Asteroid

An asteroid is a minor planet that orbits in the inner Solar System or shares its orbit with Jupiter. These are called Trojan asteroids. Asteroids are bigger than a meteoroid, at least 1 meter across. They are not planets or comets. They are made of rock, metal, or ice, and they have no atmosphere. Scientists sort them into three groups: C-type (carbonaceous), M-type (metallic), and S-type (silicaceous). Asteroids come in many sizes and shapes. Some are small piles of rubble less than a kilometer across. Ceres, a dwarf planet, is almost 1000 km wide. An object is called a comet instead of an asteroid if it grows a coma (tail) when warmed by sunlight. However, recent observations suggest that these two types of objects may be connected.

Most of the roughly one million known asteroids are found between the orbits of Mars and Jupiter, about 2 to 4 astronomical units (AU) from the Sun. This region is called the main asteroid belt. All the asteroids together have only 3% of the mass of Earth's Moon. Most asteroids in the main belt follow slightly oval, steady paths. They travel in the same direction as Earth and take three to six years to go around the Sun once.

People have watched asteroids from Earth since they were discovered in 1801. The first close-up look at an asteroid was taken by the Galileo spacecraft. Later, NASA and JAXA launched several missions to visit asteroids, and more are still underway. NASA's NEAR Shoemaker studied 433 Eros, and Dawn observed Vesta and Ceres. JAXA's Hayabusa and Hayabusa2 studied Itokawa and Ryugu and brought back samples. OSIRIS-REx studied 101955 Bennu and collected a sample in 2020. The sample was returned to Earth in 2023. NASA's Lucy, launched in 2021, will study ten asteroids. Two are from the main belt, and eight are Jupiter trojans. Also launched in 2021, NASA's Double Asteroid Redirection Test (DART) was crashed on purpose into the harmless asteroid Dimorphos in September 2022. This was a test of planetary defense technology. NASA's Psyche, launched in October 2023, will study the metal-rich asteroid Psyche. ESA's Hera, launched in October 2024, will look at the results of the DART impact. CNSA's Tianwen-2 was launched in May 2025. It will explore the co-orbital near-Earth asteroid 469219 Kamoʻoalewa and the active asteroid 311P/PanSTARRS. It will also collect samples of the loose dust and rock on the surface of Kamo'oalewa.

Pictures of asteroids that spacecraft have visited show how different they can be. The images show 433 Eros, 243 Ida with its small moon Dactyl, 1 Ceres, and 101955 Bennu. The sizes in these pictures are not to scale.

Some near-Earth asteroids could cause serious damage if they hit Earth. One well-known example is the Chicxulub impact, which is widely thought to have helped cause the Cretaceous–Paleogene mass extinction.

Terminology

Scientists now use the broad term small Solar System body. The International Astronomical Union (IAU) introduced it in 2006. It describes an object that is not a planet, a dwarf planet, or a natural satellite. This group includes asteroids, comets, centaurs, trojans, and trans-Neptunian objects.

The first asteroid found, Ceres, was at first thought to be a new planet. Other objects like it looked like points of light, similar to stars. So the astronomer Sir William Herschel suggested the word "asteroid," which means "star-like" in Greek. Before 2012, scientists took detailed pictures of some asteroids. These pictures are shown at the same scale. They include 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida with its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, and 25143 Itokawa.

In the past, small objects that orbit the Sun were sorted into comets, asteroids, or meteoroids. Anything under one meter across was called a meteoroid. The word "asteroid" has no official definition. It is often used for a rocky, irregular body that orbits the Sun and is not a planet or dwarf planet. A comet shows a tail when its ices sublimate in sunlight. Some comets slowly lose their volatile ices and become more like asteroids.

Most of the objects in this article are classical asteroids in the asteroid belt, the Jupiter trojans, and the near-Earth objects. Farther out, past Neptune, are the trans-Neptunian objects, Kuiper-belt objects, scattered-disc objects, and the Oort cloud. Ices stay solid in that cold region.

History of observations

Asteroids were discovered quite recently, even though there are so many of them. The first asteroid, Ceres, was identified in 1801. Only one asteroid, 4 Vesta, which has a fairly reflective surface, can normally be seen with the naked eye on dark nights when it is in a good position. Sometimes small asteroids that pass close to Earth can be seen briefly without a telescope. By May 2025, the Minor Planet Center had data on 1,460,356 minor planets. About 826,864 of them had enough information to be given numbers.

In 1772, German astronomer Johann Elert Bode published a number pattern based on work by Johann Daniel Titius. It became known as the Titius–Bode law, and scientists no longer accept it. The pattern seemed to fit the paths of the known planets, except for a gap between Mars and Jupiter. Bode believed a "missing planet" must be in that gap. His formula predicted it would be about 2.8 astronomical units from the Sun. In 1800, Franz Xaver von Zach asked 24 experienced astronomers, whom he called the "celestial police", to search for the expected planet. Giuseppe Piazzi, a priest in Palermo, discovered Ceres on 1 January 1801. He first thought it was a comet, but he noticed it moved slowly and fairly steadily. Later, Ceres was hidden by the Sun's bright light. The mathematician Carl Friedrich Gauss developed a method to predict where it would go. On 31 December 1801, von Zach and Heinrich W. M. Olbers found Ceres near the predicted position.

Over the next few years, the same group found 2 Pallas, 3 Juno, and 4 Vesta. No new asteroids were found until 1845, when amateur astronomer Karl Ludwig Hencke found 5 Astraea. In 1891, Max Wolf began using astrophotography to find asteroids. They appeared as short streaks on photographic plates, and this greatly increased how many were found. Even a century later, only a few thousand asteroids had been identified, numbered, and named. Astronomers found asteroids by photographing the sky in pairs, taken about an hour apart. They compared the pictures to spot an object that had moved. Once an object was identified, its position was sent to the Minor Planet Center, which checked whether it had been seen before. Its first observer could then name it, with approval from the International Astronomical Union.

Naming

Main article: Minor planet § Naming

By 1851, the Royal Astronomical Society saw that asteroids were being found so quickly that they needed a new way to be named. In 1852, de Gasparis found the twentieth asteroid. Benjamin Valz gave it a name and a number that showed its place among the discoveries: 20 Massalia. Some asteroids were never seen again after they were found. So, starting in 1892, new asteroids were listed by year and a letter. Many other systems were tried before a simple numbering system began in 1925. Today, each new asteroid gets a provisional designation, such as 2002 AT4. This name includes the year and a code for the half-month when it was found. After its orbit is confirmed, it gets a number and may later get a name, such as 433 Eros. Discoverers can suggest names, but they must follow rules set by the International Astronomical Union.

Symbols

Main article: Astronomical symbols

The first asteroids were given symbols like the ones often used for planets. By 1852, there were about two dozen, and often there were several versions of each. After the fifteenth asteroid, Eunomia, was found in 1851, Johann Franz Encke changed the 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He used a circle, with a star symbol, as the general sign for an asteroid. A number showed its order of discovery. Astronomers quickly adopted this idea. 16 Psyche, found in 1852, was the first asteroid given this numbered circle when it was discovered, though it also received a special symbol. 20 Massalia was the first asteroid without a special symbol. None were created after 37 Fides was discovered in 1855.

Formation

Main article: Origin of the asteroid belt

Many asteroids are broken pieces of planetesimals. These were small bodies in the young Sun's solar nebula that never grew big enough to become planets. Scientists think that planetesimals in the asteroid belt formed much like other objects in the solar nebula. Then Jupiter grew close to its current size. Its orbital resonances pushed most of the planetesimals in the belt out of the area. Asteroids larger than about 120 km (75 mi) across likely accreted during that early time. Smaller ones are pieces left over from collisions between asteroids. Ceres and Vesta grew large enough to melt and differentiate. This means heavy metals sank to their cores, and rocky minerals stayed in their outer crusts.

In the Nice model, many Kuiper-belt objects were caught in the outer asteroid belt, farther than 2.6 AU from the Sun. Most were later pushed out by Jupiter. The ones that stayed may be the D-type asteroids, and possibly include Ceres.

Distribution within the Solar System

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

Main article: Trojan (celestial body)

Main article: Near-Earth asteroids

Groups of asteroids orbit in the inner Solar System. The pull of other bodies and the Yarkovsky effect can change their orbits. Most known asteroids orbit in the asteroid belt, between Mars and Jupiter. Their paths are usually nearly round. The belt may hold between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) across, plus millions of smaller ones. These may be leftovers from the protoplanetary disk. In this area, Jupiter's strong gravity stopped planetesimals from joining through accretion to form planets. The belt is mostly empty, because the asteroids are spread over a very large space. The four largest objects, Ceres, Vesta, Pallas, and Hygiea, make up about 62% of the belt's mass. Ceres alone makes up 39%. The whole belt has only about 3% of the Moon's mass, while the Kuiper belt and scattered disc are over 100 times as massive. Some asteroids are called Trojans. They share an orbit with a larger planet or moon by staying at the Lagrangian points of stability, which lie 60° ahead of and behind the larger body. Most known Trojans share the orbit of Jupiter, with groups at the Greek camp and the Trojan camp.

Near-Earth asteroids, or NEAs, have orbits that pass close to Earth's orbit. As of April 2022^[update], 28,772 were known, and 878 of them are one kilometer or more across. Some NEAs are extinct comets that have lost their surface materials. Most of the others were pushed out of the asteroid belt by Jupiter's gravity. Many asteroids have natural satellites, called minor-planet moons. The asteroid 3122 Florence, about 4.5 km (2.8 mi) across, has two small moons. Radar found them during its 2017 approach to Earth. Mars has two moons, Phobos and Deimos. They may be captured asteroids, or they may have formed from an impact on Mars.

Characteristics

Asteroids range from nearly 1,000 km (620 mi) across down to rocks about 1 m (3.3 ft) wide. Anything smaller is called a meteoroid. The three largest are like tiny planets and may be leftover protoplanets. Most others are much smaller and have irregular shapes. They may be worn-down planetesimals or pieces of bigger bodies. The dwarf planet Ceres is by far the largest, at 940 km (580 mi) across. The next largest are 4 Vesta and 2 Pallas, both just over 500 km (300 mi). Vesta is the brightest of the four main-belt asteroids that can sometimes be seen with the naked eye. Ceres holds about 40% of the mass of the asteroid belt, which lies between Mars and Jupiter. As asteroids get bigger, there are fewer of them. The pattern generally follows a power law. Pictures of the largest asteroids were taken by ESO's Very Large Telescope.

See also: Largest asteroids

Ceres is the only asteroid that is round enough to count as a dwarf planet. It also has layers, like a planet. Vesta is rocky and has a large crater at its south pole called Rheasilvia. Vesta is the source of the HED meteorites that have been found on Earth. Asteroids also spin at different speeds. Few large asteroids spin faster than once every 2.2 hours. This suggests that most are loose piles of rubble.

Further information: List of fast rotators (minor planets) and List of slow rotators (minor planets)

Asteroids get darker and redder over time because of space weathering. Most of this change happens quickly, though. Most asteroids look broadly similar, even though their shapes are irregular, and many are covered in craters. Ceres has fewer large craters than scientists expected.

Scientists sort asteroids into three main groups based on their emission spectra. These are C-type (carbon-rich), M-type (metallic), and S-type (stony). Ceres seems to have a rocky core under an icy outer layer. Vesta is thought to have a nickel-iron core. Most small asteroids are piles of rubble held together loosely by gravity. Some asteroids have moons, and collisions may have created asteroid families.

Scientists think some of Earth's first water may have come from asteroid impacts. In 2009, water ice was confirmed on the surface of 24 Themis. In 2014, scientists from the European Space Agency (ESA) reported water vapor on Ceres. Asteroids also contain traces of amino acids and other organic compounds. Some scientists think asteroids may have helped bring the chemicals needed for life to Earth. In November 2019, scientists reported sugar molecules, including ribose, in meteorites.

Classification

Asteroids are grouped in two main ways: by the shape of their orbits, and by the features of their spectrum. Many asteroids are placed in groups and families based on their orbits. Groups are loose collections, while families are more closely related. Families form when a large parent asteroid broke apart long ago. Families are more common in the main asteroid belt, though a few small ones have been found among the Jupiter trojans. Kiyotsugu Hirayama first recognized main belt families in 1918, so they are often called Hirayama families in his honor. About 30–35% of asteroids in the belt belong to families. One family has also been linked to the dwarf planet Haumea. Some asteroids follow horseshoe orbits alongside Earth or another planet, such as 3753 Cruithne and 2002 AA29. The first such arrangement was found between Saturn's moons Epimetheus and Janus. Some of these asteroids become quasi-satellites for a few decades or a few hundred years. Both Earth and Venus are known to have them. Those linked to Earth or Venus, or even the hypothetical Mercury, are a special group called Aten asteroids.

In 1975, Chapman, Morrison, and Zellner created a taxonomic system. It sorts asteroids by color, albedo, and spectral shape. These features are thought to match the material on an asteroid's surface. The first system had C-types for dark, carbon-rich objects, S-types for stony objects, and U for those that fit neither. The system has since grown to include many more types, and the number keeps growing as more asteroids are studied. Two widely used systems are the Tholen classification and the SMASS classification. Both have broad groups of C, S, and X asteroids. X asteroids are mostly metal, such as the M-type. Some types are easier to detect than others, so the known counts may not match the true mix of asteroids. Also, asteroids in the same class are not always made of the same materials.

Active asteroids have asteroid-like orbits but show comet-like features, such as comae or tails. This means they lose material. They were first called main-belt comets in 2006 by David C. Jewitt and Henry Hsieh. However, not all of them are icy, and not all are found only in the main-belt. The first active asteroid, 7968 Elst–Pizarro, was discovered as an asteroid in 1979. In 1996, Eric Elst and Guido Pizarro found that it had a tail. 311P/PanSTARRS was seen by the Hubble Space Telescope to have six tails. These are thought to come from a rubble pile asteroid that spins fast enough to shed material.

NASA's Double Asteroid Redirection Test spacecraft struck the asteroid Dimorphos and made it active. This allowed scientists to study the process for the first time. Dimorphos lost about 1 million kilograms of material. The impact created a dust tail more than 10,000 kilometers long that lasted for several months. Dark comets, first identified in 2024, are asteroids that speed up in ways normally seen only in comets, yet they look like plain asteroids. The Outer Family is larger than 100 meters and has orbits that reach nearly to Jupiter. The smaller Inner Family has darker objects with nearly circular orbits. Most orbit in the main asteroid belt, except ʻOumuamua, an object from outside the Solar System.

Main article: Dark comet

Observation and exploration

See also: List of minor planets and comets visited by spacecraft and List of missions to minor planets

Before people could travel into space, scientists could only watch asteroids in the asteroid belt with large telescopes. This meant they could not see the shapes or surfaces of asteroids clearly. Even the best modern telescopes, including the Hubble Space Telescope, can show only a little detail on the largest asteroids. Scientists learn about asteroids' shapes and makeup from light curves, which show how brightness changes as an asteroid spins. They also study the light asteroids reflect. Scientists can estimate their sizes by timing when they pass in front of stars. Radar can show good details of their shapes and movements, especially for near-Earth asteroids. Patrick Michel explains that telescopes on the ground give only rough information, mostly about the thin outer layer of the surface. Spacecraft that fly near asteroids can collect much more information.

Telescopes on the ground and in space have both searched for asteroids. Space telescopes can see more of the sky because the atmosphere does not get in the way. NEOWISE observed more than 100,000 asteroids in the main belt, and the Spitzer Space Telescope observed more than 700 near-Earth asteroids. Hubble has also watched colliding asteroids, a broken-up asteroid, and an active asteroid with six comet-like tails. Space probes have visited many asteroids. The first asteroid photographed up close was 951 Gaspra in 1991. In 1993, the Galileo probe photographed 243 Ida and its moon Dactyl. Other visits included 9969 Braille, visited by Deep Space 1 in 1999, and 5535 Annefrank, visited by Stardust in 2002. In 2008, the Rosetta probe visited 2867 Šteins and 21 Lutetia. The first probe built only to study asteroids was NASA's NEAR Shoemaker. It photographed 253 Mathilde in 1997, then orbited and landed on 433 Eros in 2001. Japan's Hayabusa mission was not the first to return samples, but in 2010 it became the first to bring back samples from an asteroid's surface. That same mission studied 25143 Itokawa. Later missions include NASA's Dawn, which orbited 4 Vesta and observed the dwarf planet Ceres. Another is Hayabusa2 from JAXA, which collected samples from 162173 Ryugu. In 2023, NASA's OSIRIS-REx returned a sample from 101955 Bennu. In 2022, NASA's Double Asteroid Redirection Test (DART) deliberately hit the minor-planet moon Dimorphos of the double asteroid Didymos. The goal was to test whether a spacecraft could change an asteroid's path. NASA's Lucy is visiting several Jupiter trojans. NASA's Psyche is heading to the metallic asteroid of the same name. ESA's Hera will study the DART crater. China's Tianwen-2 will collect samples from 469219 Kamoʻoalewa and study 311P/PanSTARRS.

Asteroid mining

The idea of asteroid mining was first suggested in the 1970s. Matt Anderson says that successful asteroid mining would be a program that can pay for itself and make a profit for its investors. Asteroids might provide materials that are rare or running low on Earth. They might also provide materials for building space habitats. Heavy materials are very expensive to launch from Earth. One day, they may be mined from asteroids and used for space manufacturing and construction.

As resource depletion on Earth grows, valuable elements from asteroids become more appealing. These could be brought back to Earth for profit. They could also be used to build solar-power satellites and space habitats in space. In theory, water made from ice could refuel orbiting propellant depots. Studying asteroids could also give scientists information for the search for extraterrestrial intelligence (SETI) from an astrobiological point of view. Some astrophysicists have suggested that if advanced alien civilizations used asteroid mining long ago, signs of that activity might still be possible to detect.

Threats to Earth

See also: List of Earth-crossing minor planets

Scientists are paying more attention to asteroids whose paths cross Earth's. Over a very long time, some of these could hit Earth. The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. The first of these, 433 Eros, was discovered in 1898. Several similar objects were found in the 1930s. Astronomers began to understand that asteroids could strike Earth. Two later events made people more concerned. One was the growing acceptance of the Alvarez hypothesis, which linked an impact event to the Cretaceous–Paleogene extinction. The other was the 1994 sighting of Comet Shoemaker-Levy 9 crashing into Jupiter. Military satellites, built to detect nuclear explosions, also saw many small objects hit the upper atmosphere.

These concerns led to better sky surveys. The surveys used CCD cameras and computers linked to telescopes. By 2011, most of the large near-Earth asteroids had been found. In June 2018, the National Science and Technology Council warned that the United States was not ready for an asteroid impact. It released a plan to prepare better. In 2013, experts told the United States Congress that NASA would need at least five years of preparation before launching a mission to reach an asteroid and change its path. Scientists also study ways to change an asteroid's path. One idea is to break it apart. Another is to push it gently so it misses Earth.

Fiction

Asteroids and the asteroid belt are a popular subject in science fiction stories. In these stories, asteroids can be places where people live, sources of minerals to mine, and hazards for spacecraft travelling between places. They can also be a danger to Earth or to other planets, dwarf planets, and moons if they might crash into them.