Clark R. Chapman Southwest Research Inst. Boulder, Colorado, USA

1. Predicting and Comprehending Asteroid Impacts Fertile Ground for Applications of Statistics Clark R. Chapman Southwest Research Inst. Boulder, Colorado, USA Invited Session: Prediction of Catastrophic Events “Interface 2003”: 35th Symposium on the Interface: Computing Science & Statistics Salt Lake City, Utah 15 March 2003

2. Comets and Asteroids 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). Some of them occasionally cross the Earth’s orbit and can strike our planet...if it happens to be there at the same time. Comets Jupiter’s orbit We are Here! Asteroid Belt Sun NEOs Comets come from far beyond Jupiter Two asteroids colliding

3. The Hazard from Asteroids and Comets: Overview The Little Prince • The Earth encounters interplanetary projectiles, ranging: (a) tiny, harmless ones; (b) gigantic, destructive ones… (and everything in between). • The most dangerous ones are very rare but very destructive. Smaller impacts, with greater chances of happening soon, also merit practical concern by relevant public officials. • This extreme example of a natural disaster (tiny chances of happening, but with huge consequences) challenges a rational response by citizens and policy-makers. Meteorite punctured roof in Canon City, CO Global catastrophe

4. Sizes, Impact Frequencies of NEOs Leonid meteor shower Smallest, most frequent Second Week Boulder Dust Peekskill meteorite Huge, extremely rare Building 15 km 100 Myr K-T mass extinctor, 65 Myr ago Tunguska, 1908 Millenniunm Mountain 500,000 yr SL9 hits Jupiter 1994

5. What Do We Know About the Impact Hazard? I’ll be coming back to this! • How many asteroids and comets there are of various sizes in Earth-approaching orbits (hence, impact frequencies are known). • How much energy is delivered by an impact (e.g. the TNT equivalence, size of resulting crater). • How much dust is raised into the stratosphere and other environmental consequences. • Biosphere response (agriculture, forests, human beings, ocean life) to environmental shock. • Response of human psychology, sociology, political systems, and economies to such a catastrophe. WE KNOW THIS… Very Poorly Somewhat Very Well Very Well

6. Potential Impacts of Practical Concern

7. Visualize the Widely Different Impact Scenarios • Global, civilization-threatening horror (>2 km diam., 1 chance in 10,000 21st C.) • Regional catastrophe (e.g. tsunami destroys everything within few km of Pacific Rim)(300m-1.5km, 0.2% chance 21st C.) • Devastating local disaster (30m - 300m, 40% chance 21st C.) • Blinding Hiroshima-scale flash in sky (happens every few years) • Media hype, false alarm (happens every few months) OVER KASHMIR? OVER ISRAEL? HOW WOULD THE GENERALS RESPOND?

8. Risk vs. Scale of Impact • Annual fatalities peak for events near the global “threshold size”, about 2 km • Orange/yellow zone illustrates our range of uncertainties for agricultural disaster due to stratospheric dust • Tsunamic risk very uncertain Stratospheric Dust Tsunami

9. What Can We Do about This?What Are We Doing about It? • We can use telescopes to search for asteroids and comets that might be on a collision course with Earth during this century (Spaceguard Survey) • If one is found (among all those that we can certify as not a threat), then we could mitigate (evacuate, amass food supplies, move the asteroid so it won’t hit, etc.)

10. How Does a Scientifically Illiterate Public React to Asteroids? Here’s a case of very low statistical odds, but the potential catastrophe could destroy civilization...

11. Asteroid Impacts are “Chancy” Public Education about Probabilities We’re all familiar with people who think they will win the lottery, who build homes on the 100-year floodplain because last year’s flood was the 100-year flood according to the Army Corps of Engineers…and so on. This is not encourtaging about the possibility for rational thinking about asteroids!

12. A Royal Flush (2003 UPDATE:This statistic has changed in the last few years as we have discovered most of the mile-wide asteroids and learned that those won’t strike Earth this century: now there’s a slightly better chance of getting a Royal Flush than death-by-asteroid next year!) Odds:1 to 649,739 • It is more likely that a mile-wide asteroid will strike Earth next year than that the next poker hand you are dealt will be a royal flush.

13. Chances from Dying from Selected Causes (for U.S.A.) By terrorism (mostly due to Sept. 11th attacks)

14. 20th Century Catastrophes: We have much more to worry about than impacts! Source: John Pike (Asteroids similar) • Averaged over long durations, the death rate expected from impacts is similar to that from volcanoes. This is what we have to worry about this month

15. Fatality Rates Compared with Accidents and Natural Hazards

16. The “Scary” Case of 1997 XF11 10 5 (100000 km) 0 -5 -10 -15 -15 -10 -5 0 5 10 15 (100000 km) This Week is the Fifth Anniversary of the Scare • In March 1998, head-lines warned of pos-sible impact in 2028. • The next day, old data ruled it out…but the prediction was badly mistaken.

17. 1997 XF11 Error “Ellipse” Original “back-of-the-envelope” calculation reported in a Press Statement by the Minor Planet Center at Harvard Part of very elongated error ellipse Actual Monte Carlo calculations of very elongated error ellipse, done by Muinonen from same data available to Minor Planet Center (it took several days of computer time) Earth

18. 1997 XF11 Error Ellipse: Details

19. Returns in 2040 from 2027 Encounter with 1999 AN10 Many asteroid orbits are in physical resonances (they do not intersect Earth’s orbit randomly like particles-in-a-box). And they are moving chaotically. Within an error ellipse for a particular near-miss, there are many, tiny “keyholes” through which the asteroid could pass that would bring it back to Earth impact some years or decades hence -- so it is difficult to rule out future impacts.

20. 1998 OX4 Virtual Asteroids for January 2046 (Milaniet al., 2003) Earth Dangerous V.A. The Problem: In 1999, asteroid 1998 OX4 passed close to Earth, but telescopic positional data were sparse. Could it have passed through a keyhole that would aim it AT Earth in 2046? Monte Carlo calculations show a circumstance that gets uncomfortably close to Earth. We can accurately trace back the orbit of this “virtual asteroid”, look for it at “its” next close approach, and see if the real asteroid is actually there. If not, we’re safe from 1998 OX4.

21. Asteroid Size Distribution:How Often Impacts of Different Energies Happen

22. Two Weeks of Near-Earth Asteroid Searching by Spaceguard Survey What are the observational biases of these various search programs? How do they affect our knowledge of the size- and orbital- distributions?

23. In the Post 9/11 World... Does the remote threat of an asteroid catastrophe have anything to teach us?

24. NEO Impacts in the Context of Natural Hazards and Civil Defense • Impact hazard has similarities and dissimilarities compared with more familiar disasters • Similarities include: nature of damage partly caused by familiar forces (fire, high wind, quake, falling debris, flood) • Dissimilarities: impacts happen anywhere; no analogs to “aftershocks”; no radioactivity or enemy soldiers • Though a major impact could happen, it is much less likely than a familiar natural disaster • For every future impact that will kill thousands to hundreds of thousands of people, there will likely be hundreds of floods, typhoons, and earthquakes that will each kill just as many

25. Public Perception • While “known” to many from movies and the news, a serious impact disaster has never been experienced in recorded history. • The tiny chances and huge consequences are extremely difficult for people to relate to. • The impact hazard is “dreadful” (fatal, uncontrollable, involuntary, catastrophic, increasing…) and apocalyptic (with religious or superstitious implications for many). Public response to a real impending impact is expected to be exaggerated (e.g. Skylab falling). • Experience with news media hype and misinformation suggests we need more science literacy among journalists and citizens in general.

26. Badly Misleading News Stories in 2002 Alone • 2002 EM7 came from a “blind spot,” it was a near-miss, so the Spaceguard Survey is inadequate • Many NEOs are found departing. Goal never was to catch one just before impact. Reflects basic misunderstanding of survey approach. • 2002 NT7 “is on an impact course with Earth” (BBC, July 2002) and other hyperbole • It was a scientifically interesting case of a very small chance of impact many years from now; as usual, a few days of further observations reduced the chances to zero. • “Impact dangers less than we thought” (attributed to Brown et al., Nature, Nov. 2002) • Brown et al. studied harmless objects 1 to 10 m in size; no implications for “Tunguskas” let alone for the several km NEOs, which actually dominate the hazard, statistically.

27. Hazard Scales: The Challenge of Simply Communicating Risk to Citizens • The well-known Richter Scale has been refined over decades. • Americans wrestled last month with how to deal with “orange”. • The Torino Scale has had modest effectiveness, challenges. Terrorism Scale Richter Scale (Earthquakes) Predicted Asteroid Impact Scale

28. Comparisons of Terrorist Threat and Impact Hazard • Similarities • Threats are new, “dreadful”, poorly understood, raise fears • Few have been (or, in all likelihood, will be) killed...but many could be killed • Both strike randomly, in place and time • Dissimilarities • Terrorism is consciously done (so retribution is demanded) while impacts are an “act of God” • We can do something concrete about the impact hazard (search for the body that may impact, deflect it, or evacuate ground-zero); battling terrorism is like the “war on drugs” • We spend vast sums to battle terrorists, a few million $ annually to search for asteroids (and how much on flu vaccination programs that might save 20,000 annually?)

29. Post-September 11th Insights • We’ve “learned” to fear the unexpected in what seems like an ever more risky world -- whether or not it truly is.(Asteroids appear to be an increasing danger, even though they aren’t, due to increasing “near misses”.) • “Objective” measures of death and damage (e.g. ~3000 deaths and property damage in lower Manhattan) do not begin to predict the nature of public responses and the resulting potential losses (e.g. economic recession). • “Who was to blame for not foreseeing this kind of disaster?” (we seek villains in government, for 9/11, for the Columbia disaster…who will be blamed if an asteroid strikes?)

30. Two-tiered Approach to Dealing with Irrational Risk Responses • Public officials must be prepared to deal with disproportionate responses • The public politically demands that they do • There are real psychological, economic, and other consequences • Politicians, educators, and science journalists must endeavor to teach citizens how to evaluate more rationally the risks that affect them • Generally, fear would be reduced; rational concern would lead to constructive response • Our national and personal resources would be employed more cost-effectively

31. Current Dilemma. (FEMA has shown little interest, for example.) What should we do to develop the institutional capability to respond to a predicted or actual impact? Astronomers have tried to take the first step...