The Asteroid/Comet Impact Hazard: An Extreme Low-Probability High-Consequence Hazard Poster Presentation, 28 th Hazards Research & Applications Workshop, July 2003, Boulder, Colorado USA Clark R. Chapman Southwest Research Inst., 1050 Walnut St., Boulder CO While the chances of a mile-wide asteroid or comet striking the Earth during the 21st century is very small (about 1 in 10,000), they are not zero and the consequences could threaten the existence of civilization as we know it. Impacts by much smaller asteroids occur much more frequently. An impact that is more likely than not to occur during this century could have very serious consequences. In the fear-enhanced, post-September 11th world, responses by security or military agencies to an unpredicted impact, or even to a "near miss," could be far more dangerous than the direct effects of the explosion. For example, it is not certain that all nations threatened by nuclear attack would be able to reliably determine that an asteroid explosion in the skies above was not a nuclear attack requiring retaliation. Analysis of the impact hazard, as the end-member of a spectrum of natural hazards, provides some insight to the psychology, politics, and practicalities of dealing with an unusual, dangerous hazard. Impacts, even relatively modest ones, have some features in common with other natural disasters and/or human-caused accidents or terrorist/military actions: the manifestation of disaster generally has familiar attributes, including earthquake, tsunami, wind, shock wave, fire, falling debris, crumbling structures, etc. On the other hand, normal warning systems (e.g. missile-attack warnings, Pacific tsunami warnings) are not necessarily tuned to recognize the signatures of impacts, and the suite of potential impact damages is complex and without precedence, requiring some modifications in generic disaster response protocols. The unusual nature of this hazard is psychologically difficult for public officials to deal with. Unlike most natural hazards, there are options for mitigating the effects of an impact, or even avoiding the impact altogether. Telescopic search programs have found more than half of all Earth-approaching asteroids larger than 1 km diameter. There are proposals for searches that could find 90% of objects down to only a few hundred meters in size. When an object is found, its orbit can be determined accurately, and any possible Earth impact can be precisely predicted…or, much more likely, ruled out. If an asteroid is found headed for Earth, we will likely have decades to plan for the impact. And there are various space-borne technical options for actually changing the trajectory of the asteroid so that the impact won't happen at all. There are serious policy issues, however, concerning even the possibility of moving an asteroid. For instance, a low-thrust, long-duration "push" over the course of a year (as advocated by the B612 Foundation [ might move the nominal impact point over a country that was previously safe, before eventually changing the trajectory enough to avoid a collision anywhere on Earth. Also, for a sufficiently large asteroid or comet with short warning time, the only technology powerful enough to intervene might involve currently prohibited nuclear weapons in space. The Torino Scale. Trying to explain natural hazards to the general public in an understandable way has previously resulted in the creation of the Richter Scale (now “Magnitude” scale), the Saffir-Simpson hurricane scale, and numerous other such devices. Two currently relevant ones are shown below: (a) the much lampooned “terrorism scale” of the Homeland Security Dept. and (b) the U.S. Forest Service’s signs you can see on roads to the west of Boulder about fire danger. The Torino Scale was developed to create some public awareness of how seriously to take a future news story such as this: “Astronomers say there is a 1-in-200,000 chance that a 500-meter sized asteroid will strike Earth on April 1 st, 2028.” The scale itself is a linear, 0-to-10 scale, with associated colors and words (left). Some critics of the scale have advocated that it be 2- or 3-dimensional, but even educated lay people rarely comprehend 2-D graphs. A 2-dimensional plot (lower left) of the Torino Scale is a technical definition of how the Torino Scale values for a predicted potential impact are calculated from two quantities: the impact energy and the probability of impact. The technical version is not intended for public presentation, but for use by scientists and science communicators. Conclusions Take-Away Message: Asteroid and comet impacts may seem to be exotic, but the threat is real and potentially as serious as nuclear war. On the other hand, the chances of an impact disaster with more fatalities and consequences than large earthquakes, storms, and floods are small. Astronomical predictions are precise, so advance warning to store up food or evacuate ground zero is possible. And technology exists to actually move a threatening asteroid off its collision course. Impacts are an interesting “end-member” natural hazard. Summary: The Asteroid/Comet Impact Hazard How Big, How Often? A useful approach to communicating about the impact hazard is to compare with other hazards that people think more about. Many people fear animals like sharks and snakes, yet the annualized death toll from asteroid impacts is greater than most of those hazards. On the other hand the impact hazard is insignificant compared with disease, famine, war, and even common natural and man-made hazards (like automobile accidents and floods). Impacts range from tiny “shooting stars” (upper left) that you can see on any clear night to (clockwise) “dinosaur killers” that have only one chance in a million of striking Earth during the 21 st century. We are Here! Jupiter’s orbit Asteroid Belt Sun NEOs The processes that formed the planets 4.6 billion years ago left many small remnant objects: comets (beyond the outer planets) and asteroids (in a “belt” between the orbits of Mars and Jupiter). There are collisional, chaotic, and thermal processes that cause some smaller asteroids and their meteoritic fragments to leave the asteroid belt and orbit the Sun among the terrestrial planets. Some of them occasionally cross the Earth’s orbit and can strike our planet...if the Earth happens to be there at the same time. Impact velocities are typically 20 km/sec. By terrorism (mostly due to Sept. 11th attacks) } OVER KASHMIR? OVER ISRAEL? HOW WOULD THE GENERALS RESPOND? Different scales and frequencies of impacts of practical concern: rare, civiliza- tion-threatening impacts by multi-kilometer asteroids (top) to frequent media- scares or hype of inconsequential impacts (bottom). For each scenario, I show asteroid diameter and chance of happening somewhere on Earth this century.