devices were tested in the Plowshares program (an activity that explored the feasibility of using nuclear explosives for industrial applications), and the July 1962 Sedan nuclear test was more than 70 percent fusion (see DOE, 2000). If sufficient warning time is available, the largely fusion device can be chosen from tested designs and built with modern safety and security features.

To understand the action of a standoff nuclear explosion, and its ΔV capability, a member of the Mitigation Panel (David S.P. Dearborn, Lawrence Livermore National Laboratory) simulated the effect of a nuclear standoff detonation on homogeneous 1-kilometer-diameter NEOs with densities between 1.91 and 1.31 g/cm3. In these numerical models of a standoff burst about 150 meters above the NEO’s surface, about 40 seconds after the burst the NEO’s speed change ranged from 2.2 to 2.4 cm/s. Approximately 97.5 percent of each NEO remained intact (the NEO was held together by gravity only—it had no tensile strength), while about 2.5 percent of its mass was ejected at greater than escape speed by the rebound to the shock wave that passed through the body in reaction to the ejection of heated material. Higher porosity of the NEO will dissipate more energy, resulting in less ejecta and less speed change. The minimum speed change for a highly porous NEO is controlled by the amount of totally vaporized material. In these models this minimum ΔV is about 0.8 cm/s for an explosion with a strong neutron output. This work is preliminary, and the results provide only the scale of what can be done. NEOs come in many more sizes, shapes, and structures than what the committee could include in this simulation.

A standoff burst is usually considered the preferred approach among the nuclear options. One clear advantage is that there is no need to maneuver for a low approach speed as might be required for a surface or shallow sub-surface delivery. Neutron output associated with high fusion-to-fission ratios has many advantages including deeper neutron penetration (more impulse), high coupling efficiency, and an insensitivity to NEO composition.

Decades to Go—Small Surface Burst

Ahrens and Harris (1992, 1994) suggested using a surface or near-surface nuclear explosion. NASA’s 2006 study proposed the detonation of one or more subkiloton nuclear explosives on an NEO’s surface (NASA PA&E, 2006). In this approach the yield of the explosive must be stable and well determined. At 100 kilotons, the effect of 0.5-kiloton yield uncertainty is negligible, but not when the entire yield is 0.5 kiloton. The test base provides assurance of an effective yield with negligible uncertainty between 100 tons and 1 kiloton, but not for smaller yields. The committee notes that a rendezvous mission to implant explosives may be far more difficult than delivering a larger explosive package just above the surface.

As seen in Figure 5.2, yields between 100 and 500 tons provide significant speed increments to the body of an NEO with only modest amounts of ejecta (large amounts would be undesirable). Most of the ejected material has speeds in excess of 10 m/s, and should spread over many Earth-radii in only a year or two. The debris predicted from these models was not propagated along the sample orbits, but it is likely that the fraction of the ejecta that remains on a threatening orbit years later is no more than 10−4. As with the standoff simulations, future modeling of a more dissipative surface with very high porosity is likely to result in lower ΔV and less ejecta.

Delivering a nuclear explosive to the depth used in the simulation would be achievable with present earth-penetration technology, but it would require an approach speed equivalent to that of a rendezvous mission. Flyby speeds could be used with a fuse that fires on contact with the target and with a slightly higher-yield explosive than for rendezvous. The necessary calculations for this approach are straightforward, but current fuses would have to be upgraded to operate at the higher speeds.

Conclusions

Nuclear explosives can provide considerable protection against a potential NEO impact. This may be the only current means to prevent an impact by a large hazardous object (>500 meters in diameter) with a warning time under a decade or by a larger object (>1 kilometer in diameter) object with a warning time of several decades. With decades of warning for such large objects, the preferred approach uses a standoff detonation. Neutron output has certain advantages (Dearborn, 2004), as the energy coupling is relatively insensitive to the surface composition and density of the NEO. The simulations show that speed changes (ΔV) on the order of 2 cm/s are achievable with



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