Produced at the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Massachusetts, U.S.A.

Today saw the announcement of the discovery of the two-hundredth PHA, or "potentially hazardous asteroid". This milestone was reached with 1999 VP11, found on Nov. 7 by LINEAR, the Massachusetts Institute of Techology's Lincoln Laboratory Near-Earth Asteroid Research project. In less than three years this project has yielded more than one-third of all the PHAs, which are asteroids that can pass within 0.05 astronomical unit (5 million miles) of the earth and are intrinsically brighter than absolute magnitude 22 (suggesting that they are more than about 600 feet across). The first PHA, known as Apollo, was discovered in 1932, and all but 15 of the PHAs have been found during the past 20 years. Just half of the PHAs have been discovered since the middle of 1997, with the recent LINEAR contribution supplemented by the University of Arizona's longstanding Spacewatch project, with 12 discoveries; the Near-Earth Asteroid Tracking program of the Jet Propulsion Laboratory, with six discoveries; the University of Arizona's Catalina Sky Survey, with five; and the Lowell Observatory Near-Earth Object Program, with four. The Arizona amateur astronomer Roy Tucker has discovered two PHAs.

Mere categorization as a PHA does not imply that an object will pose any danger to the earth during the next tens of millennia and more, but this is a convenient subset of the more inclusive collection of Near-Earth Objects, most of which actually come nowhere near the earth. More than 40-percent of the PHAs have been so extensively observed that there is no way more than one or two of these could conceivably be a threat a half-millennium, say, from now. For another 30 or 40 percent the minimum distances between their orbits and that of the earth are currently too large for there to be any real possibility of an impact within the next half-millennium. Only a fraction warrant any serious examination for a possible impact during the next half-century or so. And that examination is now routinely occurring, thanks mainly to the efforts of the group at the University of Pisa. The number of actual cases found where there has been any conceivable danger during the next half century is remarkably small--four objects altogether. Knowing of these cases in a timely manner then allows special observational efforts to be made, including the search for images of these bodies in the old photographic archives. It should be no surprise that further observations will eliminate these possible dangers--almost all of the time... At least for the intrinsically brighter objects, which include those half a mile across and larger, and thanks to the dedication of an ever-increasing set of observers, professional and amateur, the process is working just fine.

Brian G. Marsden

1999 November 11


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