The terms asteroid, meteor, and meteorite are thrown about quite often. All refer to space rocks that inspire their fair share of fear in us Earthlings who suspect that, someday soon, one might land on us with enough force to wipe us all out. However, there are some key differences between these terms that begs for some distinction.
Ultimately, the difference comes down to location – i.e. whether or not they are hurling through space, or hurling through the atmosphere and impacting on the surface.
Sometimes referred to as a minor planet, or “planetoid”, the term asteroid refers to the many small celestial bodies that exist in orbit of the Sun. In general, they are rocky bodies that do not have an atmosphere, although some have a high metallic content. A few even have their own moons.
Our Solar System contains millions of asteroids, many of which are thought to be the shattered remnants of planetesimals – bodies within the young Sun’s solar nebula that never grew large enough to become planets.
Most of these asteroids are grouped together in the asteroid belt that lies between Mars and Jupiter, though there are also many that exist within the inner Solar System and are co-orbital with Jupiter. They are smaller than planets but larger than meteoroids, meaning that an asteroid can range from a few meters wide to hundreds of km wide.
Of these, the largest by far are the planetoids known as Ceres and 4 Vesta. The former, which lies between Jupiter and Mars, measures 950 km (590 mi) in diameter, is composed of rock and ice, and constitutes of the third of the mass in the asteroid belt. 4 Vesta, the second-largest body in the asteroid belt, with a mean diameter of 525 kilometers (326 mi), and holds another 9% of the belt’s mass.
Historically, the term ”asteroid” has been used to describe any body that orbits the Sun and does not have the disc of a planet or the characteristics of an active comet. However, the discovery of asteroids in the outer Solar System with volatile-based surfaces that resembled comets forced a revision of this practice. Now, the term is used to describe bodies in the inner Solar System out to the orbit of Jupiter.
A meteorite, by contrast, is a solid piece of debris that was originally an asteroid or a comet that entered the atmosphere and survived to impact the surface. Prior to impact, they are called meteoroids and become meteors when they fall through the planets atmosphere. In the process, they are heated to incandescence by air friction and form a bright trail, leading to the creation of a fireball or “shooting star”.
Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites that are largely composed of metallic iron-nickel; and, stony-iron meteorites that contain large amounts of both metallic and rocky material.
Those pieces of meteors that are not liquified by the high-energy impact with the Earth’s surface are called meteorities. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy.
A meteorite’s size can range from small to extremely large. Meteorites smaller than 2mm are classified as micrometeorites. Occasionally, an object large enough to be considered an asteroid will enter an atmosphere. Its remnants are also called meteorites if they impact the surface.
More generally, a meteorite on the surface of any celestial body is a natural object that has come from outer space. Meteorites have been found on the Moon and Mars.
In short, the difference between asteroids and meteorites all comes down to a question of location. Asteroids are always found in space. Once it enters an atmosphere, it becomes a meteor, and then a meteorite after it hits the ground. Hence, a meteorite is always going to be found on the ground. Each is made of the same basic materials – minerals and rock – and each originated in space. The main difference is where they are when they are being observed.
Meteorites are the source of a great deal of the knowledge that scientists have about the composition of other celestial bodies. Scientists have traced the origination of the meteorites found here on Earth to four other bodies: the Moon, Mars, the asteroid 4 Vesta, and the comet Wild 2. Be sure to check out those bodies as part of your research into asteroids and meteorites.
We have many great articles on the subject of asteroids and meteorites here at Universe Today, such as this general information article on asteroids, and these articles that deal with Ceres and Vesta. And here are some recent articles about the Chelyabinsk meteor that landed in Russia, as well as a 2 billion-year old Martian meteorite that contains evidence of water on Mars.