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

1. Invited Talk: Meteoroids, Meteors, and the NEO Impact Hazard Clark R. Chapman Southwest Research Inst. Boulder, Colorado, USA “Meteoroids 2007” Barcelona, Spain 9:30 a.m., Friday, 15 June 2007

2. Relationship between Meteoroids and the Impact Hazard • Astronomy and space science are esoteric subjects; only two topics have practical consequences: • “Space Weather” due to Sun-Earth interactions • Falling objects from space (asteroids, meteoroids) • While the greatest threat has been from NEOs >2 km diameter, that threat is being reduced by the Spaceguard (SG) survey search program… much of the remaining threat is from Tunguska-class impacts. • ~50% of Tunguska-class NEOs will be found by SG2 (goal 90% of NEOs >140 m) so we might well know of a threatening Tunguska impactor in advance. • In practical terms of human psychology and politics, the most likely events are of most concern, even if they are less destructive. • Therefore, attention has turned to impacts by the smallest, most frequent damaging events.

3. Sorting Solar System Bodies By Orbital Class • Inner-Earth Objects (IEOs or Apoheles) • NEAs (Atens, Apollos, Amors) • Main-Belt Asteroids (incl. Hungarias, Cybeles, Hildas, etc.) • Trojans (of Mars, Jupiter, Neptune…) • Centaurs, Scattered-Disk Objects • KBOs (Plutinos, Cubewanos) • Oort Cloud • Comets (JFCs, longer period comets) • Planetary satellites (irregular, regular) • Planets • IDPs, Meteoroids, Meteorites • “Small bodies” ~10 m - 1000 km diam. • Pluto, Eris, other large TNOs • Planets By Size

4. Meteoroid/Asteroid Numbers and Magnitudes; Impact Frequencies and Energies (Harris, 2007) • Meteoroids (upper left) are the “tail” of the distribution of larger, dangerous objects, pro-duced by col-lisions, cratering, and cometary disintegration. • Meteors and bolides are the visible mani-festation of the infrequent, rarely witnessed events that pose a real danger. • Recent revisions by Harris suggest that Tunguskas occur every few thousand years. Maybe the 1908 devastation was done by a much smaller, more frequent impact (cf. Boslough 2007).

5. Impacts of Practical Concern • Mass extinction events are too improbable to worry about • Meteorites do minor damage, hit people rarely…. But they are a minuscule fraction of the hazard from “falling objects”

6. Case Studies of Potential Impact Disasters(in Chapman 2003 OECD study) • Civilization destroyer: 2-3 km asteroid or comet impact • Tsunami-generator: ~200-300 m asteroid impacts in the ocean • ~200 m asteroid strikes land • Mini-Tunguska: once-a-century atmospheric explosion (30-40 m body) • Annual multi-kiloton blinding flash in the sky (4 m body) • Prediction (or media report) of near-term impact possibility http://www.boulder.swri.edu/clark/oecdjanf.doc • Nature of Devastation. • Probability of Happening, in 21st century. • Warning Time. • Possibilities for Post-Warning Mitigation. • After-Event Disaster Management. • Advance Preparation. What can we do now? Six case studies, exemplifying the different sizes and types of impact disasters, were discussed in these terms Cases (d), (e), and (f) involve objects of interest to meteoroid researchers.

7. “Mini-Tunguska”: Once-in-a-Century Atmospheric Explosion • Nature of Devastation. 30-40 m “office building” rock hits at 100 times speed of jetliner, explodes ~15 km up with energy of 100 Hiro-shima A-bombs. Weak structures damaged/destroyed by hurricane-force winds out to 15 km. If over land, dozens or hundreds may die, especially in poor, densely populated areas (minimal damage in desolate places). • Probability of Happening. Once-a-century, but most likely over an ocean or sparsely-populated area. • Warning Time. Very unlikely to be seen beforehand; no warning at all. • Mitigation Issues. Little can be done in advance (an adequate search system would be very costly). Rescue and recovery would resemble responses to a “normal” civil disaster. No on-the-ground advance preparation makes sense, except public education about this possibility. Mini-Tunguska

8. Annual, Multi-Kiloton Blinding Flash in the Sky • Nature of Devastation. A bus-sized boulder explodes >20 km up in the stratosphere with the energy of a small A-bomb (2 to 10 kT). The blinding flash is brighter than the Sun. No ground damage. But in a zone of military tension, such an event might be misinterpreted as an atomic attack, triggering an inappropriate response. • Probability of Happening. Annual event, somewhere on Earth. Far less likely to happen over a war zone. • Warning Time. No warning at all. • Mitigation Issues. Such events are regularly observed by U.S. Depts. of Energy & Defense, information made available to public on timescale of…???, probably not immediately to all who might be concerned. Level of knowledge among military agencies of other countries not known to me. Clearly, education about the possibilities of such events (in the context of various national military command-and-control structures) would help. OVER IRAN? OVER ISRAEL? HOW WOULD THE GENERALS RESPOND?

9. F. Prediction (or Media Report) of Near-Term Impact Possibility • Nature of the Problem. Mistaken or exaggerated media report (concerning a near- miss, a near-term “predicted” impact, etc…most likely concerning a very small NEO [large bolide]) causes panic, demands for official “action”. • Probability of Happening. Has already happened several times, certain to happen often in next decade. Most likely route for the impact hazard to become the urgent concern of public officials. • Warning Time. Page-one stories develop in hours; officials totally surprised. • Mitigation Issues. Public education, at all levels of society: in science, critical thinking, and about risk, in particular. Science education and journalism need improvement with high priority.

10. Death Threat from Impacts, by Asteroid Diameter and Location of Impact • Statistical mortality rate once Spaceguard Survey is complete in a few years (NEO Science Definition Team [SDT], 2003; tsunami data corrected by Chapman) • Current rate (many hundreds/ year) will be down to a couple hundred per year, mainly by removing threat of “Global” impactors > 2 km diameter • Dominant threat will remain for “Tunguskas,” for which there is a several-% chance this century that one will strike and kill hundreds or thousands of people. • Thus Tunguskas and their smaller cousins will dominate public interest in the impact hazard. (For nominal case) Global Worldwide Deaths (Annual) Land Tsunami Asteroid Diameter (km) How the mortality will diminish from the three kinds of impacts as the Spaceguard telescopic searches continue

11. What is the Smallest NEO that is Dangerous? Model of 30 m NEA 1998 KY26 (radar) This will be a vital issue for decision-makers • Metallic objects are not impeded much by the atmosphere, whatever their size, and are responsible for most craters <1 km diam.; but they are only ~3% of NEOs. • The 2003 SDT report considered ~50 m diameter to be the smallest truly dangerous non-metallic impactor. • Some analyses in the literature suggest that the threshold is near 30 – 40 m. • Should we then be unconcerned about deflection/ evacuation for a 25 m body? They impact ~10 times as often as 50 m impactors. • Officials will have to make decisions once the new surveys start discovering thousands of these bodies and some appear (within uncertainties) likely to impact. • We need research (physical nature of small NEOs, propagation of shock wave from high altitude, possibility of igniting flammable materials on the ground, etc.)

12. Uncertainties and Variations in Small-Size Threshold for Damage • Current theory has much uncertainty around the lower size limit (depends on nature of impactor, its velocity, and uncertain physics). • Should a person run toward a 30 m impact to study it or enjoy it (like one of my colleagues says he would do) or run away from it because it is dangerous? • Should a civil defense official evacuate people from ground-zero for a 30 m, 20 m, or 10 m predicted impact?

13. Numbers of Small NEOs Known and to be Discovered Incremental numbers: 0.5 mag. Intervals centered on listed mag. and size. Data courtesy A. Harris (June 2007) • The discovery rate for 10 m NEAs may go up 2000 times! • By the end of SG2, we will know nearly half of Tunguska-class NEAs. • We will then be tracking 2 million 30 m objects; any threatening one will demand attention, even if impact damage might be minimal. • Think of the implications for meteoroids research: a quarter-million known objects 5 m in size! H Diam. (km) Known Now SG1 (goal) SG2 (goal) No. % of Tot. No. % of Tot. No. % of Tot. 17.75 1.0 234 59 280 83 333 98 22.02 0.14 162 3.5 450 9 4000 83 24.26 0.05 147 0.09 1200 0.6 80000 40 25.36 0.03 85 0.01 640 0.08 2 million 20 27.75 0.01 17 1e(-6) 200 1e(-5) 400000 2 29.26 0.005 6 3e(-8) 30 3e(-7) 200000 0.2

14. Other Issues about Small Impacts • How dangerous are meteorite falls? (Few people have been hurt or killed so far, but the population density has increased dramatically.) • Will military establishments (e.g. the U.S. Air Force Space Command) release useful data, including lightcurves and energy estimates, on bolides observed by their “assets”? • What are realistic meta-error bars on our knowledge of the frequency of impacts by Tunguska-class and smaller objects? • Ortiz et al. poster (this meeting) show order-of-magnitude differences among acoustic, satellite, and lunar impact flux estimates • What do social scientists believe will be the reaction to discovery of an actual 5-to-30 m body predicted to strike the Earth?

15. Rubble Piles, Monoliths, Asteroidal Satellites, Cometary Fluff-balls • Approximately 20% of observed NEAs are double bodies or have satellites; another 20% appear to be contact binaries (results from Arecibo and Goldstone delay-doppler images) • Spin data indicate that NEAs >200 m diameter are mostly rubble piles (including the contact binaries), whereas NEAs <200 m in size are monoliths. • Is there any evidence from largest bolides? • Are very small NEAs binaries? • Is there any evidence from largest bolides? • What about very fragile objects? Holsapple

16. Conclusions • Large meteoroids and bolides may do little or no damage, but their brilliant impacts into the Earth’s atmosphere will be the aspect of the impact hazard that will be manifest to the public, reported by the news media, and to which officials must respond. • We must understand the threshold between the surely harmless and the possibly harmful…research on this vital question is urgently needed, before the Spaceguard-2 discoveries start overwhelming us.

18. Main-Belt Asteroid Colors:Then…and Now Hapke (1971) Chapman (1971) • Asteroid data 35 years ago like TNO data today • Disputed clusters partly OK • Trends with a,e,i convincing only after debiasing (~1975) • Matching colors/reflectance spectra to mineralogy only fair (space weathering, etc.) • Today: abundant statistics, hi-res spectra, good compos. • Colors for tens of thousands • Reflectance spectra: 1000’s • Good correspondence of taxonomy with meteorites • Relationship of NEAs to main-belt asteroids clear • Families as catastrophic collision products of (usually) homogeneous parent bodies Lessons Learned Data from Gehrels (1970) Burbine et al (2001) Ivezic et al (2002)

19. NEA Colors(Binzel et al. 2004) • S/Q type colors • Space-weathered (like M.B.) >5 km • Range from ord. chond. – M.B. <2 km • Spread of fresh to matured surfaces • Implies there may be small M.B. Q’s • NEA colors vs. M.B. • Q’s are NEAs only • More extremes • D-types (upper-rt) 10-18% of NEOs could be extinct comets • Diversity like M.B. • Outer M.B. under- represented a bit (beyond low albedo bias)

20. Size Distributions Main Belt Tedesco et al. 2005 • NEAs less “wavy” than large Main Belt ast. • TNOs have shallow slope at <20 km diam. • Comets “truncated” 0.6-4 km (Meech et al. 2004) • Separate SDs for different families/groups TNOs Bernstein et al. 2004 NEAs NASA SDT 2003

21. Geophysical Properties • Spins, shapes, satellites, masses, densities, strengths, interior structures • Most remote-sensing of surfaces reveals little about interior properties • Rapid spins = monolithic structure; do slow spins imply rubble piles? • Impact experiments, numerical modelling, scaling analysis • NEAR laser altimetry probes interior of Eros NEAR Laser Altimeter: Eros Neumann & Barnouin-Jha 2005 Holsapple 2005 Korycansky & Asphaug 2005

22. Spacecraft: Orbiters, Landers, and (soon) Sample Returns • Many fly-bys of small bodies • Significant reconnaissance • Surprises: no 2 bodies same • NEAR Shoemaker orbital mission to Eros (& landed!) • Detailed remote-sensing • Composition: ord. chondrite • Impact, landers, sample ret. • Deep Impact experiment • Contact with Itokawa soon • Awaiting sample returns by Stardust & Hayabusa • Must extrapolate physical properties measured for few visited small bodies to vast, heterogeneous population NEAR XRS data suggest Eros composition ~ ordinary chondrites Lim et al. 2005

23. Unexpected Small-Scale Geology of Eros • Flat ponds and “beaches” • Small craters absent; dominant boulders

24. Dynamics: Relationships to Physical Properties • Dynamical processes cause physical properties • Spins and axis orientations due to Yarkovsky Effect • Tidal interactions with planets/sun cause distortions and disruptions/disintegrations • Collisions and catastrophic disruptions create families, rubble pile structures, satellites (initial spins, sizes) • Physical properties elucidate dynamics • Colors help identify dynamical families • Yarkovsky/YORP effects depend on albedo, shape, thermal inertia, spin, density, etc. • Dynamical analysis can determine physical properties • Mass (hence density) • Spins (very rapid spins indicate monolith, not rubble pile) • Non-gravitational forces imply features of comet nucleus • Dynamical analysis helps us study physical processes • Specific ages for families specify rates for processes like space-weathering • How perihelia evolve and facilitate volatilization

25. NEO Impact Hazard: 99942 Apophis (2004 MN4) • In astronomy, only solar flares and impacts have major practical effects • 1:8000 chance that 320m asteroid impacts 4/13/36 (~ South Asia tsunami) • Physical properties affect: • Whether it hits “keyhole” • How Yarkovsky affects it • How we could attach to it, couple energy to divert it • How it responds to forces • How it responds to tidal forces during 2029 fly-by • Consequences of impact In the extremely unlikely event that it will hit, ground-zero will be somewhere on the red line

26. Themes and Issues • How much are we astronomersfooled by the space-weathered, impacted optical surfaces? • Can we really comprehend how processes work at near-zero gravity? • Really what are the densities, porosities, granular structures, strengths? • Are these splitting/vanishing comets “dust bunnies”? • Are M-types metallic cores? (many evidently aren’t) • Regolith-free bare rocks vs. “talcum powder” • Biased view from what penetrates our atmosphere • What are we missing? • 2003 UB313: we weren’t looking for high-inclinations • Hypotheticals: “vulcanoids”, Lou A. Frank “LAFOs” • Interstellar small bodies? • Asteroid belts/Oort clouds around other stars

27. Asteroids/ Comets: Evolving Perspectives… Traditional View ASTEROIDS Rocky, metallic, no active geology, cratered, collisional fragments, some differentiated by heating COMETS Icy, under-dense, no active geology, pristine…until they come close to the Sun, become very active, disintegrate Emerging Continuum ASTEROIDS Under-dense, rubble piles, many volatile-rich (except at surfaces), some non-impact geology, many satellites; NEAs tidally evolved COMETS Active, fluffy, evolved bodies with complex geology (impact & non-impact), easily split; precursor KBOs have satellites, interior “oceans”

28. Growing Awareness of the NEO Impact Hazard The Little Prince • Generalized fears of comets for centuries (e.g. Halley’s comet in 1910) • Dawning scientific awareness (1940s – 1970s) • NEAs can make lunar-like craters on Earth • Comet nuclei are dangerous, consolidated bodies • Shoemaker/Meteor Crater/surveys…Mariner spacecraft • M.I.T. Project Icarus: nuke an oncoming asteroid • SciFi books (“Lucifer’s Hammer”), movies (“Meteor”)(1970s) • Scientists study NEO hazard (early 1980s) • Alvarez hypothesis for K-T mass extinctions; Chicxulub • NASA Snowmass Workshop; Spacewatch survey • March 23, 1989 (“Near Miss Day”): Asclepius • Early 1990s: Congressional mandate, NASA Spaceguard and DOE Interception Workshops • 25% of public aware of NEO hazard (Slovic 1993) • Chapman & Morrison Nature paper (1994) • “Deep Impact” and “Armageddon” movies • U.S. Congress & British Parliament act Asteroid B612 Meteorite punctured roof in Canon City, CO Meteor Crater Global catastrophe