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Space Rocks
Space. It's not just a cool place to explore -- it rocks. And, space has many rocks! The rocks in space are sometimes called small bodies. Some of them are larger and are called minor planets or planetoids. They may also be known as NEOs -- Near Earth Objects. Whatever they may be called, they have much to tell us about our solar system. The three main types of space rocks are comets, asteroids and meteoroids.

Comas, Trails and Comet Tails
Comets are the "dirty snowballs" of space. They are bodies of ice, rock, gas and dust. The dirty snowball is the nucleus -- the solid center of the comet. The average comet nucleus is less than 10 kilometers (6 miles) across. A small comet is about the size of a house.
Drawing of a comet in different places as it orbits the Sun

Image to left: The comet's dust tails point directly away from the Sun. The gas tails curve toward the path of the orbit. Credit: NASA

Comets are thought to have originated in a cold area beyond Neptune and Pluto called the Oort Cloud. This cloud surrounds the solar system. Another area where comets are found is called the Kuiper (KIE per) Belt -- just past Neptune's orbit.

Comets follow a regular orbit around the Sun. They journey on a trail that may take 200 years to complete, however. And these are called short-period comets. If the comet nucleus is pulled into an orbit that brings it close to the Sun, the heat from the Sun will cause the icy nucleus to evaporate. Some of the ice and gasses start to boil off. This material forms a cloud around the nucleus. The cloud is called the coma and may be hundred of thousands of kilometers in diameter. As the comet gets closer to the Sun, the coma grows. The solar winds push the dust and gas away from the coma causing them to stream off into space to form the comet's tail. The solar winds cause the comet's tail to point away from the Sun. The tails of comets can reach 150 million kilometers in length! Each time the comet passes close to the Sun, it loses some of its material. Over time, it will break up and disappear completely.

Odd Balls
Asteroids are the oddballs of space. Whereas most heavenly bodies are spherical, most asteroids are odd-shaped. Most asteroids are made of rock, but some are made of metals such as nickel and iron. The sizes of asteroids vary. They can be as small as a boulder -- a couple of meters -- or as wide as a state -- several hundred kilometers across. The largest asteroid we've seen, Ceres, is almost as wide as Texas. It is 960 kilometers (600 miles) wide. Because of their size, asteroids are sometimes called minor planets or planetoids. Some asteroids even have moons that orbit them.

Most asteroids orbit the Sun in a region between Mars and Jupiter -- the Asteroid Belt. Sometimes, as they orbit, they venture into Earth's orbit. Scientists have found evidence that some have impacted Earth.
Up close photo of large asteroid with a small moon to the back

Image to left: Asteroid Ida has its own "moon," Dactyl. Credit: NASA

-Oids, -Ites and -Ors
There are smaller rocks in space -- meteoroids. But there are also meteorites and meteors. So what's the difference between a meteoroid, meteor and meteorite? A meteoroid is just a chunk of rock in space, usually a piece of an asteroid or comet. They are usually less than 10 meters (33 feet) wide. A meteor, or shooting star, is a streak of light that is caused by the friction when a meteoroid from space enters the Earth's atmosphere. And, meteorites are space rocks that survive the friction of the atmosphere and land on Earth.

You may see a meteor streak through the sky on any night. But, when Earth enters the orbit of a cluster of meteoroids, we have what we call a meteor shower. Earth will enter the next meteor shower, the Leonids, on November 18.

Why Space Rocks?
NASA and other scientists study space rocks to help us learn about the origin of our solar system. Asteroids are thought to be the leftovers from the formation of the inner solar system, including Earth. Comets are thought to be leftovers from the formation of the outer planets.

Finding NEOs
NASA has missions to Near-Earth Objects. NEOs are the comets and asteroids whose orbits have been nudged into Earth's path by the gravity of nearby planets. The Near-Earth Object Program will coordinate NASA-sponsored missions to find, track and classify small bodies that could approach Earth.

As of October 23, 2004, there have been 3,086 NEOs discovered. More than 700 were asteroids with diameters of 1 kilometer (.6 mile) or larger. Of these, more than 600 are classified as PHAs -- Potentially Hazardous Asteroids.
Photo of a large crater in the desert

Image to right: This meteor crater in Arizona is thought to be the result of an asteroid hitting Earth 20,000-50,000 years ago. Credit: NASA

The Big Question: Will a NEO Hit Earth?
We don't know when the next NEO impact will take place. But we can calculate the odds. The greatest danger would be from an NEO that is about 2 kilometers (1.2 miles) across. On average, one of these collides with the Earth once or twice every million years. By looking at the data of NEOs that we know about, we would be able to tell if it is going to enter Earth's path decades before it would happen.

Adapted from Near Earth Objects Program: FAQs
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