NEAR-Shoemaker’s Exploration of Eros & the Impact Hazard

1. NEAR-Shoemaker’s Exploration of Eros & the Impact Hazard Clark R. Chapman, Ph.D. Institute Scientist Department of Space Studies (Boulder, Colorado) Southwest Research Institute Invited Presentation (on behalf of Don Yeomans and David Morrison) to the ATWG Conference, Lockheed Martin, 15 May 2001, Highlands Ranch, Colorado

2. The Hazard from Asteroids and Comets • Each year, there is a 1-in-300,000 chance that a mile-wide asteroid or comet will strike the Earth. • Less than 50 percent of the objects threatening Earth have been found. Those that remain could strike without warning. • This extreme example of a natural disaster (a tiny probability, but with huge consequences) challenges a rational policy response.

3. Early asteroid-like “planetessimals” formed the inner planets and the cores of the outer planets.

9. Tunguska in Perspective

10. Impact of Fragment K of Comet Shoemaker-Levy on Jupiter • The scars of three previous impacts can be seen on the planetary disk. Image from Peter McGregor and Mark Allen, ANU 2.3m telescope.

11. What Do We Know About This Hazard? • 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 impact (such as the TNT equivalence, size of resulting crater). • How much dust is raised into the stratosphere and other environmental consequences. • Biosphere response (agriculture, forests, humans, 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

12. Classification of Hazards • High Altitude Disintegration • Projectile fragments and disperses at high altitudes (over 40 km) • Negligible surface damage • Local Effects (Blast Damage) • Projectile explodes in lower atmosphere or craters surface • Severe localized damage from blast • Global Effects (Environmental Degradation) • Short-term global scale climatic changes (such as impact winter) • Global loss of food crops leads to large-scale famine, disease, and possible breakdown of civilization • Mass Extinction (Environmental Catastrophe) • Global environmental damage on a large scale (such as K/T event) • Mass extinction (many species lost forever; nearly all humans die)

13. Chief Environmental Consequences of Impacts

14. Is Civilization Robust or Fragile? • Arguments for Fragility • Modern people are disconnected from nature, survivability • Technology is ever more specialized • People are interdependent on distant resources, other nations • If societal breakdown spawns violence, modern weaponry is very dangerous •  Arguments for Robustness • Technological refugia exist (such as bomb shelters) • Society has become experienced in disaster recovery • Technological know-how has become pervasive • Historical precedence; recovery from WWII was rapid

15. Risk vs. Scale of Impact

16. A Royal Flush Odds:1 to 649,739 • It is more likely that a mile-wide asteroid will strike Earth next year than the next poker hand you are dealt will be a royal flush.

17. 20th Century Catastrophes Source: John Pike • Averaged over long durations, the death rate expected from impacts is similar to that from volcanoes.

18. Chances from Dying from Selected Causes

20. Mitigation Options • Spaceguard Survey • 90 percent of potentially hazardous asteroids could be found telescopically within a decade; certified as safe. • Deflect by Stand-off Neutron Bomb Blast • Spacecraft and bomb technologies exist and need to be integrated and deployed, unless a threatening object were found, due to the long lead-time. • Standard Hazard Mitigation • Extrapolate civil defense, natural hazard precautions from local to world context.

22. tan  = mv/mvorbit Miss distance > 10 x Earth Radius If deflection occurs at 1 AU from Earththe necessary impluse is mvtan  = 0.0004 mvorbitTypically vorbit  20 km/s and v  8 m/s

23. Required deflection velocity as a function of time before impact. • Required impulse as a function of time before impact. Shown are capa-bilities of the Space Shuttle’s main engines, the 1st stage of the Delta II, and the NEAR spacecraft.

24. Densities of Craters and Boulders on Eros vs. Size

29. Small-Scale Features: Why? • There are boulders, blocks, even “ponds” but no thick lunar-like regolith • Eros must be different from Moon, with widespread ejecta, even net erosion (lunar regolith is churned in place) • Blocks are ejecta from big craters and/or derived in situ but not readily destroyed • J. Bell (Univ. Hawaii) invokes Yarkovsky Effect to deplete small asteroidal projectiles, hence few craters, blocks are preserved • That’s part of the story…plus seismic shaking, inhibited crater production by blocks, etc.

30. Other Eros Results from NEAR-Shoemaker • Bulk density ~2.7 g/cc (rocky but with some voids), uniform • Bulk chemistry (from X-ray) similar to ordinary chondrites (calibrations are ongoing); consistent with infrared results • Global ridge structures imply that Eros is an intact-but-shattered fragment of its parent body, not a “rubble pile” IR spectral band positions imply ordinary chondrite

31. Headline: Mile-Wide Asteroid Will Hit in 2028 Which is least likely to be correct? A. The news report is wrong due to bad journalism. B. The scientific forecaster goofed. We’re safe. C. The astronomers erred. The asteroid is tiny; most of the world is safe. D. An asteroid will hit Earth in 2028. The correct answer is D: A, B, and C are all much more likely to explain the headline.

33. 10 5 (100000 km) 0 -5 -10 -15 -15 -10 -5 0 5 10 15 (100000 km)

37. Prediction is the Event • Scientists who predict think of predictions as dry scientific results, with objective error-bars. • Users of such predictions are mobilized into action by the prediction. • The predicted event may not happen as predicted; it may or may not have consequences. The predictions always have consequences. • Predictions of emotionally laden disasters result in subjective, sometimes irrational responses. • Predictions must be made with social responsibility, whether of a potential terrorist operation or of an asteroid impact.

38. Because of Uncertainties, Future Impact Scares are Likely • There is about a 5 percent chance an object over 1 km will pass close to Earth during the 21st century. • Twice a decade, a Tungusta-sized object passes within that same distance. • Astronomers might call these “safe” encounters, but would politicians and military leaders take such a dispassionate view? • Uncertainty biases in asteroid albedoes and sizes trend toward overestimated sizing of newly found objects; expect exaggerated early reports. • Future impact scares will happen, even if mistakes are reduced. The unlikely, real disaster may be lost in the noise of false alarms.

39. When an Impact Can’t Be Ruled Out, Then What? • Rate of NEO discovery is rapidly increasing, so “near misses” will become common. • Approximate orbits can be calculated within days; accurate orbits take two or more years. • During this interval, some asteroids will seem targeted to hit Earth before orbits can be improved. • “Cry wolf” announcements can be expected from reputable astronomers, but public officials, the military, and the public are unprepared to know how to react.

40. The Torino Scale This 1-to-10 one-dimensional scale adopted by the International Astronomical Union provides a simple description to the public of the level of hazard posed by a newly discovered, potentially threatening asteroid. • Criticism: Richter Scale; the scale changes Answer: Many geological, meteorological, and space physics scales change as forecasts change. • Criticism: Scientific complexity is oversimplified. Answer: For public, keep it simple. • Criticism: Who needs it? Answer: Pedagogical tool for science journalists and teachers.

41. The Torino Scale Events Having No Likely Consequences Events MeritingCareful Monitoring Events MeritingConcern ThreateningEvents CertainCollisions

42. The Torino Scale

43. Results from SwRI Internal Research • Completed and published a chapter in Prediction. • Participated in the Torino IMPACT workshop, June 1999. • Collaborated with science writers in helping Prof. R. Binzel (MIT) develop the Torino Scale for public understanding of impact risk. • Spoke at the High Consequence Systems Surety Symposium at Sandia National Laboratory, November 1999. • Prepared invited review and paper for the Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond in Vienna, July 2000. • Prepared SwRI white paper on the impact hazard, which served as a basis for the recent AIAA/UN Workshop on International Cooperation in Space in Seville, Spain, March 2001.

45. Findings: Detection and Impact Effects • About 50 percent of an estimated 1,000 near-Earth asteroids over 1 km diameter have been found. • Last 10 percent will be more difficult to find, especially those mostly interior to the Earth’s orbit. • Near-Earth asteroids smaller than 1 km are being sampled, but most will remain undiscovered. • Long-period comets present a nearly intractable problem and may constitute a significant part of the hazard. • Ground-based searches could be augmented by space-based searches. • Analyses of environmental stages are rather crude. • So far, there is very little modeling of possible and social consequences.

47. Findings: Evaluation, Warning, and Mitigation • Existing structure is disorganized: Astronomers are just starting to learn how to communicate, but no relevant agencies are prepared to listen and act. • Asteroid deflection scenarios have been conceived, but no serious systems engineering or planning has been done to deal with various possible cases. • No known consideration by civil defense and disaster management agencies, let alone any assignment of responsibilities to scientific agencies. • No government or formal scientific advisory body has established the level of priority the impact hazard should command with respect to other national priorities.

48. Recommendations • More public education about impact hazard; public officials need to be brought up to speed. • Formal notification protocols need to be implemented concerning predicted potential impacts. • Connections must be made with national emergency management agencies and analogous international agencies. • Torino Scale needs to be clarified and utilized. • Systems studies needed on proper mix of approaches to extend the Spaceguard Survey. • Research is needed on how to couple deflection technologies to asteroids, how to avoid disruptions, and more.

49. Conclusion • Humans have the intelligence and capability to protect our civilization from this threat from the skies. • The dinosaurs failed. • Is society going to gamble and submit to fate or will it undertake a measured, rational response? • Society must educate its leaders and public officials to the facts of the impact hazard, decide what should be done, then do it.