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In June, 1996 ZetaTalk stated that Deflecting Asteroids with nukes would be ineffectual.

Does mankind now have the means to deflect such large, rapidly moving objects? Such a deflection would require a precisely placed explosive device of sufficient strength to vaporize the asteroid. Disintegration would be required because deflection is not possible in space. A nuclear explosion set off on the surface of an object in space would have minimal effect, as the explosion can expand in all directions at once. Nuclear explosions on the Earth's surface are an irresistible force meeting an immovable object - the Earth! Where the explosion can expand up or outward into the atmosphere there is only air turbulence. The portion of the explosion that is on the land side, or in the case of an underground explosion is encapsulated, is between a rock and a hot expanding place. The explosion is forced into the rock strata by the continuously expanding center of the explosion. Solid rock vaporized by nuclear explosions on the Earth's surface does not equate to the damage that would be done by one or even many nuclear devices on the surface of an asteroid. All parts of the explosion move rapidly out into space, and thus the asteroid is safely away before the nuke really gets going. A firecracker. A gnat. A sneeze. And the asteroid proceeds on its way, having only momentarily stepped aside to avoid mankind's silly experiment.
ZetaTalk: Deflecting Asteroids, written on Jul 15, 1996.

In June, 1998 a computer model on nuking asteroids showed this to be the case.

CCNet DIGEST 4 June 1998
Nukes May Not be Enough to Save earth from Asteroids
From Press Agency News
By John von Radowitz, Science Correspondent, PA NEWS

It would be far from easy to save the Earth from an incoming asteroid if the crisis depicted in the action film Deep Impact ever became reality, a study showed today. It had previously been thought that nuclear blasts could be used to deflect or break up an asteroid on collision course with Earth. But scientists have found that some types of asteroid could soak up a powerful nuclear explosion with little or no effect. Astronomer Erik Asphaug, a researcher associate at the University of California, Santa Cruz, used computer simulations to study the effects of powerful impacts on asteroids with different internal structures. And he and his colleagues found that the outcome of blasting an asteroid depends entirely on its structure.

Many asteroids are not single rocks, but are aggregates of debris left over from previous collisions. They might consist of a few large fragments held together by gravity or "rubble piles" consisting of numerous smaller pieces. The new study, published today in the journal Nature, shows that the porous nature of such asteroids would dampen down shock waves from a nuclear explosion, limiting its effects to a localised area. "It's lot more difficult to nudge these asteroids around than we had thought," said Asphaug. "More work needs to be done before we can decide whether nuclear warheads provide a viable deterrent."

The researchers created a computer model of an asteroid one mile across based on radar images ofa near-Earth asteroid called Castalina. They gave the peanut-shaped body three different internal structures: solid rock, a pair of solid rocks in close contact, and a rubble pile with pore space taking up 50% of its volume. For each of these, an impact by a house-sized rock travelling at five kilometres per second was simulated. This is equivalent to the energy produced by the atom bomb dropped on Hiroshima. In the rubble pile model, the impact shock wave died out quickly leaving a large crater in one spot, without disturbances to the rest of the asteroid. At the opposite extreme, a solid rock asteroid may shatter into many smaller pieces when blasted. The pieces form a family of smaller asteroids, or come together to form a rubble pile.

To predict the effect of a nuclear explosion on any particular asteroid, scientists would need to understand the object's internal structure, said Asphaug. There are hundreds of thousands of asteroids in near-Earth space which if they struck the planet would have as big an impact as the largest thermonuclear device ever exploded. Although the likelyhood of one of these asteroids hitting the Earth was small, the consequences would be disasterous. Asphaug said: "Asteroids are not an imminent threat, and I am far more concerened about what human beings are doing to the planet. But in case we ever identified an asteroid or comet on a collision course, it would be best to know our enemy so that we can get it before it gets us."