Tuesday, December 17, 2013

Close call asteroid....2013 XY8

XY8 was discovered on Dec. 7. If it had been on an impact trajectory, we’d have only had five days warning.

At 11:17 UTC (6:17 a.m. EST) Dec. 11, the asteroid 2013 XY8 passed the Earth at a distance of just 760,000 kilometers (470,000 miles), less than twice the distance to the Moon. It orbits the Sun every 3.3 years, swinging out about halfway to Jupiter's orbit, and coming in to just inside Earth's orbit. The current data shows it won’t come near the Earth again until 2072.

But XY8 is a good reminder that there are lots of asteroids out there, and we need to find them. And it’s also a good reminder that finding them isn’t all that easy.

Why was it discovered so late? Because even though it’s bigger than a house, it’s far away, which makes it very faint. When it passed us at closest approach it peaked at a magnitude of about 14, less than a thousandth as bright as the faintest star you can see with the naked eye. And in asteroid terms, that’s when it was pretty close by! It was even fainter when it was discovered by the Catalina Sky Survey.

Of all the things about asteroid impacts to fret over, this is what worries me the most: finding them in the first place. The bigger an asteroid is the bigger the impact, of course, but also the easier it is to find it in the first place. Bigger rocks are brighter, and we can see them more easily. The problem is that there’s a size range on the small end where they still do damage when they hit us, but are simply too small to find until they’re very close.

Chelyabinsk was like that. Although it blew up high in the atmosphere, the shock wave from the blast shattered windows and injured over a thousand people. But we had literally no warning; it was too small and too near the Sun in the sky to be found before it hit us. For small impacts like that, the first time we see the asteroid will be when it’s streaking through our air.

There are about a million asteroids of decent size that cross Earth’s path, and we’ve only charted about 10,000. That may sound like a lot, but it’s only 1 percent of what’s out there.

Asteroid 2013 XY8 animation showing it moving over the course of just over two minutes. Photo by Ernesto Guido, Nick Howes, and Martino Nicolini using the Faulkes Telescope South.

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