APS 105: Asteroids
What Is the Asteroid Belt?
The asteroid belt is a large region between Mars and Jupiter where many asteroids orbit the Sun.
Asteroids are objects made mostly of rock, metal, or a mixture of both.
They are much smaller than planets.
Even if you combined all the asteroids in the belt, their total mass would still be less than the Moon’s mass.
The belt has lots of empty space. Asteroids are usually very far apart.
Understanding AU: An astronomical unit (AU) is the average distance from Earth to the Sun.
Earth is about 1 AU from the Sun.
Mars is about 1.5 AU away.
Jupiter is about 5.2 AU away.
2. Asteroids That Come Near Earth
Scientists group asteroids based on their orbits, or paths around the Sun.
Group | Simple meaning |
|---|---|
Main-belt asteroids | Asteroids that orbit mainly between Mars and Jupiter. |
Near-Earth objects (NEOs) | Asteroids or comets whose orbits bring them within 1.3 AU of the Sun. |
Potentially hazardous asteroids (PHAs) | Asteroids large enough and with orbits close enough to Earth’s orbit to deserve careful tracking. |
“Potentially hazardous” does not mean an asteroid will hit Earth. Crossing or coming close to Earth’s orbit does not automatically cause a collision—Earth and the asteroid would have to arrive at the same place at the same time.
3. Ceres: The Biggest Object in the Belt
Ceres is the largest object in the asteroid belt.
It is about 940 kilometers (580 miles) wide.
It is classified as a dwarf planet.
Its gravity is strong enough to pull it into a nearly round shape.
It contains rock and water ice.
Its surface has craters, cracks, and bright patches of salts.
Think of gravity as pulling material toward the center from every direction. When an object is large enough, that pull can make it round.
4. What Are Asteroids Made Of?
Asteroids do not all have the same ingredients.
Material | What it means |
|---|---|
Rock | Stone and minerals. |
Metal | Mainly iron and nickel. |
Rock and metal | A mixture of both materials. |
Ice and rock | Some bodies, including Ceres, contain water ice along with rock. |
Most smaller asteroids have irregular shapes, like potatoes or uneven chunks of rock. Their gravity is too weak to pull them into balls.
5. Collisions and Craters
Over billions of years, asteroids have crashed into one another.
These collisions can:
Leave craters, which are bowl-shaped dents.
Break pieces off an asteroid.
Shatter an asteroid into many smaller pieces.
Although collisions happen, the asteroid belt is so spread out that it is not a constant traffic jam.
6. What Is a “Rubble Pile”?
A rubble-pile asteroid is a loose collection of rocks held together mainly by gravity.
Here is how one can form:
A collision breaks an asteroid apart.
Gravity pulls some of the broken pieces back together.
The pieces form a loose clump with gaps inside.
Picture a pile of rocks floating in space, with gravity keeping the rocks together.
The asteroid Itokawa is an example of a rubble pile. Its surface is covered with loose rocks and boulders.
7. How Do Pieces of Asteroids Reach Earth?
Collisions and changes in an object’s orbit can send asteroid fragments toward Earth.
The name changes depending on where the object is:
Term | Meaning |
|---|---|
Meteoroid | A small piece of rock or metal traveling through space. |
Meteor | The streak of light produced as it passes through Earth’s atmosphere—a “shooting star.” |
Meteorite | A piece that survives the trip and lands on the ground. |
Most meteorites come from asteroids. Scientists study them to learn about the materials that formed our solar system.
8. How Do Asteroids Get Their Names?
Asteroids first receive identification labels. Some later receive official names.
They can be named after:
People, including everyday people.
Places.
Mythological characters.
Other approved subjects.
Main idea to remember: Asteroids are leftovers from the formation of our solar system. Studying them helps us understand what the solar system was like billions of years ago.