DEFLECTION TECHNIQUES: WHAT MAKES SENSE?

A first step toward answering the question "what makes sense?" is to consider the deflection velocity needed to divert an asteroid (or its disrupted fragments) from impacting the Earth. A simple dimensional analysis yields that the deflection delta-v required to miss the Earth is (delta-v)(time) = (radius of the Earth), which translates to ~(20 cm/sec)/(t, years). Over a time long compared to orbit periods, the deflection delta-v is even less, ~(7 cm/sec)/(t, years). Thus with a lead-time of a decade, a delta-v of only ~1 cm/sec is required. A second important relation is that if one were to disrupt a body (say by nuclear explosion) the residual velocity dispersion of the fragments would be at least of the order of the surface escape velocity of the undisrupted body, which is ~(60 cm/sec)*(diameter in km). From these two relations, it is clear that if one were to disrupt a 1-km diameter body, within only a few months the fragments would disperse over an orbital arc large compared to the dimension of the Earth. Thus the oftcited "shotgun effect" as an argument against disruption of an asteroid on an impact course only applies for a final "death plunge" trajectory: if dispersal is achieved even a year or so in advance of the projected impact, most of the mass of the offending body would miss the Earth. For non-nuclear options, the delta-v that may be achievable is of the order of centimeters/sec, or perhaps even less, thus any such method requires at least a decade of application of force, and an even longer lead time to put it in place. Furthermore, precision tracking is required to assess the progress of the deflection, thus an active transponder on the asteroid is required in addition to the deflection "engine". For these reasons, non-nuclear deflection scenarios are of such long leadtime that the development time is short compared to the implementation time, so there is limited value in building any such system in advance of need. Only for the nuclear option is the response time short compared to the development time. However, this option leads to the "deflection dilemma", that the risk of having a deflection system "at the ready" may exceed the risk it is designed to obviate. Furthermore, short lead-time mitigation is problematic: either deflection or disruption may be difficult if the asteroid is a "rubble pile" or some exotic configuration like a contact binary object. Another often overlooked aspect of asteroid deflection is the claim that "NEAs are the easiest targets to get to" is only true if you get to choose the target. If nature chooses it for you, it may be very difficult indeed. A rendezvous with an "average" NEA is about as energetic as a rendezvous mission to Pluto, the only advantage being that it is quicker. Thus missions to rendezvous with an NEA, implant transponders, perhaps implant nuclear explosives or "green" deflection engines, is not a task for spare ICBMs or even the Space Shuttle -it will require Saturn-5 class launchers. I conclude that "paper studies" of deflection methods, or even some limited experiments like the "Deep Impact" mission, may be sensible, but building actual deflection systems in advance of demonstrated need (that is, the discovery of an asteroid on an impact course) is not advisable. Technology that would almost certainly be required for a deflection includes heavy lift launch vehicles and advanced high-performance inspace propulsion systems. Development of such technology, and exploratory missions to better understand NEAs, would have great scientific value even if never needed for asteroid deflection. DEFLECTION VELOCITIES Impact mitigation strategies can be categorized as either "deflection" or "dispersal". In the former, one envisages changing the orbit of the asteroid more or less gently by a minimal amount such that the asteroid misses the Earth. The latter envisages in one way or another blowing up the asteroid, so that the fragments disperse widely enough that most of the mass misses the Earth, at least to the point that the remaining mass intercepting the Earth causes no more than a spectacular "light show." For either method, a first consideration is how much change in velocity is necessary to miss the Earth. In simplest terms, it is just the dimension of the Earth divided by the lead-time at which the action is taken, either deflection or dispersal. If this action is applied on a time short compared to the orbital period of the asteroid then the deflection should be applied in a direction perpendicular to the direction of approach of the asteroid, and the minimum velocity is: