a radar "movie" of the 1 km size NEO 1998 WT24 rotating as it passed very near the Earth on 17 Dec 2001. WISE science team member Bob McMillan leads the ground-based optical asteroid survey Spacewatch, which is engaged in searching for these NEO's and will followup some of the more interesting WISE asteroid discoveries to provide accurate orbits and albedos.

Because WISE will be so sensitive to asteroids, its scan pattern has been designed to allow one to distinguish between asteroids and stars or galaxies by looking for motion over the period of several hours to a day spanned by the frames covering a given piece of sky.

The Spitzer Space Telescope has observed fields on the ecliptic in the infrared and sees many asteroids. The image below blinks between two epochs on a field in Taurus at 24 μm which is very close to the WISE 23 μm band. The field of view is quite close to the 47 arc-minute field of view of a single WISE frame.

These frames are correctly aligned even though most of the bright "stars" are jumping back and forth. Most of the bright objects on the ecliptic in the thermal infrared are asteroids!

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Last modified 08 Mar 2010