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In outer space, a meteoroid is a tiny rocky or metallic body.

Meteoroids are much smaller than asteroids, ranging in size from a few grains to objects of one meter in diameter. Micrometeoroids or space dust are objects that are smaller than this. The majority of them are comet or asteroid fragments, but some are collision impact debris expelled from worlds like the Moon or Mars.

When a meteoroid, comet, or asteroid hits Earth’s atmosphere at speeds of more than 20 km/s (72,000 km/h; 45,000 mph), the object’s aerodynamic heating generates a streak of light, both from the glowing object and from the trail of glowing particles it leaves behind. A meteor, sometimes known as a “shooting star,” is a natural occurrence.

When meteors reach a height of roughly 100 kilometers above sea level, they become visible. A meteor shower is a group of meteors that appear to come from the same fixed location in the sky and appear to occur seconds or minutes apart. A meteorite is the remnants of a meteoroid that has hit the ground after its surface material has been abated during its passage through the atmosphere as a meteor.

Each day, an estimated 25 million meteoroids, micrometeoroids, and other space debris enter the atmosphere, resulting in an estimated 15,000 tonnes of debris entering the atmosphere.

A meteoroid was described by the International Astronomical Union (IAU) in 1961 as “a solid object travelling in interplanetary space, having a size substantially less than an asteroid but much bigger than an atom.”

Beech and Steel suggested a revised definition in the Quarterly Journal of the Royal Astronomical Society in 1995, stating that a meteoroid should be between 100 m and 10 m (33 ft) broad. To retain the difference, Rubin and Grossman recommended a change of the prior definition of meteoroid to objects between 10 m and one meter (3 ft 3 in) in diameter in 2010, following the finding of asteroids smaller than 10 m.

The minimal size of an asteroid, according to Rubin and Grossman, is determined by what can be seen from Earth-bound observatories, therefore the line between meteoroid and asteroid is hazy. 2008 TS26 with H = 33.2 and 2011 CQ1 with H = 32.1, both with an estimated size of one meter, are two of the tiniest asteroids identified (based on absolute magnitude H) (3 ft 3 in).

The International Astronomical Union (IAU) revised its definition in April 2017, restricting size to between 30 m and one meter in diameter, but allowing for a departure for any object that causes a meteor.

Micrometeoroids and interplanetary dust are objects that are smaller than meteoroids. The name “meteoroid” is not used by the Minor Planet Center.

Extraterrestrial nickel and iron are found in almost all meteoroids. They are divided into three categories: iron, stone, and stony-iron. Chondrites are stone meteoroids that feature grain-like inclusions known as chondrules. Achondrites are stony meteoroids that lack these properties and are often generated by alien igneous activity; they contain little or no extraterrestrial iron.

The composition of meteoroids may be determined from their paths and the light spectra of the ensuing meteor as they fly through Earth’s atmosphere. Their impacts on radio waves also provide information, which is particularly valuable for daylight meteors that are normally difficult to spot. Meteoroids have been discovered to have a variety of orbits based on these trajectory studies, with some grouping in streams (see meteor showers) frequently related with a parent comet and others appearing to be erratic.

Debris from meteoroid streams may be spread into different orbits in the future. The light spectra were coupled with trajectory and light curve data to produce a variety of compositions and densities, ranging from delicate snowball-like particles with a density roughly a quarter that of ice to nickel-iron-rich solid boulders. Meteorites can also reveal information on the composition of non-ephemeral meteoroids.

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