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(meteorobs) Error in 1997 XF11 position
The jump in the miss distance from the 88-day arc to the 8 year arc is
not a jump in the minimum possible distance the object can come to the
Earth. The asteroid just happens to be further along in its orbit.
The error in the future prediction went from a very long narrow ellipsoid
aligned along the orbit (that had the Earth at the periphery) to a
much shorter and narrower ellipsoid that lay beyond the long axis of
the old prediction.
This uncertainty along the orbit is essentially the same as the mechanism
that makes meteors eventually disperse into a loop around a comet orbit,
or fireball and satellite re-entries to form a line of fragments. The
small differences in velocity of the particles (error in orbit) have their
greatest effect in relative position ALONG the path (future position
uncertainty). Eventually, these differences can amount to a whole orbit
either way resulting in an annual meteor shower if the orbits intersect
the Earth's.
1st sentence 2nd para should be "meteoroids eventually disperse". Didn't
compose off-line!
Cheers, Rob McNaught
(rmn@aaocbn.aaodot gov.au)