Last March 31 a very small asteroid called 2004 FU162 passed Earth at an altitude of only about 6500 km, the closest of any asteroid discovered by the Spaceguard Survey. From its brightness, this asteroid appears to be less than 10 m across, so that it would have exploded harmlessly in the upper atmosphere had it hit.
FU162 was discovered just hours before its closest approach by the LINEAR survey system in New Mexico, which tracked it over a 44-minute period. The fast-moving image was later identified in processing the images, but it was too late to obtain further observations since the asteroid exited from the dayside of the Earth.
Tim Spahr of the Minor Planet Center contacted Steve Chesley of the NASA NEO Program Office at JPL to try to extract an orbit, even though the data spanned only a short arc. No pre-recovery data were found, and indeed this asteroid is so small that it could not be seen by survey telescopes except when it was very close to Earth. Chesley’s orbit calculations, however, did show that this asteroid had come closer than any previously discovered. Based on this orbit, the asteroid was given an official designation and the information was released on 22 August.
Like the initially misidentified asteroid sighting of last January 13-14 of the object called AL00667, this asteroid shows the capability of LINEAR to find a few of the tiny objects that pass very close by the Earth. These are a byproduct of its survey of larger asteroids. Someday it may find one on a collision course. It is most likely, of course, is that such an object will be small like this one, and thus that it will pose no danger to the Earth. It is even more likely that such a small object will hit the Earth without being discovered, as discussed below.
Al Harris of the Space Science Institute notes that the population of NEAs with diameter of 6 m or more is a couple hundred million, with an expected impact frequency of one per several years. One missing the Earth by 2 radii (from geocenter) should be four times more frequent, or more than once per year. Thus this event is not particularly rare, except that LINEAR had the good fortune to notice it.
