Planetary Defense: Are we currently looking for our keys under the lamp post?

Planetary Defense is a serious and important endeavor and the approach taken to date is a sensible beginning. Finding and cataloging all potentially hazardous asteroids and supporting research into relevant topics required to divert such a threat to our home planet is a necessary, but far from sufficient set of activities required to ensure the survival of our species. Concentrating our efforts on the asteroid threat is relatively easy. Most asteroids move in near-circular orbits, are relatively close to the ecliptic plane and are likely to be detected as hazards many decades in advance of a potential impact. The single most likely problem that will be encountered in deflecting such a threat will be developing the political will to fund the project while there is still ample time for multiple deflection techniques to be applied successfully. While asteroid threats can be mitigated, comets are the invisible danger lurking in the vast, dark parking lot that is the outer solar system. Very few comets falling into the inner solar system will be detected more than two years before their arrival: refinement of a new comet's trajectory requires months of observation before its hazard potential can be realistically assessed and knowledge of the composition, mass and shape of the body cannot be refined sufficiently to design a deflection campaign without much more observational effort. To make matters worse, because of the highly elliptical orbits of most new comets, some of which can be far out of the ecliptic plane while a few can even be in retrograde orbits, the impact velocity of a typical comet will be significantly higher than that of an asteroid. If this increase is only a factor of two, then a typical comet carries four times the impact energy of an asteroid of similar size, though much higher multipliers are possible. The distribution of meteor stream velocities can be examined to place bounds on this threat. Finally, the time required to assemble and launch an asteroid deflection mission starting from scratch and with a high probability of success could exceed three years from mission approval. Based on the recent passage of Comet Siding Spring one must therefore conclude that a successful comet deflection mission must begin well before a hazardous comet is even detected, much less characterized as a significant threat.