Comets A comet is a very small solar system body made mostly of ices mixed with smaller amounts of dust and rock. Most comets are no larger than a few kilometers across. The main body of the comet is called the nucleus, and it can contain water, methane, nitrogen and other ices.
When a comet is heated by the Sun, its ices begin to sublimate (similar to the way dry ice “fizzes” when you leave it in sunlight). The mixture of ice crystals and dust blows away from the comet nucleus in the solar wind, creating a pair of tails. The dust tail is what we normally see when we view comets from Earth. A plasma tail also forms when molecules of gas are “excited” by interaction with the solar wind. The plasma tail is not normally seen with the naked eye, but can be imaged. Comets normally orbit the Sun, and have their origins in the Oort Cloud and Kuiper Belt regions of the outer solar system. Well-known comets include the non-periodic comets Hale-Bopp (C/1995 O1), Hyakutake (C/1996 B2), McNaught (C2006 P1), and Lovejoy (C/2011 W3). These flared brightly in our skies and then faded into obscurity. In addition, Comet Shoemaker-Levy 9 (D/1993 F2) was spotted after it had broken up after a close call with Jupiter. (The D in its proper designation means it has disappeared or is determined to no longer exist). More than a year later, the pieces of the comet crashed into Jupiter. The periodic Comet Halley (1P/Halley) is the most famous in history. It returns to the inner solar system once every 76 years. Other well-known periodic comets include 2P/Encke, which appears every 3.3 years and 9P/Tempel (Tempel 2), which was visited by the Deep Impact and Stardust probes, and makes perihelion around the Sun every 5.5 years. Facts about Comets
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Asteroids Asteroids are small, rocky solar system bodies that populate interplanetary space out to the orbit of Jupiter. There are millions of them, and they are often grouped by their composition. The planetary science community refers to them as minor planets, a general term applied to solar system bodies smaller than moons. Asteroids are mainly made of materials left over from the formation of the inner solar system words. Most of them orbit the Sun between Mars and Jupiter, although there are groups of them that orbit closer. Asteroids come in three composition classes. C-types (chondrites) are made of clay and silicate rocks. S-types are the so-called “stony” asteroids and are made mostly of silicate rocks and nickel-iron mixtures. M-types are metallic nickel-iron. These categories indicate how far from the Sun they formed in the early solar system.
Facts about Asteroids
Meteorites Earth is bombarded with millions of tons of space material each day. Most of the objects vaporize in our atmosphere, but some of the larger pieces (from pebbles to boulder-sized rocks) actually fall to the ground. Most of the objects come from asteroids, which are objects made of various types of rock and have existed since the origin of the solar system. A small rocky or metallic chunk of material that travels through space is called a meteoroid. Very small meteoroids (the size of dust) are often referred to as micro-meteoroids or space dust. These fragments may also be leftover comet debris, or were ejected in collisions between other solar system bodies such as the Moon or Mars.
As a meteoroid travels through our atmosphere, it is heated by friction. That causes it to glow, and if this happens at night, we see a long streak of light known as a meteor. If the object survives the trip and falls to Earth’s surface, it is known as a meteorite. Many of these fall into the ocean (since about 71% of Earth’s surface is covered by water). The rest fall on land, where they await discovery by meteorite hunters. Facts About Meteorites
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