1. Astronomical Doomsday Scenarios

2. Comet and Asteroid Impacts

4. Impact of Comet Shoemaker- Levy on Jupiter

7. Major Impacts on Earth Comets and asteroids do hit the earth periodically. (But not nearly as often as they used to.) The last big impact was in Tunguska, Siberia in 1908. Everything was destroyed within 30 km.

8. Major Impacts on Earth

12. Tycho

13. 160 million years ago 170 km

14. 160 million years ago

15. 109 million years ago 65 million years ago 170 km

16. Energy  m v 2 10 km asteroid at 1 m/s: Energy = 0.03 megatons 10 km asteroid at 20,000 m/s: Energy = 10,000,000 megatons ! Why are comet and asteroid collisions so destructive?

17. Frequency of Impacts

20. Near-Earth asteroid: Aphophis (2004 MN4) Close encounter with Earth on April 13, 2029 (Friday the 13th) Will be within 20,000 miles of Earth Rare event; once every 1,000 years Chance of impact in 2029 was 1/30, but now very low Impact in 2036 still possible; depends on how Earth’s gravity changes its orbit Diameter = 300 meters; catastrophic on regional scale

22. Gas and Dust Clouds

24. If Earth passes through a molecular cloud: Dust accumulates in atmosphere, blocks sunlight, causes ice age Oxygen in atmosphere combines with hydrogen from cloud to make water Gravity of cloud causes comets in outer solar system to bombard inner solar system Probably happens every 0.1-1 billion years

25. Gamma Ray Bursts

29. A gamma ray burst in the Milky Way and with its jet pointed in our direction: Would shine as brightly as the Sun for a few minutes Might also bombard the Earth with high-energy cosmic rays, eroding ozone layer Rate estimates from once every 10 million years to once a billion years Plausible mechanism for mass extinctions on Earth

30. Death of the Sun