What Are Asteroids?

Asteroids could be called minor planets or planetoids because scientists believe that asteroids were once parts of planets that tried to form millions of years ago between Mars and Jupiter.  Most of these asteroids are found in the asteroid belt between Mars and Jupiter.  Most astronomers think that there are over 50,000 asteroids in the solar system right now and over 1,150 have a diameter of 18 miles or larger.

The Size of an Asteroid

Asteroids come in many shapes and sizes.  You cannot see one with your naked eye, but you can see one through a telescope or a pair of binoculars.

The biggest asteroid known to man is named Ceres.  Ceres has a diameter of about 580 miles.  It is almost 1/3 of the size of the moon.  Italian Giuseppe Piazzi discovered it January 1, 1801.  The smallest known asteroid is called 1991 BA and is only 20 feet in diameter.

Types of Asteroids

Asteroids are divided into 3 major types depending on what they are made of and how much light they reflect.  The three types are:

C-type:  More than 75% of asteroids are this type.  They are dark and have the same chemicals as the Sun. The sun is mostly made of hydrogen, helium, and other gases that cause the sun to be so hot.  Remember, asteroids don’t have the gasses that can cause them to be hot.  (An asteroid’s temperature only reaches 100 degrees Farenhight.)

S-type:  Only about 17% of asteroids are this type.  These are somewhat bright because they are made of metallic nickel-iron combined with iron and magnesium.  That is what makes them more reflective.

M-type:  About 7% of asteroids are this type.  They are extremely bright because they are made only of nickel-iron, giving them a lot of reflective power.

Tracking Asteroids Down

In 1995, NASA and the U.S. Air Force conducted a project named Near Earth Asteroid Tracking (NEAT).  They use an observatory in Hawaii to search for asteroids in an orbit that might threaten Earth.  Astronomers have found over 200 asteroids with orbits that intersect with Earth’s orbit around the sun.  They believe also that about 1,500 are large enough to maybe cause global disaster.  The chances are small that one will hit us, though.  The best chance we have of being hit by one of these is about once every 300,000 years.  Astronomers say that tracking organizations like this could give the world notice if one will collide with Earth decades in advance.

“Asteroids.” Encarta Encyclopedia, 1996.

Astronauts Installing Experimental Camera.  <http://www.nasa.gov > Last visited: March, 2002.

Rudnyk, Marian E. “Asteroids.” World Book Encyclopedia, 2001.

Space: Everything You Need to Know and Beyond.  </J0112388> Last visited: February, 2002.

Space: Today, Tomorrow, and Always. </J0112188> Last visited: February, 2002.

Send us an email

All pictures courtesy of NASA unless otherwise noted