Asteroids, Comets and Meteors: Small Bodies of the Solar System

Between the planets lies a population of small bodies left over from the formation of the solar system. Asteroids are rocky and metallic, comets are icy and active near the Sun, and meteors are the bright streaks made when tiny fragments burn up in Earth's atmosphere. This guide explains where these objects live, how they differ, and why scientists track the ones that cross Earth's orbit.

Three names, three different objects

The words are easy to mix up, so it helps to fix them first. A meteoroid is a small piece of rock or dust moving through space. A meteor is the flash of light produced when a meteoroid enters the atmosphere and heats up. A meteorite is any surviving fragment that reaches the ground. Asteroids and comets are larger bodies that orbit the Sun, and the distinction between them is mainly about composition and behaviour near the Sun.

The asteroid belt

Most known asteroids orbit in the main belt, a broad region roughly between 2.2 and 3.2 astronomical units from the Sun, which places it between the orbits of Mars and Jupiter. The total mass of the entire belt is only about 4 percent of the Moon's mass, so the region is far emptier than science fiction often suggests. Typical asteroids are separated by hundreds of thousands of kilometres.

The largest object in the belt is Ceres, about 940 kilometres across. It is classified as a dwarf planet because it is round and orbits the Sun on its own. Vesta, at about 525 kilometres across, is the next largest and was visited by NASA's Dawn spacecraft in 2011 and 2012.

Why no planet formed there

Jupiter's gravity stirred the orbits of the material in the belt. Rather than letting dust clump into one planet, the stirring made collisions too energetic, so fragments broke apart instead of merging. Gaps called Kirkwood gaps appear where orbital periods match simple fractions of Jupiter's period, because repeated gravitational tugs remove objects from those orbits.

Asteroid types

Astronomers group asteroids by their spectral colour and composition. The three main classes are summarised below, with rounded figures from standard survey work.

Type	Main composition	Typical albedo	Share of known asteroids
C-type	carbon-rich silicates	about 0.05	roughly three quarters
S-type	silicate rock and nickel-iron	about 0.25	roughly one sixth
M-type	nickel-iron metal	about 0.15	a few percent

The shares are approximate because survey coverage and classification methods change over time. The main idea is that dark carbon-rich asteroids dominate the outer belt, while brighter rocky and metallic bodies are more common nearer the inner edge.

Comets: frozen visitors

Comets are icy bodies that release gas and dust when heated by sunlight. The solid nucleus is typically a few kilometres across. As it approaches the Sun, sublimation releases gas and dust into a glowing cloud called the coma, which can grow far larger than the nucleus itself.

Comets develop two distinct tails. The ion tail is made of charged gas, is blue, and points directly away from the Sun, driven by the solar wind. The dust tail is made of small particles pushed by sunlight pressure and curves along the comet's orbital path.

Where comets come from

Short-period comets, with orbits of less than about 200 years, usually originate in the Kuiper Belt, a region of icy bodies beyond Neptune at roughly 30 to 50 astronomical units. Long-period comets come from the Oort Cloud, a hypothetical shell of icy objects that may extend from about 2,000 to 100,000 astronomical units. Both sources hold material left over from the outer solar system's early history.

Halley's Comet

Halley's Comet is the most famous periodic comet, with an orbital period of about 76 years. It last passed closest to the Sun in 1986, and it is predicted to return around 2061. Its nucleus is about 15 kilometres long, and it has been observed at many returns since ancient times.

Meteors and meteorites

Most meteoroids are no larger than grains of sand. They strike the atmosphere at speeds from about 11 to 72 kilometres per second and begin to glow at heights near 100 kilometres. Their surfaces heat, melt and vaporise, leaving a bright trail that lasts a fraction of a second.

Showers occur when Earth passes through debris left by a comet or asteroid. The Perseids, peaking around 12 and 13 August, come from comet 109P/Swift-Tuttle and enter the atmosphere at about 59 kilometres per second. The Geminids, peaking in December, come from the asteroid-like object 3200 Phaethon.

Meteorites fall into three broad groups. Chondrites contain small rounded grains called chondrules and are the most common type. Achondrites lack chondrules and come from melted parent bodies. Iron meteorites are mostly nickel-iron and come from the cores of differentiated asteroids. Carbonaceous chondrites are especially valuable because they preserve material that is about 4.6 billion years old.

Impacts, hazards and defense

Objects that cross Earth's orbit are called near-Earth objects, or NEOs. Those with perihelion distances below about 1.3 astronomical units are tracked by survey programmes that search the sky for objects on potentially hazardous paths. Most of the largest NEOs have been catalogued, but smaller objects remain harder to find.

Impacts of different sizes have very different effects. The Chicxulub impact, about 66 million years ago, came from an object roughly 10 kilometres across and is linked to the extinction of the non-avian dinosaurs. The Tunguska event in 1908 flattened about 2,000 square kilometres of forest in Siberia from an object of roughly 50 to 60 metres, with no crater. The Chelyabinsk meteor in 2013 was about 20 metres across and exploded in the air over Russia, injuring more than 1,000 people mainly from broken glass.

Testing a deflection

NASA's DART mission in 2022 deliberately crashed a spacecraft into Dimorphos, the small moon of asteroid Didymos. The impact shortened the moon's orbital period by about 32 minutes, the first demonstration that a kinetic impactor can change the orbit of an asteroid. The result is a useful test of planetary defense methods rather than a complete solution for every threat.

Key takeaways

Asteroids are rocky and metallic remnants that mostly sit in the main belt between Mars and Jupiter. Their distribution reflects Jupiter's gravity, which kept them from forming a planet.

Comets are icy bodies that grow comas and tails when they approach the Sun. They come from the Kuiper Belt and the Oort Cloud, and their material preserves the early outer solar system.

Meteors are the visible trails of small meteoroids burning in the atmosphere, while meteorites are the rare survivors that land on the ground. Tracking near-Earth objects and testing deflection methods is how science keeps the risk of large impacts under review.