1. The Impact Hazard: Historical Perspectives
David Morrison
NASA Ames Planetary Defense Team
Workshop on Potentially Hazardous Asteroids:
Characterization, Atmospheric Entry and Risk Assessment
NASA Ames, July 7-9, 2015

2. What is the risk (hazard) from impacts?
• Comets recognized as potential impactors since Newton/Halley.
Asteroids recognized as potential impactors since early 20th century.
Risk is measured in terms of human casualties/year.
Understanding risk is a prerequisite for mitigation (planetary defense).
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3. Perspective #1: Assessing the Hazard
• During the 1990s, the standard scientific tools of sampling and statistical analysis were essential to understand the impact hazard and communicate the risk to decision makers. Chapman & Morrison and others compared the impact risk to other natural hazards, estimated the risk as a function of NEO size, and laid the foundation for establishing the Spaceguard Survey in 1998.
• Congressional language (in 1991) reflected this perspective: The Committee believes it is only prudent to assess the nature of the threat…
• More sophisticated studies of NEA populations and impact consequences by Harris, Chesley and others were used in the 2003 NASA SDT Report.
• Note that while these statistical studies provide a tool to analyze various mitigation schemes, they do not in themselves reduce the hazard.
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4. Perspective #2: Mitigating the Hazard
• The public and decision-makers are interested in warning and protection. The public-policy goal is not to refine the estimate of the risk but to identify the next impactor and do something about it.
• Changed perspective is reflected by Congress (2005): The objectives of the NEO survey program are to detect, track, catalogue, and characterize the physical characteristics of NEOs...
• The Spaceguard goal of finding 90% of the NEAs > 1 km focused on NEAs large enough to risk a global catastrophe. Such impacts dominate the risk over more frequent, smaller impacts.
• The objective is to increase public safety, not to improve scientific understanding of either NEOs or the impact risk, although these are worthy byproducts.
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5. Perspective #3: Responding to Public Concerns about Near-term Risks
• Issues that worry the public (and most decision makers) are not necessarily the greatest threats. The very rare large impacts pose the greatest hazards, but most people are more concerned about the next impact (which is likely to be small).
• Chelyabinsk has become the best-known “small” impact.
• Ideally we should design a system that detects both distant large NEAs and close small ones. But it appears that one system cannot do both jobs.
• As surveys are more complete for large NEAs, attention naturally shifts toward smaller ones.
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6. Historical Quotes
Should a comet in its course strike the Earth, it might instantly beat it to pieces. But our comfort is, the same great Power that made the Universe, governs it by his providence. And such terrible catastrophes will not happen till 'tis best they should … Benjamin Franklin (1757)
Men should be free from this fear ... for the probability of [a comet] striking the Earth within the span of a human lifetime is slim, even though the probability of such an impact occurring in the course of centuries is very great … Laplace (c. 1790)
Who knows whether, when a comet shall approach this globe to destroy it, as it often has been and will be destroyed, men will not tear rocks from their foundations by means of steam, and hurl mountains, as the giants are said to have done, against the flaming mass? - and then we shall have traditions of Titans again, and of wars with Heaven... Lord Byron (1822)
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7. Setting the Stage
First estimate of NEA impact frequency (Fletcher Watson 1941)
Meteor Crater an impact feature (Gene Shoemaker 1950)
Lunar craters are impact features (Apollo 1969)
• KT Impact Mass Extinction (Luiz Alvarez et al. 1980)
NASA Advisory Council study (Shoemaker 1981 unpublished)
Spacewatch: First CCD discovery of a NEO (Tom Gehrels 1989)
Clark Chapman & David Morrison: Cosmic Catastrophes (1989)
AIAA: Dealing with Threat of an Asteroid Striking the Earth (1990)
U.S. Congress asks NASA for Impact Threat Workshops (1990)
Glen Penfield & Alan Hildebrand find Chicxulub Crater (1990/91)
San Juan Capistrano 1st International Conference on NEAs (1991)
Newsweek Cover Story: How the World Might End (1992)
Comet S-L 9 impact with Jupiter (1994)
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8. Congressional Statement 1991
The House Committee on Science and Technology believes that it is imperative that the detection rate of Earth-orbit-crossing asteroids must be increased substantially, and that the means to destroy or alter the orbits of asteroids when they do threaten collisions should be defined and agreed upon internationally. The chances of the Earth being struck by a large asteroid are extremely small, but because the consequences of such a collision are extremely large, the Committee believes it is only prudent to assess the nature of the threat and prepare to deal with it.
NASA Authorization Bill, 1991
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9. NEA Population Estimate (Shoemaker 1981)
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10. NASA Workshops/Studies 1992
SPACEGUARD SURVEY WORKING GROUP
• Define threshold for globally catastrophic impact
• Optimize survey for greatest hazard: NEAs > 1 km
• Provide maximum warning lead time (several decades)
• Use existing CCD search technology to be cost-effective
• Program should be international, since entire planet is at risk
NEO INTERCEPTION WORKSHOP at LOS ALAMOS
• Several technologies could destroy incoming objects
• Edward Teller: We have identified a need for bigger bombs. We could develop a bomb large as 500,000 megatons, capable of disintegrating any comet or asteroid.
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11. Quantitative Estimates of Risk
Risk is defined in terms of expected average casualties per year
Risk is product of two curves: impactor population distribution (impact rates) and “kill curve” (number of casualties per impact).
Impactor population is determined from lunar impact record (crater size distribution), telescopic observations of NEAs, estimates of current (Phanerozoic) impact rates on Earth.
Greatest risk is from impactors capable of causing a global catastrophe (impact winter) with widespread crop failures, starvation, disease. Studies based on atmospheric dust loading (Toon, Pollack, Zahnle, etc.) placed threshold at about a million megatons (1-2 km diameter). For smaller impacts we can extrapolate data on direct damage from nuclear explosions (airburst & ground impacts).
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12. Interpretation of Risk
• Early hazard estimates based solely on energy of impact (e.g., size of NEA), dominated by threshold event (Impact Winter).
• If casualties are 1 billion and interval is 1 million yrs, average annual deaths are 1000/year, comparable to other natural hazards (earthquakes, volcanoes, severe storms). Hazards from smaller impacts (e.g., Tunguska) down 2 orders of magnitude.
• Impacts are catastrophic, unpredictable, uncontrollable, dreadful: Social Science research predicts major public concern (Slovic et al.)
• Public reaction initially the opposite: Impacts not understood. Considered too rare to take seriously (the giggle effect). Not worth sustained public funding. Planetary defense associated with nukes, Hollywood (Bruce Willis).
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13. The Spaceguard Survey
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14. NASA Science Definition Team 2003
Chaired by Grant Stokes (MIT)
Discussed deep survey options including ground-based (e.g., LSST), Earth-orbiting, and heliocentric platforms. Performed cost-benefit analysis on available options.
Alan Harris and Steven Chesley analyzed risk (expressed as average annual fatalities) for different populations of NEOs and completeness of surveys. Included data on population distribution over the globe, models for impact tsunamis, and various estimates of threshold for global impact damage.
Before Spaceguard, the risk was dominated by NEAs D = 1-2 km; at end of survey (90% completeness) roughly equal contributions from undiscovered 1-2 km NEAs and from NEAs D = m.
SDT estimated NEA size threshold of 140m to retire 90% of post-Spaceguard risk.
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15. Overall Hazard (from 2003 STD Report)
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16. Residual Hazard (from 2003 STD Report)
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17. NRC Revised Population (Harris 2009)
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18. NRC Revised Kill Curve (Harris 2009)
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19. Impact Hazard Revisited (Harris 2009)
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20. Impact Hazard Revisited (Harris 2009) (correcting for the discoveries of Spaceguard)
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21. Future Improvements in Hazard Models
Use finer grid of distribution of human population
Consider risk to infrastructure (power, communications, etc.) as well as people
Deal explicitly with irons, comets, other outliers
Improve understanding of impact tsunamis
• Allow for range of NEA physical properties and orbits
• Consider the special challenge of modeling very rare but catastrophic events – outside our experience base
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22. THANKS This paper reflects a community effort over 25 years
Special thanks to Clark Chapman and Al Harris for inumerable discussions and arguments over risk and its significance
Thanks to NASA and the American taxpayer, without whose support none of this would have happened.
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