Comet what may

By Phil Plait | May 4, 2006 10:18 pm

In this week’s commentary, James Randi discusses the case of one Eric Julien, who, you may recall, is saying that the Earth will be hit by a piece of the comet 73/P Schwassmann-Wachmann on May 25.

Randi gives the lowdown on this character. It was bad enough when Julien was predicting some random explosion from a piece of this comet hitting the Earth (because it won’t; the nearest chunks will be millions of kilometers away), but now he has really amped up the scenario: Julien is now claiming there will be a huge tsunami caused by the comet impact.

I read Julien’s missive on this (follow the links from Randi’s page) and while his pseudoscience is bad enough (he bases his prediction on crop circles) his science is worse. Even if (if) a piece of this comet impacted the Earth, it would certainly not create a tsunami like he says. A piece smaller than a football field would blow up high in the atmosphere and not hit the Earth’s surface. The energy from the explosion of something football-field-sized, while very large (I mean really large), would dissipate a lot before hitting the surface. It wouldn’t be anywhere near the energy needed to make a tsunami 700 feet tall. For that, you’d need a huge piece, maybe a mile across, and that would have to hit the surface of the water directly. Even then, it’s not clear it would make a big tsunami. Scientists are still trying to figure out how big a wave you’d get from an impact. The math is pretty fierce. Some scientists think the tsunami would be much smaller than shown in the movie "Deep Impact", for example.

Anyway, a regular tsunami (like the one in the Pacific in 2004) is caused when an underwater earthquake displaces the ground, which in turn displaces water. Since the ground is directly contacting the water on top of it (they are what scientists call "well-coupled") the water absorbs that energy efficiently, and you move a lot of water. An explosion above the water’s surface is not well-coupled, so it would take vast amounts of energy to make a tsunami, and for that you need a huge piece of comet. The odds of a piece that big hitting the Earth, even from this comet, is vanishingly small (that’s scientist-speak again for really really small).

I may re-read Julien’s claims this weekend if I get a chance, and maybe even do a more thorough debunking. But he’s got a whole lot of bunk to toss around… and the impact of that is just as bad as a comet’s.


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