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Planetary News: Asteroids and Comets (2002)The Close Call that was Asteroid 2002 MN24 June 2002The following selections are from the June 24, 2002 edition of NEO News, written & compiled by Dr. David Morrison. 2002 MN & Hazard Reviewby David Morrison On June 17, the LINEAR Spaceguard system discovered Near Earth Asteroid (NEA) 2002 MN, which had passed the Earth on June 15 at a distance of only 210,000 km, one of the closest asteroid flybys on record. Based on its brightness, 2002 MN has a nominal diameter of about 100 m, large enough to penetrate through the atmosphere to the surface if it struck the Earth. Considerable press interest in this object has been evident, as well it might be. It has been nearly a decade since a known asteroid passed this close. Unfortunately, however, some of the press coverage has been sensationalistic. Some either decry that the object was found after closest approach (rather than before) or express concern about the "blind spot" otherwise commonly known to astronomers as the daytime sky. It is quite true that an asteroid close to the Sun in the sky cannot be seen. However, if an NEA is approaching Earth from the daytime sky, it is likely to pass into the night sky, where it can be discovered, as this one was. Far from being a cause of concern, the discovery of NEA 2002 MN was another example of the success of the Spaceguard effort in general, and of the LINEAR program in particular. Similar concerns were expressed following the discovery of 2002 EM7 two months ago, but there is no cause for "doom and gloom" in either of these asteroids. Presumably a part of the problem is that many people do not understand the Spaceguard Survey strategy to discover and catalog NEAs long in advance of any possible threat, providing decades (or more) of warning if any is currently on a collision course. Both of these asteroids were successfully found, although they are well below the 1-km diameter that is emphasized by the current Spaceguard effort. It makes no difference if a NEA is discovered on approach or departure from the vicinity of the Earth. We don't give extra points for an approaching NEA or demerits for one that has already passed the Earth at discovery. The only effect of "blind spots", whether they be due to sunlight or moonlight or bad weather or lack of a southern hemisphere survey telescope, is to slow down the completion of the NEA catalog. Objects in blind spots will be missed until they move into a more favorable geometry, sometimes within a few days, otherwise usually within a few years. To put this latest asteroid in perspective, a 100-m asteroid hits the Earth at an average interval of a few millennia. One passes within the orbit of the Moon, however, several times per year. This has been happening throughout history. What is new is that we are now beginning to discover these objects, whereas previously they would have sped past undetected and unheralded. Following are some press comments on 2002 MN. I believe that we are making progress, in that many of the news reports on 2002 MN do accurately reflect the situation as the astronomers understand it. For a comprehensive review paper on the asteroid impact hazard now in press for the book Asteroids III, visit the NASA impact hazard website at www.impact.arc.nasa.gov. Of particular interest to some will be a revised estimate of the frequency of impacts of NEAs as a function of size. - David Morrison. Near Earth Object Information Centre
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