Near Earth Objects - DD45's Near Miss

Chris - If you have been watching the news this week then you might have noticed that the Earth had a brush with a near-Earth object. You might have been forgiven for letting it pass you by which is luckily what happened to the Earth this week. But we did have a close encounter with DD45. To tell us what DD45 was here's Professor Alan Fitzsimmons from Queens University in Belfast. Hello, Alan.

艾伦,你好。

2004 FH is the centre dot being followed by the sequence; the object that flashes by near the end is an artificial satellite.

Images obtained by Stefano Sposetti, Switzerland on March 18, 2004. Animation made Raoul Behrend, Geneva Observatory, Switzerland. (c) NASA' alt='Timelapse of Asteroid 2004 FH's flyby (NASA/JPL Public Domain)' >Chris - Tell us about this object, what is it?

Alan - It's a small asteroid. It's about between 20-40m across. It was discovered only just over a week ago, on Friday 27^thFebruary. It passed our planet by at a distance of only 72,000km on Tuesday.

Chris - That's extremely close. That's, let's put that in perspective. Satellites orbit the Earth about 25,000 miles out. That's only twice as far away as a geostationary satellite.

Alan - That's right. Occasionally we do spot these small asteroids coming past us. Objects of that size hit the earth probably about once every 2-300 years. We're not quite sure how often they hit us at the moment but they hit us on time scales of centuries.

Chris - Had this thing not been seventy thousand kilometres away and it had actually landed on the Earth what sort of damage would it have done? How would it compare with, say, the object that wiped out the dinosaurs?

Alan - Well, it's much smaller than that. The object that wiped out the dinosaurs was about ten kilometres across and had global consequences. Those objects only hit us about once ever hundred million years. An object that can cause climate change can be as small as one kilometre across, however. Even they only hit us once every million years or so. Something this size may have been similar to the object that entered our atmosphere over Tunguska in Siberia in 1908. It may have exploded low down in the atmosphere if it had entered our atmosphere and perhaps about a few kilometres up. It would have wiped out several square thousand kilometres of ground.

Chris - That's city devastating sort of level. How did we miss this kind of object? I thought we had, I was reassured to learn we had systems in place to spot these things so we could take action.

Alan - Well, it's because the systems we have in place are designed to spot the larger asteroids, the one kilometre guys and larger - the ones that would affect the entire planet. They're too small to effectively catalogue all the much smaller objects. At the moment the next generation of survey telescopes is in construction. There's something call Pan-STARRS which starts operating this year in Hawaii. Then sometime in the next decade, towards the end of the next decade something called the large synoptic survey telescope will get going in Chile. Even those telescopes won't be able to catalogue all the objects about the same size as DD45. We're just going to have to keep watching and surveying the sky.

Chris - Where did DD45 come from and given that it was so close this time round is it or is there any chance it might go round again and have another go?

艾伦-小行星的轨道上围绕太阳as everything else is in our solar system. It has an orbital period of just over 1.5 years. It's orbit just happens to have a point in it where it's very close to the Earth's orbit. Roughly once every March if the asteroid's there and the Earth is there it can come close to us. At the moment it can't hit us. The next time it will come close to us will actually be on the third of March in the year 2067 when even then it will pass by twice as far as it did this week. Over the coming centuries and thousands of years it's orbit will change slightly due to the gravitational tugs of the Earth and the other planets. It may well end up hitting us in a few thousand years' time. We don't know at the moment we haven't got enough data on it at the moment.

Chris - So unless you're Bruce Forsyth or someone who's going to live forever like that then you're probably in no danger.