NEO News (08/17/05) Asteroid experts meet at ACM-2005
Clark Chapman reports from the Asteroids/Comets/Meteors 2005 meeting, held last week in the resort town of Buzios, east of Rio de Janeiro, Brazil. This ACM-2005 is the latest of the every-three-year meetings begun in Sweden in the early 1980s. Professional astronomers gathered to discuss, in plenary sessions (no double sessions) and poster-talks, advances in the science of small Solar System bodies since the last meeting three years ago in Berlin. Most of the presentations dealt with the fundamental scientific issues concerning these remnants of the primitive "building blocks" that formed the planets 4.5 Gyr ago. Recent excitement was generated by Deep Impact's violent examination of Comet Tempel 1, and then by the reported discoveries of three more not-so-small denizens of the far reaches of the solar system, including 2003 QB313, at 97 times the distance of the Earth from the Sun -- it is almost certainly significantly larger than Pluto, perhaps deserving the monicker of "tenth planet".
Issues involving the impact hazard were treated by several speakers and even more posters at ACM 2005. Many dealt with the extraordinary circumstances of 2004 MN4, recently given a number (99942) and named Apophis -- a god with appropriate destructive attributes for this ever-metamorphosing asteroid, which once had a 1-in-37 chance of striking the Earth in 2029 and now has a 1-in-8000 chance of striking in 2036 after its spectacular close pass in 2029. [Editorial note from DM: These probabilities measure uncertainties in our knowledge of the orbit. The asteroid knows where it is going, and only in Hollywood do asteroids veer off path. But we did not know in December and still do not know today the orbit with sufficient precision to remove these uncertainties.]
Two months ago, Rusty Schweickart wrote to NASA Administrator Mike Griffin asking for a technical analysis of whether public safety requirements might mandate commitment to a transponder mission to Apophis prior to its next period of visibility in the 2012-2013 timeframe [see NEO News for 07/21/05]. At ACM, Steve Chesley of JPL's Near Earth Object Program Office presented what may be the technical underpinnings of NASA's forthcoming response. Chesley acknowledged that the usual approaches to error-analysis failed for Apophis, due to unusual circumstances: the projected 10 Earth radii miss-distance (with a conservative uncertainty of 2.5 Earth radii), calculated in late December when the "all-clear" was announced, turned out to be badly skewed when late-January radar data, and subsequent optical observations, yielded a much-closer, and visually spectacular, 5.7 plus-or-minus 1 Earth radius pass on April 13, 2029. (The misestimated uncertainty in December allowed less than 1 chance in a million of this big a change in miss-distance.) Such a large asteroid passing that close to the Earth happens only once every 1,500 years or so, according to Chesley.
Now, the real issue has become whether Apophis will pass by through the "keyhole" in 2029 that would lead to its resonant return, and Earth impact, on April 13, 2036. (G. Valsecchi explained the technical details of keyholes in his ACM talk.) The 1-in-8000 chance of impact is an unusually high probability for a 300+ meter sized object to strike the Earth in the near future. Thus it ranks at "1" on the Torino Scale, and will likely remain at that level, or even rise to "2" before the next opportunity to observe it happens about 7 years from now. (There is the possibility that radar observations will be successful early next year, but they may serve only to raise the probability of impact by narrowing the uncertainty while still including the 2036 keyhole.)
While much refinement in Apophis' orbit is expected by 2013, there remains an inherent uncertainty due to the unpredictable behavior of the Yarkovsky Effect -- an acceleration of an asteroid due to the asymmetry of solar radiation absorbed from the Sun and re-radiated on the afternoon hemisphere of a rotating body. That uncertainty will eventually be resolved during Apophis' close pass in 2021, but that might be cutting it close (if Apophis is actually on an impact trajectory) to mount a reliable deflection mission. These scenarios, being evaluated by JPL, will presumably be addressed in NASA's response to Schweickart's letter. But it already seems to Chesley that a transponder mission to Apophis will not be required prior to the expected orbital improvements in 2013. An issue that has come up in the past was the effect of perturbations by other asteroids. While it is clear that such perturbations are important for NEA's that penetrate the main asteroid belt, Chesley now concludes that for Apophis (an Aten class asteroid), this is not much of an issue.
There had been much discussion in January about whether or not it was appropriate to publish the map of possible "ground zeroes" across the Earth for the unlikely cases of impact of Apophis. At ACM, Chesley showed what others had already calculated independently, that the areas that had been at risk -- had Apophis turned out to be aimed at Earth in 2029 -- certainly would have included parts of Europe and the Middle East, as well as heavily populated parts of India and Bangladesh. (The error ellipse is exceedingly stretched, so even while it extended beyond the Moon before Christmas, its position was known to within a few hundred kilometers cross-track.) Illustrating how readily these risk zones can be calculated (or potentially miscalculated, without professional guidance), M. Krolikowska et al. (from Warsaw) presented a poster showing the narrow impact zones across the Earth of several potential impactors. In his ACM talk, G. Sitarski (also of Warsaw) showed a diagram of the zone at risk if Apophis actually does pass through the keyhole and strikes the Earth in 2036 (also see the illustration in the article by Schweickart and myself in the current, Sept.-Oct., issue of "American Scientist"). Although that narrow zone crosses Siberia and Central America, it mainly threatens a tsunami in the Pacific off the west coast of North America.
What will happen if we need to deflect Apophis to keep it from hitting the keyhole that leads back to a collision with the Earth? Chesley concluded his talk by noting that a kinetic energy deflection could be accomplished (Deep Impact style) with a 1000 kg impactor hitting at a few km/sec giving a 25 km deflection after 3 years.
Dan Durda gave a talk discussing the interesting scientific issues presented by Apophis' close pass in 2029. Due to Earth's enormous tidal forces on this body, it is certain to undergo a major change in its spin rate and probable re-arrangement of its internal structure. Durda proposes that the asteroid be outfitted with a seismic net, and that other physical studies be undertaken before, during, and after the 2029 pass, letting "nature do the work" of creating a geophysical rearrangement of the body.
F. Bernardi et al. presented a poster highlighting issues surrounding their unconventional discovery of 2004 MN4 and describing, among other things, the complex -- and highly appropriate -- reasons for naming the body Apophis. Still other ACM presentations evaluated the progress of the current Spaceguard Survey: it seems unlikely to meet its original target of discovering 90% of NEAs larger than 1 km diameter by early 2008, unless the total number is actually much less than the estimated 1100 [This total number, however, has not been revised based on recent data]. Pan-STARRS, which will begin to come on-line next year in Hawaii, will begin a new era of discovering ever-smaller NEAs. Plans for the later construction of the Discovery Channel Telescope at Lowell Observatory and the LSST were also presented. Still other talks, not specifically oriented towards hazardous NEAs, nonetheless presented information potentially relevant to the logistics of actually deflecting an NEA from Earth impact. For instance, K. Holsapple argued that small main-belt asteroids (though not necessarily NEAs) are not proven to be rubble-piles simply because of their failure to spin more rapidly than about once every couple of hours. Other speakers, however, argued that an increasing fraction of small bodies may be doubles (or even triples!), and thus more difficult to cope with.
In summary, the impact hazard was well integrated into this meeting that generally dealt with less practical, pure-science aspects of asteroids and comets.
Clark R. Chapman
Southwest Research Institute (Boulder)
16 August 2005
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