办事指南

Apocalypse postponed

点击量:   时间:2017-08-16 06:02:13

By Jeff Hecht Boston AFTER a day-long drama, in which it seemed there was an outside chance that civilisation might end 30 years from now with a catastrophic asteroid impact, astronomers declared the all clear last Thursday. Revised calculations based on data from 1990 show that on 26 October 2028 asteroid 1997 XF11 should miss the Earth by 960 000 kilometres—2.5 times farther away than the Moon. 1997 XF11 was discovered by Jim Scotti of the University of Arizona in Tucson on 6 December last year. Gareth Williams and Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, soon added it to their list of “potentially hazardous objects” which might hit the Earth. Initial calculations indicated that 1997 XF11 would miss the Earth by about 800 000 kilometres in 2028. But the inclusion of observations made on 3 and 4 March showed 1997 XF11 skimming just 42 000 kilometres above the Earth’s surface—with a small chance that it would hit us. Marsden announced the bad news in an electronic circular sent out on 11 March, trying to encourage more observations. He also asked astronomers to check their archives for any sightings from the previous occasions 1997 XF11 came within viewing distance: in 1990, 1983, 1976, 1971 and 1957. “But we didn’t think the chances were that large of finding something,” says Dan Green, who works with Williams and Marsden. So the team was surprised when Eleanor Helin of the Jet Propulsion Laboratory (JPL) in Pasadena promptly unearthed images of the 1990 encounter on photographic plates. While Marsden talked with reporters who had arrived to cover the story on 12 March, Williams recalculated the orbit, and found the asteroid would miss the Earth by 960 000 kilometres. Don Yeomans of JPL has confirmed that result. The impact of an asteroid the size of 1997 XF11, which is 2 kilometres across, would release about half a million megatons of explosive energy. That could devastate global agriculture, warns Scotti. With sufficient warning, it could be possible to nudge an asteroid away by exploding nuclear weapons a kilometre or so above its surface. Identifying all the potential threats from outer space will require a ten-year, $50-million programme,