Asteroids and meteoroids

A cartoon that I “borrowed” from a NASA website.

Asteroids and meteoroids are small rocky objects, much smaller than planets, that orbit the sun. If it is larger than 1 meter in diameter, then it is an asteroid; if it is 2 millimeters to 1 meter in size, it is a meteoroid. If it is smaller than 2 mm, it is a micrometeoroid.

Meteorites are not asteroids or meteoroids, but most meteorites are pieces of asteroids that got ejected from the asteroid when the asteroid was impacted by another asteroid. The rock is not a meteorite until it lands on Earth, although some experts define the transition from meteoroid to meteorite as when the rock’s circumsolar (heliocentric) orbit is deflected by Earth’s atmosphere.

Images of 5 asteroids, with longest dimension in yellow. All have obvious craters. Image credits: NASA
(a) Asteroid (25143) Itokawa is 535 meters long. (b) Much of the surface is covered with rocks, but about 12 craters have been identified. The author of this page is not a crater expert, but the feature in the yellow circle “looks like” a crater to him. (Note that the rock shadows are on the right side of the rocks but on the left side of the “crater.”) These images were obtained by the Japanese spacecraft Hayabusa during its close approach in 2005. Image credits: JAXA/Cecile Engrand

Meteorites do not have craters on them

Asteroids have craters formed during impact of smaller asteroids and meteoroids, but meteorites do not have craters. For small meteoroids, 90% of the mass is lost to ablation as it comes through Earth’s atmosphere. Any surface features like a crater gets erased by ablation.

The depressions are also not regmaglypts. None of these rocks has a fusion crust.