Virtual Impactors: Search and Destroy

Andrea Milani
e-mail: milani@dm.unipi.it

Steven R. Chesley
e-mail: chesley@dm.unipi.it

Andrea Boattini
e-mail: boattini@ias.rm.cnr.it

Giovanni B. Valsecchi
e-mail: giovanni@ias.rm.cnr.it

Dipartimento di Matematica, Università di Pisa
Via Buonarroti 2
56127 PISA, ITALY

IAS-Planetologia, Area di ricerca CNR
Via Fosso del Cavaliere
00133 ROMA, ITALY

revised version, November 13, 1999

Abstract:

If, for an asteroid which has been observed only over a short arc then lost, there are orbits compatible with the observations resulting in collisions, recovery would be desirable to decide if it will actually impact. If recovery is essentially impractical, as is the case for many small asteroids in the 100 m to 500 m diameter range, the next best thing is to make sure that the lost asteroid is not on a collision course. We propose a method to achieve this guarantee, with an observational effort far smaller than the one required for recovery. The procedure involves the computation of an orbit which is compatible with the available observations and, by hypothesis, results in an impact at some later encounter; this we call a Virtual Impactor (VI). The collision at some future time is a strong constraint, thus the VI has a well determined orbit. We show that it is possible to compute for each given time of observation the skyprint of the VI, that is the set of astrometric positions compatible with an impact (or a near impact). The skyprint needs to be scanned by powerful enough telescopes to perform a negative observation; once this has been done for the skyprints of all VIs, collisions can be excluded even without recovery. We propose to apply this procedure to the case of the lost asteroid 1998 OX₄, for which we have found orbital solutions with impacts in the years 2014, 2038, 2044 and 2046. Suitable observing windows are found when the VI would be close to the Earth in 2001 and in 2003, and the corresponding skyprints are small enough to be covered with very few frames. This procedure might become more and more necessary in the future, as the number of discoveries of small potentially hazardous asteroids increases; we discuss the general principles and the validation procedures that should apply to such a VI removal campaign.

Who counts the wampum of the night
To see that none is due?
--Emily Dickinson, 1859, poem 128.