How Asteroid Belts Work

By: John Fuller  | 
the main asteroid belt
The main asteroid belt is home to most of the solar system's asteroids.
Image courtesy NASA

In "The E­mpire Strikes Back," the fifth episode of the "Star Wars" films, Han Solo and his crew of fellow Rebels escape from the planet Hoth, only to fly straight into an asteroid field. The field is densely packed, and with huge, spinning rocky matter careening back and forth around the Millennium Falcon, Han Solo must deftly maneuver his spaceship to safety. Unfortunately, according to C3PO, the odds of successfully doing so are very slim -- only 3,720 to 1.

If a spaceship launched from Earth toward our solar system's asteroid belt and attempted to fly though it, would it look the same as "Star Wars," with dangerous debris flying everywhere, putting the mission in jeopardy? As it turns out, navigating through the asteroid belt wouldn't be quite as dramatic -- only a handful of asteroids - which are also considered minor planets - are big enough to cause any damage to a spacecraft, and there's much more space between them than you might think.


But that doesn't mean the main asteroid belt, located between the orbits of the planets Mars and Jupiter, is any less interesting than the field in "Star Wars." The more astronomers study the composition, activity and formation of the asteroids in their orbit around the sun, the more we understand about how the entire solar system came to be. Some theories even suggest that life on Earth started with asteroids in the planet's early stages. On the other hand, many scientists believe an asteroid caused the mass extinction of the dinosaurs and other organisms 65 million years ago.

How did the asteroid belt form, and how did it affect the rest of the solar system? What do Mars and Jupiter have to do with it, and how do their orbits affect the main belt? What about the Kuiper belt and the Oort cloud -- are they any different from the main one? Are there other asteroid belts in other solar systems like ours, or is the main belt unique? Keep reading to find out.


The Formation of the Solar System

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There are several theories that attempt to explain how the solar system began, but the most widely accepted one is known as the nebular theory. Astronomers and physicists believe the solar system started as a large, shapeless cloud of gas, dust and ice, but something disrupted the mass and set things in motion -- perhaps the explosion of a nearby star.

If you've ever watched figure skating, you may have noticed that skaters can spin much faster if they pull their arms closer to their bodies. The more concentrated their body masses are, the faster they'll be able to rotate. The same thing happened with our solar system. The hypothetical explosion squeezed the unformed gas and dust together, which began to spin faster and faster in a circle. As the sun formed in the middle, the cloud started to flatten out into a disc, sort of like a Frisbee or a pancake, with tiny dust grains making up the rest of the disc.


Eventually, dust particles began to stick together and form larger bodies called planetesimals. Even more matter flying around collided with these planetesimals and stuck to them in a process called accretion. As the bodies spun themselves and gravity brought in more dust and gas, the planetesimals accreted into protoplanets, and soon into the eight planets we currently know and love; those in the inner solar system, including Mercury, Venus, Earth, Mars, and those in the outer solar system - Jupiter, Saturn, Uranus and Neptune (sorry, Pluto - you're only a dwarf planet).

It's the area in between the fourth planet, Mars, and the fifth, Jupiter, that's important. An astronomical unit (AU) is the distance between the Earth and the sun, which is about 150 million kilometers -- astronomers use this distance as a ruler to measure other distances within the solar system and the Milky Way galaxy. Mars lies about 1.5 AU from the sun, or 225 million kilometers away. Jupiter, meanwhile, is about 5.2 AU from the sun, or 780 million kilometers away. If we subtract the two distances, there's about 3.7 AU between Mars and Jupiter, or 555 million kilometers. It seems like there's enough room between the two planets for yet another planet, right? What happened in between Mars and Jupiter during the formation of the solar system?

To find out what scientists think happened, read the next page.


The Main Asteroid Belt

2008 HowStuffWorks

So how do we explain the vast distance between Mars and Jupiter? Some astronomers have suggested that a separate planet or protoplanet actually formed between the two planets, but the impact of a high-speed comet broke up and scattered the newly formed body to create what we now know as the main or original asteroid belt.

While it's possible that comets and other large objects were flying around the solar system and breaking up material during the early stages, most scientists accept a much simpler theory -- asteroids are leftover matter from the solar system's formation that never successfully came together as one planet. But how come nothing came together?


If you look at Jupiter's mass, you'll notice it's extremely large. People refer to it as a gas giant for good reason -- while the Earth's mass is about 6x10^24 kilograms, Jupiter's mass is estimated to be 2x10^27 kilograms. It's a much closer relative to our sun than to rocky planets like Earth or Mars.

Jupiter's massive size would be enough to disturb the rocky matter that fell in between it and Mars -- its strong gravitational pull would cause any potential protoplanets to collide and break apart into smaller bits. We're then left with a large, spread-out collection of asteroid bodies that orbits around the sun in the same direction as Earth -- the main asteroid belt. With its center around 2.7 AU from the sun, the belt separates Mars and the other rocky planets from the massive, cold gas giants like Jupiter and Saturn.

For a closer­ look at asteroids within the belt, see the next page.


Asteroid Characteristics

Asteroid 951 (Gaspra) as seen by the Galileo spacecraft in 1991.
Image courtesy NASA

The majority of the asteroids in the main asteroid belt fall under three categories:

C-type (carbonaceous) - These make up about 75 percent of all known asteroids. C-type asteroids are actually thought to be similar in composition to the sun, just without hydrogen, helium and other combustible material. They're very dark and absorb light easily, and you can locate them on the outer edges of the main belt.


S-type (silicaceous) - These make up about 17 percent of all known asteroids. Their composition is mainly metallic iron and iron-magnesium silicates, and they're found in the inner edge of the main belt.

M-type (metallic) - The remaining 8 percent of the asteroids are made of metallic iron and are found in the middle region of the main belt.

Asteroids orbit around the sun in the same direction as the Earth, typically in a slightly elliptical pattern. They rotate simply, much like the Earth, except over a much short period of time -- anywhere between one hour and a day, depending on their size. Interestingly, most asteroids larger than 200 meters spin very slowly, no faster than once every 2.2 hours. This led astronomers to assume that bigger asteroids are very loosely held together because of constant bombardment from other asteroids. If they spin any faster, they'll break apart and fly out into space. It's suggested that asteroid 253 (Mathilde) is about as dense as water, even though it's 52 kilometers wide.

Many people might be surprised to learn that most asteroids in the main belt are only the size of a pebble. Despite the vast amount of space it takes up, astronomers estimate the total mass of the entire main asteroid belt to be less than 1/1,000th the mass of the Earth, or less than half the size of the moon. Sixteen asteroids have diameters of 240 kilometers or larger, the largest of which is Ceres, which has a diameter of about 1,000 kilometers.

Are all asteroids in our solar system in the main belt, or are there other bodies that share the space between Mars and Jupiter? And what about other asteroids belts out there? See the next page to go beyond the main belt.


Main-Belt Comets and Other Belts

Although a main-belt comet behaves like regular comets by emitting a tail of gas and dust, its orbit is more like an asteroid's.
Pedro Lacerda/University of Hawaii

On Nov. 26, 2005, graduate student Henry Hsieh and Professor David Jewitt of the University of Hawaii made a startling discovery. Upon looking through an 8-meter Gemini North Telescope on the dormant volcano Mauna Kea, the two noticed a mysterious asteroid, Asteroid 118401, emitting comet-like dust. When they looked at two separate comets, they realized these three objects were neither asteroids nor comets, but a completely new category of comets -- main-belt comets.

Comets are simply big lumps of ice and dust moving through space. Heat from the sun causes the ice to evaporate, and a trail of gas and dust is left behind as the object moves through space -- that's why comets have tails. The orbit of a main-belt comet, however, is much different from that of a regular comet, which usually circles the sun in a tilted, highly elliptical fashion like a stretched-out rubber band. Instead, a main-belt comet travels a fairly circular, level orbit, much like an asteroid.


The biggest revelation coming from the discovery of main-belt comets is the possibility that an icy asteroid may have crashed into Earth and provided it with life. Astronomers originally believed ice from regular comets provided the Earth with its water, but recent discoveries have proved that comet water doesn't have much in common with our planet's water. If asteroidal water is anything like ours, main-belt comets may provide us with important insights into the Earth's formation and even our own existence.

Another discovery made in the same year suggests that there are other belts out there. Astronomers at NASA located what may be a massive asteroid belt around HD69830, a star 41 light years away that's closely related to our sun. This asteroid belt is either the same as our solar system's belt -- a collection of debris that was unable to form into a large body -- or the early stages of a new solar system. If it's the latter case, observing the belt may help us better understand the important process of planetary formation [source: National Geographic News].

To learn lots more about asteroids, space and space exploration, see the next page.


Asteroid Belts FAQ

How big is the asteroid belt?
The main asteroid belt is located between Mars and Jupiter. There's about 3.7 AU between Mars and Jupiter, or 555 million kilometers.
What planets are the asteroid belt between?
The asteroid belt lies between Mars and Jupiter.
What is the Kuiper Belt and what is found there?
The Kuiper Belt is similar to the main asteroid belt in that it's another disc-shaped collection of leftover debris from the solar system's formation. The big difference is that it extends much farther out into space: It begins past Neptune at 30 astronomical units (AU) and reaches as far as 50 AU.
Why is the Kuiper Belt important?
The Kuiper Belt is important because it is often referred to as the "final frontier" of our solar system, and it becomes increasingly difficult to measure the sizes of objects within or past this area.
What shape is the asteroid belt?
The asteroid belt is a torus-shaped or disc-shaped region.

Lots More Information

Related HowStuffWorks Articles

More Great Links


  • "Asteroids." NASA.
  • "General properties of asteroids." University of Tennessee.
  • "Main asteroid belt." Sol Station: 2006.
  • "Mysteries of the solar nebula." California Institute of Technology, NASA Jet Propulsion Laboratory: July 20, 2001.
  • "New class of comets may be the source of Earth's water." University of Hawaii Press: March 23, 2006.
  • Handwerk, Brian. "Asteroid belt discovered around our sun's 'twin.'" National Geographic News: April 21, 2005.
  • Hsieh, Henry. "Main-belt comets." University of Hawaii Press: March 23, 2006.