Two kinds of small body
When the planets formed about 4.6 billion years ago, not everything ended up in a planet. Countless smaller pieces stayed behind, and they are still orbiting the Sun. Two broad groups are the best known. Asteroids are mostly rocky or metallic. Comets are mostly a mix of ice, dust and rock. Both are small compared with planets, and both are valuable because many of them have changed relatively little since the early solar system.
The distinction is real but not perfectly sharp. Some objects classed as asteroids have been seen to shed dust like a comet, and some old comets have lost most of their ice and look much like asteroids. It is more accurate to think of a spectrum than two sealed boxes.
What asteroids are
Asteroids are lumps of rock and metal, ranging from boulders to bodies hundreds of kilometers across. Their shapes are usually irregular because their gravity is too weak to pull them into a sphere. Some are solid, while others appear to be loose piles of rubble held together by gravity.
Most known asteroids lie in a wide band between Mars and Jupiter called the main belt. Early in the solar system, Jupiter’s strong gravity stirred up the region and prevented the material from gathering into a planet. The belt is spread thinly, so it is mostly empty space. Other groups of asteroids travel in orbits that bring them near Earth, and these are tracked closely.
Asteroids are often sorted by their composition. Some are dark and carbon-rich, others are stony, and a smaller number are rich in metal. The differences reflect where and how each one formed, and in some cases whether it was once part of a larger body that melted and later broke apart.
What comets are
A comet is sometimes described as a dirty snowball, although many scientists prefer the phrase “icy dirtball” because it is mostly dust and rock held together with frozen water, carbon dioxide and other ices. The solid part, the nucleus, is typically only a few kilometers across.
Comets look dramatic only near the Sun. As one approaches, heat turns ice directly into gas, which carries dust away. This forms a fuzzy cloud called a coma and often two tails. The dust tail tends to curve along the orbit, while the gas tail points away from the Sun because it is pushed by the solar wind. Far from the Sun a comet is just a dark, frozen nucleus.
Where they come from
Asteroids mostly come from the inner solar system, while comets originate in the cold outskirts. Short-period comets, which return within a couple of centuries, are generally thought to come from the Kuiper Belt beyond Neptune. Long-period comets, which can take thousands of years or more to return, are thought to come from the Oort cloud, a hypothesized distant shell of icy bodies that has not been directly observed.
Gravitational nudges from planets or passing stars can send an icy body inward. After many passes near the Sun, a comet loses material and may break apart or fade. This is why comets are considered short-lived compared with the age of the solar system.
Meteors and meteorites
Small bits of debris often turn up in everyday conversation, so the terms are worth separating. A meteoroid is a small piece of rock or dust in space. A meteor is the streak of light it makes while burning up in the atmosphere. A meteorite is a piece that survives and lands. Many meteor showers happen when Earth passes through the dusty trail left by a comet. Most meteorites are fragments of asteroids.
Why scientists study them
Planets have been heated, melted and reshaped, which erased much of their early record. Small bodies were mostly spared, so they act like time capsules. Studying their chemistry helps researchers test ideas about how the planets formed. Some meteorites even contain grains that formed before the Sun, linked to the explosions of earlier stars such as supernova explosions that enriched the original gas cloud.
Small bodies may also matter for Earth’s history. Impacts have shaped the planet’s past, and a large impact about 66 million years ago is widely linked to the end of the dinosaurs, as shown in the layers recorded through fossil formation and the rock record. Some scientists also explore whether comets and asteroids delivered some of Earth’s water and organic compounds, although how much they contributed is still debated.
There is a practical side too. Tracking near-Earth asteroids helps identify any that could one day threaten the planet. Large impacts are rare, and no known large object is on course to hit. Test missions have shown that nudging an asteroid’s path is possible in principle.
Visiting small bodies
Robotic spacecraft have flown past, orbited and landed on both kinds of object, and some have brought samples back to Earth. These trips are part of a broader effort in deep space exploration. They are well suited to small worlds because the weak gravity makes landing gentle, and much of the work depends on advances in space robotics. Close-up images have shown surprising surfaces, from boulder-strewn rubble piles to jets of dust escaping a comet.
Ongoing space agency missions continue to target small bodies, and new discoveries come from sky surveys as well as spacecraft. Because exact numbers of known objects change constantly, it is safest to treat any total as a snapshot.
A simple way to remember the difference
Think of asteroids as the rocky leftovers from the warmer, inner region and comets as the frozen leftovers from the cold outer region. Both are fragments of the original building material, and by studying them scientists hope to read the early chapters of the story that planets themselves have overwritten.