Article Posted: April 19, 1999
Following is a file of information on the near-Earth asteroid called 1999 AN10, discovered by the MIT-USAF LINEAR telescope on 13 January 1999. Dynamicists Andrea Milani, Steven Chesley, and Giovanni Valsecchi carried out an analysis of its orbit, which involves resonances with the Earth and permits close encounters with the Earth over the next several hundred years.
On 26 March 1999, these authors requested several colleagues to look at their manuscript and check the general validity of their calculations of the orbit of this asteroid. They wrote, in part: “The subject of this paper is such that we consider essential that its content be reviewed by the most qualified experts before it is made public. This paper has been submitted to a scientific journal. We do not want the content of this paper to reach the non-scientific media until it has been carefully reviewed. . . . Note that it would be unwise to hurry with a public announcement for three good reasons. First, we have established that there is no risk of impact until 2039, and even then the probability of impact is well below the background level. Second, the asteroid is now almost impossible to observe, and even if it were observed new astrometric positions taken now would not contribute significantly to the improvement of the orbit. Third, the issues raised by this case are indeed very complex. . . Please note that we had no obligation to submit our paper to this highly unusual refereeing procedure: we felt this as a moral obligation. We are asking you to carefully examine our paper looking for every possible fault in our arguments, but with respect for our work and for our scientific priority. . . . We intend to make the paper available on our web server on April 6 unless some of you can point out to some reason not to. Thus you should send us your comments, criticisms, and whatever queries you have, as soon as possible. In particular if there is some fundamental flaw in our arguments we would like to know before making any information publicly available.”
Several of the colleagues they addressed responded with detailed technical commentary, but none disagreed with the basic conclusion that this asteroid poses no significant threat of Earth impact for at least the next 40 years. Thus, Milani and his co-authors posted the manuscript on their website early on 6 April, as they had indicated they would do. About a week later the manuscript was circulated to a larger group of experts at the request of the International Astronomical Union. These informal technical referees also agreed with the conclusions concerning the exceedingly low probability of an impact with Earth.
Subsequently to this Web posting, the case of asteroid 1999 AN10 became widely known and has stimulated considerable discussion on the Internet and in the international press. The remainder of this message reproduces some of the commentary related to this asteroid and the mode of release of information. Everything that appears here has already been made public on other websites. The material is drawn together here as a reference on a subject of general public interest dealing with the probability of asteroid impact and of the best way such information should be made available to the public.
David Morrison (19 April 1999)
1) ABSTRACT OF THE TECHNICAL PAPER CLOSE EARTH APPROACHES OF ASTEROID 1999 AN10: RESONANT AND NON-RESONANT RETURNS
Andrea Milani, Steven R. Chesley Dipartimento di Matematica, Universit? di Pisa Via Buonarroti 2 56127 PISA, ITALY Giovanni B. Valsecchi IAS-Planetologia Area di ricerca CNR Via Fosso del Cavaliere 00133 ROMA, ITALY March 26, 1999
Abstract: The Earth passes very close to the orbit of the asteroid 1999 AN10 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 +/- 12 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
2) OFFICIAL COMMENTS FROM THE INTERNATIONAL ASTRONOMICAL UNION
From the IAU website The International Astronomical Union Working Group on Near Earth Objects (WG NEO) provides, as a service to the international astronomical community, voluntary expert review of reports that might have implications for possible future Earth impacts. The review process was first used in April 1999 in the case of newly discovered mile-wide asteroid 1999 AN10. NEOs with orbits that permit close encounters or even collisions with the Earth are of considerable interest to scientists who compute asteroid orbits. As a consequence of their frequent close encounters with the Earth or other planets, it is difficult to predict their orbits with high precision for more than a century or so into the future. One such object is 1999 AN10, discovered by the MIT-USAF LINEAR search program on 13 January 1999. A detailed analysis of the orbit of 1999 AN10 was completed by researchers Andrea Milani, Steven R. Chesley and Giovanni B. Valsecchi in March 1999. Their paper, which has been submitted for publication in a technical journal, includes an examination of the potential risk of 1999 AN10 hitting the Earth in the next several decades. They conclude that, while there is some uncertainty in the exact orbit of this NEO following its next close planetary encounter in August 2027, the chances of its actually hitting the Earth in the next 40 years are minuscule — the authors estimate that the chance of impact is of order 1 in a billion (1 in a thousand million), which they indicate is 10,000 times less than the chance that the Earth will be struck by some as-yet-undiscovered similar-sized NEO in any one year. The IAU’s Working Group on Near-Earth Objects has formed an ad hoc committee, with widely international expert membership, whose members are available to review predictions of impact hazards if so requested. This committee functions similarly to the referees of most technical journal articles in reviewing the predictions, and it also keeps the appropriate IAU officials completely informed about any such predictions. The technical paper by Milani and colleagues has been subject to such informal review during the first two weeks of April 1999, and it is the consensus of the reviewers that the work is accurate and of the highest scientific quality. The IAU reviewers also note that the chances of impact by NEO 1999 AN10 during the time-span considered in this paper are negligible compared to the risks we run continuously of being struck by one of man similar size NEOs that have not yet been discovered. Like asteroid 1997 XF11, which was widely discussed in the press in March 1998, this asteroid does not pose any significant danger to the Earth on the time scale of the next several decades. Astronomers will continue to search for new NEOs and to track the orbits of those already discovered, especially when, like 1999 AN10, their orbits bring them close to the Earth. But this object, as demonstrated in the technical paper by Milani and his colleagues, should not evoke any particular public concern. Thus, the reviewers from the WG NEO agree with the authors in ruling out any danger to Earth from 1999 AN10 in the next forty years. The object will be followed closely over the next several years in order to define the longer-term properties of its orbit more accurately, as will be the case with numerous other, similar objects that will be continue to be discovered over the next several years as NEO searches intensify and orbital computation methods improve.