Did an extraterrestrial impact cause the Ice Age extinctions? If so, this could be one more motivation to get our act together in terms of finding and managing these things.

The journalist who wrote this article needs to do more fact checking next time. The article states that a hydrogen bomb has 100 to 1,000 megaton yield. This is incorrect. Typical yield of a U.S. weapon is 220-250 kilotons. The Soviet bombs were typically 500 kilotons, with some of the gravity bombs (carried by bomber planes) going up to 5 megaton yield.

It is mistakes that this that make question the whole article itself.

Um, kurt, you're off by several orders of magnitude. Hydrogen bombs are fusion bombs and do indeed have a yield in the 10 to 100 megaton range; they require a 220-250 kiloton fission bomb just to trigger the fusion bomb in the first place.

There was a 20 megaton "Cheyenne Killer" warhead supposedly mounted on a variant of the SS-18.

I think we had some 10 megaton bunker busters mounted on Titan IIs.

The Soviet superbomb was about 50 MT. It was not deployed. The biggest US bomb in the arsenal was the 25 Mt Mk-41. So the author of piece missed it by more than an order of magnitude. Once you get past these yields, you just end up blowing out the atmosphere into space faster rather than destroying more stuff to paraphrase Edward Teller.

I also guess they found a new use for those old discredited TAPS codes Sagan et al used to frighten the populace into disarmament.

Ed,

I stand by my point.

The largest bomb ever made, but not deployed, was the Soviet Tsar bomb 57MT that was detonated in 1961. Yes, there were gravity bombs on B-52s that went up to 25 MT and the Soviets had some in this range. However, there were not very many of these. Most of the arsenals were in the 250-500 kT range.

The largest practical yield for a fission device is around 40 kT. Anything over this yield is fusion.

Check out:

http://www.cdi.org/nuclear/database/nukestab.html

http://www.johnstonsarchive.net/nuclear/index.html

Back to the original point of the news release ...

I think it's interesting that as near as I can tell the age of the impact seems to be a bit older than the usual date given for the dieout of the megafauna, 11,700 years bp. Could it be that the impact didn't kill *enough* humans, compared to the critters they were accustomed to hunting? That the number of humans that survived exceeded the diminished carrying capacity of the North American ecosystems?

There's an interesting critique of the idea here:

http://impact.arc.nasa.gov/news_detail.cfm?ID=176

Yes, this theory appears to be rather ridiculous.

Thought they tested fission devices considerably greater than 40 kT - there was some concern that the hydrogen bomb would not work or would be impractical.

I suppose all of the fission cores in the U.S. arsenal are part fusion in that they are tritium "boosted." The idea was to miniaturize the weapons as much as possible and make use of MIRV's -- a lot of precisely-targeted small weapons are more potent than a few crudely-targeted large ones. This tritium boosting is an issue with the "decay" of the U.S. arsenal -- didn't they shut down production of tritium?

On the other hand, Richard Rhodes described the hydrogen bomb as more fission than anything else with fusion providing the neutrons. Not only do you have a fission trigger, there is that plutonium spark plug in the middle of the fusion assembly, and the U238 jacket to convert fast fusion neutrons into fission yield. The U238 jacket is not to make the bomb "dirty" although it does that -- in its absence, you would have something more akin to the "neutron bomb" with diminished explosive power but enhanced radiation effects.

Well, if the impact or detonation was on or over the several thousand feet thick Laurentide ice sheet, would there necessarily be a crater? On the other hand, wouldn't there be clear evidence in Greenland ice cores?

and the U238 jacket to convert fast fusion neutrons into fission yield.

It's my understanding that some (most?) US thermonuclear bombs actually have a 235U jacket in the secondary. The fast neutron fission cross section of 235U is about twice that of 238U, IIRC, so this increases the yield/weight ratio, particularly in smaller warheads.