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The Infrared Space Observatory Deep Asteroid Search
A total of six deep exposures (using the astronomical observationtemplate CAM01 with a 6" pixel field of view) through the ISOCAM LW10filter (IRAS band 1, i.e., 12 μm) were obtained on a ~15' squarefield centered on the ecliptic plane. Point sources were extracted usingthe technique described in 1999 by Désert et al. Two knownasteroids appear in these frames, and 20 sources moving with velocitiesappropriate for main-belt asteroids are present. Most of the asteroidsdetected have flux densities less than 1 mJy, that is, between 150 and350 times fainter than any of the asteroids observed by IRAS. These dataprovide the first direct measurement of the 12 μm sky-plane densityfor asteroids on the ecliptic equator. The median zodiacal foreground,as measured by ISOCAM during this survey, is found to be 22.1+/-1.5 mJypixel-1, i.e., 26.2+/-1.7 MJy sr-1. The resultspresented here imply that the actual number of kilometer-sized asteroidsmay be higher than several recent estimates based upon observations atvisual wavelengths and are in reasonable agreement with the statisticalasteroid model. Using results from the observations presented here,together with three other recent population estimates, we conclude thatthe cumulative number of main-belt asteroids with diameters greater than1 km is (1.2+/-0.5)×106. Based on observations with theInfrared Space Observatory (ISO), an ESA project with instruments fundedby ESA Member states (especially the PI countries: France, Germany, theNetherlands, and the UK) with the participation of ISAS and NASA.
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