Meteoroid

A meteoroid is a small, solid object moving through interplanetary space. These objects are significantly smaller than asteroids but larger than individual atoms. They serve as important pieces of evidence for understanding our Solar System. Scientists categorize them based on their size and what they are made of. Most meteoroids are fragments from comets or asteroids. Others are pieces of debris ejected from bodies like the Moon or Mars during collisions.

As a meteoroid interacts with Earth, it undergoes several distinct transformations. When it enters Earth's atmosphere, the visible streak of light it creates is called a meteor. If many meteors appear to come from the same point in the sky, it is a meteor shower. If the object survives the intense heat and ablation of its surface, it becomes a meteorite. A meteorite is the part of the original object that survives the atmospheric passage and impacts the ground.

Meteoroids are classified into three main groups based on their chemical composition. Most contain extraterrestrial nickel and iron. The three primary types are iron, stone, and stony-iron. Some stone meteoroids contain small, grain-like inclusions called chondrules, which makes them chondrites. Other stony meteoroids lack these features and are called achondrites. Achondrites are typically formed from extraterrestrial igneous activity and contain very little iron.

Defining the exact size of a meteoroid has been a complex task for astronomers. In 1961, the International Astronomical Union (IAU) defined them as objects smaller than asteroids but larger than atoms. In 1995, researchers Beech and Steel proposed a range between 100 micrometers and 10 meters. Later, in 2010, Rubin and Grossman suggested a range between 10 micrometers and one meter. This was to maintain a distinction from asteroids, which can be quite small. In April 2017, the IAU adopted an official revision limiting the size to between 10 micrometers and one meter.

These objects travel through space at many different velocities. Most meteoroids moving through Earth's orbital space average about 20 kilometers per second. The fastest ones move at about 42 kilometers per second, which is the escape velocity from the Sun. When a meteoroid meets Earth's atmosphere head-on, the combined speed can reach 71 kilometers per second. For example, on January 17, 2013, a one-meter comet entered the atmosphere over California and Nevada. It collided with the atmosphere at a speed of approximately 72 kilometers per second.

Every day, an estimated 25 million meteoroids and micrometeoroids enter Earth's atmosphere. This results in about 100 tons of material entering the atmosphere every year. While many are tiny, larger collisions can be quite significant. On October 6, 2008, an asteroid was observed on a collision course with Earth. It entered the atmosphere the next day and struck northern Sudan. This was the first time a meteoroid had been tracked in space before impacting Earth.

Meteoroid impacts create lasting changes on the surfaces of celestial bodies. On objects like the Moon or Mercury, which have little atmosphere, they create enduring impact craters. These craters are the dominant geographic features on many small moons and asteroids. On Earth, geological processes like tectonics or erosion can bury or transform these craters over time. Sometimes, a very energetic collision can vaporize the entire impactor. In other cases, the heat can melt terrestrial material into objects called tektites.