[X] My Assistant

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A small lake in Siberia may be a secondary impact feature from the famous 1908 Tunguska event:

http://www.blackwell-synergy.com/doi/full/...2.x?cookieSet=1

[dvandorn]

The answer is sort of "all of the above." The factors that contribute to a bolide explosion include:

Angle of Attack: A shallow entry allows for a lot more heating time. A more direct trajectory straight towards the ground creates a higher heating pulse, but for a far shorter amount of time. On a steep-angle trajectory, the impactor reaches the ground very quickly after encountering the upper atmosphere and is usually still "frozen" in the middle when it strikes. But shallow-angle impactors have time to heat through far more effectively, even though the peak heat pulse is less.

Composition: It's likely that only bodies containing volatiles will actually explode dramatically in the air prior to impact. A large stony body will simply ablate as you suggest. No matter how long it is heated, the worst that will happen to such a stony body is that it will come apart at maximum heat load and/or aerodynamic stress. Such a break-up can look a little like an explosion, but the energy is all kinetic. If the impactor is a cometary fragment, however, with frozen volatiles within, those volatiles can heat up as the bolide travels through the atmosphere. Let's say a lot of the impactor is made up of methane ices and clathrates -- and in the lower atmosphere, the body begins to break up as hundreds of tons of now-flammable methane and other hydrocarbon products are released into a white-hot plasma trail surrounded by fire-feeding oxygen. It goes kablooey... As evidence, we have the Tagish Lake meteor fragments, which are some of the few examples of what (we think) is a cometary fragment that have been recovered (the pieces were actually fine-grained clays, not stone or metals like most meteorites). Not coincidentally, the Tagish Lake body exploded in mid-air.

Speed: This isn't as important of a factor, in that a shallow entry angle will usually slow a bolide to relatively slow speeds by the time it heats enough to explode. But a higher-energy entry will actually be less likely to cause an explosion in that it can reduce the heating time, as compared to the heating time endured by a slower impactor traveling along the same trajectory. A lot of this depends on whether the body's vacuum perigee is a positive or negative number at the time it hits the upper atmosphere.

Now, I'm not a meteorite expert, but I play one on the Internet... However, I think I have the basics correct, here.

-the other Doug