It's relatively hard to look for this information in Wikipedia; the page on Effects of nuclear weapons covers it best.

@WetSavasnnaAnimal aka Rod Vance, it might be worth adding an explanation of how a nuclear explosion in space would be different, since there would be no surrounding matter.

@BillN Exactly so. In theory, if one could devise a chemical explosive - or even a fuel - with many orders of magnitude greater energy density than those that actually exist, the result of igniting (or burning) such a material would be much the same as a nuclear blast, in terms of the heat, shockwave and mushroom cloud.

@S.McGrew Well, there would still be the surrounding matter of the bomb itself - don't forget that airburst nukes are even more effective than "groundburst", despite the fact that air is much more sparse (and just a tiny mass compared to the bomb). Space-based nuclear explosions will be much different, yes, but not in the "produce large amounts of high kinetic energy plasma" department. The main difference is in what happens after that phase.

@Luann, Right: if an atomic bomb were used to deflect an asteroid, its main effect would be to heat and vaporize a portion of the surface of the asteroid. That material would expand (explode) and the reaction would push the asteroid away from the locus of the bomb explosion. An extremely high-powered laser could be used to accomplish the same thing.

You could add that most nuclear weapons are designed to be triggered in the air, a thousand feet or more above the target. That is done so that most of the "surrounding matter" that is heated by the nuclear excursion will be air. The result is a much larger blast and fireball than you would get if the weapon were triggered at ground level or, below ground.

My favorite example of how quickly that radiation heats things up is the Rope trick effect, where a captive test nuke heats up the ropes to the point of incandescence.

@Luann I'm now wondering whether it would be worthwhile to add cladding of some sort to a nuke designed specifically for space warfare, just to capture more of the initial radiation burst and convert it to explosive force ... probably this is the sort of design detail that's not available in unclassified sources, alas.

@Bill N: Chemical explosions, or at least some of them - the ones used as propellants, like gunpowder - involve a second factor, the conversion of the solid/granular propellant to a larger volume of gas.

Pointedly you get a shockwave out of high-enough velocity impacts, too. Once there is enough energy released in the presence of matter in a small enough space and time, the sequence of events leading up to a shockwave is essentially inevitable. This is observed, for instance, in meteors and meteorites, and can also be produced on a smaller scale in the lab.

@jamesqf : Yes. This effect is also present in the nuclear case as well - the bomb materials heat up and form a gas which also expands violently, albeit at far higher temperatures (10-100 MK instead of the 1 kK or so of a chemical burst). The difference is that energy transport , at least initially, from the bomb material to the atmosphere, is driven primarily in the nuclear case by radiative transport, while in the chemical case by direct contact. That doesn't mean the direct pressure has no effect at all, though. The reason for this is the rapid increase by the fourth-power law, compared

to the power developed by direct pushing. At a temperature of 1 MK, the radiation is one trillion times more intense than at 1 kK, and at 100 MK it is ten quadrillion times more intense. Naturally this radiation level is sustainable only for the briefest of times given the energy supply, and thus results in a near-instant deposition of most of the bomb energy into the surrounding atmosphere by radiation out to the optical depth at those wavelengths (mostly soft X-ray).

For that matter, technically speaking radiative transport also occurs in a chemical explosion too because the chemical fireball does emit and some of that is at wavelengths at which the atmosphere is absorptive, it's just that it is negligible in terms of transferring bomb energy to the atmosphere owing to the much lower temperature. So both mechanisms exist in both types of explosion, but which one is predominant is different. Also, the peak at 1-2 kK is in the SWIR/MWIR bands and actually atmosphere is pretty transparent there; so the amount deposited in absorptive bands is much less.

For asteroid impact, temperatures are more intermediate - about 100 kK at max for a fast strike (to get 1 MK or more on collision would I believe require impact speeds exceeding the escape speed from the Solar System, though these calculations are not simple and I've only played with some very simplistic models.). Radiative transport will thus be about 100 million times larger than for a chemical explosion, but up to one trillion times less than for a nuclear explosion.

(Correction: at 100 MK the radiation is actually 10^20 (100 quintillion!) times more intense than 1 kK.)

@dmckee ...which I get the impression was one of the main clinchers for the Chicxulub site being accepted as the source of Alvarez's proposed bolide explanation of the KT boundary; it was the presence of shocked quartz in the area, which had only ever before been seen at nuclear explosion sites.

@The_Sympathizer: I think we're talking about two different things here. You're talking about expansion of gasses (or solid materials evaporating) due to intenst heat. I mean the actual production of gas by chemical decomposition, for instance in an airbag inflator, sodium azide decomposes & produces a large volume of nitrogen gas (2NaN3 -> 2Na + 3N2). The heat generated is (mostly) an unwanted byproduct. At the extreme, you have things like air rifles that work just by expansion of compressed gas.