Smaller Objects in the Solar System
Asteroids are small, airless rocky worlds revolving around the sun that are too small to be called planets. They are also known as planetoids or minor planets. In total, the mass of all the asteroids is less than that of Earth's moon. But despite their size, asteroids can be dangerous. Many have hit Earth in the past, and more will crash into our planet in the future. That's one reason scientists study asteroids and are eager to learn more about their numbers, orbits and physical characteristics. Most asteroids lie in a vast ring between the orbits of Mars and Jupiter. The main asteroid belt holds more than 200 asteroids larger than 60 miles long. Nearly all asteroids are irregularly shaped, although a few are nearly spherical.
A comet is an icy body that releases gas or dust. They are often compared to dirty snowballs, though recent research has led some scientists to call them snowy dirtballs. Comets contain dust, ice, carbon dioxide, ammonia, methane and more. Astronomers think comets are leftovers from the gas, dust, ice and rocks that initially formed the solar system about 4.6 billion years ago. Comets orbit the sun but are believed to be located in an area called the Oort Cloud, far beyond the orbit of Pluto.
Meteorites vary in size from a few millimetres across to several feet in diameter. Meteorites are rarely round in shape. They are typically irregular, with rounded edges. The surface of a freshly fallen meteorite will appear black and shiny due to the presence of a fusion crust, the result of frictional heating and abrasion of the outer surface of the rock as it passes through the Earth's atmosphere. Most meteorites are believed to originate in the asteroid belt between Mars and