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It missed us

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Ken Tapping, March 11, 2009

In the sky this week...

> Venus is still unmissable in the western sky.

> Saturn rises in the early evening. Although far less spectacular than Venus to the naked eye, it looks a lot more interesting through a telescope.

> The Moon will be Full on the 10th: "The Moon of Meltwater".


Around 13:44 Greenwich Mean Time on Monday, March 2nd, we came close to being hit by a rocky object between 20 and 50 m across. It passed only 72,000 km from us, which is only about a sixth of the distance to the Moon, and less than twice the distance of the geosynchronous satellite belt, where our TV broadcasting satellites and other important satellites are located. If it had hit us, the impact would have been like the one that took place at Tunguska, Siberia, in 1908. This produced an explosion that flattened some 80 million trees over an area of 2000 square kilometres and rattled glasses on shelves in Paris, thousands of kilometres away. This is equivalent to a thermonuclear bomb with a power of 10 to 15 Megatonnes of TNT. 

While not in the same class as some impacts in the more distant past, the only factor that saved us from a major disaster was that the impact took place in a remote area. If it had happened in Western Europe or the Eastern Seaboard of North America it could have killed many millions and caused widespread devastation. Monday's near miss was not the record for nearness. As far as we know, that was in March 2004, when an object designated 2004 FU162 passed within 6,500 kilometres. Although only about 6 m across, it was moving at tens of kilometres a second and if it had impacted in a populated area it would have caused a lot of damage. These days we fully appreciate the threat posed by objects whose paths around the Sun cross the Earth's orbit. There are telescopes searching for them and a UN Working Group on Near-Earth Objects looking at the problem.

Monday's object, designated 2009 DD45, was spotted on the preceding Saturday by the Siding Spring Observatory in Australia. Small dark grey or blackish objects lit only by sunlight are very hard to spot, which is the main reason for our having only a couple of days' notice in this case. However, even with more notice it is not clear what we could do to avoid a disaster, apart from to determine where on Earth the object is likely to impact, and evacuate people within a few kilometres of the impact point.

The science fiction movies of intrepid astronauts flying to threatening objects and blowing them up are likely to remain science fiction. Doing that just replaces one big threat with a million smaller ones. Changing the orbit of millions of tonnes of rock is not easy, and the fragments will simply continue along the original path, posing an even bigger threat. The only technically feasible option at the moment is to identify threats years in advance, so that we have time to send a small spacecraft to the body to attach a small rocket motor. Using the body's material as fuel, it will apply enough of a nudge over time to change the orbit enough to avoid an impact. Obviously a couple of days' notice would not be enough to do that. We have a lot of work to do by way of better detection methods, orbit determination techniques and then to come up with viable ways to deal with these threats in a timely fashion. Monday's event was Mother Nature reminding us how important this is.

Ken TappingKen Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,