What is an asteroid? Space rocks and the asteroid belt, explained
By the WhyCosmos team · Last updated:
An asteroid is a rocky or metallic object, smaller than a planet, that orbits the Sun. More than a million are known, most of them in the asteroid belt between Mars and Jupiter. They are leftovers from the formation of the solar system about 4.6 billion years ago, so they preserve clues to how the planets were built.
Asteroids get a dramatic press: films show crowded rock fields, and headlines warn of near misses. The real picture is calmer and more interesting. This page explains what asteroids are, what the asteroid belt actually looks like, how meteors and meteorites fit in, and what scientists are doing to make sure an asteroid never takes Earth by surprise.
How to explain it to your child
Explain it to an 8-year-old
Asteroids are rocky lumps that travel around the Sun, a bit like planets but much smaller. Think of them as leftover building blocks from when the planets were made, a very long time ago. Most of them float around the Sun in a wide ring called the asteroid belt, between Mars and Jupiter.
Explain it to a 13-year-old
Asteroids are small solar system bodies made of rock, metal or carbon-rich material that never came together into a planet, largely because Jupiter's strong gravity kept stirring them up. Their make-up and orbits are a record of the early solar system, which is why space agencies have flown spacecraft to them and even brought samples back to Earth for study.
Key facts
| What it is | A rocky or metallic body orbiting the Sun, smaller than a planet |
|---|---|
| Known so far | More than a million |
| Main home | The asteroid belt, between Mars and Jupiter |
| Largest object in the belt | Ceres, about 940 km across, now classed as a dwarf planet |
| Largest true asteroid | Vesta, about 525 km across |
| Do asteroids make their own light? | No; they reflect sunlight |
| Near-Earth asteroids | Tracked by astronomers so their future paths can be predicted |
| Planetary defence test | NASA's DART spacecraft changed an asteroid's orbit in 2022 |
Is the asteroid belt crowded?
Not like it looks in films. The belt holds a huge number of objects, but they are spread across an enormous volume of space, so far apart that spacecraft routinely cross it without steering around anything. NASA's Pioneer 10 was the first to fly through it, in 1972 and 1973, and many missions have done so safely since.
The belt is also surprisingly light. If you gathered every asteroid in it into one body, the result would have only a few percent of the mass of our Moon.
What is the difference between an asteroid, a meteor and a meteorite?
| Word | What it is | Where it is |
|---|---|---|
| Asteroid | A rocky or metallic body, from a few metres to hundreds of kilometres across | Orbiting the Sun |
| Meteoroid | A small piece of rock or dust, usually less than a metre across | Travelling through space |
| Meteor | The streak of light, a "shooting star", when a meteoroid burns up in the air | In Earth's atmosphere |
| Meteorite | A piece that survives the fall and lands | On the ground |
| Comet | An icy body that grows a glowing tail near the Sun | Orbiting the Sun |
So a shooting star is not a star at all. India has a famous reminder of what bigger meteorites can do: Lonar Lake in Maharashtra fills a crater made by a space-rock impact. And one of the best meteor showers of the year, the Geminids in mid-December, comes from dust shed by an asteroid, 3200 Phaethon, rather than a comet.
Can an asteroid hit Earth?
Yes, and small ones do all the time: tiny pieces enter the atmosphere every day and almost all burn up harmlessly. Large impacts are very rare. The best known happened about 66 million years ago, when an asteroid roughly 10 km wide struck near what is now the Yucatán Peninsula in Mexico, an impact linked to the extinction of the dinosaurs.
The good news is that astronomers now track near-Earth asteroids and calculate their orbits decades ahead. NASA states that no known asteroid larger than 140 metres has a significant chance of hitting Earth in the next 100 years. The asteroid Apophis, about 340 metres wide, will pass within about 32,000 km of Earth on 13 April 2029, closer than some satellites, but it will not hit.
How are scientists protecting Earth from asteroids?
In September 2022, NASA deliberately crashed its DART spacecraft into Dimorphos, a small moon of the asteroid Didymos. The impact shortened Dimorphos's orbit by about 32 minutes: the first time humanity had intentionally changed the motion of a natural object in space. It showed that a well-aimed spacecraft could nudge a dangerous asteroid off course, given enough warning. The European Space Agency's Hera spacecraft is following up to study the result in detail.
The goal of planetary defence is not to make anyone afraid. It is to find hazards early enough that they can be dealt with calmly.
What can asteroids teach us?
Because asteroids have changed little in 4.6 billion years, they are some of the best records of how the solar system began. Japan's Hayabusa2 brought samples home from the asteroid Ryugu in 2020, and NASA's OSIRIS-REx returned samples from Bennu in 2023, which scientists have found to contain some of the chemical building blocks of life. NASA's Psyche spacecraft is on its way to a metal-rich asteroid that may be part of the core of an early planet.
The planets and the scale of the solar system are the first three lessons of Astronomy 101, the WhyCosmos live course for ages 8 to 16.
Questions parents and kids ask
An asteroid is a rocky or metal lump, smaller than a planet, that travels around the Sun. Most are leftovers from when the planets formed.
Ceres, about 940 km across, is the largest object in the asteroid belt, but it is now classed as a dwarf planet. The largest body still called an asteroid is Vesta, about 525 km across.
Asteroids are mostly rock and metal and look like points of light. Comets contain much more ice; when they come close to the Sun the ice turns to gas and they grow a glowing coma and tails.
Not any known large one in the foreseeable future. NASA says no known asteroid larger than 140 metres has a significant chance of hitting Earth in the next 100 years, and astronomers keep watching for new ones.
An asteroid roughly 10 km wide that struck near the Yucatán Peninsula in Mexico about 66 million years ago, leaving the Chicxulub crater. The impact is linked to the extinction of the dinosaurs.
Some do. The first was found when the Galileo spacecraft photographed the asteroid Ida in 1993: a tiny moon, Dactyl, was spotted in the images in 1994. Dimorphos, the target of NASA's DART mission, is a moon of the asteroid Didymos.
Your child asks questions like this every week.WhyCosmos runs live online astronomy classes for ages 8–16, taught by real astrophysicists — where questions like these are the curriculum.
Related terms in this glossary
Sources and further reading
NASA: Asteroids · NASA: DART mission · NASA: Apophis · NASA JPL: Center for Near Earth Object Studies