Recommendation: Immediate action is required to ensure the continued operation of the Arecibo Observatory at a level sufficient to maintain and staff the radar facility. Additionally, NASA and the National Science Foundation should support a vigorous program of radar observations of NEOs at Arecibo, and NASA should support such a program at Goldstone for orbit determination and the characterization of physical properties.

For both Arecibo and Goldstone, continued funding is far from assured, not only for the radar systems but for the entire facilities. The incremental annual funding required to maintain and operate the radar systems, even at their present relatively low levels of operation, is about $2 million at each facility (see Chapter 4). The annual funding for Arecibo is approximately $12 million. Goldstone is one of the three deep-space communications facilities of the Deep Space Network, and its overall funding includes additional equipment for space communications.


“Mitigation” refers to all means of defending Earth and its inhabitants from the effects of an impending impact by an NEO. Four main types of defense are discussed in this report. The choice of which one(s) to use depends primarily on the warning time available and on the mass and speed of the impactor. The types of mitigation are these:

  1. Civil defense. This option may be the only one feasible for warning times shorter than perhaps a year or two, and depending on the state of readiness for applying an active defense, civil defense may be the only choice for even longer times.

  2. “Slow-push” or “slow-pull” methods. For these options the orbit of the target object would be changed so that it avoided collision with Earth. The most effective way to change the orbit, given a constraint on the energy that would be available, is to change the velocity of the object, either in or opposite to the direction in which it is moving (direct deflection—that is, moving the object sideways—is much less efficient). These options take considerable time, on the order of decades, to be effective, and even then they would be useful only for objects whose diameters are no larger than 100 meters or so.

  3. Kinetic impactors. In these mitigation scenarios, the target’s orbit would be changed by the sending of one or more spacecraft with very massive payload(s) to impact directly on the target at high speed in its direction, or opposite to its direction, of motion. The effectiveness of this option depends not only on the mass of the target but also on any net enhancement resulting from material being thrown out of the target, in the direction opposite to that of the payload, upon impact.

  4. Nuclear explosions. For nontechnical reasons, this would likely be a last resort, but it is also the most powerful technique and could take several different forms, as discussed in the report. The nuclear option would be usable for objects up to a few kilometers in diameter.

For larger NEOs (more than a few kilometers in diameter), which would be on the scale that would inflict serious global damage and, perhaps, mass extinctions, there is at present no feasible defense. Luckily such events are exceedingly rare, the last known being about 65 million years ago.

Of the foregoing options, only kinetic impact has been demonstrated (by way of the very successful Deep Impact spacecraft that collided with comet Tempel-1 in July 2006). The other options have not advanced past the conceptual stage. Even Deep Impact, a 10-kilometer-per-second impact on a 6-kilometer-diameter body, was on a scale far lower than would be required for Earth defense for an NEO on the order of 100 meters in diameter, and it impacted on a relatively large—and therefore easier to hit—object.

Although the committee was charged in its statement of task with determining the “optimal approach to developing a deflection capability,” it concluded that work in this area is relatively new and immature. The committee therefore concluded that the “optimal approach” starts with a research program.

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