bound together by gravity. It was found to have a bulk density of about 2 g/cm3 (Abe et al., 2006), that is, a porosity near 40 percent. Some asteroids, such as Eros, have densities near that of solids but are probably heavily fractured (Britt et al., 2002). However, “2001 0E84” is a large (approximately 1-kilometer-diameter) body rotating so rapidly that it must be very strong and is therefore not very porous; “(6187) 1986 DA” is essentially a solid iron NEO.1 All other known fast-spinning bodies are small (<200 meters in diameter). There are also low-density objects, like Asteroid Mathilde, on which observed craters suggest a very porous surface with larger efficient shock dissipation. The bulk density of cometary nuclei is likely less than 1 g/cm3.
NEOs have a wide range of shapes, sizes, and densities. The bulk density of those asteroids for which it is known is comparable with that of materials used in nuclear-effects simulations (e.g., gravel ≈ 1.5 g/cm3 and gravel with sand ≈ 1.9 g/cm3). The sophisticated computer simulations discussed here were used to model one of many possible structures, a 1-kilometer-diameter structure with a high-density core of 2.63 g/cm3 surrounded by a 250-meter-thick surface layer of 1.91 g/cm3.
Experimental results indicate that high porosity can significantly reduce the shock strength and rebound of shocked material (Holsapple, 2004). The impulse from a given energy coupled into a porous surface is lower than it would be for a nonporous solid, and the ejecta is reduced. A complete and adequate crushing model is necessary to determine the shock effects on a porous body. High-porosity dissipative surfaces lead to quantitatively similar uncertainties for both nuclear explosives and kinetic impactors, and an impactor mission to study asteroid structure would provide useful data for both approaches.
The limited set of conditions studied in the simulation described below begin to examine uncertainties in important physical properties in order to lead to an understanding of the application of nuclear explosions to NEO orbit change. They are not exhaustive, and there is much more to learn about the effects of shape, spin, and structure. Except for NEOs 10 kilometers in diameter or larger, it is generally likely that nuclear explosives can provide a more-than-large-enough ΔV, with little material loss and with essentially no danger of fragmentation.
In the nuclear standoff scenario, the short burst of energy from a nuclear explosive is used to strongly heat a thin layer of an NEO’s surface. As this layer accelerates away from the NEO, the NEO’s main body recoils in the opposite direction and, if this “back reaction” of the NEO is large enough, the NEO’s path is altered to avoid collision with Earth. A nuclear explosion in space radiates most of its energy as x rays and gamma rays or as fast-moving neutrons. The proportion of x rays to neutrons is a function of the nuclear reactions that predominate in the explosion. For a given yield, fusion reactions produce more neutrons than do fission explosives. Neutrons offer an advantage for the standoff scenario because they penetrate about 1,000 times deeper into the NEO’s surface than do x rays and thus can heat a larger volume of material, giving a stronger impulse because more mass is ejected above escape speed. Neutron penetration is also nearly independent of the NEO’s composition for atoms between carbon and iron in the periodic table. Large amounts of hydrogen in the surface (such as in comets or asteroids with hydrated minerals) more strongly limit neutron penetration.
The area of the NEO’s surface that is heated by a standoff nuclear explosion depends on the distance between the asteroid and the point of detonation; the depth of penetration depends on the distance between the surface and the detonation point. Thus, detonation close to the surface heats only a small area close to the explosion, whereas more distant explosions spread their energy over a larger area of the asteroid. The neutrons penetrate most deeply vertically underneath the explosion and, because of the increased distance, penetrate less deeply at other places.
A detailed simulation of energetic neutrons incident on granite (Bedrossian, 2004) found that more than 70 percent of the incident energy was deposited in the granite (efficient deposition). More than 30 percent of the incident energy was deposited into a depth of about 15 centimeters. The energy required to convert rock into hot (more than 10,000 kelvin) plasma is high: 10 kilotons of TNT converts about 4,000 tons of surface material into plasma expanding at more than 2 km/s (Dearborn, 2004). The high efficiency of the deposition and relatively deep penetration of neutrons reduce the necessary neutron yield to near 100 kilotons of TNT-equivalent. High-fusion