The question of the “average size” of a meteorite is complex because the vast majority of extraterrestrial material reaching Earth is not what most people envision. A meteorite is a fragment of rock or metal from space that survives its fiery passage through the atmosphere and lands on the planet’s surface. The size of these space rocks, originally called meteoroids, ranges from microscopic dust grains to objects the size of small asteroids. This enormous variation means the statistical average is quite misleading, requiring a distinction between the constant flux of cosmic dust and the rare, large impactors.
The True Statistical Average: Micrometeorites
The true statistical average of extraterrestrial material is microscopic, represented by micrometeorites. These tiny particles are typically smaller than one millimeter in diameter, often measuring only a few hundred micrometers across. Micrometeorites are abundant, with an estimated 30,000 to 60,000 tons of this material falling onto Earth each year.
This immense volume means that micrometeorites collectively account for the majority of the mass accreted by Earth from space. They survive atmospheric entry because their small size slows them down before they can completely vaporize. While many melt and resolidify into glassy spheres, others survive largely unmelted, offering scientists pristine samples of early solar system material.
Scientists collect these particles by filtering meltwater from Antarctic ice or by gathering them from deep-sea sediments, environments largely free of terrestrial contamination. These samples reveal that the most common size, based on mass distribution, peaks at around 200 micrometers. These microscopic specks are not the objects the public typically considers meteorites.
The Typical Size of Recovered Specimens
The meteorites recovered and cataloged as specimens are fundamentally different from micrometeorites. They are the remnants of larger space rocks that created visible meteors or fireballs, and are the objects most people imagine when they think of a meteorite. Most recovered specimens are relatively small, ranging from a few grams to a few kilograms in mass.
A typical recovered stony meteorite, the most common type, often weighs between 100 grams and 10 kilograms, with many specimens being softball-sized or smaller. The most common mass range for stony meteorites is around 128 to 256 grams, translating to a diameter of approximately 4 to 5 centimeters. The original meteoroids that produce these fragments are significantly larger, often losing over 95% of their initial mass through ablation and fragmentation during atmospheric entry.
The largest recovered single meteorite, the Hoba meteorite in Namibia, is an iron specimen weighing about 60 tons and measuring nearly three meters across. This massive size is an extreme outlier. The typical recovered fragment is a small, dense piece of rock that has cooled to form a characteristic fusion crust on its surface.
The Rarity of Large Meteorite Impacts
At the opposite end of the size spectrum are the large objects that cause significant events upon entering Earth’s atmosphere. These impactors, which are meter-sized and larger, are exceedingly rare compared to the daily influx of smaller material. Objects with diameters of about 20 meters, like the one that caused the 2013 Chelyabinsk event, enter the atmosphere only about twice every century.
The Chelyabinsk impactor, approximately 17 to 20 meters in size, exploded high in the atmosphere, causing a massive airburst that released energy equivalent to about 500 kilotons of TNT. Larger objects, such as the 30-meter-diameter body responsible for the 1908 Tunguska event, can flatten forests over vast areas without creating a traditional impact crater. The rarity of these events increases dramatically with size.
An impactor large enough to cause a catastrophic, global effect, such as the one that created the Chicxulub crater, is estimated to have been 10 to 15 kilometers in diameter. Such large-scale collisions happen only once every tens of millions of years. For objects larger than a kilometer, capable of causing continental-scale devastation, the impact frequency is estimated to be only once every million years.