Colleagues: This is a follow-up to my note yesterday about the non-statistical nature of an asteroid search carried out for the purposes of planetary defense. You may be interested in the following "sound bite" statements that I have found can help to make these and related points. (Reactions are welcome, especially if you don't agree with any of these statements.) David Morrison ================================== In the real world, impacts are deterministic: There either is or is not an asteroid that will collide with Earth during this century. Asteroids do not change orbits capriciously except in Hollywood. We already know the statistics and the risks -- what we need now is to search for individual objects, one at a time. The public requirement is to find the next impactor before the Earth collides with it. Many science objectives can be met with statistical samples, but planetary defense requires a complete survey. The public does not care if the annual risk of dying from an impact is one in a million or one in two million -- they want to know if it will happen to them. When you cross a busy intersection, you don't ask what the statistics are on pedestrian accidents -- you look to see if a car or truck is headed toward you. There is no last-minute warning system: Either we discover an asteroid and predict its impact decades or centuries in advance, or else it strikes without warning -- the first you will know is when the sky lights up and the ground starts to shake. The individual risk is dominated by objects large enough to produce a global environmental disaster -- one that places you at risk anywhere on the planet. The most likely impact is a small one, but the most likely impact to endanger you (or any individual) is a big one. A large country (like Russia, China, USA) is more likely to be hit, but its citizens are no more at risk than those of a small country. We have a choice: Deal with the small impacts that hit more frequently but pose very low risk, or deal with the large ones that are infrequent but are more likely to kill you. (The current Spaceguard Survey deals with the large ones that pose the greatest risk) You have more to fear from the residual 10% of big ones that Spaceguard will not have found by 2008 than from the sum of all the smaller asteroids. Small cometary impacts (<1 km) are a negligible hazard for the simple reason that there are few if any such small comets. The probability of being killed by an asteroid is very low, but it is larger than the chances of being killed by many other things (e.g., defective food or drugs, or leakage from nuclear power plants) that governments take very seriously. Impacts are the extreme example of a hazard of very low probability but very great consequences. Impacts are the only known natural hazard that could destroy civilization. Asteroid impacts are the only known natural hazard that can be eliminated entirely by appropriate application of technology. -- +++++++++++++++++++++++++++++++++++++++++++ David Morrison, NASA Ames Research Center 240-1 Tel 650 604 5094; Fax 650 604 4251 david.morrison@arc.nasa.gov or dmorrison@arc.nasa.gov website: http://astrobiology.arc.nasa.gov website: http://nai.arc.nasa.gov website: http://impact.arc.nasa.gov LSST LSST LSST LSST LSST Mailing List Server LSST LSST LSST LSST LSST LSST LSST LSST This is message 80 in the lsst-general archive, URL LSST http://www.astro.princeton.edu/~dss/LSST/lsst-general/msg.80.html LSST http://www.astro.princeton.edu/cgi-bin/LSSTmailinglists.pl/show_subscription?list=lsst-general LSST The index is at http://www.astro.princeton.edu/~dss/LSST/lsst-general/INDEX.html LSST To join/leave the list, send mail to lsst-request@astro.princeton.edu LSST To post a message, mail it to lsst-general@astro.princeton.edu LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST