New orbital calculations indicate that the NEA called 2004 FU162 came the closest of any asteroid discovered by the Spaceguard Survey. From its brightness, this asteroid appears to be less than 10 m across, so that it would have exploded harmlessly in the upper atmosphere had it hit (to reach the surface, it would have required a diameter of about 100 m). The previous close-distance record was held by asteroid 2004 FH, which came within 50,000 km (see news item for March 23, 2004) FU162 was discovered on March 31, just hours before its closest approach, by the LINEAR survey system in New Mexico, which tracked it over a 44-minute period. The fast-moving image was later identified in processing the images, but it was too late to obtain further observations since the asteroid exited from the dayside of the Earth. Tim Spahr of the Minor Planet Center contacted Steve Chesley of the NASA NEO Program Office at JPL to try to extract an orbit, even though the data spanned only a short arc. No pre-recovery data were found, and indeed this asteroid is so small that it could not be seen by survey telescopes except when it was very close to Earth. Chesleys orbit calculations, however, did show that this asteroid had come closer than any previously discovered NEA. Based on this orbit, the asteroid was given an official designation and the information was released on 22 August. Like the initially ambiguous asteroid sighting of last January 13-14 of the object called AL00667 (see news item for February 19, 2004), this asteroid shows the capability of LINEAR to find a few of the tiny NEAs that pass very close by the Earth. These are a byproduct of its survey of larger asteroids. Someday it may find one on a collision course. It is most likely, of course, is that such an object will be small like this one, and thus that it will pose no danger to the Earth. It is even more likely that such a small object will hit the Earth without being discovered, as discussed below. Al Harris of the Space Science Institute notes that the population of NEAs with diameter of 6 m or more is a couple hundred million, with an expected impact frequency of one per several years. One missing the Earth by 2 radii (from geocenter) should be four times more frequent, or more than once per year. Thus this event is not particularly rare, except that LINEAR had the good fortune to notice it. Following is an account of FU162 from the New Scientist (on-line). David Morrison ======================== ASTEROID SHAVES PAST EARTH'S ATMOSPHERE Jeff Hecht 23 August 04 NewScientist.com news service The closest observed asteroid yet to skim past the Earth without hitting the atmosphere, was reported by astronomers on Sunday. The previously unknown object, spanning five to 10 metres across, has been named 2004 FU162. It streaked across the sky just 6500 kilometres - roughly the radius of the Earth - above the ground on 31 March, although details have only now emerged. The MIT Lincoln Laboratory's asteroid-hunting LINEAR telescope in Socorro, New Mexico,US, observed the new object four times over a 44-minute period, several hours before its closest approach in March. Lincoln astronomers, who have discovered over 40,000 asteroids and comets since 1980, quickly recognised the object came exceptionally close, and posted their findings for confirmation on a web page run by the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics. However, by the time it was posted the object had moved into the daytime sky, so follow-up observations were impossible and the listing was quickly removed. A search for prior observations yielded no results. Despite having only four positions for the object, Steven Chesley of NASAs Jet Propulsion Laboratory was able to calculate its orbit because it was moving rapidly across the sky. He also calculated that the encounter with the Earth shifted the asteroid's orbit closer towards the Sun. Previously orbiting the Sun once a year in an orbit that ranged as far inside the Earth's orbit as outside, 2004 FU162 now has a nine-month orbit centred closer to Venus than the Earth. The Minor Planet Center published Chesleys results on Sunday in its electronic circular. "This was an extraordinarily close encounter and so the orbital change was quite extraordinary. 2004 FU162 was deflected by about 20 degrees because of the Earth's gravity. I've never seen anything like that before," Chesley told New Scientist. The previous record for the closest asteroid approach to Earth was set on 18 March by an object called 2004 FH which missed the Earth by about 40,000 kilometres. That was a much larger object, around 30 metres in diameter - big enough to produce a one-megaton explosion in the atmosphere. Although it was likely to have exploded so high that the energy would have dissipated harmlessly. The smaller 2004 FU162 would have burned up as a fireball ending with a smaller explosion, had it ventured into the Earths atmosphere. |
