Clark R. ChapmanSouthwest Research Inst.Boulder, Colorado, USA

Clark R. ChapmanSouthwest Research Inst.Boulder, Colorado, USA

““Meteoroids 2007”Meteoroids 2007”Barcelona, SpainBarcelona, Spain

9:30 a.m., Friday, 15 June 20079:30 a.m., Friday, 15 June 2007

““Meteoroids 2007”Meteoroids 2007”Barcelona, SpainBarcelona, Spain

9:30 a.m., Friday, 15 June 20079:30 a.m., Friday, 15 June 2007

Invited Talk:Invited Talk:

Meteoroids, Meteors, and Meteoroids, Meteors, and the NEO Impact Hazardthe NEO Impact Hazard

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.

Sorting Solar System Bodies

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 Orbital ClassBy Orbital ClassBy Orbital ClassBy Orbital Class

By SizeBy Size

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).

100

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910111213141516171819202122232425262728293031

10-1 102 105 108

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This report (2007)Stuart 2001Harris 2002Brown et al. 2002,annual bolide eventConstant power lawfrom 2003 SDT reportDiscovered to 10/31/06

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Impact Energy, MT

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”

Case Studies of Potential Impact Disasters (in Chapman 2003 OECD study)

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

Six case studies, exemplifying the different sizes and types of impact disasters, were discussed in these terms

a.a. Civilization destroyer: 2-3 km asteroid Civilization destroyer: 2-3 km asteroid or comet impactor comet impact

b.b. Tsunami-generator: ~200-300 m Tsunami-generator: ~200-300 m asteroid impacts in the oceanasteroid impacts in the ocean

c.c. ~200 m asteroid strikes land~200 m asteroid strikes land

d.d. Mini-Tunguska: once-a-century Mini-Tunguska: once-a-century atmospheric explosion (30-40 m body)atmospheric explosion (30-40 m body)

e.e. Annual multi-kiloton blinding flash in Annual multi-kiloton blinding flash in the sky (4 m body)the sky (4 m body)

f.f. Prediction (or media report) of near-Prediction (or media report) of near-term impact possibilityterm impact possibility

Cases (d), (e), and (f) involve Cases (d), (e), and (f) involve objects of interest to meteoroid objects of interest to meteoroid researchers.researchers.

http://www.boulder.swri.edu/clark/oecdjanf.doc

“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

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?

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.

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 Thus Tunguskas and their smaller cousins will dominate smaller cousins will dominate public interest in the impact public interest in the impact hazard.hazard.

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Asteroid Diameter (km)

Tsunami

Land

Global(For nominal case)

How the mortality will diminish from the three kinds of impacts as the Spaceguard telescopic searches continue

What is the Smallest NEO that is Dangerous?

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.)

Model of 30 m NEA 1998 KY26 (radar)

Model of 30 m NEA 1998 KY26 (radar)

This will be a vital issue for decision-This will be a vital issue for decision-makersmakers

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?

Numbers of Small NEOs Known and to be Discovered

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 9822.02 0.14 162 3.5 450 9 4000 8324.26 0.05 147 0.09 1200 0.6 80000 4025.36 0.03 85 0.01 640 0.08 2 million 2027.75 0.01 17 1e(-6) 200 1e(-5) 400000 229.26 0.005 6 3e(-8) 30 3e(-7) 200000 0.2

Data courtesy A. Harris (June 2007)Incremental numbers: 0.5 mag. Intervals centered on listed mag. and size.

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?

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

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.

Main-Belt Asteroid Colors:Then…and Now

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

Hapke (1971)Chapman (1971)

Ivezic et al (2002)

Data from Gehrels (1970)

Burbine et al (2001)

Les

son

s L

earn

ed

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)

Size Distributions

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

Main BeltMain Belt

TNOsTNOs

NEAsNEAs

Bernstein et al. 2004

Tedesco et al. 2005

NASA SDT 2003

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

Holsapple 2005

Neumann & Barnouin-Jha 2005

Korycansky & Asphaug 2005

NEAR Laser Altimeter:

Eros

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

Lim et al. 2005

NEAR XRS data suggest Eros composition ~ ordinary chondrites

Unexpected Small-Scale Geology of Eros

Flat ponds and “beaches”

Small craters absent; dominant boulders

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

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 impactIn the extremely unlikely event

that it will hit, ground-zero will be somewhere on the red line

Themes and Issues

How much are we astronomers fooled 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

Asteroids/ Comets: Evolving Perspectives…

ASTEROIDSASTEROIDS

Rocky, metallic, no active geology, cratered, collisional fragments, some differentiated by heating

COMETSCOMETS

Icy, under-dense, no active geology, pristine…until they come close to the Sun, become very active, disintegrate

Traditional View

ASTEROIDSASTEROIDS

Under-dense, rubble piles, many volatile-rich (except at surfaces), some non-impact geology, many satellites; NEAs tidally evolved

COMETSCOMETS

Active, fluffy, evolved bodies with complex geology (impact & non-impact), easily split; precursor KBOs have satellites, interior “oceans”

Emerging Continuum

Growing Awareness of the NEO Impact Hazard

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

The Little Prince

Meteorite punctured roof in Canon City, CO

Global catastrophe

Asteroid B612

Meteor Crater