How Asteroids Formed

01 The Leftovers of Creation

The solar system formed approximately 4.6 billion years ago from a vast, slowly rotating cloud of gas and dust known as the solar nebula. Under its own gravity, the cloud collapsed inward, flattening into a rotating disk, the protoplanetary disk, with the nascent Sun at its center. In the inner, hotter regions of the disk, volatile compounds like water, methane, and ammonia were vaporized and swept outward by radiation pressure and the solar wind. What remained were refractory materials: silicates, metals, and other high-temperature compounds. These are the building blocks of the rocky worlds, and the asteroids are the pieces that were never assembled into planets.

The process of building planets from the disk is called accretion. Tiny dust grains collided and stuck together electrostatically, forming larger clumps. These clumps grew into meter-sized boulders, then kilometer-sized planetesimals, then Moon-sized protoplanets. Over tens of millions of years, the protoplanets swept up most of the material in their orbital paths, growing into the terrestrial planets: Mercury, Venus, Earth, and Mars. But not every planetesimal was consumed. In the region between Mars and Jupiter, gravitational interference from massive Jupiter prevented the remaining planetesimals from coalescing into a single planet. Jupiter’s gravity stirred the bodies in this zone, increasing their relative velocities and causing destructive collisions rather than constructive accretion. The result was a population of fragmented bodies that never grew larger than a dwarf planet: the asteroid belt.

Today, the asteroid belt contains millions of bodies orbiting the Sun between approximately 2.2 and 3.2 astronomical units. Despite its depiction in popular culture as a dense field of tumbling rocks, the asteroid belt is mostly empty space. The total mass of all asteroids combined is less than five percent of the mass of the Moon, spread across a volume larger than the space between Earth and Mars. Most asteroids are small. The largest, Ceres, is roughly 940 kilometers in diameter and accounts for about a third of the belt’s total mass by itself.

02 The Three Families

Asteroids are broadly classified into three compositional types based on their spectral properties, the way they reflect sunlight at different wavelengths. C-type asteroids, carbonaceous, are the most common, comprising roughly 75 percent of the known population. They are dark, with albedos typically below 0.05, meaning they reflect only about five percent of the light that hits them. Their composition is dominated by carbon compounds, hydrated minerals, and volatiles. They preserve the most primitive material in the solar system, relatively unaltered since their formation. Ceres, the largest asteroid, is a C-type body, as is Pallas, the third largest.

S-type asteroids, silicaceous, make up about 17 percent of the population. They are brighter and redder than C-types and are composed primarily of silicate minerals, including iron and magnesium silicates. These are the bodies that have been heated, differentiated, and in some cases partially melted. The second-largest asteroid, Vesta, is an S-type, and its surface shows evidence of lava flows from an early period of volcanic activity. Vesta is the only asteroid known to have undergone significant igneous processing. It has a layered internal structure, with a crust, mantle, and iron core, that mirrors the structure of the terrestrial planets.

M-type asteroids, metallic, are the rarest at roughly 8 percent of the known population. They are composed largely of iron-nickel alloys and are believed to be the exposed metallic cores of larger bodies that formed, differentiated, and were then shattered by catastrophic collisions. These fragments preserve the interior composition of planetesimals that were destroyed billions of years ago. The asteroid Psyche, approximately 220 kilometers in diameter, is the largest M-type and the target of a NASA orbiter mission launched in 2023. Studying Psyche is the closest astronomers can get to examining a planetary core, since the cores of Earth, Mars, and Venus are buried beneath thousands of kilometers of rock and inaccessible to direct observation.

03 Jupiter’s Disruptive Hand

The reason the asteroid belt exists at all is Jupiter. With a mass two and a half times that of all other planets combined, Jupiter’s gravitational influence extends across the entire outer solar system, and its effects are felt even in the asteroid belt. In the early solar system, as Jupiter grew by accreting gas from the protoplanetary disk, it migrated inward slightly before reversing course and settling into its current orbit. This migration, now called the Grand Tack model, had profound consequences for the asteroid belt.

As Jupiter moved inward, its gravitational perturbations stirred up the planetesimals in the region between Mars and the Sun, exciting their orbital eccentricities and inclinations. Bodies that had been on nearly circular, coplanar orbits were flung onto elliptical, inclined paths. When two such bodies collided, their relative velocities were high enough that the collisions were destructive rather than constructive. Instead of sticking together to form larger bodies, they fragmented. The asteroid belt became a grindstone, a region where planetesimals were broken down faster than they could grow.

Jupiter also created regions of gravitational stability and instability throughout the belt. At certain distances from Jupiter, known as mean-motion resonances, an asteroid’s orbital period is a simple fraction of Jupiter’s. These resonant orbits are unstable: repeated gravitational kicks from Jupiter eventually eject the asteroid from the belt entirely. The most famous of these is the Kirkwood gap at about 2.5 astronomical units, where an asteroid would complete exactly three orbits for every one of Jupiter’s. These gaps are nearly empty because any asteroid that wanders into them is quickly scattered. The result is a belt with a complex structure of populated zones and depleted zones, all sculpted by the gravity of the largest planet.

04 Rubble Piles and Solid Rock

Asteroids are not all solid bodies. In fact, most of them are not. Observations over the past three decades, particularly from spacecraft missions and radar imaging, have revealed that many asteroids are what planetary scientists call rubble piles: loose aggregations of fragments held together by their own weak self-gravity rather than by material strength. These bodies have bulk densities far lower than the rocks they are made of, indicating that they contain significant void space. Itokawa, visited by Japan’s Hayabusa mission in 2005, is a classic rubble pile: a peanut-shaped collection of boulders and gravel with an estimated porosity of roughly 40 percent.

The rubble-pile structure is a direct consequence of the asteroid belt’s collisional history. When two planetesimals collide at high velocity, the impact can shatter both bodies. If the resulting fragments are gravitationally bound, they reaccumulate into a new body composed of the debris. This reaccumulated body is a rubble pile. Its fragments are not cemented together; they are simply resting on one another. The escape velocity from the surface of a rubble pile is so low that a person standing on one could launch a rock into space by hand. The body holds together only because the gravitational attraction between the fragments, weak as it is, exceeds the outward forces acting on it.

Some asteroids are solid monoliths. These are typically smaller bodies, under a few hundred meters across, that are fragments of larger asteroids shattered by collisions. Unlike rubble piles, they are held together by the material strength of their rock or metal. When such a monolithic asteroid is hit by another body, it can break apart entirely, producing a swarm of smaller fragments, some of which eventually reaccumulate into new rubble piles. The cycle of collision, fragmentation, and reaccumulation has been operating for 4.6 billion years and continues today. The asteroid belt is not a static population but a dynamic system, constantly grinding itself down and rearranging its constituents.

05 Differentiated Worlds

A few asteroids are not primordial fragments at all but the re-exposed interiors of larger bodies that once existed. During the early solar system, some planetesimals grew large enough to melt internally, driven by the heat from radioactive isotopes embedded in their rocky material. In the first few million years of solar system history, the short-lived isotope aluminum-26, which has a half-life of about 717,000 years, was abundant in the protoplanetary disk. Its radioactive decay released enough heat to melt planetesimals larger than roughly 20 kilometers in diameter. Once molten, these bodies underwent differentiation: dense metallic iron sank to the center to form a core, while lighter silicates floated upward to form a mantle and crust.

Vesta is the surviving example of such a differentiated body. With a diameter of about 525 kilometers, it is the second-largest asteroid and the only one that has been visited by a dedicated orbiter mission, NASA’s Dawn, which arrived in 2011 and spent 14 months mapping its surface. Dawn’s data confirmed that Vesta has a layered internal structure: an iron core roughly 220 kilometers in radius, a mantle of the mineral olivine, and a basaltic crust. Its surface bears the scars of two enormous impact basins at the south pole, each roughly 400 and 500 kilometers across, that were created by collisions with other large bodies. These impacts ejected enormous quantities of material, much of which fell to Earth as a specific class of meteorites called HEDs (howardites, eucrites, diogenites). The chemical link between Vesta and HED meteorites, established through spectroscopic comparison, allows scientists to study Vesta’s deep interior directly, through meteorite specimens in laboratories.

Ceres, the largest asteroid at 940 kilometers, is a different case. It is large enough to be roughly spherical under its own gravity, which earned it classification as a dwarf planet in 2006 alongside Pluto. Dawn visited Ceres after Vesta, arriving in 2015 and remaining in orbit until 2018. Ceres is a C-type body, ice-rich, with a crust composed of hydrated minerals and a possible subsurface ocean of briny water. Bright deposits of sodium carbonate on its surface, discovered by Dawn, are consistent with the recent exposure of liquid water from below. Ceres may still have an active subsurface ocean, making it one of several solar system bodies where liquid water persists outside Earth.

06 Near-Earth Asteroids

Not all asteroids remain in the main belt. Through gravitational interactions with Jupiter and the inner planets, asteroids are continuously injected into the inner solar system, where they may cross the orbits of Earth, Venus, or Mars. These near-Earth asteroids, or NEAs, are the population that poses a potential impact hazard to Earth. They are also the population most accessible to spacecraft missions and, potentially, to future mining operations. The approximately 35,000 known near-Earth asteroids range in size from a few meters to a few kilometers. The largest known NEA, Ganymed, has a diameter of about 32 kilometers, though most are far smaller.

NEAs are delivered from the main belt through a combination of resonances and the Yarkovsky effect, a subtle force produced by the way a rotating asteroid absorbs and re-radiates solar energy. Because an asteroid’s afternoon side is warmer than its morning side, the thermal radiation it emits is not perfectly symmetric. The tiny asymmetry produces a continuous thrust that, over millions of years, can shift the asteroid’s orbit enough to move it into a resonance with Jupiter, which then ejects it from the belt. The process is slow, but the asteroid belt has been feeding bodies into the inner solar system for billions of years, and it continues to do so today.

Earth is hit by asteroidal material constantly. Most of it is in the form of dust and small pebbles that burn up in the atmosphere as meteors. Objects large enough to survive atmospheric entry and reach the surface as meteorites arrive at a rate of roughly one per square kilometer per million years. Larger impacts, from bodies tens of meters across, occur roughly once per century. The 2013 Chelyabinsk event, in which a roughly 20-meter asteroid exploded over Russia, injured approximately 1,500 people and was a reminder that the asteroid population is not merely a scientific curiosity but a factor in the safety of life on Earth.

07 What Asteroids Tell Us

Asteroids are the most accessible samples of the early solar system. Unlike planets, which have been reshaped by billions of years of geological activity, many asteroids preserve material that is essentially unchanged from the time of their formation. Meteorites, which are fragments of asteroids that have fallen to Earth, contain inclusions older than any rock on the planet’s surface. The calcium-aluminum-rich inclusions found in some carbonaceous chondrite meteorites have been dated to 4,567 million years ago, establishing the age of the solar system itself. These tiny inclusions, some less than a centimeter across, are the oldest solid objects known to exist.

Spacecraft missions to asteroids have transformed the field. The Hayabusa2 mission returned samples from the C-type asteroid Ryugu in 2020, and the OSIRIS-REx mission returned samples from the B-type asteroid Bennu in 2023. Both asteroids are carbon-rich rubble piles that have never been thermally altered since their formation. Laboratory analysis of the returned material has revealed the presence of amino acids and other organic compounds, supporting the hypothesis that asteroids may have delivered the raw ingredients for life to the early Earth. The connection between asteroid composition and the origin of biological chemistry is one of the most active areas of modern planetary science.

The asteroids are not merely leftovers. They are a record of the conditions in which the solar system formed, a catalog of the materials from which planets are built, and a continuing source of raw material arriving at Earth’s surface. They are also, potentially, a resource for future space exploration. The same metallic and volatile materials preserved in asteroids for billions of years could be extracted and used by a spacefaring civilization. The study of asteroids sits at the intersection of planetary science, astrobiology, and the long-term future of the human relationship with the solar system, a span of inquiry that stretches from the oldest solid matter in the solar system to the practical question of what we will build from it next.