58 The Asteroid Belt-Mid

The Asteroid Belt FOCUS Question What is the Asteroid Belt, and how did it form?

What Is an Asteroid? Our solar system has one star, eight planets, and many moons. But did you know there are trillions of other objects traveling around the Sun? Trillions! Some of these objects are asteroids—rocky space bodies that orbit the Sun. They are too small to be planets, and they have no atmosphere, or air. Most are irregular in shape. They are not spheres, like the Sun and planets. Their surfaces may be craggy and marked by craters. Asteroids have been around since our solar system formed 4.6 billion years ago. They are the "leftovers" from those early days. Do You Know? Asteroids may be as small as a car (a few meters across) or as large as a small moon.

Short Days, Long Years One day is the time it takes an object in space to make one complete rotation, or spin. Asteroids spin quickly. A day on an asteroid may last for just a few hours! A year is the time it takes an object in the solar system to make one complete orbit of the Sun. Most asteroids have longer years than Earth because they are farther from the Sun. Look at the chart below to see how long a day and a year are on some well-known asteroids. Compare them to a day and year on Earth. ASTEROID ORBITS AND ROTATIONAL PERIODS Earth Eros Gaspra Ida Lutetia Day (Rotation) 24 hours 5 hours 7 hours just over 4 hours just over 8 hours Year (Orbit) 365 days 643 days just over 3 Earth years nearly 5 Earth years nearly 4 Earth years

Rockin in the Ring Millions of asteroids hang out in the Asteroid Belt. This area of space lies between Mars and Jupiter. How did they get there? When the solar system was forming, dust and gas swirled around the Sun. Some of the dust and gas collided and clumped together. Once the clumps became large enough, their gravity pulled in even more of the debris. These growing clumps became the planets and their moons. Once Jupiter formed, its massive gravity tugged on the smaller, leftover clumps. This kept them from forming into larger bodies. These leftover bits and pieces are asteroids. Wowser! All the asteroids in the Asteroid Belt together would make an object far smaller than the Moon.

Notable Asteroids The first object spotted in the Asteroid Belt was Ceres, in 1801. It is much larger than other Belt objects. Because of its size, Ceres is now classified as a dwarf planet, like Pluto. NASA's Galileo probe studied an asteroid named Ida (EYE-duh) in 1993. It found a surface peppered with craters. More surprising was a small, orbiting moon. The moon, called Dactyl (DAK-tul), may be a piece of Ida that broke away in an impact with another asteroid. Radar images show that Kleopatra is shaped like a dog bone! It has two moons. It is more like a pile of rubble stuck together than one solid rock. The moons may have broken away from the spinning asteroid.

How to Avoid Asteroids Some asteroids travel out of the Belt. If they get too close to a planet, they may crash into it. An asteroid chunk 20 meters (66 ft.) wide fell to Earth over Russia on February 15, 2013. It blew up in the air. The powerful blast broke windows and damaged buildings. More than one thousand people were hurt. Scientists use telescopes to track the orbits of nearby asteroids. We may be able to knock large ones off track before they reach Earth. Word Wise A small piece of an asteroid that could fall to Earth is called a meteoroid. If it enters Earth's atmosphere, it is called a meteor. Once it lands on Earth, we call it a meteorite.