What Happens When an Impact Takes Place?

1. Impacts, Evolution, and EthicsDavid Morrison, NASA Astrobiology Institutehttp://impact.arc.nasa.govhttp://neo.jpl.nasa.gov

2. What Happens When an Impact Takes Place? Meteors & Bolides (up to 5 MT) • Great fireworks, no damage Tunguska-class (15 MT) impact • Damage similar to large nuclear bomb (city-killer) • Average interval: 1000 years • Minor risk relative to other natural disasters Global catastrophe (> 1 million MT) • Global environmental damage, threatening civilization • Average interval: 1 million years • Unique risk relative to other natural disasters Mass Extinction (> 100 million MT) • Global environmental catastrophe (nearly instantaneous) • Average interval: 100 million years • Unique ability to kill most living things on Earth

3. The KT Mass Extinction Comet or asteroid 15 km diameter • Energy 100 million megatons • Chicxulub crater 200 km diameter in Yucatan Immediate environmental catastrophe • Blast and tsunami extend thousands of km • Back-falling hot debris create global firestorm • Long-lived stratospheric dust blocks light Mass Extinction • Sharply defined by marine forams (90% extinct) • More than half of all fossil-forming species go extinct, animal and plant, land and sea • All dinosaurs plus majority of mammal species go extinct Critical evidence • Extraterrestrial material (e.g., iridium) and shocked crystals in boundary layer, globally distributed • Largest impact crater on Earth, simultaneous with extinction • Apparently instantaneous extinctions (not gradual)

4. 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

5. Hazard of Globally Catastrophic Impacts Kills more than 1.5 billion people • We define "globally catastrophic" this way Energy threshold calculated to be near 1 million MT • Primary global effect is from stratospheric dust and smoke • Average interval roughly 1 million years • Primarily from Near Earth Asteroids (NEAs), with population estimated at 1100 • Risk is similar to other natural hazards (earthquakes, severe storms) Unique in capacity to destabilize civilization • Qualitatively different from that of other natural hazards • Can be compared with global nuclear war • Only known natural hazard that can destroy civilization • Much more frequent than mass extinctions

6. Terrestrial Impact Frequency Hiroshima year Tunguska century Tsunami danger ten thousand yr. Global catastrophe million yr. K/T billion yr. 0.01 1 100 10,000 million 100 million TNT equivalent yield (MT)

7. Comparison with Other Risks • Statistical risk of death from impacts is about 1 in 4 • million per year, or about 1:50,000 lifetime risk • Much less (in U.S.) than auto accidents, shootings • Comparable with other natural hazards (e.g. earthquakes, floods) • Near threshold for hazards most people worry about • Well above threshold for government regulation • Severity of disasters (billions of people killed) is greater than any other known hazard we face • Apparently unique in its threat to civilization • Places this disaster in a class by itself • Average interval between major disasters (million yrs) is larger than for other hazards • Causes some to question credibility of hazard • Difficult to gain political support (easy to defer action)

8. The Problem with Statistics • The impact hazard is the most extreme example of risks with very low probability but very large consequences. Such low probabilities are difficult to grasp (after all, there has been no major impact disaster in all of recorded history). Statistically, it could happen any time, but that doesn’t help us mitigate the risk. • The hazard that concerns us -- a possible impact within the next few centuries -- is actually a deterministic threat. It either will or will not happen. If it will happen, the asteroid is already on a collision course, and it will pass close to the Earth repeatedly (every few years) before it hits. • We must look to see whether or not the Earth will be hit in the next few centuries. That is the rationale for the Spaceguard Survey -- to find any specific asteroid that is likely to hit the Earth, and to provide decades or centuries of warning. • If we look, we can have plenty of warning. If we don’t carry out a survey, we will have no warning and no opportunity to mitigate the disaster, when eventually it happens.

9. Spaceguard Survey • Spaceguard Survey originally proposed by NASA panel in 1992 • Additional support from US Congress in 1995 • Adopted as NASA goal in 1998 (in collaboration with USAF) • Survey Objective: Discover and track 90% of the Near Earth • Asteroids (NEAs) with diameter greater than 1 km within • ten years (by 2008) • Estimated number of NEAs larger than 1 km: approximately 1100 • Number discovered through end of 2003: 750 • Estimated completion date (to 90%): 2008-2010 • Focuses on impacts that could have global consequences • Will provide decades of warning of future impacts

10. Spaceguard Survey • Most asteroids are being discovered by MIT-Lincoln Lab (LINEAR) search program • Uses two US Air Force 1-m telescopes with NASA support

11. Beyond Spaceguard • Spaceguard Survey will have eliminated 90% of the impact risk by end of this decade • To eliminate 99% of risk, we must extend survey to fainter asteroids (down to 150 m diameter) • Bigger telescopes cost more money -- eliminating the next 9% is more expensive than dealing with the first 90% • Proposed Large Synoptic Survey • Telescope (LSST) will do the • job while also advancing • astrophysics -- is it worth it?

12. Planetary Defense • Impacts are the only natural hazard that can, in principle, be eliminated. • We could develop the technology to change asteroid orbits • Should we develop this technology now? • Or wait until specific threat is identified? • Should the U.S., as the world’s only • superpower, assume responsibility, • or should this be an international effort? • How much should we spend to protect • our planet? Who can we trust with this • responsibility? How do we ensure that • asteroid defense systems are not • misused?

13. Societal & Ethical Issues: Basics • Do we Have the Right to Interfere in the Evolutionary Process? • Impacts & mass extinctions have been part of Earth’s history • for 4 billion years • Adaptive radiation & speciation follow mass extinctions • We owe our existence to the Chicxulub impact • We might be replaced by something “better” • If Answer is “YES,” We Ask How Best to Protect our Planet • and Ourselves

14. Societal & Ethical Issues: Extended Surveys? • Should we extend asteroid surveys to sub-km impactors, • down to the limit of penetration of the atmosphere? • This would be consistent with an imperative for governments • to make an effort to identify and protect their populations • from preventable disasters. However, his would be • considerably less cost-effective than the current Spaceguard • Survey, since we would need to spend at least an order of • magnitude more funds to protect against a risk that is at • least an order of magnitude smaller than that of NEAs • larger than 1 km. However, it has been recommended by two • NRC Panels and a recent NASA Science Definition Team.

15. Societal & Ethical Issues: Who Is in Charge? Who should be in charge -- of possible extensions of the Spaceguard Survey or testing of defensive systems? Is NASA the correct agency? Or the DoD? To date, there is no official position or plan that allocates responsibility. This issue is sometimes raised among astronomers, who ask “Who should I call if I discover an asteroid on a collision course with the Earth?” Should civil defense and disaster relief agencies be planning to deal with the aftermath of an impact explosion that occurs without warning? Today, no warning would be expected for sub-km impacts. Who should assume responsibility in planning for mitigation if such a disaster should occur?

16. Societal & Ethical Issues: Internationals Why is the United States alone? How important is international participation? While the impact hazard has been discussed internationally by the United Nations, the Council of Europe, the Organisation for Economic Co-operation and Development, the International Astronomical Union, and the International Council of Scientific Unions, no concrete action has been taken. UK did a nice government study but ignored all the recommended actions. Perhaps it is the proper role of the only superpower to assume unilateral responsibility for the protection of our planet from cosmic impacts?

17. Societal & Ethical Issues: Self Defense? • Should we develop technologies for deflecting asteroids? • To date, essentially no funds have been spent for this purpose. • Many would argue that it is prudent to begin such research • before an actual threat is identified. Others argue that defense • systems are themselves inherently risky and should not be • developed in the absence of a specific threat. • Should we test asteroid deflection technologies? Teller argued • that the experience gained in planning an international test • project would be invaluable if and when we faced the real thing -- • especially if the options for defense included nuclear explosives. • The recent proposal by the B612 Foundation for a first test of a • space tug represents such an experimental approach.

18. Societal & Ethical Issues: Sub-km Impacts • Which impacts (if any) do not require mitigation, and who will • make the decision? Consider a 100 m NEA that will impact • in the ocean -- will assurance from the science community • that there is no danger satisfy the public? • Suppose that a land impact is predicted and there are small • cities in the target area. Who will decide whether an effort • should be made to deflect the asteroid? Who will pay for it? • Suppose a sub-km impactor is identified and a decision is made • to change the orbit, but in this process the impact point crosses • Nations B, C, and D, which were originally not at risk. Who • can be trusted to carry out the deflection maneuver? And • who is responsible if the effort is only partly successful?

19. Societal & Ethical Issues: Trust Will the public trust either scientific judgments or the decisions of public officials? If an asteroid is discovered with an initial well-publicized non-zero chance of collision, and subsequent observations ultimately convince the scientific community that it will miss by a very small margin, will the public believe them? Or suppose an asteroid is on a collision course but the scientists estimate that it is only 40 m in diameter and will disintegrate harmlessly at high altitude. Will this be believed? Even if the scientists are trusted, is the public likely to support continued and perhaps accelerated government spending to protect the Earth from asteroids? It is difficult to sustain interest and support in the absence of known threats, and there has never been an asteroid impact in a populated area in all of recorded history.