Following is a news announcement from the NASA/JPL NEO Program Office plus two newspaper stories dealing with the close pass of asteroid 2004 FH. This event is another tangible indication of the success of LINEAR program and of the Spaceguard Survey. Asteroids this size have been coming this close roughly once every 2 years, and one hits the Earth every few decades, but for the first time we are now able to detect some of these small visitors. Perhaps this information will help us to develop a better sense of our cosmic environment, where space rocks go sailing past the Earth at high speed every day. Many reporters have asked what would be the consequences if this asteroid had hit the Earth. This is difficult to answer, since its nominal size of 30 m lies near the lower limit for penetration of the atmosphere. A variety of models have been calculated for the effects of the atmosphere on incoming projectiles, and these were reviewed for the recent NASA Science Definition Team (SDT) report on sub-kilometer asteroids. The Tunguska impactor was a stony (asteroidal) object about 60 m in diameter. While it was significantly decelerated in the atmosphere, it still produced a very destructive explosion of about 10 megaton energy when it disintegrated at an altitude of 6-8 km. In contrast, the energy of a 30 m stony asteroid at the same speed is about 1 megaton. The studies reviewed by the SDT indicated that, for a fixed composition and entry speed, the damage falls off very rapidly for sizes smaller than Tunguska, going to zero for energies below about 2 megatons. The drop-off is so fast toward smaller sizes because two effects add: the energy is smaller and the explosion takes place at higher altitude. If severe ground damage is defined by a blast overpressure of 4 psi (pounds per square inch), then Al Harris notes: What is happening is that at 3 MT, the overpressure falls under 4 psi at a distance of around 15 km from the blast center, and the blast center is around 10 km or so up, so the intersection with the ground is around 100 sq. km. If you drop the size to 1 MT, the numbers are reversed: the blast is around 15 km up and the radius out to 4 psi overpressure is around 10 km, so nothing happens on the ground, even right below it. The diameter for this 2-3 megaton limiting case is near 40 m with the proviso that some local damage might be done to poorly built structures even below 4 psi pressure. The larger source of uncertainty is the nature of the projectile itself. Penetration and explosive energy will differ significantly depending on the density and composition of the asteroid, with a rare iron object even making it to the surface and forming a small crater. In addition, the astronomers do not measure the actual diameter of the asteroid, but rather its apparent brightness. The diameter is inferred from the brightness based on an assumed reflectivity or albedo. Thus, even if we felt confident that a 30 m diameter stony asteroid would do no harm, it does not follow that we can be sure that asteroids 2004 FH would not be dangerous. Statistically, such asteroids do not constitute a significant threat, but that does not mean that we can ignore the possible danger posed by a poorly characterized object for which we have measured only its brightness. David Morrison ================================================ RECENTLY DISCOVERED NEAR-EARTH ASTEROID MAKES RECORD-BREAKING APPROACH TO EARTH Steven R. Chesley Paul W. Chodas NASA's Near Earth Object Program Office Wednesday, March 17, 2004 A small near-Earth asteroid (NEA), discovered Monday night by the NASA-funded LINEAR asteroid survey, will make the closest approach to Earth ever recorded. There is no danger of a collision with the Earth during this encounter. The object, designated 2004 FH, is roughly 30 meters (100 feet) in diameter and will pass just 43,000 km (26,500 miles, or about 3.4 Earth diameters) above the Earth's surface on March 18th at 5:08 PM EST (2:08 PM PST, 22:08 UTC). On average, objects about the size of 2004 FH pass within this distance roughly once every two years, but most of these small objects pass undetected. This particular close approach is unusual only in the sense that scientists know about it. The fact that an object as small as asteroid 2004 FH has been discovered now is mostly a matter of perseverance by the LINEAR team, who are funded by NASA to search for larger kilometer-sized NEAs, but also routinely detect much smaller objects. Asteroid 2004 FH's point of closest approach with the Earth will be over the South Atlantic Ocean. Using a good pair of binoculars, the object will be bright enough to be seen during this close approach from areas of Europe, Asia and most of the Southern Hemisphere. Scientists look forward to the flyby as it will provide them an unprecedented opportunity to study a small NEA asteroid up close. ============================================ 100-FOOT ASTEROID TO MAKE CLOSEST-EVER EARTH FLY-BY TODAY: NO RISK POSED The Associated Press, 17 March 2004 SAN DIEGO - A 100-foot diameter asteroid will pass within 26,500 miles of Earth on Thursday, the closest-ever brush on record by a space rock, NASA astronomers said Wednesday. The asteroid's close flyby, first spied late Monday, poses no risk, NASA astronomers stressed. "It's a guaranteed miss," said astronomer Paul Chodas, of the near-Earth object office at NASA's Jet Propulsion Laboratory. The asteroid, 2004 FH, is expected to make its closest approach at 2:08 p.m. PST, streaking over the southern Atlantic Ocean. It should be visible through binoculars to stargazers across the southern hemisphere, as well as throughout Asia and Europe, said astronomer Steve Chesley, also of JPL. Professional astronomers around the globe scrambled Wednesday to prepare for the flyby, which could provide an unprecedented chance to get a close look at the asteroid, he added. The asteroid will pass within the moon's orbit. Similarly sized asteroids are believed to come as close to Earth on average once every two years, but have always escaped detection. "The important thing is not that it's happening, but that we detected it," Chesley said. Astronomers found the asteroid late Monday during a routine survey carried out with a pair of telescopes in New Mexico funded by the National Aeronautics and Space Administration. Follow-up observations on Tuesday allowed them to pinpoint its orbit. "It immediately became clear it would pass very close by the Earth," Chesley said. Astronomers have not ruled out that the asteroid and our planet could meet again sometime in the future. If the two were to collide, the asteroid likely would disintegrate in the atmosphere, Chesley said. ================================= SMALL ASTEROID BYPASSES EARTH BY JUST 26,500 MILES Guy Gugliotta The Washington Post, 19 March 2004 WASHINGTON A 100-foot-wide space rock yesterday missed Earth by only 26,500 miles, the closest asteroid ever detected by astronomers before it actually made its approach. The harmless flyby occurred at 2:08 p.m. Pacific time. "We figure that on average something this size hits the Earth every two to three years," said astronomer Paul Chodas, of NASA's Near-Earth Object Program. "We've never detected them out in space, either approaching or receding. This is the closest." Chodas said astronomers from the Minor Planet Center, in Cambridge, Mass., registered the asteroid, which was detected by telescopes in New Mexico late Monday, and gave it the designation 2004 FH. It was visible through binoculars in the Southern Hemisphere as well as throughout Asia and Europe, said astronomer Steve Chesley, of the Jet Propulsion Laboratory in Pasadena, Calif. Chodas described 2004 FH's detection as "almost an inevitable result" of the NASA sponsored Spaceguard Survey Program, a 10-year effort begun in 1998 to monitor and map space objects that come within 120 million miles of the sun. Earth is 93 million miles from the sun. Astronomers have found 2,653 objects to date. Yesterday's space rock qualified as a "small asteroid," Chodas said, and simply would have burned up albeit spectacularly if it had entered the atmosphere. Of far greater interest are asteroids one-half mile in diameter or larger, and NASA has catalogued 693 of these, which Chodas reckoned to be 60 to 70 percent of the 1,200 total. "Anything 40 or 50 meters (130 feet to 165 feet) or larger will probably make it to the (Earth's) surface," Chodas said. "Something smaller will break up." Perhaps Earth's most famous recent encounter with a space rock occurred in 1908, when a comet or asteroid estimated between 200 and 300 feet [should be an asteroid about 200 feet -- DM] in diameter exploded five miles above ground near Tunguska, in Russian Siberia, with the force of a 40-megaton bomb [should be smaller, probably 10-15 megaton DM], incinerating [should be toppling or flattening DM] the forest for 25 miles around. "We think something like this may happen once every 500 years," Chodas said. The Near Earth Object Program seeks to detect such dangers years before they approach Earth, so something can be done. Meteor showers, by contrast, normally feature baseball-sized or pea-sized objects, Chodas said. |
