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The Rise And Fall Of Near-Earth Asteroid 2001 VK5

NEO News - 2001 VK5 
David Morrison (dmorrison@mail.arc.nasa.gov)
December 7, 2001

I would like to report briefly on some recent actions of the International Astronomical Union Working Group on NEOs concerning the new NEA 2001VK5, discovered on 11 November 2001 and initially announced through Minor Planet Electronic Circular 2001-V49. This example illustrates the operations of the IAU technical review process as well as the generally excellent cooperation of NEA observers and theorists in dealing with a new asteroid that initially appears to be on an orbit that could lead to a subsequent impact with the Earth. The great majority of such cases, of course, will turn out to pose no threat as additional observations become available. This is such an example.

Andrea Milani and his team in Pisa calculated an orbit for VK5 and noted that the initial results with such a short arc yielded an enormous number of "Virtual Impactors (VIs)"; their algorithms detected more than 1,000 separate impact pathways, for impacts between 2002 and 2080. The list was made progressively available on the web, as their computers were producing the output, starting on 14/11/2001; in the same day the SGF Central Node launched a "new campaign" to ensure adequate followup. This object was reobserved on 16/11/2001. By the late hours of 17/11, Milani had calculated a new list of VIs, with fewer individual impactors but with more significant probabilities, including a November 2011 Virtual Impactor had a probability (computed with uniform probability density) of 4.6e-7. This object was of estimated H magnitude 17.6 and with an average impact velocity could generate an explosion with energy greater than 100,000 MT. This corresponded to a value on the Palermo technical scale of -1.85, already a noteworthy value. Moreover, there were another 14 VIs identified with possible impacts between 2014 and 2056. At about the same time Steve Chesley reported that the JPL Sentry automatic collision monitoring system, which is in continuing development, also autonomously detected the impacting solutions of VK5 reported by Milani. This NEA was in the Category 1 class on both the Torino Scale and the Palermo Scale, although everyone recognized that these calculations were based on a short arc, and that new astrometry would almost surely resolve the uncertainties within a few days.

Milani requested that the WGNEO activate its technical review process for 2001VK5, which I did on 19 November, writing "I concur with your decision to submit this information to the IAU WGNEO Technical Committee for review at this time, since it has such a complicated orbit that it is especially important to obtain independent verification of the virtual impactor possibilities. In the meantime the asteroid remains visible in a dark sky, and I have no doubt that additional data will be coming in. This parallel request for both new observational data and a peer check of the calculations is in my opinion entirely appropriate. Depending on the events of the next 72 hours the IAU may (or may not) wish to make a formal statement . . . I personally urge that this be kept as low-key as possible until additional data are acquired that may resolve the risk of impact."

The members of the IAU Technical Review Team (Don Yeomans, Paul Chodas, Steve Chesley, Karri Muinonen, Giovanni Valsecchi) did an excellent job of providing independent analysis of the orbit, confirming (using different methods) Milani's calculations of the probability of impact. At the same time, with the active cooperation of both the Minor Planet Center (Brian Marsden) and the Spaceguard =46oundation Central Node (Andrea Carusi), additional observations were obtained and given immediately to the orbit calculators. By 21 November (the end of the 72 hour period for the IAU Technical Review), the possibility of an impact in 2011 had been eliminated by new data, but there still remained several VIs in the out years. In view of the continuing input of data, we decided to defer any announcement from the IAU until after the USA Thanksgiving holiday. Milani posted the current information on his website but also did not make any public announcement.

By 25 November, orbit calculations based on new observations had effectively eliminated all of the virtual impactor possibilities, and 2001VK5 had dropped back to a category zero in the Torino impact scale.

I believe that this case is an example of the proper functioning of the IAU Technical Review of orbital calculations and of the efforts of the MPC and SGF to solicit and coordinate additional observations of a high-priority target. The fact that this international collaboration was carried out without publicity provides a model for future cases of short-arc NEAs that are initially flagged as potential impactors but that drop back into the background as more observations are accumulated. I expect there will be many more such examples as new NEAs continue to be discovered at a high rate.

David Morrison
Chair, IAU WGNEO


NEO News is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, the International Astronomical Union, or any other organization. For additional information, please see the website: http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.

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