Tuesday, January 26, 2010
Only nuclear weapons can save us from a large asteroid impact?
"Nuclear explosives constitute a mature technology, with well-characterized outputs. They represent by far the most mass efficient method of energy transport and should be considered as an option for NEO mitigation. Nuclear explosives provide the only option for large NEOs (> 500 meters) when the time to impact is short (years to months), or when other methods have failed and time is running out. The extensive test history of nuclear explosives demonstrates a proven ability to provide a tailored output (the desired mixture of x rays, neutrons, or gamma rays) and dependable yields from about 100 tons to many megatons of TNT-equivalent energy. Coupled with this test history is an abundance of data on the effects of the surface and subsurface blasts, including shock generation and cratering.
Various methods have been proposed for using nuclear explosions to reduce or eliminate an NEO threat; for a given mass of the NEO the warning time is a primary criterion for choosing among them.
With decades of warning, the required change in velocity (ΔV) from the explosion is millimeters to a centimeter per second and can be met for NEOs many kilometers in diameter. This range of values is much less than the 25 to 50 cm/s escape velocity from moderate to large (500 to 1000 meter) bodies, so it is reasonable to assume that such a small ΔV would not lead to the target’s fragmentation or to excessive ejecta (i.e., debris thrown off the object). This expectation is met in hydrodynamic simulations presented here that show that nuclear explosions can provide ΔV from 0.7 to 2.4 cm/s, for payload masses less than a ton (including the nuclear device’s fuse and environmental cocoon). In models of NEOs with surface densities as in terrestrial environments, nearly 98 percent of a body remains bound as a single object through only its own weak gravity. The small amount of ejecta expands over the decades to form a large cloud of low-density debris, reducing its posed threat by another factor of 104 to 105. The amount of the ejecta depends on the surface porosity. As in the case of kinetic impacts, a dissipative, low-density surface will reduce the amount of ejecta, thus reducing the ΔV."
You can read the entire NRC report at:
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