An asteroid is a small planet-like object that is not a planet or a comet. It is larger than a meteoroid (at least 1 meter in size) and orbits the Sun either in the inner Solar System or shares Jupiter’s orbit (called Trojan asteroids). Asteroids are made of rock, metal, or ice and have no atmosphere. They are divided into three main types: C-type (carbon-rich), M-type (metallic), or S-type (rocky). Asteroids vary greatly in size and shape, from small groups of rocks less than 1 kilometer wide to Ceres, a dwarf planet nearly 1,000 kilometers in diameter. 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 show that some objects may have features of both asteroids and comets.

Of the about one million known asteroids, most are found between the orbits of Mars and Jupiter, roughly 2 to 4 astronomical units (AU) from the Sun. This region is called the main asteroid belt. The total mass of all asteroids combined is only 3% of the Moon’s mass. Most asteroids in the main belt follow slightly oval-shaped, stable orbits, moving in the same direction as Earth and taking 3 to 6 years to orbit the Sun.

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 Eros, and Dawn observed Vesta and Ceres. JAXA’s Hayabusa and Hayabusa2 collected samples from Itokawa and Ryugu. OSIRIS-REx studied Bennu and brought a sample back to Earth in 2023. NASA’s Lucy, launched in 2021, is studying ten asteroids, including two from the main belt and eight Jupiter trojans. NASA’s DART mission, launched in 2021, intentionally crashed into the asteroid Dimorphos in 2022 to test technology for protecting Earth from potential impacts. NASA’s Psyche, launched in 2023, will study the metallic asteroid Psyche. ESA’s Hera, launched in 2024, will examine the effects of the DART mission. China’s CNSA launched Tianwen-2 in 2025 to explore the near-Earth asteroid 469219 Kamoʻoalewa and the active asteroid 311P/PanSTARRS, collecting samples from Kamoʻoalewa.

Near-Earth asteroids could cause serious damage if they hit Earth. A famous 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 the term "small Solar System body," which refers to objects in the Solar System that are not planets, dwarf planets, or natural satellites. This group includes asteroids, comets, and other recently discovered objects. The IAU stated that 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 discoveries of similar objects appeared as points of light, like stars, but moved across the sky differently. This led astronomer Sir William Herschel to suggest the term "asteroid," which comes from the Greek word asteroidēs, meaning "star-like." During the early 1800s, the terms "asteroid" and "planet" were sometimes used interchangeably.

Traditionally, small Solar System bodies were classified as comets, asteroids, or meteoroids, with objects smaller than one meter called meteoroids. The term "asteroid" has never been officially defined but is often used to describe irregularly shaped, rocky objects that orbit the Sun and are not classified as 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 asteroids later showed comet-like activity, and 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" in popular media, but the term is increasingly used 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, most known asteroids stayed near or within Jupiter’s orbit. However, in 1977, astronomers found objects like 2060 Chiron that lived farther from the Sun than Jupiter. These were called centaurs. In 1992, 15760 Albion was discovered beyond Neptune’s orbit, leading to the identification of trans-Neptunian objects. Beyond these 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 exist in the cold outer regions of the Solar System, where ices remain solid. If centaurs or trans-Neptunian objects moved closer to the Sun, their ices would turn into gas, and they would be classified as comets.

Kuiper-belt objects are called "objects" to avoid labeling them as asteroids or comets. They are likely made of materials similar to comets, though some may resemble asteroids. Most do not have the stretched-out orbits typical of comets, and many are larger than traditional comet nuclei. Recent discoveries, such as the Stardust probe’s analysis of comet dust, suggest that the differences between comets and asteroids may not be as clear-cut as once thought, indicating a range of characteristics rather than strict categories.

In 2006, the IAU created the category of dwarf planets for the largest minor planets—those massive enough to be shaped like an oval by their own gravity. Only the largest asteroid, Ceres, which is about 975 kilometers wide, is classified as a dwarf planet. Asteroids are sometimes described as "undifferentiated," while dwarf planets are "differentiated," meaning their interiors are layered or structured differently.

History of observations

Asteroids were discovered fairly recently, even though there are many of them. The first asteroid, called Ceres, was found in 1801. Only one asteroid, 4 Vesta, is usually visible to the naked eye in dark skies when it is in the right position. Occasionally, small asteroids that pass close to Earth may briefly be seen without the help of a telescope. 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, except for a gap between Mars and Jupiter. Bode’s formula predicted the existence of a planet with an orbit about 2.8 astronomical units (AU) from the Sun. This prediction was supported when William Herschel discovered Uranus near the expected distance beyond Saturn. 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 discovered Ceres on January 1, 1801. He was looking for a star in a catalog but found a moving object instead. Piazzi observed Ceres 24 times, the last time on February 11, 1801, when illness stopped his work. He shared his discovery with two astronomers in January 1801, calling it a comet but noting its slow, steady movement. In April, he sent full observations to three astronomers, and the results were published in September 1801.

By that time, Ceres had moved out of view because Earth’s position changed, making it hard to find again. Mathematician Carl Friedrich Gauss, then 24 years old, calculated Ceres’ orbit and predicted its location. On December 31, 1801, von Zach and another astronomer found Ceres near the predicted spot. At 2.8 AU from the Sun, Ceres fit the Titius–Bode law well. 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 more asteroids: 2 Pallas, 3 Juno, and 4 Vesta (discovered in 1807). No new asteroids were found until 1845. In 1830, amateur astronomer Karl Ludwig Hencke began searching for asteroids. In 1845, he discovered 5 Astraea, the first new asteroid found in 38 years. Gauss named it. By 1851, 15 asteroids had been found. In 1868, James Craig Watson discovered the 100th asteroid, and the French Academy of Sciences honored three top asteroid hunters with a commemorative medallion.

In 1891, Max Wolf used astrophotography to find asteroids, which showed up as short streaks on long-exposure photos. This method greatly increased the number of discoveries. Wolf alone found 248 asteroids, starting with 323 Brucia, while only about 300 had been found before that. Many more asteroids were known to exist, 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, asteroid discovery followed a four-step process. First, a wide-field telescope or astrograph photographed a section of the sky, with pairs of images taken one hour apart over several days. Second, the images were viewed through a stereoscope, where moving objects appeared to float above the background stars. Third, a moving object’s position was measured precisely using a digitizing microscope, compared to known star locations.

These first three steps did not officially discover an asteroid. Instead, the observer found an "apparition," given a provisional designation with the discovery year, a letter for the half-month, and a letter-number for the sequence (example: 1998 FJ 74). The final step was sending the data to the Minor Planet Center. Computer programs there determined if the apparition matched earlier observations to form a single orbit. If confirmed, the object received a catalog number, and the first observer with a calculated orbit was named the discoverer. The name was then approved by the International Astronomical Union.

Naming

In 1851, the Royal Astronomical Society noticed that asteroids were being discovered so quickly that a new system was needed to group or name them. In 1852, when de Gasparis found the twentieth asteroid, Benjamin Valz gave it a name and a number to show its order among discoveries, such as 20 Massalia. Sometimes asteroids were found but not seen again. Starting in 1892, new asteroids were listed using the year and a capital letter to show the order in which their orbits were calculated and recorded 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 discovered in 1893, so after 1893Z came 1893AA. Many variations of these methods were tested, including systems that used years and Greek letters in 1914. A simple numbering system based on the order of discovery 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 an alphanumeric code showing the half-month of discovery and the order within that half-month. 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 sometimes skip the number entirely or mention it only once when repeating a name in writing. Discoverers can suggest names for asteroids, 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 about two dozen asteroid symbols, often with 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 used by astronomers, and the next asteroid found, 16 Psyche in 1852, was the first to be labeled this way at the time of its discovery. 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 objects 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 pushed most planetesimals out of the asteroid belt. Computer models 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 this early period, while smaller ones were created from collisions between asteroids before or after Jupiter's disruption. Ceres and Vesta grew large enough to become hot and separate into layers, with heavy metals sinking to the core and rocky materials forming the outer layer.

In the Nice model, many Kuiper-belt objects were moved into the outer asteroid belt, which is more than 2.6 AU from the Sun. Most of these objects were later pushed out by Jupiter, but some may remain as 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 orbits are changed by the gravity of other Solar System objects and by the Yarkovsky effect. Important groups include:

Most known asteroids orbit in the asteroid belt between Mars and Jupiter. These asteroids usually have orbits that are not very stretched out. The belt is thought to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter, and many more smaller ones. These asteroids may be leftover pieces from the early Solar System, and in this area, the process of forming planets was stopped by Jupiter's strong gravity.

Despite common beliefs, the asteroid belt is mostly empty. The asteroids are spread over such a large area that it would be unlikely to hit one without careful aiming. However, hundreds of thousands of asteroids are already known, and the total number may be in the millions or more, depending on the size limit. Over 200 asteroids are larger than 100 km, and studies using infrared light show there are between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. Most known asteroids have brightness levels between 11 and 19, with an average of about 16.

The total mass of the asteroid belt is estimated to be 2.39 × 10 kg, which is about 3% of the Moon's mass. The Kuiper Belt and Scattered Disk have more than 100 times the mass of the asteroid belt. 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.

Trojan asteroids share an orbit with a larger planet or moon but stay far away because they orbit in two stable points called Lagrangian points, L4 and L5, which are 60° ahead of and behind the larger body.

In the Solar System, most known Trojan asteroids share Jupiter's orbit. They are split into two groups: the Greek camp at L4 (ahead of Jupiter) and the Trojan camp at L5 (behind Jupiter). More than a million Jupiter Trojan asteroids larger than 1 km are thought to exist, with over 7,000 currently recorded. So far, nine Mars Trojans, 28 Neptune Trojans, two Uranus Trojans, two Earth Trojans, and one temporary Venus Trojan have been found. Simulations suggest Saturn and Uranus likely do not have any original Trojan asteroids.

Near-Earth asteroids (NEAs) have orbits that pass close to Earth's orbit. Those that cross Earth's path are called Earth-crossers. As of April 2022, 28,772 NEAs were known, with 878 being larger than 1 km.

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

Some asteroids have natural 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 m across. These moons were discovered using radar imaging during the asteroid's 2017 close approach to Earth.

NEAs are grouped based on their orbit measurements:

• Atiras or Apoheles have orbits entirely inside Earth's orbit. Their farthest point from the Sun (aphelion) is less than Earth's closest point (0.983 AU).
• Atens have orbits that cross Earth's path. Their average orbit distance (semi-major axis) is less than 1 AU, and their farthest point is more than 0.983 AU.
• Apollos have orbits that cross Earth's path. Their average orbit distance is more than 1 AU, and their closest point to the Sun (perihelion) is less than 1.017 AU.
• Amors have orbits entirely outside Earth's orbit. Their closest point to the Sun is more than 1.017 AU but less than 1.3 AU. Some Amor asteroids cross Mars' orbit.

Scientists are not sure whether Mars' moons, Phobos and Deimos, are captured asteroids or formed from an impact on Mars. Both moons are similar to carbon-rich C-type asteroids in their appearance, brightness, and density. One idea is that they may be captured main-belt asteroids. Their orbits are nearly circular and lie in Mars' equatorial plane, which would require a mechanism to adjust their original highly elliptical orbits and tilt. However, Mars' thin atmosphere may not have been strong enough to slow down a large object like Phobos. Another possibility is that Phobos formed in orbit after Mars formed, rather than forming at the same time as Mars.

Another idea is that Mars once had many small bodies like Phobos and Deimos, which may have been sent into orbit by a collision with a large object. However, Phobos' low density and the presence of materials found on Mars suggest it may have formed from material ejected by an impact on Mars, similar to how Earth's Moon is thought to have formed.

Characteristics

Asteroids come in many sizes, from about 1,000 km (620 mi) for the largest down to rocks as small as 1 m (3.3 ft). 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 believed to be leftover early planet-like objects. Most asteroids are much smaller and have irregular shapes. These are likely broken pieces of larger objects or early planet-like bodies that were damaged by collisions.

The largest asteroid is Ceres, with a diameter of 940 km (580 mi). The next largest are 4 Vesta and 2 Pallas, each with a diameter of just over 500 km (300 mi). Vesta is the brightest of the four largest asteroids and can sometimes be seen with the naked eye. Occasionally, a near-Earth asteroid may also be visible without special equipment, such as 99942 Apophis.

The total mass of all objects in the asteroid belt, located between Mars and Jupiter, is estimated to be (2394 ± 6) × 10 kg, or about 3.25% of the Moon’s mass. Ceres alone makes up 938 × 10 kg, or about 40% of this total. Adding the next three largest asteroids—Vesta (11%), Pallas (8.5%), and Hygiea (3–4%)—brings the total to about 60%. The next seven largest asteroids increase this to 70%. The number of asteroids increases rapidly as their sizes decrease.

Larger asteroids are fewer in number. Most asteroids larger than about 120 km (75 mi) are ancient, surviving from the early solar system. Smaller asteroids are usually pieces broken off from larger ones. The main belt likely had 200 times more asteroids in the past than it does now.

The three largest asteroids in the main belt—Ceres, Vesta, and Pallas—are intact early planet-like objects that share traits with planets. They are unusual compared to most asteroids, which are irregularly shaped. The fourth-largest asteroid, Hygiea, is nearly round but may not have layers inside like other large asteroids. Together, the four largest asteroids make up about five-eighths of the asteroid belt’s total mass. Ceres and Vesta alone make up half of this.

Ceres is the only asteroid that is shaped by its own gravity and is classified as a dwarf planet. It has a higher brightness than other asteroids and may have a layer of ice on its surface. Like planets, Ceres has layers: a crust, a mantle, and a core. No rocks from Ceres have been found on Earth.

Vesta also has layers inside, but it formed outside the frost line in the solar system, so it has no water. Its surface is mostly made of basaltic rock and minerals like olivine. Vesta has a large crater at its southern pole called Rheasilvia and an ellipsoidal shape. Vesta is the source of the Vestian family of asteroids and the HED meteorites, which make up 5% of all meteorites found on Earth.

Pallas is unusual because it spins on its side, with its rotation axis tilted at a steep angle. Its composition is similar to Ceres, with high levels of carbon and silicon, and may have layers inside. Pallas 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 origin of this family. Observations suggest Hygiea may have been broken apart during a collision and later reformed. It has a nearly round shape, which may indicate it was once in balance under its own gravity or was reformed after a collision.

Large asteroids may not have natural satellites because most satellites of main belt asteroids are believed to form from collisions, creating loose piles of debris. This suggests that many large asteroids are made of loose debris from past collisions.

Measurements show that large asteroids have a limit to how fast they can spin. Few asteroids larger than 100 meters have a rotation period shorter than 2.2 hours. If an asteroid spins faster than this, the force pushing outward on its surface would overcome gravity, causing loose material to escape. This suggests most large asteroids are loose piles of debris from collisions.

Asteroids become darker and redder over time due to exposure to space conditions. However, most of this change happens quickly, within the first 100,000 years, making it hard to use color measurements to determine an asteroid’s age.

Except for the four largest asteroids (Ceres, Pallas, Vesta, and Hygiea), most asteroids are similar in appearance but irregular in shape. For example, 253 Mathilde, a 50 km (31 mi) asteroid, is a loose pile of debris covered in large craters. Observations of 511 Davida, a 300 km (190 mi) asteroid, also show a rough, cratered surface. Medium-sized asteroids like Mathilde and 243 Ida have thick layers of loose material covering their surfaces. Of the four largest asteroids, Pallas and Hygiea are not well understood. Vesta has cracks around a large crater at its south pole but is otherwise round.

The Dawn spacecraft found that Ceres has many craters but fewer large ones than expected. Models suggested Ceres should have 10 to 15 craters larger than 400 km (250 mi) in diameter. The largest confirmed crater on Ceres, Kerwan Basin, is 284 km (176 mi) wide. Scientists think the lack of large craters is due to the slow flattening of impacts over time.

Asteroids are classified based on their light reflection patterns. Most fall into three groups: C-type (carbon-rich), M-type (metallic), and S-type (stony). The composition of asteroids varies, and in most cases, it is not fully understood. Ceres appears to have a rocky core covered by an icy layer. Vesta is thought to have a nickel-iron core, an olivine mantle, and a basaltic crust. Hygiea may be the largest undifferentiated asteroid, with a composition similar to carbon-rich meteorites, but it could have been broken apart and reformed after a collision. Some asteroids are remnants of early planet-like objects, rich in rock and metal. Most small asteroids are loose piles of debris held together by gravity, while the largest are

Classification

Asteroids are often grouped based on two main factors: the shape of their orbits around the Sun and the way they reflect light.

Many asteroids are grouped or classified together because of similarities in their orbits. Large groups are usually named after the first asteroid discovered in that group. These groups are loosely connected, while families are more tightly linked and formed when a large asteroid broke apart in the past due to a collision. Families are more common and easier to find in the main asteroid belt, though some smaller families have been found among Jupiter’s trojan asteroids. These groups were first identified in 1918 by Kiyotsugu Hirayama and are often called Hirayama families.

About 30–35% of asteroids in the main belt are part of families, which are believed to have formed from past collisions. Some families are even linked to objects like the dwarf planet Haumea.

Some asteroids follow unusual horseshoe-shaped orbits that share the same path as Earth or another planet. Examples include 3753 Cruithne and 2002 AA29. This type of orbit was first observed between Saturn’s moons Epimetheus and Janus. These objects can temporarily act as temporary satellites for a few decades or centuries before returning to their original orbit. Earth and Venus are known to have temporary satellites.

Objects that share orbits with Earth, Venus, or even Mercury are a special group of asteroids called Aten asteroids. These objects can also be found near outer planets.

In 1975, scientists created a system to classify asteroids based on their color, brightness, and light patterns. These features are thought to show what materials make up the asteroid’s surface. The system originally had three categories: C-types for dark, carbon-rich asteroids (75% of known asteroids), S-types for rocky (silica-rich) asteroids (17% of known asteroids), and U for those that did not fit into the other two. Over time, more types have been added as more asteroids are studied.

Today, the two most common classification systems are the Tholen and SMASS systems. The Tholen system, created in 1984 by David J. Tholen, 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 three main groups: C, S, and X asteroids. X-type asteroids are mostly metallic, such as M-type asteroids. There are also smaller groups within these categories.

The number of asteroids in each category does not always match how common those types are in space. Some types are easier to find, which can affect the totals.

Early classifications were based on guesses about an asteroid’s composition, but these guesses are not always accurate. This has caused confusion because different classifications may not always match the same materials. Even asteroids in the same group may have different compositions.

Active asteroids are objects that orbit 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 because not all active asteroids are icy or limited to the asteroid belt.

The first active asteroid discovered was 7968 Elst–Pizarro. It was found as an asteroid in 1979 but later shown to have a tail in 1996, earning the name 133P/Elst–Pizarro. Another example is 311P/PanSTARRS, which was observed by the Hubble Space Telescope to have six comet-like tails. Scientists believe these tails form when material is ejected from the asteroid due to fast spinning.

In 2022, NASA’s DART spacecraft crashed into the asteroid Dimorphos, making it an active asteroid. This mission provided the first direct observation of how an asteroid can become active after an impact. The collision caused Dimorphos to lose about 1 million kilograms of material and create a dust tail that stretched over 10,000 kilometers.

Dark comets are asteroids that move in ways similar to comets but do not have tails or glowing areas. They were first identified in 2024. Two groups have been found: the Outer Family, which includes larger objects with orbits near Jupiter, and the Inner Family, which includes smaller, darker objects with nearly circular orbits. All dark comets orbit in the asteroid belt, except for ʻOumuamua, an interstellar object that showed non-gravitational movement but looked like an asteroid.

Observation and exploration

Until space travel was possible, objects in the asteroid belt could only be seen using large telescopes. Their shapes and surfaces were unknown. Today, the best ground-based telescopes and the Hubble Space Telescope can see only a little detail on the largest asteroids. Scientists can guess some information about asteroid shapes and materials by studying how their brightness changes as they spin (called light curves) and by analyzing their light colors (spectral properties). Sizes can be estimated by timing how long an asteroid blocks the light of a star (called a star occultation). Radar imaging can show asteroid shapes and how they move, especially for asteroids near Earth. Spacecraft that fly by asteroids can give much more information than telescopes. Missions that bring back samples from asteroids help scientists learn about the loose material on their surfaces (regolith).

Asteroids are small and dim, so ground-based observations (GBO) can only provide limited information. Using ground telescopes, scientists can measure how bright an asteroid appears (visual magnitude). This can be used to estimate the asteroid's size. Light-curve measurements from GBO can help scientists guess how long an asteroid takes to spin, its tilt, and its rough shape. Spectral data from visible and near-infrared light shows what materials are on an asteroid's surface. However, this only tells about the top layer (a few micrometers thick). As planetologist Patrick Michel explains:

Near-Earth asteroids that come close to Earth can be studied better with radar. Radar can show details like craters and rocks on the surface. Observatories like Arecibo in Puerto Rico and Goldstone in California use radar to study asteroids. Radar also helps scientists learn how asteroids move and spin.

Both space and ground telescopes search for asteroids. Space telescopes can find more asteroids because they avoid Earth's atmosphere and can see more of the sky. NEOWISE found over 100,000 main-belt asteroids, and the Spitzer Space Telescope found over 700 near-Earth asteroids. These observations give rough sizes but little detail about surface features like regolith depth, composition, or how loose the material is.

The Hubble Space Telescope has studied asteroids, such as watching asteroids collide, break apart, or show comet-like tails. It also observed asteroids chosen for missions.

According to Patrick Michel:

The first close-up photo of an asteroid was taken in 1991 of 951 Gaspra, followed by 243 Ida and its moon Dactyl in 1993. These were imaged by the Galileo probe on its way to Jupiter. Other asteroids visited by spacecraft include 9969 Braille (by Deep Space 1 in 1999), 5535 Annefrank (by Stardust in 2002), 2867 Šteins and 21 Lutetia (by Rosetta in 2008), and 4179 Toutatis (by China's Chang'e 2 in 2012).

NASA's NEAR Shoemaker was the first probe to orbit and land on an asteroid. It photographed 253 Mathilde in 1997, orbited 433 Eros, and landed on it in 2001. In 2005, Japan's Hayabusa studied 25143 Itokawa and returned samples to Earth in 2010. NASA's Dawn orbited 4 Vesta for a year and studied Ceres for three years.

Hayabusa2, launched by JAXA in 2014, studied 162173 Ryugu and returned samples to Earth in 2020. It is now heading to a new target in 2031. NASA's OSIRIS-REx, launched in 2016, collected samples from asteroid Bennu and delivered them to Earth in 2023. It will now study asteroid Apophis in 2029.

In 2021, NASA's DART mission crashed into Dimorphos, a moon of the asteroid Didymos, to test ways to change an asteroid's path. The mission was successful, shortening Dimorphos's orbit by about 32 minutes.

NASA's Lucy, launched in 2021, will fly by seven Jupiter trojans. It has already visited asteroids Dinkinesh and Donaldjohanson. NASA's Psyche, launched in 2023, will study the metallic asteroid Psyche in 2029.

ESA's Hera, launched in 2024, will study the results of DART's impact, measuring the crater and how much force was transferred.

China's Tianwen-2, launched in 2025, will explore asteroid Kamoʻoalewa and 311P/PanSTARRS, collecting samples from Kamoʻoalewa.

• Asteroid-dedicated space probes
• Hayabusa2
• Dawn
• Lucy
• Psyche

• JAXA's DESTINY+ will study the Geminids meteor shower's parent body, 3200 Phaethon, and other objects. It is planned for 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 make money for its investors." Scientists believe asteroids could provide materials that are rare or nearly gone on Earth, or materials needed to build structures in space. Heavy and costly materials that are hard to send from Earth might one day be mined from asteroids and used for making things in space.

As Earth’s 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 space homes, becomes more appealing. Hypothetically, water made from ice found on asteroids could be used to fuel spacecraft in space.

From a scientific viewpoint, looking for resources on asteroids might help in the search for signs of 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

Scientists are increasingly interested in finding asteroids that travel close to Earth’s orbit. These objects could, over time, possibly hit Earth. The three main groups of near-Earth asteroids are called Apollos, Amors, and Atens.

One well-known near-Earth asteroid, 433 Eros, was discovered in 1898. During the 1930s, scientists found several similar objects. In the order they were discovered, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and 69230 Hermes. Hermes came as close as 0.005 AU to Earth in 1937. This event helped scientists understand the possibility of an asteroid hitting Earth.

Two major events later increased concern. First, scientists accepted the Alvarez hypothesis, which suggests a large asteroid impact caused the Cretaceous–Paleogene extinction. Second, in 1994, scientists observed Comet Shoemaker-Levy 9 collide with Jupiter. Additionally, the U.S. military shared information that its satellites, designed to detect nuclear explosions, had recorded hundreds of impacts by objects 1 to 10 meters in size.

These discoveries led to the creation of advanced surveys using special cameras and computers linked to telescopes. By 2011, scientists estimated that 89% to 96% of near-Earth asteroids larger than 1 kilometer had been found. As of October 29, 2018, the LINEAR system alone had discovered 147,132 asteroids. Among these, 19,266 near-Earth asteroids were identified, including nearly 900 larger than 1 kilometer.

In June 2018, the National Science and Technology Council warned that the United States is not ready for an asteroid impact. It released the "National Near-Earth Object Preparedness Strategy Action Plan" to improve preparedness. In 2013, experts told the U.S. Congress that NASA would need at least five years to prepare for a mission to change an asteroid’s path.

Different methods to avoid asteroid collisions have trade-offs based on cost, risk, and technology. Techniques to change an asteroid’s course include deflection or breaking it apart. These methods use energy sources like kinetic, nuclear, or solar/thermal energy. Strategies are divided into two main types: fragmentation and delay.

Fragmentation aims to break the asteroid into pieces small enough to burn up in Earth’s atmosphere. Delay uses the fact that Earth and the asteroid move in space. If the asteroid’s arrival is delayed or advanced by a short time, it may miss Earth. Earth moves about 12,750 kilometers in 425 seconds, or seven minutes. Adjusting the asteroid’s path by this amount could prevent a collision.

"Project Icarus" was created in 1967 as a plan to avoid a collision with asteroid 1566 Icarus. The plan used Saturn V rockets, which were not tested until after the plan was made. Each rocket carried a nuclear warhead and guidance systems. Warheads would be detonated 30 meters from the asteroid to change its path. If the asteroid was destroyed or its course changed, later missions might be adjusted or canceled. The final rocket would launch 18 hours before the predicted impact.

Fiction

Asteroids and the asteroid belt are common features in science fiction stories. In these stories, asteroids may serve as places where humans could live, sources of valuable minerals, dangers for spaceships traveling between two locations, and potential dangers to life on Earth or other planets, dwarf planets, and moons if they collide with these places.