Asteroids are the rocky and metallic leftovers of planet formation — bodies that never grew into worlds. Most are smaller than a city; a handful are the size of a country. Together they keep a record of the first few million years of the Solar System, written in stone and metal.
What is an asteroid?#
An asteroid is a small, rocky or metallic body that orbits the Sun and, unlike a comet, shows little or no activity — no glowing coma, no tail. Astronomers group asteroids with other small bodies under the umbrella term minor planet, and more than a million of them are now known.
The vocabulary around them is easy to blur. Here is how the terms differ:
| Term | What it means |
|---|---|
| Asteroid | A small rocky or metallic body orbiting the Sun, largely inactive. |
| Comet | An icy body that releases gas and dust when it nears the Sun, forming a coma and often a tail. |
| Minor planet | The formal umbrella term for asteroids and similar small bodies catalogued by the Minor Planet Center. |
| Dwarf planet | A body massive enough to be rounded by its own gravity that has not cleared its orbital neighbourhood. Ceres, the largest body in the main belt, is one. |
| Meteoroid | A small fragment in space, from grains of dust up to about a metre across. |
| Meteor | The streak of light produced when a meteoroid burns up in Earth's atmosphere. |
| Meteorite | A fragment that survives its fall and reaches the ground. |
The first asteroid ever found was Ceres, discovered by Giuseppe Piazzi on 1 January 1801. It was reclassified as a dwarf planet in 2006 but is still counted among the minor planets.
Where they live#
Asteroids are not spread evenly through the Solar System. Gravity — above all Jupiter's — has sculpted them into distinct populations.
| Region | Distance from the Sun | What defines it | In our catalogue |
|---|---|---|---|
| Main belt | about 2.1–3.3 AU | Between Mars and Jupiter; home to the great majority of known asteroids. | Vesperine, Nyxara |
| Hungarias | 1.78–2.0 AU | A group at the inner edge of the belt on steeply inclined orbits. | Ilyra |
| Near-Earth asteroids | perihelion below 1.3 AU | Orbits that bring them close to Earth's path; split into Atiras, Atens, Apollos and Amors. | Helion Drift, Lumen Ward |
| Hildas | about 3.7–4.2 AU | Held in a 3:2 resonance with Jupiter: three orbits for every two of Jupiter's. | Peregrine Ash |
| Jupiter Trojans | about 5.2 AU | Share Jupiter's orbit, clustered around the L4 and L5 Lagrange points 60° ahead of and behind the planet. | Thalassa Veil |
Beyond Jupiter lie the Centaurs and the trans-Neptunian objects of the Kuiper belt — icy worlds outside the scope of this catalogue.
Despite the cinematic image of a crowded belt, the space between asteroids is vast. Every spacecraft sent to Jupiter and beyond has crossed the main belt without needing to steer around a single rock. The total mass of the belt is only a few percent of the Moon's, and Ceres alone carries roughly a third or more of it.
Entries
14
Near-Earth
4
By spectral class
- C-typeCarbonaceous3
- S-typeSilicaceous5
- M-typeMetallic2
- V-typeBasaltic1
- X-complexMixed / enstatite1
- D-typePrimitive red2
By orbit class
- Outer main belt3
- Inner main belt2
- Middle main belt2
- AtiraNEA1
- AtenNEA1
- ApolloNEA1
- AmorNEA1
- Hungaria1
- Hilda1
- Jupiter Trojan1
How big are they?#
Sizes span an enormous range. Ceres is about 940 km across and Vesta, the brightest asteroid seen from Earth, about 525 km. Below them the numbers climb steeply: for every large body there are thousands of small ones, down to boulders a few metres wide.
- Largest in the belt
- ≈ 940 km
- Ceres, a dwarf planet
- Largest asteroid
- ≈ 525 km
- Vesta, by mean diameter
- Known minor planets
- 1,000,000+
- And rising every year
- Catalogue range
- 0.31–24.5 km
- Kestrel to Thalassa Veil
Measuring an asteroid's size is harder than it sounds. Most appear as single points of light, and their brightness depends both on size and on how reflective the surface is. A small, bright body and a large, dark one can look identical. An asteroid with absolute magnitude H = 18, for example, could be about 0.7 km across if its surface is bright or about 1.5 km if it is as dark as charcoal. The relationship is explained in Orbits explained, and surface reflectivity in Spectral classes.
How fast do they spin?#
Asteroids rotate, and their changing brightness as they turn — the lightcurve — reveals their rotation period and hints at their shape. Most turn once every few hours to a day or so.
- The spin barrier. Very few asteroids larger than a few hundred metres spin faster than about once every 2.2 hours. Faster, and loose material would fly off the equator — strong evidence that many asteroids are rubble piles, loose aggregates held together mainly by gravity rather than solid monoliths.
- Sunlight changes spin. Absorbed sunlight re-emitted as heat produces a tiny torque — the YORP effect — that can slowly speed up or slow down a small asteroid's rotation over millions of years.
- Slow and tumbling rotators. Some bodies take days or weeks to turn once, and some tumble rather than spin cleanly around one axis.
In our catalogue, rotation periods run from 2.4 hours for Kestrel to 15.7 hours for Sable Lantern. Entries faster than 3 hours carry a "Fast rotator" trait; those slower than 12 hours, "Slow rotator".
How asteroids are discovered and designated#
Today almost all asteroids are found by automated sky surveys. Wide-field telescopes photograph the same patch of sky several times a night; software looks for points of light that move against the fixed stars. Long-running surveys such as the Catalina Sky Survey, Pan-STARRS, ATLAS and the Zwicky Transient Facility account for most near-Earth asteroid discoveries of recent decades, and the Vera C. Rubin Observatory is expected to add discoveries on an unprecedented scale.
Detection and report
Observers report positions of the moving object to the Minor Planet Center (opens in a new tab) (MPC), which operates under the auspices of the International Astronomical Union and collects asteroid observations from around the world.
Provisional designation
A new object receives a provisional designation built from the year and the half-month of discovery. In
2004 MN4,Mmarks the second half of June 2004 andN4means it was the 113th object designated in that half-month (the letterNis the 13th in a 25-letter sequence that skipsI, plus four full cycles of 25).Orbit refinement
Further observations — often across several oppositions, sometimes from archival images — pin the orbit down until its future positions can be predicted reliably.
Numbering
With a secure orbit, the MPC assigns a permanent number.
2004 MN4became(99942).Naming
The discoverers may then propose a name, which must be approved by the IAU's Working Group Small Bodies Nomenclature.
(99942)was named Apophis. Official names cannot be bought.
Why asteroids matter#
Time capsules#
Many asteroids have changed little since the Solar System formed about 4.6 billion years ago. Most meteorites come from asteroids, and they give laboratories direct access to that early material — including the oldest solids yet dated.
Sample-return missions#
Three missions have brought asteroid material back to Earth, each confirming links between spectral classes and meteorites:
| Mission | Agency | Target | Type | Sample on Earth |
|---|---|---|---|---|
| Hayabusa | JAXA | (25143) Itokawa | S-type | 2010 |
| Hayabusa2 | JAXA | (162173) Ryugu | C-type | December 2020 |
| OSIRIS-REx | NASA | (101955) Bennu | B-type, within the C-complex | September 2023 |
The Itokawa grains matched ordinary chondrite meteorites, the most common kind to fall on Earth. Both Ryugu and Bennu turned out to be carbon-rich rubble piles, and their samples contain water-bearing minerals and organic compounds.
Planetary defence#
In 2022, NASA's DART spacecraft deliberately struck Dimorphos, the small moon of the asteroid Didymos, and shortened its orbit by about half an hour — the first demonstration that a spacecraft impact can measurably change an asteroid's path. Surveys that find and track near-Earth asteroids early are the foundation of that work.
Worlds in their own right#
NASA's Dawn mission orbited Vesta and then Ceres, revealing a giant impact basin at Vesta's south pole and bright salt deposits on Ceres. Missions on their way today include Psyche, heading for a large metal-rich asteroid, and Lucy, bound for the Jupiter Trojans.