1. Notes 2-4 Asteroids, Meteors and Comets 2/26/09

2. Asteroids Asteroids are metallic, rocky bodies without atmospheres that orbit the Sun but are too small to be classified as planets. Tens of thousands of asteroids congregate in the so-called main asteroid belt: located between the orbits of Mars and Jupiter. Although only a few hundred are known it is estimated that a few thousand asteroids orbit in what is known as the Lagrange points which are 60 degrees in front and behind Jupiter along its orbital path. These are known as Trojan asteroids.

4. The largest known asteroid, Ceres, is also the first discovered one. Ceres was discovered in 1801 and is about 600 miles in diameter. To the right is an image of Ceres taken from the Hubble Space telescope. Asteroids range in size from the great Ceres down to the size of pebbles. Sixteen asteroids have been discovered with diameters of 150 miles or more. Asteroids

5. This picture shows the sizes of the original three dwarf planets (Eris, Ceres, and Pluto) as compared to Earth. It also shows Pluto's large moon Charon (and its two small moons Nix and Hydra) and Eris's moon Dysnomia to scale. The image also shows Earth's Moon (Luna) and the planet Mars for comparison.

6. It is estimated that the total mass of all asteroids would comprise a body approximately 930 miles (1,500 kilometers) in diameter -- less than half the size of the Moon. Most asteroids are irregularly shaped due to multiple collisions with other asteroids. Asteroids For example, the asteroid 216 Kleopatra is about the size of New Jersey and is shaped like a dog bone.

7. The majority of asteroids fall into the following three categories: C-type (carbonaceous): Includes more than 75 percent of known asteroids. They are very dark. Composition is thought to be similar to the Sun, depleted in hydrogen, helium, and other volatiles. C-type asteroids inhabit the main belt's outer regions. S-type (silicaceous): Accounts for about 17 percent of known asteroids. They are relatively bright. Composition is metallic iron mixed with iron- and magnesium-silicates. S-type asteroids dominate the inner asteroid belt. M-type (metallic): Includes many of the rest of the known asteroids. They are relatively bright. Composition is apparently dominated by metallic iron. M-type asteroids inhabit the main belt's middle region. Asteroids

8. Meteors are also known as shooting stars. A shooting star happens when a small (pea sized) piece of interstellar dust or rock enters the Earth’s atmosphere at a high speed. The high speed though the atmosphere causes enormous air pressure in front of the meteor which causes the air to heat up and burn the meteor as it streaks though the sky. Most meteors burn up in the atmosphere long before they come close to the ground. A meteor shower happens when the Earth moves through the dust trail left behind the path or a comet. Meteors

9. If a meteor is large enough it will get much farther through the atmosphere without burning up. When it gets down to where the air is much more dense (5-7 miles up) it will fracture and possibly explode and burn much brighter, this is known Meteorites as a fireball. Some fireballs do not completely burn up before they strike the ground. Meteors strike the ground more often than you might think, probably about 2 or 3 per day worldwide. However, not all meteor impacts are dramatic events like those depicted in the movies. A meteorite is a meteor that has struck the ground.

10. The Peekskill meteor of 1992 was captured on 16 independent videos and then struck a car. Documented as brighter than the full Moon, the spectacular fireball crossed parts of several US states during its 40 seconds of glory before landing in Peekskill, New York. The resulting meteorite, pictured here, is composed of dense rock and has the size and mass of an extremely heavy bowling ball.video

11. When large meteors hit the ground they leave behind craters. The earth has far fewer craters on our surface than other bodies in space (such as the moon) because of weather erosion hiding the evidence of earlier impacts. Some impact craters still Meteorites remain very much intact on Earth. Also, we have found and studied meteorite fragments.

12. This is Meteor Crater located near Winslow, Arizona is the result of a nickel-iron meteorite that fell on earth 49,000 years ago. It was 150 feet across (only!), weighed roughly 300,000 tons, and was traveling at a speed of Meteorites 40,000 miles per hour. The force generated by its impact was equal to the explosion of 20 million tons of TNT. Meteor crater is ¾ of a mile across and 550 ft deep.

13. A widely accepted theory blames the impact 65 million years ago of an asteroid or comet at least 6 miles (10 kilometers) in diameter for mass extinctions among many life forms, including the dinosaurs. A large impact such as this one would cause a huge amount of dust and debris to be thrown into the atmosphere Meteorites causing the sun’s light to be blocked for years from reaching the surface of the Earth, killing plants and eventually animals.

14. The existence of comets has been known about since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC. As of 1995, 878 comets have been cataloged and their orbits at least roughly Comets calculated. Comets are sometimes called dirty snowballs or "icy mudballs". They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.

15. nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids; coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus. Comets hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen; dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye; ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

16. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Halley), stay within the orbit of Pluto for a Comets significant fraction of their orbits. Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.

17. As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the solar system by a close encounter (esp. with Jupiter). Comets By far the most famous comet is Comet Halley which visible every 75-76 years. It was last seen in 1986 and will return in 2061. It is the only short period comet that is visible to the naked eye. Comet Shoemaker-Levy 9 was a "big hit" for a week in the summer of 1994.

18. Comets From July 16 through July 22, 1994, pieces of an object designated as Comet P/Shoemaker-Levy 9 collided with Jupiter. This is the first collision of two solar system bodies ever to be observed. Comet Shoemaker-Levy 9 consisted of at least 21 discernable fragments with diameters estimated at up to 2 kilometers.