Near Earth Objects (NEOs) are small solar system bodies that are infamous because their orbits take them near earth's orbit; sometimes they pass dangerously close to the earth or even collide with it.
Congress has mandated a study of NEOs precisely because they are the source of potentially catastrophic impacts. There are estimated to be over ten thousand NEOs large enough to be worrisome, but so far only about half of them have been spotted in optical surveys, and only a few dozen of these have well determined sizes.
SAO astronomers Joe Hora, Giovanni Fazio, Tim Spahr, and Howard Smith, together with five of their colleagues, have used the Infrared Array Camera (IRAC) on the Spitzer Space Telescope to measure the diameters and albedos (reflectivities) of three of the smallest NEOs ever studied, the smallest one being only about 160 meters across. For comparison, the Tunguska event in Russia in 1908 was first thought to have been caused by an approximately 500-meter asteroid, although some new estimates of the devastation suggest the asteroid could have been smaller than 100 meters.
In addition to measuring the size and other properties of these NEOs, the new results support the models of their infrared, mineral properties. They also demonstrate the power of the IRAC instrument to detect small and faint NEOs; the IRAC instrument team is led by Fazio. NASA has announced that Spitzer, after its liquid cryogens are gone next spring, will continue to operate for another two years with its only instrument being the IRAC camera. Measurement of the sizes and other properties of many more NEOs will be an important project for the so-called "Warm Spitzer Mission" to undertake.