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Re: (meteorobs) 1997 XF11 in 2028
The circumstance with P/Swift-Tuttle (the Perseid comet) is not identical.
The future calculations for it are complicated by non-gravitational
motions and an apparent anomaly in the southern hemisphere astrometry
from the previous return. The orbit wasn't necessarily in good shape
and the formal uncertainties allowed the Earth to fall with the positional
uncertainty at the next return. However, it would appear, and Marsden may
not agree) that the southern hemisphere astrometry from the 1800's is in
error and having observed the object at large heliocentric distance on the
way out this passage, the orbit can be determined more accurately without
such concern for non-gravitational forces (which it turns out are very
small). It was then realised that the close approach at the next return
could not include a collision with the Earth.
Nobody should get worried about 1997 XF11. The formal uncertainty does
include an Earth impact, but the probability is somewhat less than 1%.
Should the object be precovered on an old plate, the uncertainty will
drop dramatically, and by May, the continued astrometry will drop the
uncertainty further. It may however require another 2 years to have
sufficient data to say yes or no, but if the predictions are equivocal
then we will have to await the really close approach a few years later.
That should nail it.
Marsden is in a difficult situation. If the calculations suggest a possible
collision with the Earth, no matter how slim (and I don't include predictions
made from poor orbits), he really must make some sort of announcement, with
the uncertainty made clear. Then no criticism can be made of "covering up"
the information (I've already had that put to me about 1997 XF11, XF must
mean X-Files!) and astronomers are alerted to the need for further
astrometry and the search for prediscovery images can be done in ernest.
Cheers, Rob McNaught
(rmn@aaocbn.aaodot gov.au)