Most meteoroids are made of rock, metal, or a mixture of both. The rock portion is dominated by silicate minerals like olivine and pyroxene, while the metal portion is an alloy of iron and nickel. The exact recipe depends on where the meteoroid came from: a shattered asteroid, a comet, or even the surface of Mars or the Moon. Officially, the International Astronomical Union defines a meteoroid as any natural solid object roughly between 30 micrometers and 1 meter across, moving through interplanetary space.
The Three Main Types
Meteoroids fall into three broad composition categories: stony, iron, and stony-iron. Stony meteoroids are by far the most common, making up the vast majority of material that enters Earth’s atmosphere. Iron meteoroids are far rarer but much more likely to survive the trip to the ground intact, which is why iron meteorites are overrepresented in museum collections. Stony-iron meteoroids split the difference, with a roughly 50:50 ratio of rock to metal.
These categories reflect the meteoroid’s origin. Iron meteoroids are fragments of the metallic core of a large asteroid that once heated up enough to separate into layers, much like Earth has an iron core beneath its rocky mantle. Stony meteoroids come from the outer layers of such bodies, or from asteroids that never differentiated at all. Stony-iron meteoroids originate from the boundary zone between core and mantle.
What Stony Meteoroids Contain
Stony meteoroids are primarily made of silicate minerals, with olivine and pyroxene being the most abundant. Olivine is a greenish mineral rich in iron and magnesium; pyroxene is a related mineral with a slightly different crystal structure. On some olivine-dominated asteroids, olivine makes up 68 to 93% of the surface material. These are the same minerals found in Earth’s upper mantle, but in meteoroids they often preserve textures and chemistry that date back to the earliest days of the solar system.
The most common stony meteoroids are called chondrites, named for tiny spherical grains called chondrules embedded in them. These millimeter-sized globules look like half-buried eggs on a broken surface. They formed when molten droplets cooled rapidly in the cloud of gas and dust that became our solar system, roughly 4.5 billion years ago. Some chondrites are packed with chondrules (up to 80% by volume), while others, particularly the carbon-rich varieties, contain far fewer or none at all.
Even “stony” meteoroids contain significant metal. The most metal-rich ordinary chondrites (H-group, about 45% of all ordinary chondrites) carry 15 to 20% iron-nickel metal by mass. L-group chondrites hold 7 to 11%, and LL-group chondrites just 3 to 5%. All chondrites contain 1.0 to 1.8% nickel overall, a concentration far higher than typical Earth rocks.
Achondrites are a second class of stony meteoroid. They lack chondrules because they formed on bodies where volcanic or melting processes destroyed those primitive textures. Their mineral makeup, density, and texture can look so similar to Earth rocks that they’re the hardest meteorites to identify in the field. Some achondrites originated on Mars or the Moon, blasted into space by ancient impacts.
Iron-Nickel Metal
Iron meteoroids are nearly 100% metal, typically 70 to 95% iron, 5 to 30% nickel, and small amounts of cobalt (0.2 to 2%). Trace elements like titanium, chromium, and manganese appear at levels below 0.05%. Many also contain an iron sulfide mineral called troilite.
The nickel content is what makes meteoritic iron distinctive. Industrial iron rarely contains that much nickel unless it’s a specialty high-nickel steel. When iron meteoroids cool extremely slowly over millions of years inside an asteroid core, the iron and nickel separate into two interlocking crystal structures. Slicing and etching an iron meteorite reveals these as Widmanstätten patterns, geometric bands that are impossible to replicate artificially and never appear in stony meteorites. They’re essentially a fingerprint of deep-space cooling rates.
Stony-Iron Meteoroids
Stony-iron meteoroids come in two varieties. Pallasites contain chunks of olivine suspended in a matrix of iron-nickel metal, creating what many consider the most visually striking of all meteorites. When sliced thin, the olivine crystals glow translucent green or gold against the metallic background. Mesosiderites are more chaotic mixtures of iron-nickel metal and the silicate mineral pyroxene, jumbled together by violent collisions between asteroids.
Carbon, Water, and Organic Molecules
Carbonaceous chondrites are a special subgroup of stony meteoroids that contain carbon-rich compounds and water-bearing minerals. They carry a surprising inventory of organic chemistry: carboxylic acids, amino acids, amines, sugar-related compounds, and even nucleobases (the building blocks of DNA and RNA). Researchers have identified roughly 35 different amino acid varieties in some specimens, ranging from two to five carbon atoms in length.
These meteoroids also contain hydrous minerals called phyllosilicates, formed when water interacted with rock on the meteoroid’s parent body billions of years ago. This means the water wasn’t just sitting on the surface; it chemically altered the rock itself. The presence of both water-bearing minerals and organic molecules in carbonaceous chondrites is one reason scientists study them for clues about how the ingredients for life were delivered to early Earth.
NASA’s OSIRIS-REx mission returned samples from asteroid Bennu in September 2023, the largest asteroid sample return by a U.S. mission. Bennu is a carbonaceous body, and its pristine samples are now being analyzed for bulk chemical composition. Because this material has never been exposed to Earth’s atmosphere or weather, it offers a cleaner picture of what primitive meteoroid material actually looks like before contamination.
Density Varies Enormously
The density of a meteoroid depends heavily on its composition and internal structure. Cometary dust particles tend to be extremely porous and fluffy, with densities as low as 300 kg/m³ for some meteor shower particles (about a third the density of water). Interplanetary dust particles show a two-peaked distribution, clustering around 600 kg/m³ and 2,000 kg/m³. At the heavy end, some micrometeorites reach densities of 5,700 kg/m³, approaching the density of solid iron. For context, water is 1,000 kg/m³ and iron is about 7,800 kg/m³.
This range tells you something important: not all meteoroids are solid chunks of rock or metal. Many, especially those shed by comets, are loosely packed aggregates of tiny grains with a lot of empty space inside. A cometary meteoroid can be more air than substance, which is why it burns up completely in the atmosphere and never reaches the ground.
What Happens During Atmospheric Entry
When a meteoroid hits Earth’s atmosphere at speeds between 12 and 70 km/s, friction heats its surface to temperatures of 2,000 to 12,000 K. This melts and strips away the outermost layer over a period of about 5 to 40 seconds, forming a thin glassy coating called a fusion crust. The crust is chemically different from the interior because the molten surface exchanges oxygen and other elements with the atmosphere during those extreme seconds. Iron in the crust can evaporate and redeposit with an altered isotopic signature, and atmospheric oxygen gets mixed into the melt.
The fusion crust is typically only a millimeter or two thick. Everything beneath it remains largely unaltered, which is why freshly fallen meteorites are so valuable to scientists. Crack one open and you’re looking at material that has been essentially unchanged for 4.5 billion years, wrapped in a paper-thin shell of atmospheric fire.