Meteorite

Full definition

Strictly speaking, a meteorite is an object that previously had three successive names: asteroid or cometary fragment in space, then meteoroid once it became small enough, then meteor during its luminous descent through the atmosphere, and finally meteorite once it has landed. The transition from 'meteor' to 'meteorite' happens at 30-50 km altitude: if the fragment hasn't vaporized by then, it ends its journey in free fall, at subsonic speed (a few hundred meters per second), before landing.

The vast majority of recovered meteorites are fragments of main-belt asteroids, chipped off their parent body in collisions and drifting for millions of years on orbits perturbed by Jupiter before crossing Earth. A few dozen are lunar in origin (blasted off the Moon by impacts and later captured by Earth's gravity) and a few dozen more are martian — the latter especially precious, as they are the only physical samples of Mars we have while we wait for the Mars Sample Return mission (planned for the 2030s).

In the lab, meteorites are identified by several signatures: a black fusion crust (glassy, 1-2 mm thick, produced by surface vaporization during descent), an irregular often blunted shape, abnormally high density for size, and traces of chondrules (the oldest class, chondrites, contains small silicate spherules formed in the early solar nebula 4.567 billion years ago).

Each meteorite is thus a time machine: it carries within its minerals the chemical and isotopic signature of the protoplanetary disk from which the Solar System formed, frozen for 4.56 billion years.

Numbers, structure and classification

The Meteoritical Bulletin Database lists over 75,000 catalogued meteorites (end of 2024), but this figure has exploded since we began systematically combing Antarctica (1976-present, ANSMET program) and certain hot deserts (Sahara, Oman) where black rocks stand out visually against pale sand.

The total annual mass reaching the ground is estimated at 10 to 100 tonnes — a high-sounding figure, though the overwhelming majority consists of micrometeorites (< 1 mm) falling everywhere on the planet, all the time, even found in urban gutters. True 'observed falls' (meteorites whose descent was seen before being picked up) number only about fifty per year.

Records. The largest single meteorite ever found is Hoba, in Namibia: a 60-tonne iron block (2.7 m × 2.7 m × 0.9 m), fallen 80,000 years ago and never moved since — too heavy. The largest moved fragment is Cape York (Greenland), including 'Ahnighito' (31 tonnes, on display at the American Museum of Natural History in New York). The oldest documented European fall is Ensisheim (Alsace, November 7, 1492) — 127 kg, still kept in the Ensisheim church today.

Mineralogical classification. Three main families:

• Stony meteorites (94%) → chondrites (undifferentiated, rich in chondrules, the most primitive material) and achondrites (differentiated, from the crust or mantle of already-structured parent bodies — includes lunar and martian meteorites). • Iron meteorites (5%) → iron-nickel alloys (5-20% Ni), from the metallic core of differentiated planetesimals later shattered. • Stony-iron meteorites (1%) → pallasites (iron + green-yellow olivine, spectacular when sliced) and mesosiderites.

The different types

In detail, each great family subdivides further.

Chondrites (~87% of falls). The oldest and most informative cosmic witnesses. Carbonaceous chondrites (CC — groups CI, CM, CV, CK, CO, CR) contain up to 3% carbon and pre-biotic amino acids. One of the most famous, Allende (Mexico, February 8, 1969), yielded CAI refractory inclusions dated to 4.567 billion years — the oldest known Solar System solids. Ordinary chondrites (H, L, LL) make up the majority of recent falls. Enstatite chondrites (EH, EL) are chemically close to primitive Earth.

Achondrites (~7% of falls). Inherited from differentiated parent bodies. Include the HED group (Vesta: howardites, eucrites, diogenites — link confirmed by the Dawn mission), the aubrites, the ureilites, and the two star subgroups: lunar (about a hundred catalogued) and martian (SNC: shergottites, nakhlites, chassignites — about 350 known).

Irons (~5% of falls, but heavily over-represented in museums due to their resistance to weathering). Widmanstätten structure visible when polished and etched with nitric acid: a magnificent pattern of kamacite-taenite bands that only forms after extremely slow cooling (1 K per million years) in a planetary core. Physical proof that differentiated worlds with metallic cores existed before their collisional destruction.

Lunar and martian fragments. Ejected by large impacts, they wander interplanetary space before being captured by Earth's gravity. To confirm them, we compare isotopic composition to Apollo samples (Moon) or in-situ measurements from rovers and landers (Mars).

How do we observe them?

Recovering a meteorite requires either luck (observed fall), patience (systematic prospecting in deserts or Antarctica), or a surveillance camera network.

Observed falls. When a fireball is witnessed and filmed by multiple observers or stations, we can triangulate its atmospheric trajectory, reconstruct its pre-atmospheric orbit, and compute the likely fall zone (the 'strewn field'). The FRIPON network (France, ~100 cameras), the Desert Fireball Network (Australia) and the Global Meteor Network fully automate this process. Landmark case: the Winchcombe meteorite (UK, February 28, 2021), a 600 g CM2 carbonaceous chondrite recovered 12 hours after the fall — the first carbonaceous chondrite to land in the UK, precious because little altered by moisture.

Antarctic prospecting. The ANSMET program (Antarctic Search for Meteorites) and its Japanese and European equivalents bring back hundreds of specimens each year from the Antarctic plateau, where blue ice reveals meteorites fallen over the last tens of thousands of years. Hot deserts (Sahara, Oman, Atacama) are also very productive.

Sample return. The modern strategy: rather than wait for them to fall, go get them directly from their parent body. Hayabusa (JAXA, 2010) and Hayabusa2 (2020) delivered respectively a few milligrams from 25143 Itokawa and 5.4 g from 162173 Ryugu. OSIRIS-REx (NASA) returned 121.6 g from 101955 Bennu in September 2023. These in-situ samples are perfectly contextualized — we know the orbit, global mineralogy, sampling site — unlike 'wild' meteorites.

And the amateur? You can find micrometeorites at home. Filter gutter dust, pass a magnet over it (to concentrate iron-nickel spheres), then examine under a microscope (40-100×). Jon Larsen popularized the method — several thousand urban micrometeorites have been lab-confirmed that way. See also our astronomy calendar for shower peaks favorable to fireball observation.

Not to be confused with

The confusions are classic and worth settling.

Meteoroid, meteor, meteorite. Same root, three states: the object before, in space (meteoroid), the luminous phenomenon during descent (meteor), the fragment on the ground (meteorite). The same stone changes name depending on where it is in its journey.

Tektite. A natural glass, often black or olive green, sometimes mistaken for a meteorite. But tektites are NOT extraterrestrial: they are terrestrial rocks melted and ejected by a meteorite impact, then cooled in flight. Moldavites (Central Europe) come from the Ries impact crater (Germany, 14.8 Ma). Scientifically fascinating, but terrestrial.

Meteorwrong (pseudo-meteorite). Many people bring 'strange rocks' to their local natural-history museum that turn out to be industrial slag, terrestrial magnetite, or weathered basalt. The misleading signs: dark crust, irregular surface, magnetic attraction. A good lab distinguishes in minutes: nickel isotopic chemistry and Widmanstätten structure settle the debate.

Micrometeoroid and cosmic dust. Bodies under 1 mm. If they reach the ground, they are technically micrometeorites. They fall everywhere and make up most of the extraterrestrial mass flux received by Earth.