Asteroid

An asteroid is a minor planet. It is larger than a meteoroid and orbits the Sun. It is not a planet or a comet. Asteroids can be rocky, metallic, or icy. They range in size from small rocks to Ceres, which is very wide.

Most asteroids are found in the main asteroid belt. This belt is between Mars and Jupiter. Astronomers have studied asteroids for over 200 years. Space missions from agencies like NASA and JAXA have visited asteroids. These missions have given us close-up views and samples from asteroids like Itokawa, Ryugu, and Bennu. They help us learn about our solar system.

Terminology

The term asteroid was first used by astronomer Sir William Herschel. He found objects that looked like stars but moved in a different way. These objects orbit the Sun. They are not planets or comets. They are made of rock, metal, or ice and come in many sizes.

In 2006, scientists called the International Astronomical Union (IAU) made a new term, small Solar System body. This term includes asteroids, comets, and other small objects in space. Today, most people still use the word asteroid for rocky objects that orbit the Sun. These are especially found between the planets Mars and Jupiter. The largest asteroid, Ceres, is so big that it is now called a dwarf planet.

History of observations

Asteroids were discovered not too long ago. The first one, Ceres, was found in 1801. Normally, only one asteroid, 4 Vesta, can be seen without a telescope on a very dark night when it is in the right place. As of May 2025, scientists knew about over 1.4 million minor planets, with about 826,864 having enough details to get a special number.

Discovery of Ceres

In 1772, a German astronomer named Johann Elert Bode thought there might be a planet between Mars and Jupiter because of a pattern he saw in how far planets are from the Sun. In 1800, some astronomers started looking for this missing planet. One of them, Giuseppe Piazzi, found Ceres on January 1, 1801. At first, he thought it was a comet but then learned it was something different. After Piazzi got sick, a mathematician named Carl Friedrich Gauss figured out a way to predict where Ceres would go. It was found again in December 1801, which proved Bode’s idea was right.

Further search

More asteroids were found in the years that followed. It wasn’t until 1845 that another asteroid, 5 Astraea, was found. The search got faster in 1891 when Max Wolf started using pictures of the sky to find asteroids, which helped spot them more easily. Still, only a few thousand were known for a long time.

19th and 20th centuries

In the past, finding asteroids meant taking pictures of the sky at different times and looking for what moved. If something moved, scientists measured it and decided if it was part of an orbit. If it was, it got a special number and a name.

Naming

Main article: Minor planet § Naming

When astronomers started finding many asteroids, they needed a better way to name them. In the past, each asteroid had a special name and number. For example, the twentieth asteroid found was named 20 Massalia.

Today, new asteroids get a temporary name. This name includes the year they were found and a special code. After scientists learn the asteroid's path, it gets a number, like 433 Eros, and may also get a name chosen by the person who found it.

Symbols

Main article: Astronomical symbols

The first asteroids had special symbols, like the ones used for planets. But as more asteroids were found, astronomers began using a simple circle with a number. This made it easier to keep track of all the asteroids.

Formation

Main article: Origin of the asteroid belt

Many asteroids are pieces left over from the early days of our Solar System. They were small bodies called planetesimals that never grew big enough to become full planets. When the planet Jupiter grew very big, its strong gravity moved most of these planetesimals away. Some asteroids, like Ceres and Vesta, grew large enough to change shape.

In models of our Solar System's history, some objects from far away were pulled into the asteroid belt by Jupiter's gravity. These might be a special group of asteroids called D-type asteroids.

Distribution within the Solar System

See also: List of minor-planet groups, List of notable asteroids, and List of minor planets

Different groups of asteroids move around in the inner Solar System. Their paths change because of the gravity of other objects and the Yarkovsky effect.

Asteroid belt

Most known asteroids are found in the asteroid belt, between the orbits of Mars and Jupiter. This belt may hold many asteroids bigger than one kilometer across. The belt is mostly empty space, with asteroids far apart. The total mass of the asteroid belt is small, about 3% of the Moon's mass. The four largest objects — Ceres, Vesta, Pallas, and Hygiea — make up most of the belt's mass.

Trojans

Main article: Trojan (celestial body)

Trojans are asteroids that share an orbit with a bigger planet, staying in special spots called Lagrangian points. In our Solar System, most known trojans move with Jupiter, split into two groups: one ahead of Jupiter and one behind it. A few trojans have been found sharing the orbits of Mars, Neptune, Uranus, and Earth.

Near-Earth asteroids

Main article: Near-Earth asteroids

Near-Earth asteroids have paths that come close to Earth. Some cross Earth's path. Many of these asteroids come from the asteroid belt, pushed out by Jupiter's gravity. A few near-Earth asteroids have tiny moons orbiting them.

Moons of Mars

Main article: Moons of Mars

Main articles: Phobos (moon), and Deimos (moon)

It is not sure if Mars’s moons, Phobos and Deimos, are captured asteroids or formed from material thrown up by a crash on Mars. Both moons look like some types of asteroids, but their paths and makeup also suggest they may have come from Mars itself.

Characteristics

Asteroids are rocky or metallic objects that orbit the Sun. They come in many sizes, from tiny rocks just 1 meter across to the dwarf planet Ceres, which is almost 1,000 km wide. Most asteroids are shaped irregularly, but the biggest ones are more round.

The largest asteroid is Ceres, followed by Vesta and Pallas. These big asteroids have different materials and structures. Some asteroids might have water ice and organic compounds on their surfaces, which scientists think could have helped bring water and the building blocks of life to Earth.

Classification

Main article: Active asteroid

Main article: Dark comet

Asteroids can be grouped by how they move around the Sun and what they are made of. Some groups are named after the first asteroid found in that group. These groups help scientists learn about asteroids.

Scientists also classify asteroids by their color and brightness, which can tell us about their surface materials. The most common types are C-types, which are dark and carbon-rich, and S-types, which are stony or rocky. There are many other types, and scientists keep studying and classifying more asteroids.

Active asteroids look like comets because they sometimes show trails, but they orbit like asteroids. Dark comets are another special group of asteroids that act like comets but do not show trails.

Observation and exploration

Until we could travel through space, we could only see objects in the asteroid belt with big telescopes. Their shapes and terrain were a mystery. Today, modern telescopes and space observatories like the Hubble Space Telescope can show us some details about the largest asteroids. Scientists study these objects using light changes, radar images, and visits from spacecraft to learn about their sizes, shapes, and what they are made of.

Spacecraft missions have given us close-up views and even samples from asteroids. Early missions like Galileo and NEAR Shoemaker took pictures of asteroids up close. More recent missions, such as Hayabusa and OSIRIS-REx, collected samples to bring back to Earth for study. These missions help scientists learn about the makeup and history of asteroids in our solar system.

Asteroid mining

Asteroid mining is an idea from the 1970s about getting valuable materials from asteroids. These materials could be rare or running out on Earth, or useful for building things in space, like homes for astronauts or satellites that provide power.

As we use more of Earth's resources, getting materials from asteroids might become very useful. For example, water from icy asteroids could be used to refuel spacecraft. Scientists also think studying asteroids could help us learn about space.

Threats to Earth

See also: List of Earth-crossing minor planets

Scientists watch for asteroids that might one day come close to Earth. These are called near-Earth asteroids, and the three main groups are the Apollos, Amors, and Atens.

We have learned much about these asteroids using special cameras and telescopes. By 2018, we had found most of the bigger ones — those one kilometer or larger. There is still more work to do to keep Earth safe.

Fiction

Asteroids and the asteroid belt are often found in science fiction stories. They can be places where people might live, sources of useful minerals, or dangers for spaceships flying through space. Sometimes, they are shown as possible threats to Earth and other planets if they crash into them.