Small bodies: asteroids, comets, meteoroids and meteorites

At a glance

Counts come from the Minor Planet Center (MPC) “Latest Published Data” census page, which carries no explicit “as of” stamp (its newest listed circular batch is dated 14 April 2026); we read it on 29 October 2026. ESA’s NEO Coordination Centre gave 42,238 NEAs and 124 near-Earth comets in its September 2026 newsletter. Distances: NASA Kuiper belt and comet fact pages.

Sources: [1] [2] [3]

Definitions and naming

Small bodies are the Solar System objects that are neither planets, dwarf planets nor moons: asteroids, comets and the smaller debris they shed. An asteroid is a rocky or metallic body that shows no gas activity; the largest, Vesta, is about 530 km across, while the smallest catalogued ones are only metres wide. A comet is an icy body that, when it nears the Sun, develops a gaseous atmosphere and tails. The boundary is blurred: some asteroids shed dust and some comets go dormant.

A meteoroid is a rock in space ranging from a dust grain to a small asteroid. When it enters the atmosphere and glows it becomes a meteor (“shooting star”); one brighter than Venus is a fireball. Any fragment that survives to reach the ground is a meteorite.

Names follow rules set by the International Astronomical Union and administered by the MPC. A newly found minor planet receives a provisional designation made of the year, a letter for the half-month of discovery and a letter (plus, if needed, a number) for its order within that half-month: 2024 YR4 is an object found in the second half of December 2024. Once its orbit is secure it is numbered and its discoverer may propose a name, as with (98943) Torifune. Comet designations carry a prefix: P/ for periodic comets, C/ for non-periodic ones and I/ for interstellar objects, as in 3I/ATLAS.

Sources: [1] [4] [5] [6]

The asteroid belt, Kirkwood gaps, Trojans and spectral classes

Most asteroids orbit between Mars and Jupiter. NASA estimates that the main belt holds 1.1–1.9 million asteroids larger than 1 km, yet the combined mass of all asteroids is less than that of the Moon.

The belt is not uniform. A histogram of semi-major axes shows the Kirkwood gaps, nearly empty bands where an asteroid’s orbital period would be a simple fraction of Jupiter’s (the 3:1, 5:2, 7:3 and 2:1 resonances). Repeated tugs pump up eccentricities and clear objects out; the JPL main-belt distribution diagram shows the gaps clearly.

Trojans share a planet’s orbit, clustering around the two stable Lagrange points that lead and trail it. Jupiter’s Trojans are by far the largest such population: 16,737 are catalogued, and NASA notes they may rival the main belt in number.

Reflectance spectra and albedo sort asteroids into classes. C-type asteroids, the most common, are dark and rich in clays and silicates; S-type (stony) bodies mix silicates with nickel-iron; M-type (metallic) objects appear dominated by nickel-iron.

Sources: [1] [4]

Near-Earth objects and planetary defence

A near-Earth object (NEO) is an asteroid or comet with perihelion below 1.3 au. NASA’s CNEOS divides near-Earth asteroids into four groups: Atiras, whose orbits lie entirely inside Earth’s (a < 1.0 au, aphelion < 0.983 au); Atens, Earth-crossers with a < 1.0 au; Apollos, Earth-crossers with a > 1.0 au and perihelion < 1.017 au; and Amors, which approach from outside (1.017 < q < 1.3 au). A potentially hazardous asteroid has a minimum orbit intersection distance with Earth of 0.05 au or less and absolute magnitude H ≤ 22, roughly 140 m or larger. The MPC census lists 3,552 Atens, 22,068 Apollos and 17,238 Amors (it does not list Atiras separately).

The first full-scale deflection test was NASA’s DART, which struck Dimorphos, the moonlet of asteroid Didymos, on 26 September 2022. Its orbital period fell from 11 h 55 min to 11 h 23 min (32 ± 2 min), over 25 times the 73-second success threshold. A 2026 Science Advances study found that the binary’s roughly 770-day solar orbit shortened by about 0.15 s (University of Illinois summary). ESA’s Hera, launched on 7 October 2024 (ESA), is surveying the aftermath: as of October 2026 it is performing braking manoeuvres, with close-proximity operations due to begin in early November 2026 and its CubeSats Juventas and Milani to be released in early 2027. NASA’s infrared NEO Surveyor telescope is in integration and testing for launch no earlier than September 2027 (JPL, May 2026).

Risk is communicated with two scales. The Torino scale (adopted by the IAU in 1999) is an integer 0–10 public-communication scale combining impact probability and consequence: 0 means no hazard, 1 a routine discovery, 2–4 encounters meriting astronomers’ attention, 5–7 threatening events and 8–10 certain collisions. The Palermo scale is logarithmic and compares an event’s probability with the background impact risk; values above 0 merit concern, while values below −2 have no likely consequences (CNEOS).

Asteroid 2024 YR4, discovered in December 2024 by the ATLAS station in Chile and estimated at 40–90 m, became the first object to reach Torino level 3; the only object ever rated higher is Apophis, which reached level 4 in 2004. ESA’s estimate of an Earth impact on 22 December 2032 peaked at 2.8% on 18 February 2025 (ESA blog); within a week further observations had cut it to 0.004% (NASA). The chance of a lunar impact then rose to 4.3%, until JWST observations on 18 and 26 February 2026 ruled it out: the asteroid will pass the Moon at about 21,200 km.

Sources: [1] [2] [7] [8] [9] [10]

Interactive lab: what if an asteroid hits?

The lab opens on the Chelyabinsk 2013 preset: this stony body, about 19 m across, carried roughly 500 kilotons of energy and, as the lab predicts, broke up in the air, so the crater row only shows what would have happened had it reached the ground. Raise the diameter to 140 m, the rough threshold for potentially hazardous asteroids, and see how both the energy and the average interval between such impacts grow. The Chicxulub preset runs the same calculation for a 10 km body.

Sample return and asteroid missions

Japan’s Hayabusa made the first asteroid sample return, from Itokawa. Hayabusa2 launched on 3 December 2014, reached asteroid Ryugu on 27 June 2018, touched down on 22 February and 11 July 2019 and landed its capsule on 6 December 2020. In its extended mission it flew past asteroid Torifune (2001 CC21) on 5 July 2026, about 744 m from the asteroid’s centre; preliminary estimates give a mean diameter of about 540 m (JAXA, July 2026). After Earth swing-bys in 2027 and 2028 it is scheduled to reach 1998 KY26 in 2031.

NASA’s OSIRIS-REx launched on 8 September 2016, arrived at Bennu on 3 December 2018, collected material on 20 October 2020 and dropped its capsule over Utah on 24 September 2023; the bulk sample weighs 121.6 g. Analyses published in January 2025 found 14 of the 20 amino acids used in terrestrial proteins, all five nucleobases, abundant ammonia and 11 minerals that form as brine evaporates (NASA); in December 2025 the sugars ribose and glucose were reported. This is not evidence of life, but of prebiotic chemistry. Renamed OSIRIS-APEX, the spacecraft will rendezvous with Apophis shortly after that asteroid’s close pass on 13 April 2029, about 32,000 km above Earth’s surface (NASA Apophis facts).

Lucy (launched 16 October 2021) flew by Dinkinesh on 1 November 2023 and Donaldjohanson on 20 April 2025; its Trojan tour runs from Eurybates and its moon Queta (12 August 2027) to the binary Patroclus–Menoetius (3 March 2033). Psyche (launched 13 October 2023) passed Mars at about 4,609 km on 15 May 2026 and reaches its metal-rich target in August 2029 (JPL).

Sources: [11] [12] [13]

Comets and interstellar visitors

A comet’s solid nucleus, typically a few kilometres across, is ice mixed with dust under a dark organic crust. Near the Sun, ices sublimate into a coma that can extend hundreds of thousands of kilometres, and two tails form: a curved dust tail and a straight ion (gas) tail pointing away from the Sun. Short-period comets orbit in under 200 years; long-period comets can take up to 30 million years. Short-period Jupiter-family comets derive mainly from the Kuiper belt beyond Neptune, while long-period comets come from the Oort cloud, an unobserved shell about 100,000 au away.

ESA’s Rosetta (launched 2 March 2004) arrived at comet 67P/Churyumov–Gerasimenko on 6 August 2014, delivered the Philae lander on 12 November 2014 and ended its mission on 30 September 2016. It detected molecular oxygen and nitrogen, the amino acid glycine and phosphorus, and found that the comet’s water has a different “flavour” from Earth’s oceans.

Three interstellar objects are known. 1I/ʻOumuamua, found by Pan-STARRS1 on 19 October 2017, was highly elongated and up to about 400 m long; 2I/Borisov, found by Gennady Borisov on 30 August 2019, was a clearly active comet. 3I/ATLAS was reported on 1 July 2025 by the ATLAS telescope in Rio Hurtado, Chile, with pre-discovery images back to 14 June. It moved at about 221,000 km/h at discovery, reached perihelion on about 30 October 2025 at roughly 1.4 au, passed no closer than about 1.8 au from Earth, and Hubble constrained its nucleus to between 440 m and 5.6 km. In 2026 JWST detected methane in it, the first mid-infrared chemical fingerprint of an interstellar object (NASA).

Sources: [3] [5] [14]

Meteoroids, meteors and meteorites

About 44,000 kg of meteoritic material falls on Earth every day. Meteorites are grouped as stony, iron and stony-iron. Stony meteorites split into chondrites, primitive rocks containing millimetre-sized chondrules that never melted, and achondrites, which come from bodies that melted and differentiated (the HED group is linked to Vesta). Irons come from metallic cores. Of the 1,944 meteorites approved by the Meteoritical Society in 2025, 1,336 were ordinary chondrites, 86 lunar and 24 Martian (Meteoritical Society).

A fall is a meteorite seen to arrive and collected soon after; a find is recovered later without an observed fall. Most falls are stony, but irons are over-represented among finds because they are dense and easy to distinguish from terrestrial rocks. Only 15 of the meteorites approved in 2025 were falls.

Meteor showers occur when Earth crosses a debris stream. The August Perseids come from comet 109P/Swift–Tuttle; the December Geminids, peaking on 13–14 December, are unusual in coming from the asteroid (3200) Phaethon, sometimes described as a “rock comet”. Camera fireball networks such as Australia’s Desert Fireball Network triangulate bright meteors to reconstruct their orbits and locate fallen fragments.

Sources: [6]

Going further: data archives

The Minor Planet Center, the IAU body for small-body astrometry, assigns designations and publishes orbits. JPL’s Small-Body Database gives orbital and physical parameters for each object, and CNEOS publishes close-approach tables, the Sentry impact-risk list and discovery statistics.

Mission data sit in the NASA PDS Small Bodies Node. Meteorite classifications are in the Meteoritical Bulletin Database, and the International Meteor Organization collects meteor observations and fireball reports.

Sources: [1] [7]

Known vs open questions

Established

• Kinetic impact works: DART shortened Dimorphos’s orbit by 32 minutes and measurably nudged the Didymos system’s orbit around the Sun.
• Bennu samples contain amino acids, all five nucleobases and sugars, and their parent body once held salty liquid water.
• 2024 YR4 poses no significant risk to Earth or the Moon in 2032; Apophis poses no impact risk for at least 100 years.

Open

• How much of the DART momentum boost came from ejecta, and what is Dimorphos’s internal structure? Hera’s measurements should answer this.
• Was Earth’s water delivered mainly by asteroids, comets or neither?
• Is Psyche an exposed planetary core or something else?
• How common are interstellar objects?
• How many hazardous NEAs of 140 m and larger remain undiscovered?