An asteroid is a small object that is larger than a meteoroid (at least 1 meter in size) and is not a planet or a comet. These objects orbit the Sun either in the inner part of the Solar System or share Jupiter's orbit (called Trojan asteroids). Asteroids are made of rock, metal, or ice and do not have an atmosphere. They are grouped into three main types: C-type (carbon-rich), M-type (metallic), or S-type (rocky). Asteroids vary greatly in size and shape, from small piles of rubble less than a kilometer wide to Ceres, a dwarf planet nearly 1,000 kilometers across. A body is called a comet, not an asteroid, if it develops a tail (called a coma) when heated by the Sun. However, recent studies suggest that the difference between asteroids and comets is not always clear.

There are about one million known asteroids, most of which are located between the orbits of Mars and Jupiter, roughly 2 to 4 astronomical units (AU) from the Sun. This area is called the main asteroid belt. All asteroids combined have a total mass equal to about 3% of the Moon's mass. Most asteroids in the main belt have slightly oval-shaped orbits that are stable and take between three and six years to complete one orbit around the Sun, moving in the same direction as Earth.

Asteroids have been observed from Earth since their discovery in 1801. The first close-up view of an asteroid was taken by the Galileo spacecraft. Later, NASA and JAXA launched several missions to study asteroids. NASA's NEAR Shoemaker studied the asteroid Eros, and the Dawn mission observed Vesta and Ceres. JAXA's Hayabusa and Hayabusa2 missions collected samples from the asteroids Itokawa and Ryugu. OSIRIS-REx studied Bennu, collecting a sample in 2020 that was returned to Earth in 2023. NASA's Lucy mission, launched in 2021, will examine ten asteroids, including two in the main belt and eight Jupiter trojans. NASA's DART mission, launched in 2021, intentionally crashed into the asteroid Dimorphos in September 2022 to test technology for planetary defense. NASA's Psyche mission, launched in October 2023, will study the metallic asteroid Psyche. ESA's Hera mission, launched in October 2024, will analyze the effects of the DART impact. China's CNSA launched Tianwen-2 in May 2025 to explore the near-Earth asteroid 469219 Kamoʻoalewa and the active asteroid 311P/PanSTARRS, collecting samples from Kamoʻoalewa's surface.

Near-Earth asteroids could cause major damage if they collide with Earth. A well-known example is the Chicxulub impact, which is believed to have caused the Cretaceous–Paleogene mass extinction.

Terminology

In 2006, the International Astronomical Union (IAU) introduced a new term, "small Solar System body," which is now preferred. This term describes objects in the Solar System that are not planets, dwarf planets, or natural satellites. It includes asteroids, comets, and other recently discovered types. According to the IAU, the term "minor planet" may still be used, but "small Solar System body" is now the preferred term.

Historically, the first asteroid discovered, Ceres, was initially thought to be a planet. Later, other similar objects were found. At the time, these objects appeared as points of light like stars but could be distinguished from stars by their movement across the sky. This led astronomer Sir William Herschel to suggest the term "asteroid," which comes from the Greek word asteroidēs, meaning "star-like" or "star-shaped." During the early 1800s, the terms "asteroid" and "planet" (not always specified as "minor") were sometimes used interchangeably.

Traditionally, small objects orbiting the Sun were classified as comets, asteroids, or meteoroids. Objects smaller than one meter were called meteoroids. The term "asteroid" has never been officially defined but is often used to describe irregular, rocky objects that orbit the Sun and do not meet the IAU's definitions for planets or dwarf planets. The main difference between asteroids and comets is that comets develop a coma (a tail) when their surface ices turn into gas due to sunlight. Some objects first classified as minor planets later showed comet-like activity, while some comets eventually lose their ices and become asteroid-like. Comets usually have more stretched-out orbits than most asteroids, and highly stretched-out asteroids may be dormant or extinct comets.

Minor planets beyond Jupiter's orbit are sometimes called "asteroids," especially in popular media. However, it is now more common to use "asteroid" only for objects in the inner Solar System. This article will focus mainly on classical asteroids, such as those in the asteroid belt, Jupiter trojans, and near-Earth objects.

For nearly 200 years after Ceres was discovered in 1801, all known asteroids mostly stayed near or inside Jupiter's orbit. However, a few, like 944 Hidalgo, traveled farther out for part of their orbit. In 1977, 2060 Chiron was discovered, the first object that permanently stayed beyond Jupiter. These are now called centaurs. In 1992, 15760 Albion was found, the first object beyond Neptune (other than Pluto). Many similar objects were later discovered and are now called trans-Neptunian objects. Beyond them are Kuiper-belt objects, scattered-disc objects, and the distant Oort cloud, which is thought to be the main source of dormant comets. These objects live in the cold outer regions of the Solar System, where ices remain solid and comet-like bodies show little activity. If centaurs or trans-Neptunian objects move closer to the Sun, their ices would turn into gas, and they would be classified as comets.

Kuiper-belt objects are called "objects" partly to avoid classifying them as asteroids or comets. They are mostly made of materials similar to comets, though some may be more like asteroids. Most do not have the highly stretched-out orbits of comets, and the ones discovered so far are larger than typical comet nuclei. Studies of dust collected by the Stardust probe suggest that the difference between comets and asteroids may be more of a gradual change than a clear divide.

In 2006, the IAU created the category of dwarf planets for the largest minor planets—those massive enough to be shaped by their own gravity. Only the largest object in the asteroid belt, Ceres (about 975 km wide), has been classified as a dwarf planet. Asteroids are sometimes described as "undifferentiated," while dwarf planets are "differentiated," meaning their interiors are layered.

History of observations

Although there are many asteroids, they were discovered more recently. The first one, Ceres, was found in 1801. Only one asteroid, 4 Vesta, which has a surface that reflects light well, is usually visible to the naked eye in dark skies when it is in the right position. Occasionally, small asteroids passing close to Earth may be briefly seen without the aid of telescopes. As of May 2025, the Minor Planet Center had information about 1,460,356 minor planets in the inner and outer Solar System. About 826,864 of these had enough details to receive numbered names.

In 1772, German astronomer Johann Elert Bode, using a formula suggested by Johann Daniel Titius, published a rule called the Titius–Bode law. This rule was later proven incorrect. It predicted the orbits of known planets except for a gap between Mars and Jupiter. Bode believed a planet might exist in this gap. His formula suggested a planet would orbit the Sun at about 2.8 astronomical units (AU), or 420 million kilometers. The discovery of Uranus by William Herschel near this predicted distance supported the rule. In 1800, Franz Xaver von Zach, editor of the German journal Monatliche Correspondenz, asked 24 astronomers to search for the missing planet. Though they did not find Ceres, they later discovered asteroids 2 Pallas, 3 Juno, and 4 Vesta.

One of the astronomers chosen for the search was Giuseppe Piazzi, a Catholic priest in Sicily. Before joining the group, Piazzi found Ceres on January 1, 1801. He was looking for a star listed in a catalog but noticed a moving object instead. He thought it might be a comet. Piazzi observed Ceres 24 times, the last on February 11, 1801, when illness stopped his work. He shared his discovery with two astronomers in January 1801, describing it as a comet but noting its slow, steady movement. In April, he sent his full observations to Oriani, Bode, and French astronomer Jérôme Lalande. His findings were published in September 1801.

By then, Ceres had moved out of view due to Earth’s orbit around the Sun. Its position near the Sun made it hard for others to find. By the end of 1801, Ceres should have been visible again, but its exact location was hard to predict. Mathematician Carl Friedrich Gauss, then 24 years old, created a method to calculate its orbit. In weeks, he predicted where Ceres would be and shared his results with von Zach. On December 31, 1801, von Zach and another astronomer found Ceres near the predicted spot. At 2.8 AU from the Sun, Ceres matched the Titius–Bode law. However, Neptune, discovered in 1846, was 8 AU closer than predicted, leading most scientists to believe the law was a coincidence. Piazzi named the object Ceres Ferdinandea, honoring Sicily’s patron goddess and King Ferdinand of Bourbon.

Over the next few years, von Zach’s group found three other asteroids: 2 Pallas, 3 Juno, and 4 Vesta (discovered in 1807). No new asteroids were found until 1845. Amateur astronomer Karl Ludwig Hencke began searching for asteroids in 1830. In 1845, he found asteroid 5 Astraea, the first new asteroid discovered in 38 years. Gauss named it. By the end of 1851, 15 asteroids had been found. In 1868, when James Craig Watson discovered the 100th asteroid, the French Academy of Sciences honored three successful asteroid hunters—Karl Theodor Robert Luther, John Russell Hind, and Hermann Goldschmidt—by engraving their faces on a commemorative medallion.

In 1891, Max Wolf used astrophotography to find asteroids. This method, which captured moving objects as streaks on photographic plates, greatly increased discovery rates. Wolf alone found 248 asteroids, starting with 323 Brucia. Before this, only slightly more than 300 asteroids had been discovered. Many more existed, but some astronomers ignored them, calling them "vermin of the skies." Even a century later, only a few thousand asteroids had been identified, numbered, and named.

In the past, discovering asteroids followed a four-step process. First, a region of the sky was photographed using a wide-field telescope or astrograph. Pairs of photos were taken, usually one hour apart, over several days. Second, the photos were viewed under a stereoscope. A moving object in orbit around the Sun would appear slightly shifted compared to background stars. Third, once a moving object was identified, its position was measured precisely using a digitizing microscope, compared to known star locations.

These first three steps do not confirm an asteroid’s discovery. The observer only finds an "apparition," which receives a temporary name made up of the discovery year, a letter for the half-month of discovery, and a letter-number for its order (example: 1998 FJ 74). The final step is sending observations to the Minor Planet Center. Computer programs there check if the apparition matches earlier sightings to determine the object’s orbit. If confirmed, the object gets a catalog number, and the first observer with a calculated orbit is credited as the discoverer and given the right to name it, with approval from the International Astronomical Union.

Naming

In 1851, the Royal Astronomical Society noticed that asteroids were being found so quickly that a new way to group or name them was needed. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz gave it a name and a number to show its order among all discovered asteroids, called 20 Massalia. Sometimes, asteroids were found but not seen again. To solve this, starting in 1892, new asteroids were listed with the year and a letter showing the order in which their orbits were calculated that year. For example, the first two asteroids found in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all asteroids found in 1893, so after 1893Z came 1893AA. Many variations of these methods were tested, including using years with Greek letters in 1914. A simple system that numbered asteroids in the order they were discovered was finally used in 1925.

Today, newly discovered asteroids are given a temporary name, such as 2002 AT 4, which includes the year of discovery and a mix of letters and numbers to show the half-month of discovery and the order within that time. Once an asteroid’s orbit is confirmed, it is given a number, and later it may also be given a name, like 433 Eros. The official naming rule uses parentheses around the number—such as (433) Eros—but parentheses are often left out. Informally, people may skip the number entirely or only mention it once when repeating a name later. Names can be suggested by the discoverer, following rules set by the International Astronomical Union.

The first asteroids discovered were given special symbols, similar to those used for planets. By 1852, there were 24 asteroid symbols, which often had many different versions. In 1851, after the fifteenth asteroid, Eunomia, was found, Johann Franz Encke changed the system for the 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ). He introduced a circle, a symbol used for stars, as the standard symbol for asteroids. The circle was numbered in the order of discovery to identify specific asteroids. This method was quickly accepted by astronomers. The next asteroid found, 16 Psyche in 1852, was the first to be labeled this way when it was discovered. However, Psyche also had an iconic symbol, as did a few other asteroids found later. 20 Massalia was the first asteroid not given an iconic symbol, and no new symbols were created after the 1855 discovery of 37 Fides.

Formation

Many asteroids are broken pieces of planetesimals, which are small bodies in the early solar system that never became planets. Scientists believe that planetesimals in the asteroid belt developed similarly to other objects in the solar nebula until Jupiter reached its current size. At that time, gravitational forces from Jupiter's orbit pushed away over 99% of the planetesimals in the belt. Simulations and differences in how fast asteroids spin and their surface features suggest that asteroids larger than about 120 km (75 mi) in diameter formed during the early solar system, while smaller asteroids are pieces from collisions between larger asteroids during or after Jupiter's disruption. Ceres and Vesta grew large enough to melt and separate layers, with heavy metals sinking to their centers and rocky materials remaining in their outer layers.

In the Nice model, many Kuiper-belt objects were moved to the outer part of the asteroid belt, which is more than 2.6 AU from the Sun. Most of these objects were later pushed out by Jupiter, but some that remained may be D-type asteroids, and possibly include Ceres.

Distribution within the Solar System

Various groups of asteroids have been found orbiting in the inner Solar System. Their paths are affected by the gravity of other objects in the Solar System and by the Yarkovsky effect. Important groups include:

Most known asteroids orbit in the asteroid belt between Mars and Jupiter. These asteroids generally move in orbits that are not very stretched out. The belt is believed to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in size, as well as many smaller ones. These asteroids may be leftover pieces from the early Solar System, and in this region, the process of forming planets from smaller objects was stopped by Jupiter’s strong gravity.

Although often shown as crowded, the asteroid belt is mostly empty. The asteroids are spread over such a large area that hitting one by accident would be unlikely. However, hundreds of thousands of asteroids are known, and the total number is estimated to be in the millions or more, depending on the size of the smallest asteroids counted. Over 200 asteroids are larger than 100 km, and surveys using infrared light have found between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. The brightness levels of most known asteroids range from 11 to 19, with the middle value around 16.

The total mass of the asteroid belt is estimated to be 2.39 × 10^21 kg, which is about 3% of the Moon’s mass. The Kuiper Belt and Scattered Disk are much larger, with a combined mass over 100 times greater. The four largest objects in the asteroid belt—Ceres, Vesta, Pallas, and Hygiea—make up about 62% of the belt’s total mass, with Ceres alone accounting for 39% of that.

Trojans are groups of asteroids that share an orbit with a larger planet or moon but do not collide with it. They remain in stable points called Lagrangian points, L4 and L5, which are located 60° ahead of and behind the larger body.

Most known Trojans in the Solar System orbit Jupiter. These Trojans are divided into two groups: the Greek camp at L4 (ahead of Jupiter) and the Trojan camp at L5 (behind Jupiter). Scientists believe more than a million Jupiter Trojans larger than 1 km exist, with over 7,000 currently recorded. Smaller numbers of Trojans have been found around Mars, Neptune, Uranus, Earth, and Venus. Simulations suggest Saturn and Uranus likely do not have any original Trojans.

Near-Earth asteroids (NEAs) are asteroids whose orbits come close to Earth’s. Those that cross Earth’s path are called Earth-crossers. As of April 2022, 28,772 NEAs were known, with 878 larger than 1 km in diameter.

A few NEAs are extinct comets that lost their icy surfaces. However, having a faint or temporary comet-like tail does not always mean they are classified as near-Earth comets, making the classification unclear. Most NEAs are pushed out of the asteroid belt by Jupiter’s gravity.

Many asteroids have natural satellites, or moons. As of October 2021, 85 NEAs were known to have at least one moon, with three having two moons. The asteroid 3122 Florence, a large potentially hazardous asteroid with a diameter of 4.5 km, has two moons measuring 100–300 meters across. These moons were discovered using radar imaging during Florence’s 2017 close approach to Earth.

NEAs are grouped based on their orbit characteristics:

• Atiras or Apoheles: These asteroids orbit entirely inside Earth’s path. Their farthest point from the Sun (aphelion) is closer than Earth’s closest point (perihelion, 0.983 AU). This means their average distance from the Sun (semi-major axis) is also less than 0.983 AU.
• Atens: These asteroids have an average distance from the Sun (semi-major axis) less than 1 AU but cross Earth’s orbit. Their farthest point from the Sun (aphelion) is greater than Earth’s perihelion (0.983 AU).
• Apollos: These asteroids have an average distance from the Sun (semi-major axis) greater than 1 AU but cross Earth’s orbit. Their closest point to the Sun (perihelion) is less than Earth’s farthest point (aphelion, 1.017 AU).
• Amors: These asteroids orbit entirely outside Earth’s path. Their closest point to the Sun (perihelion) is farther than Earth’s aphelion (1.017 AU), but they remain within 1.3 AU of the Sun. Some Amors may cross Mars’ orbit.

It is unclear whether Mars’ moons, Phobos and Deimos, are captured asteroids or formed by an impact on Mars. Both moons share similarities with carbon-rich C-type asteroids, including their color, brightness, and density. One theory suggests they may have been captured from the asteroid belt. However, their nearly circular orbits and alignment with Mars’ equatorial plane suggest a mechanism, such as atmospheric drag or tidal forces, to adjust their orbits. Another idea is that Phobos and Deimos formed from material ejected by an impact on Mars, similar to how Earth’s Moon is believed to have formed. Evidence, such as the presence of minerals found on Mars in Phobos and its low density, supports this impact theory.

Characteristics

Asteroids come in many different sizes. The largest are about 1,000 km (620 mi) wide, while the smallest are just 1 m (3.3 ft) across. Objects smaller than 1 m are called meteoroids. The three largest asteroids are similar to small planets. They are mostly round, have layers inside, and are thought to be leftover pieces from when the solar system formed. Most asteroids are much smaller and have irregular shapes. These are likely pieces of larger objects that were broken apart by collisions.

The largest asteroid is Ceres, which is 940 km (580 mi) wide. The next two largest are Vesta and Pallas, each about 500 km (300 mi) wide. Vesta is the brightest asteroid that can sometimes be seen without a telescope. Some near-Earth asteroids can also be seen without special tools, like Apophis.

The total mass of all asteroids in the belt between Mars and Jupiter is about 3.25% of the Moon's mass. Ceres makes up about 40% of this mass. Vesta, Pallas, and Hygiea together make up about 60%, and the next seven largest asteroids bring the total to about 70%. There are many more small asteroids than large ones.

Larger asteroids are less common than smaller ones. Most asteroids larger than about 120 km (75 mi) are very old and formed when the solar system began. Smaller asteroids are usually pieces broken off from larger ones. The number of asteroids in the belt was probably 200 times greater in the past.

The four largest asteroids—Ceres, Vesta, Pallas, and Hygiea—are special. They are more like small planets than the typical irregular-shaped asteroids. Ceres is the only asteroid that is a dwarf planet because it has a shape influenced by its own gravity. It may have ice on its surface and has layers like a planet. No pieces from Ceres have been found on Earth.

Vesta also has layers inside, but it formed outside the frost line, so it has no water. It is made mostly of basaltic rock and minerals like olivine. Vesta is the source of meteorites called HED meteorites, which make up 5% of all meteorites found on Earth.

Pallas is unusual because it spins on its side, like Uranus. It has a composition similar to Ceres, with high amounts of carbon and silicon. It is the source of the Palladian family of asteroids.

Hygiea is the largest carbon-rich asteroid and lies close to the plane of the solar system. It is the largest member of the Hygiean family of asteroids. Unlike Vesta, no large crater has been found on Hygiea that could explain the family’s origin. Observations suggest Hygiea may have been broken apart and reformed after a collision.

Large asteroids may not have natural satellites because satellites are usually formed from collisions, creating loose piles of debris. Studies show that most asteroids larger than 100 meters cannot spin faster than 2.2 hours without losing surface material. This suggests many large asteroids are loose piles of debris from past collisions.

Asteroids become darker and redder over time due to space weathering, but most color changes happen quickly, making it hard to use color to determine their age.

Most asteroids, except the four largest, look similar if they are irregularly shaped. For example, Mathilde and Davida are covered in craters and have rough surfaces. Vesta has a large crater but is mostly round. Ceres has many craters but fewer large ones than expected. Scientists think this is because the surface slowly smooths out over time.

Asteroids are grouped by their light patterns into three main types: C-type (carbon-rich), M-type (metallic), and S-type (rocky). Most asteroids are not fully understood. Ceres has a rocky core and icy layers. Vesta has a metal core, a layer of olivine, and a basaltic surface. Hygiea may be made of simple materials but could have layers. Many asteroids are loose piles of debris, while the largest are likely solid. Some asteroids have moons or are paired with other asteroids, which may form from collisions.

Classification

Asteroids are often grouped based on two factors: the paths they follow around the Sun, and how they reflect light.

Many asteroids are organized into groups or families based on their orbital paths. Large groups are named after the first asteroid discovered in that group. Groups are loosely connected, while families are more tightly linked and formed when a large asteroid broke apart in the past. Families are more common and easier to find in the main asteroid belt, though some small families have been found among Jupiter’s trojans. These families were first identified by Kiyotsugu Hirayama in 1918 and are often called Hirayama families.

About 30–35% of asteroids in the main belt belong to families, which are believed to have formed from past collisions. One family is linked to the plutoid dwarf planet Haumea.

Some asteroids have special horseshoe-shaped orbits that share paths with Earth or other planets. Examples include 3753 Cruithne and 2002 AA 29. The first known horseshoe orbit was discovered between Saturn’s moons Epimetheus and Janus. These objects sometimes act as temporary quasi-satellites for decades or centuries before returning to their original paths. Earth and Venus are known to have quasi-satellites.

Objects that share orbits with Earth, Venus, or even Mercury are a special type of Aten asteroid. These objects can also be linked to outer planets.

In 1975, a system for classifying asteroids was created by Chapman, Morrison, and Zellner. This system uses color, brightness, and light patterns to determine surface composition. The original system had three categories: C-types (dark, carbon-rich asteroids, 75% of known asteroids), S-types (rocky asteroids, 17% of known asteroids), and U (asteroids not fitting C or S). Over time, more categories were added as more asteroids were studied.

Today, the most common classification systems are the Tholen and SMASS systems. Tholen’s system, created in 1984, used data from an eight-color survey and divided asteroids into 14 categories. The SMASS system, developed in 2002, expanded this to 24 categories. Both systems include C, S, and X types. X-type asteroids are mostly metallic, like M-type asteroids. Other smaller categories also exist.

The distribution of asteroid types in observations does not always match their true abundance in space. Some types are easier to detect, which affects the numbers seen.

Early classifications were based on guesses about asteroid composition, but these guesses are not always accurate. This has caused confusion, as different classifications are used. While asteroids of different types likely have different materials, asteroids in the same group may not share the same composition.

Active asteroids have orbits like asteroids but show comet-like features, such as tails or glowing areas. These objects were first called main-belt comets in 2006, but this name can be misleading. Some active asteroids are not icy or limited to the main belt.

The first active asteroid discovered was 7968 Elst–Pizarro. Found as an asteroid in 1979, it was later observed to have a tail in 1996 and named 133P/Elst-Pizarro. Another example is 311P/PanSTARRS, which has six comet-like tails. These tails may form when fast-spinning asteroids release material.

NASA’s DART mission made the asteroid Dimorphos an active asteroid by crashing into it. This event showed that impacts can create active asteroids. Observations found that Dimorphos lost about 1 million kilograms of material, creating a dust plume and a long dust tail that lasted months.

Dark comets, first identified in 2024, are asteroids that move in ways similar to comets but lack visible tails or glowing areas. Two groups have been found: the Outer Family, with objects larger than 100 meters and orbits near Jupiter, and the Inner Family, with smaller, darker asteroids in nearly circular orbits. All dark comets orbit in the main asteroid belt except ʻOumuamua, an interstellar object that showed non-gravitational movement.

Observation and exploration

Until space travel was possible, objects in the asteroid belt could only be seen using large telescopes. Scientists could not learn much about their shapes or surfaces. Today, the best ground-based telescopes and the Hubble Space Telescope can see some details on the largest asteroids, but not much. Scientists can guess some information about asteroid shapes and materials by studying how their brightness changes as they spin and by looking at how they reflect light. Sizes can be estimated by timing how long an asteroid blocks a star’s light. Radar imaging helps scientists learn about asteroid shapes and how they move, especially for asteroids near Earth. Spacecraft that fly by asteroids can collect much more information than telescopes. Missions that bring back samples help scientists study the surface material of asteroids.

Asteroids are small and hard to see from Earth, so information from ground-based observations is limited. Ground telescopes can measure how bright an asteroid appears, which helps estimate its size. By studying how brightness changes over time, scientists can guess how fast an asteroid spins and its rough shape. Spectral data, which shows how light reflects off an asteroid, helps scientists learn about its composition. However, this data only shows the top layer of the surface. Patrick Michel explains that radar can study asteroids in more detail, especially those near Earth. Radar can show features like craters and rocks on the surface. Observatories like Arecibo in Puerto Rico and Goldstone in California use radar to study asteroids.

Both space and ground telescopes search for asteroids. Space telescopes find more objects because they avoid Earth’s atmosphere and can see more of the sky. NEOWISE found over 100,000 asteroids in the main belt, and the Spitzer Space Telescope found over 700 near-Earth asteroids. These observations give rough sizes but not much about surface details like material type or structure.

The Hubble Space Telescope has studied asteroids, such as watching them collide, break apart, or have comet-like tails. It also helped plan missions to study specific asteroids.

The first close-up photo of an asteroid was of 951 Gaspra in 1991, then 243 Ida and its moon Dactyl in 1993. These were taken by the Galileo spacecraft on its way to Jupiter. Other spacecraft, like Deep Space 1 and Stardust, visited asteroids during their missions.

NASA’s NEAR Shoemaker was the first spacecraft to orbit and land on an asteroid. It studied 253 Mathilde in 1997 and landed on 433 Eros in 2001. The Japanese Hayabusa probe studied 25143 Itokawa, brought back samples to Earth in 2010, and was the first sample-return mission. NASA’s Dawn spacecraft studied 4 Vesta and the dwarf planet Ceres.

Hayabusa2, launched by Japan in 2014, studied 162173 Ryugu, brought back samples in 2020, and is now heading to a new target in 2031. NASA’s OSIRIS-REx studied asteroid 101955 Bennu, brought back samples in 2023, and will now study asteroid Apophis.

NASA’s DART mission tested how to change an asteroid’s path by crashing into Dimorphos in 2022. This mission was successful, as it changed Dimorphos’s orbit. NASA’s Lucy mission will fly by seven Jupiter trojans. Psyche, launched in 2023, will study a metallic asteroid. ESA’s Hera mission, launched in 2024, will study the results of DART’s impact.

China’s Tianwen-2, launched in 2025, will study asteroids 469219 Kamoʻoalewa and 311P/PanSTARRS. It will collect samples from Kamoʻoalewa.

Key asteroid missions include Hayabusa2, Dawn, Lucy, Psyche, and JAXA’s DESTINY+, which will study the Geminids meteor shower’s parent body and other objects. DESTINY+ is planned for launch in 2028.

Asteroid mining

The idea of mining asteroids was first suggested in the 1970s. Matt Anderson describes successful asteroid mining as "a program that can support itself financially and earn money for its investors." Some scientists believe asteroids could provide materials that are rare or no longer available on Earth, or materials needed to build living spaces in space. Materials that are heavy and costly to send from Earth might one day be obtained from asteroids and used for making things in space.

As Earth’s natural resources become harder to find, the idea of taking valuable materials from asteroids and bringing them back to Earth for profit, or using space resources to build solar power satellites and living spaces in space, is becoming more appealing. It is also possible that water from ice on asteroids could be used to refuel stations in space.

From the viewpoint of studying life in space, looking for resources on asteroids might help scientists search for signs of intelligent life beyond Earth. Some scientists think that if advanced alien civilizations mined asteroids long ago, evidence of their activities might still be visible today.

Threats to Earth

There is growing interest in finding asteroids that have orbits crossing Earth's path, which could, over time, collide with Earth. The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens.

The near-Earth asteroid 433 Eros was discovered in 1898, and the 1930s saw many similar objects found. In order of discovery, these included 1221 Amor, 1862 Apollo, 2101 Adonis, and 69230 Hermes, which came within 0.005 astronomical units (AU) of Earth in 1937. Scientists began to understand the possibility of Earth being hit by asteroids.

Two events in later years raised concern: the growing support for the Alvarez hypothesis, which suggests an asteroid impact caused the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also shared information that its satellites, designed to detect nuclear explosions, had recorded hundreds of impacts by objects 1 to 10 meters wide in Earth's upper atmosphere.

These discoveries led to the creation of highly effective surveys using charge-coupled device (CCD) cameras and computers connected directly to telescopes. By 2011, it was estimated that 89% to 96% of near-Earth asteroids larger than 1 kilometer in diameter had been found. By 29 October 2018, the LINEAR system alone had discovered 147,132 asteroids. Among all surveys, 19,266 near-Earth asteroids had been found, including nearly 900 larger than 1 kilometer (0.6 miles) in diameter.

In June 2018, the National Science and Technology Council warned that the United States is not ready for an asteroid impact and released the "National Near-Earth Object Preparedness Strategy Action Plan" to improve readiness. According to expert testimony in the U.S. Congress in 2013, NASA would need at least five years to prepare for a mission to change an asteroid's course.

Different methods to avoid asteroid collisions have trade-offs based on factors like cost, risk, and technology. Techniques to change an asteroid's path can be grouped by type, such as whether they break the object apart (fragmentation) or delay its arrival (delay). Fragmentation aims to split the object into smaller pieces that miss Earth or burn up in the atmosphere. Delay uses the fact that Earth and the asteroid move in space. Since Earth is about 12,750 km (7,920 mi) in diameter and travels at about 30 km/s (19 mi/s), it moves one diameter every 425 seconds, or about seven minutes. Delaying or advancing the asteroid’s arrival by this amount can cause it to miss Earth.

"Project Icarus" was one of the first plans created in 1967 as a backup for a potential collision with asteroid 1566 Icarus. The plan used the Saturn V rocket, which had not yet flown when the plan was made. Six rockets would be launched at different times before impact, each carrying a 100-megaton nuclear warhead and modified Apollo spacecraft for guidance. The warheads would explode 30 meters from the asteroid’s surface to change its path or break it apart. Later missions would adjust based on the asteroid’s movement. The final rocket would launch 18 hours before impact.

Fiction

Asteroids and the asteroid belt are common features in science fiction stories. Asteroids can appear in these stories in several ways: as places where humans might live, as sources of valuable materials like minerals, as dangers for spaceships traveling between two locations, and as a danger to life on Earth and other planets that have life, as well as to moons and other celestial objects if they hit these places.