Stones from the sky?

1. Stones from the sky? In the Aristotelian model of the Universe, planetary orbits are separated by crystalline spheres. It is consequently impossible for stones to fall from heaven (a view also held by Newton).

2. In 1795 farm labourers in Yorkshire (N.England) reported that during a severe thunderstorm a stone had fallen from the sky and buried itself in the ground. One farm labourer had been so close that he was hit by mud and debris. The stone created an impact crater about 1 m in diameter, and had to be dug out of the ground.The local squire (Capt. Topham) exhibited the stone in London (entrance fee = 1 shilling), and provided testimonials from locals who had heard or seen it fall. Sir Joseph Banks (the President of the Royal Society) compared it to other imputed “meteorites” from Italy, India, and Paraguay, and concluded that stones could, indeed, on occasion fall from the sky. Captain Edward Topham Topham’s monument to the meteorite (1799)

3. Asteroid/Comet Impacts Geog 312 Ian Hutchinson

4. TOPICS1. The ThreatDirect and Indirect Effects2. Risk AnalysisCalculating the probabilities3. Protection?NASA to the rescue?

5. Asteroids • Asteroid orbits continuously modified by gravitational perturbation of asteroid belt. • About 2000 asteroids currently have orbits that cross that of Earth (= NEO’s :Near Earth Objects). • Orbital trajectories of 200 NEO’s are known; i.e. the paths of 90% of the asteroids that threaten Earth are unknown. • Largest NEO’s have diameters of about 8 km; the orbits of about 35% of asteroids >5 km diameter are known.

6. Asteroid size-frequency relations

7. Comets • About 10-20% of comets (piles of rubble and ice with tail =“coma”) are in Earth-crossing orbits. • Some 700 long-period comets (T>200 yrs) known. • Periodic comets (T≤200 yrs) - 95% have lost their coma (= “stealth comets”) 25 known, 1500 > 1 km diameter may exist. • Our first warning is likely to be their initial entry into Earth’s atmosphere.

8. Effects • Direct (predominantly local)Impact crater plus blast-wave and firestorm • Indirect effects (may be global)Dust veil (large impactors)Acid rain (large impactors)Tsunami (oceanic impacts)

9. Impactors • <10 m diameter - burnup in atmosphere. • Category 1: 10-100 m diameter - disintegrate in atmosphere; exploding fragments create “airburst” (e.g. Tunguska event). • Category 2: 100 m - 1 km diameter - capable of striking surface, forming impact craters, effects local (e.g. Meteor Crater, AZ). • Category 3: > 1 km in diameter may cause severe global effects (e.g. Chicxulub impactor, Mexico)

10. Impact craters on Mercuryindicative of the protective effects of Earth’s atmosphere

12. Category 1: Tunguska • 50-60 m diameter stony meteor? exploded in June 1908 above central Siberia. Energy release ~ 10-30 MT TNT (~1 000 - 3 000 Hiroshima bombs) • Radius of destruction: 25 km (= 2 000 km2). • Recorded by seismograms in Irkutsk and barograms in London.

13. First photos of Tunguska fireball were taken by a Russian expedition in 1920’s, more than a decade after the event.

14. 1200 m wide, 180 m deep Category 2:Meteor (a.k.a. Barrington) Crater, AZ. Impact occurred about 50 000 years ago; it is likely that all plant and animal life within 10 km of the impact site was vapourized.

15. Category 3

16. Category 3 Crater 10 - 15x diameter of impactor Veil of dust in atmosphere for months/years Reduced sunlight Reduced photosynthesis Lowered global temperature Polar and temperate areas uninhabitable Food chain collapses

17. Category 3 Very high temperatures at impact site Firestorm spreads from impact site N2 in atmosphere burns Intense smokefrom firestorm: reduced sunlight, etc. Nitric acid produced; acidic precipitation Reduced photosynthesis; food chain collapses

18. Rock hammer for scale Sandstone Tertiary Clay Coal Cretaceous Shale Asteroid impact dust deposit (clay layer) marking K-T boundary at 65 Ma BP in Colorado, 2500 km from impact site.

19. Hazardclassification The scale value PS is given by PS = log10 [PI / (fB. DT)], where PI is the impact probability of the event in question and DT is the time until the potential event, measured in years. The annual background impact frequency, fB = 0.03 . E-4/5 is the annual probability of an impact event with energy (E, in megatons of TNT) at least as large as the event in question. The Palermo scale was developed to categorize potential impact risks. Intended for use by specialists.

20. Hazardclassification The scale was devised by delegates to an international symposium in Torino (Turin; Italy) in 1999 as a means of communicating risk to the public.

21. Potential impactor: (2002 NT7:Feb 01/2019?) Initial reports based on on only a handful of observations of NT-7’s orbit in 2002 2002 NT7 is 2 km in diameter

22. The NT7 scare [2002] * * based on an assumed initial velocity of 25 km/s

23. VK184 will cross the Earth’s orbit four times between AD2048 and 2057. The probability of impact during this time is 0.0001 (~1:10,000) Extremely unlikely to collide with Earth in this period Current* top three NEOs(ranked by Palermo scale) N.B. 2002 NT7 no longer features on the list of potential impactors. * as of Aug. 15, 2008 (http://neo.jpl.nasa.gov/risk/)

24. What is the probability that an inhabited area or city will be hit?

25. Computing annual probability of impacts (Tunguska ~300 yr recurrence; = 0.003 annual probability) Tunguska

26. Impact probability (P) where: P = P(D) * P(A) D = projectile diameter; P(D) = annual frequency of projectile D; P(A) = probability of hitting target ; = area of target/surface area of Earth

27. Annual probability (P) of a “Tunguska event*” impacting: 1) an inhabited area (10% Earth area)P = 0.003t * 0.1 = 1 : 3 300 2) a city (1% Earth area) P = 0.003t * 0.01 = 1 : 33 000 3) Fraser lowlands(0.01% Earth area) P = 0.003t * 0.00001 = 1 : 33 million *assumes 300 yr return interval for Tunguska event (estimates range from 50-500 yr recurrence)

29. Tunguska impact area from a local perspective

31. NT7

32. Oceanic Impacts:the tsunami hazard Tsunamis reach all amphi-oceanic areas within 10 hours of impact

33. airbursts NT7 after Ward and Asphaug (2000)

34. Simulation of 500 m diameter asteroid impact into 5 km deep ocean vi = 20 km /sri = 3.3 g/cm3 t = 25 s After Crawford and Mader (1998)

35. Ocean impact tsunami Source:www.lanl.gov/worldview/news/tsunami.mov (Stephen Ward)

36. after Ward and Asphaug (2000) vi =impactor velocity; ri = impactor density; h = water depth

37. 1000-year probabilities (%) of impact tsunami exceeding critical wave height at typical coastal and mid-ocean sites in the Pacific Ocean after Ward and Asphaug (2000)

38. “Barriers” =ridges “Fingers of God” N. America Impact tsunamis: bathymetric effects Impact site =abyssal canyons; up to five-fold increase in wave height at coastline Africa Europe

39. Deep Impact Project • NASA detonated a 370 kg impactor (= 5 T of dynamite) in a near-Earth comet (9P/TEMPEL-1) on July 4, 2005. • The primary purpose was to study cometary structure (which proved to be less icy and dustier than expected), but the experiment may illustrate the effects of trying to deflect or fragment such objects before they reach Earth. View of the nucleus of the comet 9P/Tempel-1 from impactor • BUT - is it advisable to create numerous projectile fragments?

40. Spacewatch Project • Initiated at the University of Arizona in early 1980’s, the Spacewatch project involves automated searches of the sky for 20 nights per month for new asteroids (particularly NEOs) and short-period comets. Now includes cooperative efforts with other observatories in North America, Europe and Australia.

41. Topic One Graphics courtesy of: University of Bologna, NASA, Don Davis, US Geological Survey, Natural Resources Canada http://fernlea.tripod.com/ woldcottage.html