The asteroid belt contains solar system remnants

Asteroid belt: Ring of thousands of small white dots, diffuse at inner and outer edges, between orbits of Mars and Jupiter.
Artist’s concept of our solar system from the sun to the 5th planet, Jupiter. In this illustration, the asteroid belt is the white donut-shaped cloud. Image via Wikimedia Commons.

Meet the asteroid belt, a place in our solar system where small bodies – mostly rocky and some metallic – orbit the sun. Sometimes scientists call these little worlds minor planets. One (Ceres) is technically a dwarf planet. These objects move mostly between the orbits of our solar system’s 4th planet, Mars, and 5th planet, Jupiter. Astronomers once thought they were leftovers of a rocky planet that Jupiter’s gravity tore apart long ago. Now they think differently. They think the asteroids are likely simply remnants from the formation of our solar system 4.6 billion years ago.

The word asteroid means starlike. Asteroids got this name because, when astronomers first discovered them in the early 1800s, they thought they looked like stars. And yet their movement was separate from stars. Because they are closer to us, they move against the starry backdrop. This showed asteroids to be something other than stars.

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Asteroids by the millions

While the graphic may make it seem like the asteroid belt is teeming with debris, if you lumped all the material together it would only create a body smaller than Earth’s moon.

The asteroid belt contains objects that vary wildly in size. It has 1 to 2 million asteroids more than half a mile (about a km) across. Plus, the asteroid belt contains untold millions of smaller ones, some probably no bigger than pebbles. In 1801, the astronomer Giuseppe Piazzi discovered the first asteroid, which is also the biggest object in the asteroid belt. It is 1 Ceres, which measures some 587 miles (945 km) across. The International Astronomical Union has reclassified Ceres from an asteroid to a dwarf planet.

A round gray world with many craters. In one crater, there are 2 very bright spots.
Ceres is one of several main-belt objects that spacecraft have visited. This composite image is from the Dawn spacecraft, still the only earthly spacecraft to have orbited first one body in our solar system (Vesta, 2011 to 2012), then gone on to orbit a second (Ceres, arrived 2015). The 2 bright spots in Occator Crater are salt deposits. Image via NASA.

Distances in the asteroid belt

Outer space is vast. And thus, despite there being many millions (possibly billions) of objects in the asteroid belt, the average distance between them is 600,000 miles (about 1 million km). This means that spacecraft can fly through the asteroid belt without colliding with any asteroids. (Although, obviously, a chance collision is never completely out of the realm of possibility and bad luck.) The asteroid belt is certainly nothing like the densely packed fields depicted in fantasies such as “Star Wars.”

Standing on any asteroid in the belt, you would likely be unable to see any other asteroids, because of their distance.

The asteroid belt lies between 2.2 and 3.2 astronomical units (AU) from our sun. One AU is the distance between the Earth and sun. So the width of the asteroid belt is roughly 1 AU, or 92 million miles (150 million km).

Its thickness is similarly about 1 AU.

A slightly oblong, roundish rocky body with a lot of craters on it.
Asteroid 4 Vesta – discovered in 1807 – is the 2nd-biggest object in the main belt after Ceres. The Dawn spacecraft orbited Vesta from July 2011 to September 2012. You can see a towering mountain at this asteroid’s south pole – more than twice the height of Mount Everest – at the bottom of the image. The set of 3 craters known as the snowman are at the top left. Image via NASA.

Asteroids in and out of the main belt

We often call the asteroid belt the main belt to distinguish it from other, smaller groups of asteroids in the solar system such as the Lagrangians and Centaurs in the outer solar system.

What scientists once thought was a homogeneous belt they now know to be slightly more complicated. There are different and distinct zones within the main-belt asteroids. This is especially true at its edges, where astronomers now recognize the Hungaria group at the inner edge and the Cybele asteroids at the outer. Toward the middle of the belt there is the highly inclined Phocaea family.

In addition, astronomers have established that the age of asteroids in the main belt also varies. They’ve now classified several asteroid groupings by their age, including the Karin family, a group of about 90 main-belt asteroids that share an orbit and may have come from a single object some 5.7 million years ago. And there is the Veritas family, from about 8.3 million years ago. A very recent group is the Datura family, dating from a collision just 530,000 years ago.

A very oblong, cratered rocky asteroid.
Asteroids tend not to be round. That’s because they don’t have enough internal mass to have strong enough gravity to pull themselves into the shape of a ball. 433 Eros, shown here, is famous as the 1st asteroid ever orbited by a spacecraft, NEAR Shoemaker, in 1998. Eros is a main-belt asteroid, even though its orbit crosses that of Mars. It doesn’t quite reach Earth’s orbit. Image via NASA.

Bottom line: The asteroid belt is a region of our solar system – between the orbits of Mars and Jupiter – where many small bodies orbit our sun.

February 7, 2023

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