PARIS, July 21 — The International Astronomical Union last revised its official definition of a meteoroid in April 2017, setting the size range between 30 micrometers and one meter, while allowing a deviation for objects that cause a meteor. That definition was the product of decades of debate among astronomers. The body first defined a meteoroid in 1961 as a solid object moving in interplanetary space, considerably smaller than an asteroid and considerably larger than an atom.
In 1995, researchers proposed a narrower window of 100 micrometers to 10 meters across. Then, in 2010, after the discovery of asteroids below 10 meters in diameter, another proposal suggested objects between 10 micrometers and one meter.
The IAU’s 2017 compromise settled on the current limits.
What Meteoroids Are Made Of
Almost all meteoroids contain extraterrestrial nickel and iron. Scientists classify them into three main groups: iron, stone, and stony-iron. Stone meteoroids that contain chondrules are called chondrites.
Those without chondrules are achondrites, typically formed from extraterrestrial igneous activity and containing little or no iron. Composition is inferred through multiple methods: studying the trajectory and light spectra of meteors, analyzing radio signals, and examining meteorite fragments recovered on the ground.
A meteorite is the remains of a meteoroid that survives atmospheric ablation and impacts the surface. Most meteoroids originate from the asteroid belt, where planetary gravity perturbs their orbits. Others are cometary particles that produce meteor showers.
Some meteoroids are collision debris ejected from the Moon or Mars.
Daily Influx and Meteor Showers
An estimated 25 million meteoroids, micrometeoroids, and other space debris enter Earth’s atmosphere each day. That adds up to roughly 15,000 tonnes of material per year. Objects smaller than meteoroids are classified as micrometeoroids or space dust.
The visible passage of a meteoroid, comet, or asteroid entering the atmosphere is called a meteor. When multiple meteors appear seconds or minutes apart from the same fixed point in the sky, it is known as a meteor shower.
These events are often linked to cometary debris streams.
Blurring the Line With Asteroids
The distinction between meteoroids and asteroids has grown fuzzy as telescopes improve. Some asteroids as small as one meter have been detected from Earth. These detections blur the boundary between the two classes, though the definition maintains a one-meter upper limit for meteoroids.
Beyond that limit, objects are classified as asteroids. The overlap means that some bodies could be labeled either way depending on when and how they are observed.
For now, the established framework remains the 2017 revision. Researchers continue to refine their understanding of these objects, their origins, and their potential to reach the ground. The standard definitions may evolve again as survey telescopes reveal more small near-Earth objects.
