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Comments on Potentially Hazardous Asteroid 1999 AN10

Asteroid 1999 AN10 made the news recently because, according to a group of researchers in Italy, there is a remote possibility that it could collide with the Earth in the year 2039. Writing in a scientific paper submitted for publication, researchers Andrea Milani, Steven R. Chesley and Giovanni B. Valsecchi say that the chance of a collision in 2039 is exceedingly small, only about one in a billion, but they add that the asteroid's orbit will remain threateningly close to the Earth's orbit for many centuries to come.

Although the threat posed by 1999 AN10 must certainly be taken seriously, the probability of impact for this object is so miniscule that the authors of the paper felt no great urgency to inform the press of the new calculations, and the other NEO scientists reviewing the paper agreed with this policy. To put it into perspective, consider that the probability of 1999 AN10 impacting in 2039 is tens of thousands of times less than the probability of an undiscovered asteroid of equivalent size hitting the Earth during the same 40-year period. Furthermore, in just a few months, 1999 AN10 will be observed again, as it moves back into the nighttime sky, and the new data will, in all likelihood, completely eliminate the possibility of impact in 2039. Researchers should then be able to start examining the possibility of impacts after 2039.

As it turned out, the Milani et al. paper was publicized not by the authors, but by a third party who found it accidentally on one of the author's web pages; the authors were not even consulted before their results were publicized. An internet debate ensued on such issues as why the results had not been made public, and whether or not the paper had been peer-reviewed to ensure accuracy. The reasons for not making the results public have already been described: basically, there was no great urgency to publicize a one-in-a-billion-chance impact 40 years from now, when even that remote a possibility will likely disappear in a few months.

On the issue of peer review, Milani and his colleagues followed a commendable course. The authors distributed their paper to qualified experts more than a week before placing the paper on their web page, seeking confirmation of their results. Our group at JPL examined the paper and saw no major flaws. We have also confirmed the existence of the impact scenario for 2039, and we confirm that the probability of impact in 2039 is about one in a billion.

Paul W. Chodas
Research Scientist
NEO Program Office

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