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Comets, Asteroids, and Meteors Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 17, 2011

Myriad Meteoroids

The largest known asteroid, Ceres, measures hundreds of kilometers in diameter. The smaller we go, the more asteroids there are, in general. There isn’t any definitive limit at which we say, “This thing is an asteroid, but if it gets 1 milligram less massive, then we’ll call it a meteoroid.” In a general way, we can say that if it’s too big to be called a boulder, then it’s an asteroid; boulders and rocks in space can be called meteoroids .

Even then, it’s not that simple. Meteoroids and comets are substantially different. Comets consist of rocky and icy material combined, but meteoroids have essentially no ice. Some meteoroids are stony, something like the granite we know on Earth. Others are mixtures of stony and metallic stuff. Still other meteoroids are mostly metal, largely iron. Some resemble pieces of amber or glass.

How do we know what meteoroids are made of? They’re too small to be observed through telescopes; space probes have never been sent specifically to visit them, although some of them have struck our space vehicles. We know about meteoroids because they often fall to Earth’s surface. Technically, when a meteoroid enters Earth’s atmosphere, it becomes known as a meteor . If a meteor survives to reach the surface, either as one object or as fragments of the original meteor, then it becomes a meteorite . Meteorites are abundant on the continent of Antarctica, where the ice preserves them, and where they are easy to differentiate from Earth’s surface.

There are believed to be more rock-sized meteoroids than boulder-sized ones, more pebble-sized ones than rock-sized ones, and more sand-grain-sized ones than pebble-sized ones. Then, as we keep getting smaller, we have to call them micrometeoroids . In the extreme, they can be called interplanetary dust or meteoric dust .

The asteroid belt between Mars and Jupiter is home to a huge number of “space rocks,” but there are plenty of such objects that orbit outside this zone. Some orbit the Earth instead of the Sun. It is believed that there are myriad rock-sized objects in orbit around each of the planets, as well as around the major moons of the planets, including our own Moon. There is also evidence that some rock-sized objects are entirely independent of the Solar System and that they come into our little corner of the Universe as a result of the movement of the Sun within the Milky Way galaxy. Technically, a space rock becomes a meteoroid when it attains a solar orbit such that it has the potential to be pulled into the Earth by our planet’s gravitation.

Practice problems of this concept can be found at: Comets, Asteroids, and Meteors Practice Problems

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