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A. W. Harris (Space Science Institute)
On the night of January 13, a "one night stand" (ONS) asteroid (AL00667), observed by the LINEAR survey the previous night, was identified as possibly being on an imminent impact trajectory with the Earth, some 30 hours after the initial ephemeris was posted on the IAU Minor Planet Center NEO Confirmation Page. The solution was not in error, an impacting orbit was indeed consistent with the data, but the observation arc was too short and observational uncertainties too great to make a definite determination. In fact, the object turned out to be a fairly distant NEA not passing particularly close to the Earth at all. What happened was that the apparent motion, actually almost entirely due to geocentric apparent motion of a distant object, lined up very closely with the diurnal topocentric parallactic apparent motion a nearby object would have if coming straight at the Earth. This raises the question of whether a distant main-belt asteroid detected with only a short arc can mimic a nearby object on a collision course. I have examined this hypothesis and conclude that such confusion can exist for a few percent of detections with observational errors of the order of 1 arcsecond over a one-hour period, as is typical of LINEAR observations. The "lesson of AL00667" is that it was unusual only because the nominal solution happened to be so provocative, and it was an unusual (NEA) object to begin with. If every apparently main-belt ONS detection were subjected to the same scrutiny over the entire range of allowed solutions, cases of possible impactor trajectories would likely be very common. Or stated the other way around, if there were an asteroid "with our name on it" about to impact the Earth, there is a substantial chance it would not be recognized, even if it were imaged a few nights out.
The author(s) of this abstract have provided an email address for comments about the abstract: harrisaw@colorado.edu
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Bulletin of the American Astronomical Society, 36 #2
© 2004. The American Astronomical Soceity.