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Asteroid 1998 OX4 Virtual Impactors

June 29, 1999

To the participants of the Torino "IMPACT" Workshop:

In the last session of the meeting we announced that the small asteroid 1998 OX4 had the possibility of impacting the Earth, although with a very low probability. In the time since we left Torino we have found some other possible collision solutions for this NEA. (For such announcements, see the impact risk home page of the NEODyS online information system at http://newton.dm.unipi.it/neodys/risk.html.) Unfortunately this asteroid is lost, and the recovery appears almost impossible with the present telescope resources and technologies. We were very unhappy with this unpleasant conclusion, and therefore immediately after the meeting we begun work on a possible solution.

We can now announce that we have found a solution to the problem raised by 1998 OX4; the same method could be applied to other similar cases of lost potential impactors which might be discovered in the future. In short, the idea is to replace a full recovery campaign, requiring an inordinate amount of observational resources, with a targeted search for only those, among the possible orbital solutions for 1998 OX4, which lead to impacts; we call these the Virtual Impactors (VIs). We have shown that it is practical to launch a campaign of negative observations to show that the VIs do not actually exist, and that this can be accomplished using a very limited amount of telescope and observer time. Thus the impact risk could be eliminated, even without actually recovering the lost asteroid.

This result is announced in a paper now submitted for publication, with the title "Virtual Impactors: Search and Destroy," by Milani, Chesley, Boattini and Valsecchi. The preprint is available online from

http://copernico.dm.unipi.it/~milani/virimp/

(HTML version for WWW browsers)

http://copernico.dm.unipi.it/~milani/preprints/virimp.ps

(PostScript version for laser printers).

Yours

Andrea Milani, Steven Chesley, Andrea Boattini, Giovanni Valsecchi

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