** Next:** 1 The 2027 encounter

# CLOSE EARTH APPROACHES

OF ASTEROID 1999 AN :

RESONANT AND NON-RESONANT RETURNS

**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**

### Abstract:

**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.**

*Andrea Milani Comparetti *

Tue Apr 6 19:21:11 MET DST 1999