http://www.space.com/scienceastronomy/asteroid_tally_020723.html<br /><br />True Tally of Asteroids Probably at High End of Estimates<br />By Robert Roy Britt<br />space.com<br />23 July 2002<br /><br />The continued rapid pace of discovery for large asteroids in relatively<br />close proximity to Earth suggests there may be more of them than some<br />scientists have predicted. The speculation, from one of the astronomers who<br />helps count the rocks, does not imply a significantly increased threat to<br />Earth, but it does extend a long-running debate over just how many of these<br />space rocks exist.<br /><br />Large Near Earth Objects (NEOs) are asteroids bigger than 1 kilometer (0.62<br />miles) that loosely inhabit the region of the solar system through which<br />Earth orbits. More than 600 of these large NEOs have been found.<br /><br />Not a single one is known to be on a collision course with the planet.<br /><br />However, the big, Sun-orbiting hunks of metal and stone are of great<br />interest to scientists because of the potential for one being discovered<br />that could hit us down the road. Such an impact would cause devastation on a<br />global scale, possibly pushing humans into a Dark Age existence. Experts say<br />the scenario might be avoided by mounting a mission to deflect or destroy an<br />asteroid known to be targeting us.<br /><br />Various research groups from around the world have taken their best shots in<br />recent years at calculating how many 1-kilometer and larger NEOs are out<br />there. Estimates have ranged from 700 to 2,000.<br /><br />Astronomers frequently cite 1,000 as the best estimate these days. Another<br />popular count, however, is 1,200. Both prognostications are based on<br />detailed studies and have error margins of plus or minus 100 or 200<br />potentially deadly giant boulders.<br /><br />If either of the leading estimates is correct, then just more than half of<br />all NEOs have been found.<br /><br />To Brian Marsden, director of the Minor Planet Center where asteroids are<br />officially catalogued, logic dictates that the pace of discovery ought to be<br />slowing down, because there are now fewer NEOs that remain to be detected<br />and hence the chances of finding each one is reduced.<br /><br />But the pace has not slowed.<br /><br />"Since the new discoveries are indeed still coming along at about 100 per<br />year ... the number is probably not 1,000 minus anything, but more likely at<br />least 1,200 or so," Marsden told SPACE.com.<br /><br />If Marsden is correct, astronomers would not be overly surprised. But his<br />logic does represent an unexpectedly simple method of suggesting that the<br />lowball estimates are probably not correct.<br /><br />Meanwhile, SPACE.com has learned of a new estimate. It comes from NASA<br />researcher David Morrison and his colleagues and will be presented in a<br />forthcoming book titled "Asteroids III" (University of Arizona Press).<br /><br />Morrison, senior scientist at the NASA Astrobiology Institute at the Ames<br />Research Center, said via e-mail that it's "pretty clear that the number is<br />not substantially less than 1,000." His team prefers 1,100, plus or minus<br />100 -- which, Morrison points out, encompasses the possibility of 1,200<br />suggested by Marsden.<br /><br />Alan Harris is a co-author of the chapter in "Asteroids III" that deals with<br />the impact odds. Harris is also researcher at NASA's Jet Propulsion<br />Laboratory, and he spends a lot of his time making and analyzing these<br />estimates. Harris said the discovery rate continues to be high in part<br />because detection methods have improved. It's very difficult to know, he<br />said, how strong this effect is versus the possibility that there are simply<br />more asteroids out there than expected.<br /><br />"A population of 800 is implausible in the light of present discovery rates<br />and the number already discovered," Harris said. One thousand is plausible,<br />he added, as is 1,200. "I have to say though that I would be less surprised<br />if the real population is greater than 1,200 than that if it is less than<br />1,000."<br /><br />Ultimately, the exact number is of little consequence. The important thing<br />is simply whether or not one of them is headed our way.