Asteroid

An asteroid is a minor planet—an object larger than a meteoroid (thus 1 meter or larger) that is neither a planet nor an identified comet—that orbits within the inner Solar System or is co-orbital with Jupiter (Trojan asteroids). Asteroids come in different types, like rocky, metallic, or icy, and they vary greatly in size—from small piles of rocks to Ceres, which is almost 1000 km wide and considered a dwarf planet.

Most asteroids are found in the main asteroid belt, a region between the orbits of Mars and Jupiter. This belt contains about one million known asteroids, but all together they only make up 3% of the mass of Earth's Moon. These asteroids orbit the Sun in stable paths, taking three to six years to complete one journey.

Astronomers have been studying asteroids for over 200 years, and several space missions have visited them. These missions, led by agencies like NASA and JAXA, have provided close-up views and even brought back samples from asteroids like Itokawa, Ryugu, and Bennu. These missions help scientists understand the history of our solar system and how to protect Earth from any potentially dangerous asteroids.

Terminology

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

In 2006, a group of scientists called the International Astronomical Union (IAU) introduced a new term, small Solar System body, to include asteroids, comets, and other small objects in space. Today, most people still use the word asteroid to talk about rocky objects that orbit the Sun, especially those 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 relatively recently. The first one, Ceres, was found in 1801. Normally, only one asteroid, 4 Vesta, can be seen without a telescope under dark skies when it is in the right position. As of May 2025, scientists had information about over 1.4 million minor planets, with about 826,864 having enough details to be officially numbered.

Discovery of Ceres

In 1772, German astronomer Johann Elert Bode suggested there might be a planet between Mars and Jupiter based on a pattern he noticed in the distances of planets from the Sun. In 1800, a group of astronomers began searching for this missing planet. One of them, Giuseppe Piazzi, discovered Ceres on January 1, 1801. He first thought it was a comet but later realized it was something new. After Piazzi fell ill, mathematician Carl Friedrich Gauss developed a method to predict Ceres’ movement. It was rediscovered in December 1801, confirming Bode’s idea.

Further search

More asteroids were found over the next few years. It wasn’t until 1845 that another asteroid, 5 Astraea, was discovered. The rate of discovery sped up in 1891 when Max Wolf began using photographs to find asteroids, which made it easier to spot them. Even so, only a few thousand were known for many years.

19th and 20th centuries

In the past, finding asteroids involved taking photographs of the sky at different times and comparing them to see what moved. If something moved, it was measured and reported to scientists who determined if it was part of an orbit. If so, it received an official number and name.

Naming

Main article: Minor planet § Naming

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

Today, new asteroids get a temporary name that includes the year they were found and a special code. Once scientists are sure they know the asteroid's path, it gets a number, like 433 Eros, and may also receive a name chosen by the person who found it.

Symbols

Main article: Astronomical symbols

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

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 to its current size, its strong gravity shook up the area, scattering most of these planetesimals away. Some asteroids, like Ceres and Vesta, became large enough to change shape and separate into different layers.

In models of our Solar System's history, some objects from far out 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

Various groups of asteroids orbit within the inner Solar System. Their paths are influenced by the gravity of other objects and by the Yarkovsky effect.

Asteroid belt

Most known asteroids are found in the asteroid belt, located between the orbits of Mars and Jupiter. This belt is thought to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer in diameter. The belt is mostly empty space, with asteroids spread far apart. The total mass of the asteroid belt is small, about 3% of the mass of the Moon. 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 larger planet, staying in stable positions called Lagrangian points. In our Solar System, most known trojans orbit with Jupiter, divided into two groups: one ahead of Jupiter and one behind it. Fewer 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 orbits that pass close to Earth. Some cross Earth's path. As of April 2022, 28,772 near-Earth asteroids were known, with 878 being larger than one kilometer in diameter. These asteroids often come from the asteroid belt, pushed out by Jupiter's gravity. Some 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 uncertain whether Mars’s moons, Phobos and Deimos, are captured asteroids or formed from material ejected by an impact on Mars. Both moons share similarities with certain types of asteroids, but their orbits and composition also suggest they may have formed from Martian material.

Characteristics

Asteroids are rocky or metallic objects that orbit the Sun. They vary greatly in size, from small rocks just 1 meter across to the dwarf planet Ceres, which is almost 1,000 km in diameter. Most asteroids are irregularly shaped, but the largest ones are more round.

The biggest asteroid is Ceres, followed by Vesta and Pallas. These large asteroids have different compositions and structures. Some asteroids may 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 based on 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 understand how asteroids formed and moved.

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 as well, and scientists continue to study and classify more asteroids.

Active asteroids look like comets because they sometimes show trails or fuzzy edges, but they orbit like asteroids. These objects were first discovered in 1979 and have been studied ever since. Dark comets are another special group of asteroids that act like comets but do not show trails or fuzzy edges. They were first identified in 2024.

Observation and exploration

Until the age of space travel, objects in the asteroid belt could only be observed with large telescopes, their shapes and terrain remaining a mystery. Today, modern telescopes and space observatories like the Hubble Space Telescope can reveal some details about the largest asteroids. Scientists study these objects using light changes, radar images, and spacecraft visits to learn about their sizes, shapes, and compositions.

Spacecraft missions have provided close-up views and even samples from asteroids. Early missions like Galileo and NEAR Shoemaker photographed 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 understand 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 and even find signs of old, advanced civilizations that might have mined asteroids long ago.

Threats to Earth

See also: List of Earth-crossing minor planets

Scientists are very interested in finding asteroids that might one day hit Earth. These are called near-Earth asteroids, and the three main groups are the Apollos, Amors, and Atens.

We have learned a lot about these asteroids thanks to special cameras and telescopes. As of 2018, we had discovered most of the bigger ones — those one kilometer or larger. However, there is still work to do to make sure we are ready if one ever heads our way.

Fiction

Asteroids and the asteroid belt are popular in science fiction stories. They can be places where people might live, sources of valuable minerals, dangers for spaceships traveling through space, or even threats to Earth and other planets if they crash into them.