Andrea Milani, Steven R. Chesley
Dipartimento di Matematica, Università di Pisa
Via Buonarroti 2
56127 PISA, ITALY
E-mail: milani@dm.unipi.it, chesley@dm.unipi.it
Giovanni B. Valsecchi
IAS-Planetologia
Area di ricerca CNR
Via Fosso del Cavaliere
00133 ROMA, ITALY
E-mail: giovanni@ias.rm.cnr.it
March 26, 1999
The Earth passes very close to the orbit of the asteroid
1999 AN twice per year, but whether or not this asteroid can
have a close approach depends upon the timing of its passage across
the ecliptic plane. The uncertainty of this timing grows with time: by
2027 it is
days. Among the possible orbital solutions there are
some that undergo a close approach in August 2027, but no impact is
possible. However, the period of the asteroid may be perturbed in such
a way that it returns to an approach to the Earth at either of the
possible encounter points. We have developed a theory which
successfully predicts the 25 possible such returns up to 2040. We have
also identified 6 more close approaches resulting from the cascade of
successive returns. None of these encounters can result in an impact,
except one in August 2039: the probability that the true asteroid
actually follows a collision course for that date is less than the
probability of being hit by an undiscovered asteroid within any given
day. Because of this extremely chaotic behaviour there is no way to
predict all possible approaches for more than a few decades after any
close encounter, but the orbit will remain dangerously close to the
orbit of the Earth for about 600 years.