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P. W. Chodas, D. K. Yeomans (JPL/Caltech)
Asteroid 1997 XF11 received much notoriety in March 1998 when for a time, orbit solutions indicated that it would make a remarkably close approach to the Earth on October 26, 2028. The miss distance calculated from these orbit solutions was less than one quarter of a lunar distance, and possibly even smaller, making it easily the closest-ever predicted approach of a minor planet to the Earth. The fairly large size of the asteroid, probably over a kilometer across, also made the object notable. Interest in this object spread rapidly when initial reports to the press suggested that a collision in 2028 could not be ruled out. But, in fact, a complete analysis of the observations available on March 11 (an 88-day data arc) shows that the probability of impact in 2028 was very tiny, essentially zero. When XF11's position uncertainty ellipse is plotted in the plane perpendicular to the geocentric velocity vector, the ellipse is seen to be extremely elongated, over 1000 times longer than its width. This elongation is due to the fact that the position uncertainty along the orbit grows linearly with time over the 30-year prediction period, while uncertainties perpendicular to the orbit vary only periodically. The great length of the uncertainty ellipse makes it difficult to predict a precise miss distance, but the narrow width of the ellipse allows the computation of a likely minimum possible miss distance, about 28,000 km in the case of XF11's passage in 2028. As it turns out, on March 12, additional pre-discovery images of the asteroid were found, which greatly strengthened the orbital solution and moved the predicted close approach out to an unremarkable 980,000 km. But these observations were not needed to rule out the possibility of a collision in 2028. We also investigate the close Earth approaches of 1997 XF11 and associated uncertainties for a few decades beyond the year 2028.