a ½-kilometer-diameter NEO with the gravity tractor; new, heavy-lift launchers such as the Ares cargo launcher might allow delivering 5 times more massive impactors. Multiple impactors provide robustness against random failures and the opportunity to fine-tune the results by varying the number of impacts. Even a single impactor that could be launched within 6 months of discovery might change the orbit of a 100-meter-diameter NEO, the size that is near the upper limit for the use only of civil defense mitigation, with a warning time of only 1 to 2 years.


Finding: Kinetic impactors are adequate to prevent impacts on Earth by moderate-sized NEOs (many hundreds of meters to 1 kilometer in diameter) with decades of advance warning. The concept has been demonstrated in space, but the result is sensitive to the properties of the NEO and requires further study.

NUCLEAR METHODS

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 in diameter) 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 (see Appendix E). Coupled with this test history is an abundance of data on the effects of 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 several 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 1,000-meter-diameter) 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 1 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.

Alternatively, when the time to projected impact is short (i.e., years rather than decades), it may be impossible to apply a sufficient ΔV without fragmentation, but the limiting factor is assembly and launch. A nuclear package with a new fuse (i.e., a fuse that is not designed for terrestrial use) and a new container requires a cylinder about 1 meter in length and 35 centimeters in diameter, with a mass under 220 kilograms. The longest lead-time item for incorporating such a device in a rocket system is the development of a container to deliver the device and a fusing system capable of operating with the timing constraints required by the spacecraft velocities near impact with the NEO. Specifications for a nuclear bus could be the same as those for a kinetic-impactor mission, but it would be very challenging to construct and integrate with the booster rocket and the nuclear package in under a year. This “latency time” between the decision to act and the launch can be reduced dramatically (perhaps as much as 100-fold) by designing and testing these critical components in advance of discovering a hazardous NEO.

Models and Uncertainties

Nuclear outputs are well determined from tests. Just as with kinetic impactors, the greatest uncertainty in their use lies in the NEO response—the uncertainty relating particularly to current understanding of shock propagation through low-density material and of the large variety of NEO structures and behavior upon impact that could be encountered. Consider as examples: Asteroid Itokawa, like many asteroids, appears to consist of rubble weakly



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