Asteroids

2. Asteroids

3. In general, asteroids are small

6. Asteroids are found by looking for moving objects (streaks) in long exposure photographs

8. A Short History of Asteroids • 1000 km Ceres was discovered in 1801 • 600 km Pallas was discovered in 1802 • Juno and Vesta were discovered in the 19th Century -- all the rest in the 20th Century • Officially, there are about 7000 known asteroids, most tiny and less than 100 km across, but as many as 100,000 might be out there • Even 100,000 spread out over an 18,000,000 mile orbit means that they are rather rare

9. Was the asteroid belt once a planet that has since been destroyed? • If all the asteroids were assembled into a planet, it would have a tiny diameter of only 1500 km, or about 12% Earth’s diameter. • The combination of the pull of the Sun’s gravity and Jupiter’s gravity keeps the asteroids “stirred up” enough to keep anything from coalescing. • Most of the original asteroids are gone!

11. Asteroid Ida and its tiny moon, Dactyl

12. Close-up of asteroid Ida Only 12 meters across

14. Asteroids exist outside the asteroid belt • Trojan asteroids in front and behind Jupiter • Apollo asteroids which cross Earth’s orbit about the Sun • Kuiper asteroids (Kuiperoids) exist beyond the orbit of Neptune • these Kuiperoids might not be rocky asteroids at all, but rather, icy comets

15. Comets

16. Comet West Comet Kohoutek

17. Comets lack tails until they enter the inner solar system

18. Comets often have two tails:a thin ION tail and a curving DUST tail

20. Anatomy of a comet

21. 15 km long by 8 km wideComet Halley nucleus

22. Comets seem to come from two possible places • Oort Cloud • Reservoir of long period comets that might only come through the solar system once in billions of years and can come from any direction • Kuiper Belt • Reservoir of short period comets that come through the solar system regularly and only come in along the plane of the ecliptic

24. The Kuiper Belt of comets spreads from Neptune out 500 AU from the Sun

25. Known Kuiper Belt Objects

26. Kuiper Belt Object 1993SC - these images were taken 4.6 hours apart

27. Pluto: The biggest Kuiper belt object?

29. Comet orbits are altered by gravitational interactions with planets

30. Small rocky debris peppers the solar system • meteors • falling stars • shooting stars • bolides • fireballs each are caused by small rocks colliding with Earth’s atmosphere and heating up due to friction with the air

35. Meteorites are space debris that land intact

36. Meteorite Types • Stony meteorites • look much like ordinary rocks • Iron meteorites • heavy and composed on iron and nickel minerals • Stony-iron meteorites • contain roughly equal amounts of rock and iron • rare ones are carbonaceous chondrites which have never melted and contain amino acids - one of the building blocks of life

37. Impact craters and meteor showers mark remnants of space debris on Earth Arizona crater is some 50,000 years old

38. The 1908 Siberean Tunguska mystery provides evidence of catastrophic collisions

39. Henbury, Australia

40. Popigai, Russia

41. A large asteroid’s impact with Earth may well have killed off the dinosaurs 65 million years ago

44. Gravity Field of Chicxulub Impact Crater

45. Could a killer asteroid strike Earth soon? • Yes, but it is extremely unlikely • Extinction-level impacts probably occur only once every 50-100 million years (the last big one was 65 million years ago) But: • Smaller impacts happen much more often!

46. Homework #2 review Good: • Nice sketches! • Descriptions: “like a thumbnail” • Most described phases & motion Not as good: • Not to scale, no time & date, stars • Didn’t answer “Why?” • Confusion about diurnal motion (east to west) vs. lunar orbit (west to east)