Dearborn said he was inspired to study the nuclear option a decade or so ago when he heard other researchers tell the media that nuclear weapons wouldn’t work against asteroids. “That’s just not true,” he said in an interview, saying he was offended by their “indefensible” claims.

Until two years ago, Dearborn worked on the problem on his own time. But since 2012 he said he and a colleague have had a small Livermore discretionary grant, amounting to perhaps several hundred thousand dollars, for their work on asteroids. He estimated that about a dozen scientists at the weapons labs have worked on the problem, but none is dedicated to it full time.

Nuclear explosives could work in two scenarios, he said. Where adequate time exists to divert the oncoming boulder, meaning a decade or longer, a “standoff” nuclear blast could knock it off course. When the time to impact is just a few years, he says, it would be too late for deflection but a carefully executed nuke strike would prevent most damage.

“You fragment it with enough force so that the pieces spread out ... [and most] miss the Earth,” he said. Small bits of rock would burn up in the atmosphere, or fall as dust. “Fragmentation may reduce a catastrophe to an inconvenience,” he said in a lecture he gave to high school students at Lawrence Livermore, a video of which was posted on the web in 2010.

Dearborn says there is that no need to develop new weapons and therefore no testing would be required. “The current, existing devices that we have provide all of the energy that is necessary,” he said. But he acknowledged that a large asteroid close to hitting Earth would probably require a weapon with the yield of about a megaton, one million tons of TNT, roughly the largest in the current U.S. arsenal.

Teller’s vision of placing these devices in orbit would not be practical, he said, because they would require periodic maintenance and would have to constantly shuttle between Earth and Space. He said major uncertainties remain, because some asteroids are like solid rocks, others like a handful of pebbles held together by gravity, and still others are porous like pumice. Different materials would react differently to being hit with either a nuclear explosive or an impactor—a crucial issue, because if one broke up in large pieces and remained on a dangerous path, the Earth could still get pummeled.

Melosh and Dearborn disagree, with Melosh asserting that since no large, Earth-killing, near-term threats are on the horizon, “the remaining smaller objects can be dealt with by non-nuclear means, kinetic detection being the most straightforward” and technically advanced.

“I think that the need for deflecting very large objects that might require nuclear detonations is waning and that a reevaluation of realistic needs is very much in order,” Melosh said.