2. Meteoroids, Meteors, Meteorites Meteoroids--small rocky or metallic particles throughout the solar system Meteors-- entering Earth’s atmosphere, friction causes heat and disintegration- -Meteorites--remnants surviving the fall to hit the ground

3. Where Do Meteorites Come From? Some meteorites are primitive leftovers of the formation of the solar system, possibly from asteroids. Some meteorites travel on highly elliptical orbits from the asteroid belt to cross the Earth’s path, due to collisions between asteroids. Many “meteor showers” are associated with comets, but most of these are small particles that burn up in the atmosphere.

4. Comet’s path Earth Meteor showers are caused by Comets! Many “meteor showers” are associated with comets. The dust tail contains small particles evaporated from the comet. These particles remain in orbit about the Sun. If the Earth passes through the “dust cloud”, then meteors may be seen.

5. Iron 4% (from core of differentiated asteroids) Stony 95% (from surface of differentiated asteroids) Stony-iron (from undifferentiated asteroids) Rare class of Stony Carbonatious Chondrites- C, Si, Mg covered with dark carbon organic compounds, maybe primordial Material. Types of Meteorites Meteors are: Early solar system leftovers

6. Meteorite Types Stony 95% Most common Rocky, carbon, silicates usually

7. Irons 4 % of all are nickel-iron Iron meteorites often have a pattern of ‘thumbprints’ caused by fiery re-entry The hot molten iron is pushed back by the air and solidifies. Iron meteorite cut, polished and etched with dilute acid.

8. Carbonaceous A specialty of some carbonaceous meteorites is the evidence that they have been processed by liquid water. We believe the parent meteorite bodies must have been sufficiently warm and pressured at some period in history for liquid water to flow.

9. The Barringer crater, also called the “Meteorite Crater” in Arizona was formed by the impact 50,000 years ago of a 300,000 ton iron meteorite. The crater is 550 feet deep and nearly a mile wide. About 30 tons of iron have been recovered from the surrounding plains.

10. Happy Hunting Ground: The Antarctic Antarctic ice sheets have proved to be very fruitful grounds for hunting meteorites. They are particularly well suited to identifying falls: the fragments stand out clearly against the blue ice. Picture: (left) NASA/JSC (right) Dr Ursula Marvin, Smithsonian Astrophysical Observatory

11. Ages of Rocks The technique used today to measure the ages of rocks is radioactive or radioisotope dating. The advent of radioactive dating of rocks once and for all set geological timescales on an absolute, as opposed to a relative basis.

12. Some of the falling rocks from space survive the trip through the atmosphere…

13. Origin Of some Meteorite

14. Large Meteor over the Tetons (1972) Aphelion distance 2.3 AU Diameter 3 to 10 m Seen at height of ~50 km – skipped out of atmosphere

15. Asteroids

16. Main Belt The so-called Main Belt of asteroids lie between the orbits of Mars and Jupiter, with semi- major axes 2.2 to 3.3 AU.

17. Facts About Asteroids There are about 150,000 asteroids cataloged. In spite of what this diagram might imply, the asteroid belt is relatively empty. Total mass < Moon The average distance between any 2 is more than 1 million km.

18. Asteroids, like planets, have orbits close to ecliptic plane, small tilt, and nearly circular orbits. Asteroid orbits As Jupiter formed it stirred up velocities in what would become the asteroid belt. Higher velocities meant planetesimals destroying each other rather than accreting. Asteroids are basically chunks of rock left over from the formation of the solar system.

19. Asteroid Composition Three types based on appearance and Composition C : carbon & silicates Very dark, primitive (old) S : stony M : metal (iron) Remnants of the core of shattered larger body We probably now know almost all of the asteroids larger than 25 km across, and 50% of the ones down to 10 km in size. No carbon, lighter appearance Abundance of silicates

20. Orbits: Gaps In the main belt, orbital distances are not distributed evenly. The gaps in the orbital distances are known as resonance, or Kirkwood gaps. The resonance effect is the gravity of Jupiter: the tugs from Jupiter ’s gravity eventually altered the orbit.

21. Outside The Main Belt Outside the main belt, the gravity of Jupiter makes most nearby orbits unstable. There are five Lagrangian points, but in terms of asteroids, the L4 and L5 points equidistant from Jupiter and the Sun are most important. These are regions of gravitational stability for small bodies in the fields of two larger bodies.

22. The first was named Hektor and all others named after heroes of the Trojan War. Hence, these asteroids are named Trojan Asteroids. The L4 and L5 points of Jupiter are occupied by hundreds of asteroids. They are primitive bodies, trapped there since the birth of Jupiter.

23. Near-Earth Objects Only about 1% of asteroids cross the Earth’s orbit, but we are very interested in them! The first one discovered was Apollo in 1948: for this reason Earth-crossing asteroids are called Apollo asteroids. The largest Earth-crosser is Eros (30 km).

24. Meeting Asteroids Up-Close Spacecraft have made close-flybys of 4 asteroids, and even landed on one! The first two significant encounters were due to the Galileo spacecraft, en route to Jupiter, which made flybys of : Gaspra The NEAR-Shoemaker spacecraft made two encounters: A flyby of Mathilde in 1997 ( Mathil-de) Orbited and finally landed on Eros in 2000.

25. 951 Gaspra S-type (Gasp-ra) Gaspra on October 29, 1991. The high- resolution image was taken at a distance of 5300 km. Gaspra measures 19x12x11 km The highly irregular shape indicates massive collision(s) in the past which nearly destroyed it. More than 600 small craters are visible here, from 100-500m in size.

26. 243 Ida and Dactyl S-Type Galileo encountered Ida on August 28, 1993, finding an irregular body 58x24x21 km in size. The main discovery was that Ida is accompanied by a small moon, Dactyl, the first natural satellite of an asteroid ever discovered.

27. NEAR Shoemaker The Near-Earth Asteroid Rendezvous (NEAR) spacecraft, was launched in 1996, to encounter, orbit and land on asteroid 433 Eros. It was later named Near Shoemaker. The spacecraft continued to operate for more than a week on the surface.

28. Close up of Eros

29. The Last photo before the hard landing

30. The Big One Do we ever get hit? Yes! The questions are when and by what?

31. Classify events by size –Small –Medium –Large Earth Impacts Small: –Happens all the time, most are very tiny (‘pea’ sized) –Will burn up or break up in the atmosphere –Meteors! (“Shooting stars) 3-5/hr on a typical night 100 tons per day Meteor showers & storms From the trail of a comet, each “shower” happens once every year

32. Earth Impacts: Medium Medium: 50 m - 1 km across at top of atmosphere Only one every century Causes “severe local damage” Two great examples from ‘recent ’ history. The Tunguska event The Berringer Crater, in Arizona

33. The impactor was probably a comet 30 m across. (1) The Tunguska event Happened in 1908,Tunguska, Siberia, Russia, flattened a forest,where 2,150 square km flattened, and 1/2 of the area was burned. Early in the morning the object was visible with a tail 800 km long. The object vaporized at an altitude of 6 km, leaving no crater. Herds of reindeer burned, a few human fatalities. Night skies in Western Europe were bright enough to read a book by the light.

35. (2) The Berringer Crater, in Arizona Happened 50,000 years ago The crater: 1,200 m across,200 m deep The impactor? An iron meteorite, 100 m across Going 40,000 mph! Explosion = 20 million tons of TNT a moderate atomic bomb

36. The Berringer crater, Arizona

37. Earth Impacts: Large Large: more than 1 km across at top of atmosphere –One every few million years –Severe global effects –More than 2 km can cause mass extinction –Most recent: 65 million years ago

38. The K Impact Clay layer of that age over much of world has high Iridium levels –Iridium is rare in earth’s crust, but is common in asteroids and (sort of) in comets 60 % of all species on Earth disappeared, including the dinosaurs, 65 my ago, at the end of the Cretaceous (K) period.

39. The K extinction event The impactor: Probably a comet Size: ~ 10 km across Energy released: 100 million million tons of TNT (5 million atomic bombs) Some day..... possible

41. Comets - History Barer of Bad News 66 – Fall of Jerusalem 451 – Rise of Attila the Hun 1066 – Norman conquest of England 1456 – Turks invade Europe; Pope prays for deliverance from “the Turk, and the comet” Once believed to be atmospheric phenomena Tycho Brahe proved they were not atmospheric 1700 & 1758 – Edmond Halley predicts return of Halley’s comet, & comet returns on schedule 1835 & 1910 – Mark Twain & Halley’s comet Usually get one bright comet per decade

42. Famous Comets - Halley’s Comet Comet of 1577

43. A comet is a great blob of ice and dust, a dirty snowball. A comet far from the sun has only a nucleus. The ice ball may be anywhere from 1 mile to a dozen miles in diameter. As the comet gets closer to the sun, solar heat vaporizes more and more of the frozen gases, and it spews out into space, blowing some dust out also. The gas and dust form a form a hazy cloud around the nucleus called the Coma.

44. –Coma (~ 1 million km) Lots of gas and dust produced by jets from nucleus Coma forms only when comet passes inside ice line –Nucleus (~ 1 km – 20 km) Dirty snowball (icy composition) Lots of dust and gas Jets ignite when they hit sunlight.

45. Comet Nuclei Thought to be ice, but mostly Hydrogen is observed. Darker than coal. Jets of gas and dust venting from core are observed.

46. Comets Comets often have 2 tails pointing away from the sun Type 1 is the gas tail – straight with emission lines. (Ion ) Type 2 is the dust trail – curved with reflection spectra tails, may be > AU long Ion Tail Dust Tail Coma (Nucleus too small to see)

48. Comets have the most eccentric (elliptical) orbits of any solar system object,they spend most of their time far from Sun. Comet orbits Prograde orbit is in same direction as planets orbit and Retrograde orbit is in opposite direction.Comet orbits are equally divided between prograde and retrograde. Comet's orbital plane can have any tilt (inclination) relative to ecliptic plane.

50. Comets Are of Two Kinds Long Period Comets seen only once; periods of at least 100,000 yr come in from all angles originate just outside of the Solar System (the Oort Cloud), Short Period Comets periods of years to centuries more closely confined to the ecliptic plane originate mainly in the Kuiper Belt (Ki-per) ices some what processed by the Sun Example: Halley’s comet thought to be from the Solar Nebula material

51. Where do comets come from? Solar System, 39 AU Kuiper Belt, 50-200 AU Oort Cloud, 1-2 Light years

52. Short period comets-Kuiper Belt (Ki -per ) These objects are typically 10 to 100 km in diameter, Hundreds are now known. In 1951 Gerard Kuiper speculated that there was material farther out than Pluto, from which certain comets originated.

53. Most (original) orbits have aphelions of >1000 AU Passing stars deflect comets in from the cloud The Oort Cloud

54. Shoemaker-Levy-9 in 1993 SL-9 passed close to Jupiter (within Roche limit) in July 1992 First real opportunity to watch large impacts happen – and with ~1 year advance notice. Impacts into atmosphere – not into surface Gravitational forces pulled the comet apart into fragments Each fragment on slightly different orbit, headed back towards Jupiter Impacts on Jupiter over period 16 – 22 July 1994

55. SL-9: Impacts on Jupiter

57. Comets near the Sun

58. The End ( A possibility)