if short term probability is low, the magnitude of a larger asteroid impact is infinite because it risks total extinction
IRWIN I. SHAPIRO et al in 10,( Harvard-Smithsonian Center for Astrophysics, Chair FAITH VILAS, MMT Observatory at Mt. Hopkins, Arizona, Vice Chair MICHAEL A’HEARN, University of Maryland, College Park, Vice Chair ANDREW F. CHENG, Johns Hopkins University Applied Physics Laboratory FRANK CULBERTSON, JR., Orbital Sciences Corporation DAVID C. JEWITT, University of California, Los Angeles STEPHEN MACKWELL, Lunar and Planetary Institute H. JAY MELOSH, Purdue University JOSEPH H. ROTHENBERG, Universal Space Network, Committee to Review Near-Earth Object Surveys and Hazard Mitigation StrategiesSpace Studies BoardAeronautics and Space Engineering BoardDivision on Engineering and Physical Sciences, THE NATIONAL ACADEMIES PRESS, http://www.fas.harvard.edu/~planets/sstewart/reprints/other/4_NEOReportDefending%20Planet%20Earth%20Prepub%202010.pdf)
Our planet inhabits a hazardous environment. Earth is continually bombarded by cosmic objects. Luckily for us, most are very small and cause no harm to life. Some, however, are large and cause considerable harm. Evidence of these collisions, large and small, is abundant, from the dense defacement of Mercury and the Moon to the craters festooning the surfaces of even small asteroids. Although impacts of cosmic objects on Earth have occurred since its very formation, humanity has been at best dimly aware of these events until very recently. Only two centuries ago it was widely doubted that objects orbiting the Sun could or would collide with Earth. In general, we cannot predict precise times and locations of future impacts, but can make statistical statements about the probability of an impact. Objects larger than about 30 meters in diameter probably strike Earth only about once every few centuries, and objects greater than about 300 meters in diameter only once per hundred millennia. Even objects only 30 meters in diameter can cause immense damage. The cosmic intruder that exploded over Siberia in 1908 may have only been a few tens of meters in size; yet this explosion severely damaged a forest of more than 2,000 square kilometers. Had an airburst of such magnitude occurred over New York City, hundreds of thousands of deaths might have resulted. Assessing risk is difficult primarily due to lack of sufficient data. Our best current estimates are given in Chapter 2, where the risk is presented with its dependence on impactor size and associated average impact frequency, along with damage estimates in terms of lives and property. Figure 1.1 illustrates the estimated frequency of NEO1 impacts on Earth for a range of NEO sizes. For impactor diameters exceeding about 2 to 3 kilometers, world-wide damage is possible, thus affecting all of humanity and our entire living space (the minimum size at which impactors can cause global devastation is still uncertain). While exceedingly rare, the consequences of such a collision are enormous, almost incalculable. This presents the classic “zero times infinity” problem: nearly zero probability of occurrence, but nearly infinite devastation per occurrence.
Impact Outweighs - Probability > Timeframe
Asteroid impacts outweigh any more frequent threats. Their timeframe arguments are logically flawed.
Huebner et al. 09 [W.F. Huebner, L.N. Johnson, D.C. Boice, P. Bradley, S. Chocron, A. Ghosh, P.T. Giguere, R. Goldstein, J. A. Guzik, J. J. Keady, J. Mukherjee, W. Patrick, C. Plesko, J.D. Walker, K. Wohletz, 2009, published in Astronomicheskii Vestnik, 2009, Vol. 43, No. 4, pp. 348–356. PN]
Others have argued that there are many natural disasters such as hurricanes, typhoons, tornados, earth- quakes, tsunamis, and volcanic eruptions that occur much more frequently than impacts of cosmic objects with the Earth. Thus it would behoove us to concentrate our attention on avoiding catastrophes from such events. Three facts undermine this argument. Firstand perhaps most important, it considers only the probability aspect of risk assessment. Risk assessment requires that likelihood and consequence be convolved. Because the potential consequence of PHOs is so great, consideration of probability alone is not appropriate (Chap- man and Mulligan, 2002). Second, prevention of catastrophic loss from other natural events is based almost entirely on early warning systems. Similarly, such warning systems for PHO detection are in place and are being further improved to identify smaller objects, down to about 140 m in size. Third, there are no known or likely forceful countermeasures for other natural hazards. In sharp contrast it is within our reach to develop countermeasures against the collision of a cosmic object with Earth. All the elements of the technology to defend our planet are in hand; we need only tailor and deploy the technologies to meet the need. !n the discussion that follows, we illustrate a comprehensive plan for countermeasures against collisions of asteroids and comet nuclei with Earth.
Resist the temptation to prefer immediate impacts over seemingly longer term ones, prefer probability
Chapman No Date(Clark R. , planetary scientist, first editor of Journal of Geophysical Research- Planets, PhD MIT, The asteroid impact hazard and interdisciplinary issues, http://www.boulder.swri.edu/clark/icsupb05.doc., Google Scholar, NC)
In my own discussionsof the impact hazard in public forums during the past two decades, I have learned several things (not all surprising) about perceptions of this issue, both by lay people and scientists: * There is a common tendency for people to think of long "waiting times" before the next impact rather than in terms of "chances" of a disaster in the near-term. For the same reason people will build in a hundred-year floodplain, thinking (especially in the aftermath of an actual flood) that a flood won't happen for a hundred years, many people believe that an urgent responseto the NEA threat isn't required: we can let the next generation deal with it. Yet many people buy lottery tickets (or avoid very low-probability hazards) with odds of winning (or dying) that are much lower than the chances of a large NEA impact happening this decade. * People have enormous difficulty judging consequences of different degrees. It is very difficult for me to communicate the differences between a civilization-killing impact and a mass-extinction event (although it would take a thousand of the former impacts to equal one of the latter). Should/will people consider 100 deaths per year (roughly the statistically averaged threat from NEAs) to be serious or not? We live in a society that can become very concerned about the life of a single individual highlighted by the news, yet remain oblivious to the plight of millions in a different context. At the peak of the Rwanda genocide killings, newspaper headlines were instead dominated for a week by the impact hazard (when Comet Shoemaker-Levy 9 fragments were crashing into Jupiter). American society felt that "the world had changed" when ~3000 people died on 11 Sept. 2001, yet the ~3000 American traffic fatalities in Sept. 2001 went unnoticed. Since a large NEA impact has never been witnessed, it is difficult to predict how seriously even properly informed people might react to such a predicted impact. * People are inclined to visualize the problem as involving an NEA that is on its way in and the way to deal with it is to "blow it up" shortly before it hits. The picture of an NEA orbiting the Sun countless times (and for decades, centuries, or longer) before it hits -- all the while remaining in our cosmic neighborhood, where it is accessible by spacecraft -- is difficult to get across.