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Fragment of the Imagination

April 13, 2006

Yahoo!News does it again.

A few months back, this "news" syndicator had a totally bogus article about a "chaos cloud" that was going to doom us all. The original source? The Weekly World News. Like it took a rocket scientist to figure out that was garbage.

Evidently, Yahoo@News is trying to top their own dumbosity. This time, they are reporting -- as straight news -- a story that on May 25 of this year a comet fragment will impact the Earth. Their source? the Exopolitics Institute, which studies UFOs, crop circles, and no doubt the Tooth Fairy and Bat Boy.

So what's the story? This guy, Eric Julien, who claims he is (and for all I know, may actually be) an ex-air traffic controller in France, says he has tracked (using "NASA simulations" which means he went on the web and poked around with orbital simulators) a fragment of a comet called P73 Schwassmann-Wachmann, a comet that was discovered in 1930. Comets are giant hunks of ice and rock, and when they get near the Sun the ice turns into a gas and creates the long tail. Sometimes the ice is what holds the comet together, so as it sublimates the comet can break apart. Schwassmann-Wachmann recently did this, fragmenting into many smaller chunks. These chunks can drift away from the main mass of the comet (if there is a main mass left), following different orbits over time.

image by Jim Scotti of the comet breaking up in 1995 However, the fragments of Schwassmann-Wachmann are all still pretty close together. And even if they weren't it would hardly matter-- closest approach of the comet will occur not on May 25 but around May 14, when it will be about 10 million kilometers from Earth-- about 6 million miles away. While that's close as comet passes go, that's still a long way. When asked if it will collide with Earth, Near-Earth Object expert Don Yeomans said, "Goodness, no. The closest fragment will be about six million miles away--or twenty-five times farther than the Moon."

But don't let the fear mongers hear that! Here is what the Yahoo!News article says:

Using NASA simulations of the comet's path, Julien concludes that impact is likely around May 25 precisely when the comet crosses the Earth's ecliptic plane. While the first fragment will cross at approximately 10 million miles, lagging fragments threaten to collide. While astronomers have stated that the comet poses no direct threat, Julien argues that some fragments are too small to observe. Astronomers have predicted possible meteor showers indicating some cometary debris will enter the atmosphere.
This is marginally true. Note the word "possibly" in the last sentence. We might get some meteors from this, but it's doubtful due to the great distance. But the next quotation is the money quote:
Julien argues that the kinetic energy of even a 'car sized' fragment will impact the Earth with devastating effect.

Bzzzzzt! Wrong. Call a fragment like this 3 meters across. It would have a mass of 7 tons, and at an impact velocity of 16 kilometers per second (the speed of the comet as it passes us) the impact energy would be the somewhat respectable equivalent of 450 tons of TNT. That's about 0.02 times the energy released by the Hiroshima atomic bomb.

That may sound like enough to do some damage, but it wouldn't. Ice is not very solid. When a small chunk enters the Earth's atmosphere, the energy of entry causes it to heat rapidly, and it explodes. But it does so while it is still very high over the ground, 50 miles or so. Some estimates are that a chunk of ice or rock a meter across enters the Earth's atmosphere once a month! Yet we don't even notice. While something 3 meters across would make a bigger bang (27 times as big, in fact*), it's still pretty small -- just half the Hiroshima bomb. That's big enough to see, but still too high up to have any real effect on the ground. No sound, no worries, and certainly no "devastating effect".

And remember, that's only if a fragment actually hits us. Which almost certainly one won't.

Julien goes on, tying this event to the Bush Administration, Iran, and (of course!) crop circles. 'nuff said. So duh, we're in no danger.

Chalk another phantasm up to the extremist UFO crowd. But I expect that from such groups as the Expolitics Institute; their stock-in-trade is a proud and utter denial of reality. A tour of their site should show you that. But again, Yahoo!News calls themselves a news organization, and passing along nonsense that is guaranteed to scare a lot of people is negligent at best.

Do I sound ticked? Damn straight I'm ticked. I am firing off a letter to Yahoo!News about this. Maybe I can convince them to talk to a real scientist first before disseminate utter bilge like this sort of story.

Oops! I originally said it would be eight times as big, because for some reason I was thinking the radius doubled. But it really triples, and volume goes as the cube of the radius. So 3x3x3=27. That's probably big enough to see, whereas 8 times is marginal. Interestingly, I would still expect there would be at least one explosion like this per year, yet no one ever sees them! Maybe they really are hard to see, but it's hard to understand why. I'll have to look into this...

©2008 Phil Plait. All Rights Reserved.

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