In this post I go down a rocky road of clarifying some of the often confused terminology relating to asteroid and comet impacts and the associated phenomena of meteoroids, meteors and meteorites. Much of the confusion is due to misunderstanding and misuse of the terms in the media and movies, but somewhat surprisingly you can still find examples of misuse in the scientific literature.
In a subsequent post I discuss the real impact risks associated with near-Earth potentially hazardous objects, giving real-life examples and discussing some of the simulations being run by NASA.
And if that isn’t enough, a further post provides a synopsis of over 70 movies and TV shows that depict meteoroids, meteors and meteorites on the big or small screen. Many are disaster movies featuring the threat of an extinction level event in a tale of asteroid vs Earth (and indeed a movie with that exact same title is on the list), but sometimes the screenwriter opts for the asteroid, comet, meteoroid, meteor or meteorite to have less disastrous consequences.
The official definition
The International Astronomical Union (IAU) sets out certain definitions relating to meteor astronomy. This includes clarifying the difference between meteoroids, meteors and meteorites, as well as other related terms. The IAU also acknowledges the persisting confusion not only by the general public but also in the scientific literature.
The three terms meteoroid, meteor and meteorite are on the face of it easy enough to distinguish: a meteoroid is the solid part of a natural body passing through the atmosphere and the physical source of the material that feeds the bright phenomenon we see as a meteor, with the surviving part reaching the surface as a meteorite. See the IAU website for more information. I also include a glossary of terms used in this post at the end of this post.
Note, however, that meteoroids and meteorites refer to natural objects, whereas meteors can result from the passage of any object, natural or artificial, travelling through the atmosphere – whether asteroid, comet, satellite, space capsule, alien probe, and so on.
There is also a distinction based on location and sequence: meteoroids travel into the atmosphere from interplanetary space; meteors occur in the atmosphere; and meteorites reach the ground as the end product.
Meteorites and meteor wrongs
In the 1965 movie, Voyage To The Prehistoric Planet, which interestingly is set in 2020 so worth a watch just to see what filmmakers in 1965 thought we would be up to now, one of a trio of spaceships en route to colonize Venus is struck and destroyed by an asteroid – and provides a classic case of confusing the terms “asteroid” and “meteorite”. There are some wonderful wistful moments where the astronauts ponder the events befalling them, with quotes like: “There is no fair or unfair – to a meteorite, you get hit, you die.”
Well, yes, with an asteroid or meteoroid that is probably true. But with a meteorite, who knows? There is, after all, the apocryphal Legend of The Nakhla Dog who was struck by a meteorite, but which could just as easily be called Schrödinger’s Dog, seeing as no-one seems truly to know the outcome of the impact of that particular martian meteorite event.
A fire in the sky
Depending on the brightness of a meteor, it may be called a fireball, bolide or superbolide and be accompanied by an airburst. On a planetary body with no atmosphere, there is no meteor, and in that case there may be an impact flash as the object hits the ground (like the one seen during a 2019 lunar eclipse). The following images show meteors of different sizes:
The Chelyabinsk meteor (2013) was a superbolide, caused by a 20 metre undiscovered asteroid.
The Kamchatka bolide (2018) was caused by an 10 metre undiscovered asteroid, observed above the clouds over the Bering Sea by Japanese and US meteorological satellites and picked up by ground-based infrasound and seismic detectors.
The Botswana meteor (2018) was a fireball, caused by a 2 metre asteroid (2018 LA) discovered 8 hours prior.
The pre-atmospheric entry energy of the Chelyabinsk meteoroid, most of which was absorbed by the atmosphere, was equivalent to 400 kilotons of TNT, more than twice that of Kamchatka, and a thousand times that of Botswana. None of these meteoroids were on the list of Potentially Hazardous Asteroids (PHAs), all going undetected beforehand. You can read more about the Chelyabinsk event further down in this article.
This is confusion, am I confusing you?
If you dig deeper into the definitions of meteoroids, meteors and meteorites described above, the biggest confusion and one that can make your head go supernova is meteoroid, which involves a definition based on size…sometimes. The size range of incoming space rocks is, of course, continuous and the IAU has set “by agreement” that meteoroids measure “roughly” between 1 metre and 30 microns; any smaller than that and we are talking interplanetary dust particles (IDPs). But what about something larger than 1 metre?
Anything larger than 1 metre tends to be debris from asteroids, rather than comets. So what do we call those incoming objects – asteroid fragments, or comet fragments if they happen to make it, or large meteoroids, perhaps? Asteroid 2018 LA that caused the Botswana meteor was only 2 metres in size, which roughly meets the criteria of a meteoroid. What about the 10 metre Kamchatka and 20 metre Chelyabinsk objects? The answer is that although the IAU has agreed that a meteoroid has a certain size, when you spot your meteor, it is caused by a meteoroid, irrespective of the size of the object causing it.
At this point, some might say the definition of a meteoroid is about as clear as the dark fusion crust or the dark desert patina that covers many a meteorite find or fall. And you are not alone in that, because the IAU – the very ones who concocted the definition of the meteoroid – think so too, as you can read in their notes here. The quirk in their definition of size is in the third blob under “remarks to meteoroid.”
Finds and falls
In this vast oasis of space rocks at least we know that, regardless of the perpetrator’s size, the piece on the ground associated with the event is always called a meteorite.
But it doesn’t end there: meteorites are also defined as falls and finds.
Meteorite falls are what are collected when the meteoroid and/or meteor are tracked through the atmosphere and the falls are observed. The Chelyabinsk meteorites are falls because the meteor was tracked to where the meteorites fell. The meteoroid and subsequent meteor associated with asteroid 2008 TC3 (whose impact in 2008 had been predicted beforehand) were tracked to Sudan, spawning the Almahata Sitta meteorite falls. Another famous fall, of course, is the martian Nakhla meteorite of 1911, the one that may or may not have vaporised a dog.
Meteorite finds are what you come across by chance, strolling through the desert, trekking across Antarctica, or remotely driving a rover across Mars, for example. Or if you have a job that takes you down into the ground, like in a quarry. Quarries are the perfect place to go and see a clean vertical section of millions of years of rock with no expensive equipment required (just permission, sometimes). Some important scientific discoveries have been made that way, in particular in a number of Swedish quarries, where a whole host of meteorites that rained down during the so-called Ordovician Meteor Event half a billion years ago are linked to a concurrent period of bio-diversification that took place on Earth (which you can read more about here).
So that is the end of my article that aims to clarify the difference between meteoroids, meteors and meteorites.
Want to read more about asteroids, comets, meteoroids, meteors and meteorites?
My next article Real and Simulated Asteroid and Comet Impacts discusses potential meteoroid impact threats to Earth and the simulations being run by NASA to mitigate a catastrophe.
If you want to read about the difference between asteroids and comets, read my article Asteroid vs Comet.
If you are looking for a nice long read about asteroid taxonomies, try my article Asteroid Classification, which runs through the multitude of perplexing asteroid classification systems that have been devised over the last half a century. You’ll need to commit to get through it.
On a lighter note, if you have ever wondered what asteroids, comets, meteoroids, meteors and meteorites smell like (and why beauty companies have named perfumes, potions and powders after them) read my article Scent from Heaven.
Finally, if you want to watch some asteroid and comet disaster movies, I review 70 of them in my article Lights, Camera and Asteroid!
Airburst. The explosion in the atmosphere of a meteoroid that distributes the disintegrated material and releases energy over a wide area. If the incoming object fragments into significant sized pieces in the airburst, there may be an impact crater; if it disintegrates, there won’t be. The ground directly beneath the airburst is called the hypocentre or ground zero.
Asteroid. A small body in the Solar System composed of rock and metal, also known as a minor planet (as the IAU prefers). Most are located in the Asteroid Belt between Mars and Jupiter. A large asteroid is sometimes called a planetoid. A small asteroid (less than 1 metre across) is sometimes called a meteoroid. Asteroids that orbit near Earth are called near-Earth asteroids (NEAs). Those that orbit strictly within the orbit of Earth are called Atiras. Those that orbit strictly outside the orbit of Earth but within that of Jupiter are called Amors and may be Mars-crossers. Atens and Apollos are Earth-crossers (the distinction being their semi-major axis). Asteroids that co-orbit with a larger body are called Trojans. Other collections include Centaurs and Damocloids.
Asteroid Spectral Type. Asteroids are classified by surface composition, according to characteristics in their reflectance spectra at visible and near infrared (VNIR) wavelengths. The class can be any of (A, B, C, D, E, F, G, K, L, M, O, P, Q, R, S, T, V, X), depending on the classification scheme used. For example, the Chelyabinsk meteoroid was likely S type (like the Flora family) and 2012 DA14 (367943 Duende) appears to be L type. You can read about the different asteroid classification schemes here and here.
Bolide. An extremely bright meteor, having an apparent magnitude of −14 or brighter. For comparison, a bit brighter than a full moon.
Comet. A small icy body in Solar System composed of a loose collections of ice, dust and rock, usually on a highly eccentric elliptical orbit, that produces a tail and coma as it nears the Sun. Most comets reside in the outer Solar System, but not all.
Fall. A meteorite that is recovered subsequent to a meteor being observed during transit through the atmosphere, or eyewitness accounts of an observed impact on the ground. Most recovered meteorites are falls. They are highly sought-after by scientists because they are fresh and unaltered and therefore provide important information about the composition of other bodies in the Solar System.
Find. A meteorite that is found without the meteor being tracked or impact observed. Being unobserved, finds may be found thousands of years after having fallen and will be subject to weathering effects on Earth. This will cause alteration to the minerals in terms of oxidation and replacement by secondary minerals, such as clays, oxides, and evaporites.
Fireball. A particularly bright meteor, having an apparent magnitude of –4. For comparison, Venus is –4 .4.
Impact Flash. The light produced during a meteoroid impact on a body that lacks an atmosphere. No atmosphere, no meteor.
Meteor. The bright light produced when the exterior of a meteoroid is abraded during its high speed entry into and through Earth’s atmosphere. The term actually refers to all associated physical phenomena including light, heat, ionization, and shock.
Meteorite. The surviving part of a meteoroid that isn’t vaporised during its transit through the atmosphere. It is called a meteorite regardless of which planetary body it is found upon. It is designated from where it came (e.g. martian meteorite if from Mars; lunar meteorite if from the Moon; terrestrial meteorite if from Earth). Meteorites have a classification system which you can read about here. A terrestrial meteorite was unknowingly found on the Moon by the Apollo astronauts, who brought it back embedded in a lunar rock (you can read about that here).
Meteoroid. A small space rock, smaller than an asteroid and larger than interplanetary dust (i.e. less than 1 metre and larger than 30 microns). It could be a fragment of an asteroid or comet, or a collisional fragment of a planet or moon. Note that the solid body that causes a meteor, if it is natural body, is defined as a meteoroid regardless of its size.
Meteor Shower. The effect seen from Earth as it passes through a meteoroid stream, such as the dust tail of a comet, creating a debris trail of particles burning up in the atmosphere. Sometimes called a shooting star, but it has as much to do with a star as does the effect of your so-called star sign on your personality.
Near Earth Object (NEO). A small Solar System body with an orbit that brings it close to Earth, meaning a perihelion less than 1.3 AU. It could be a near-Earth asteroid (NEA) or a near-Earth comet (NEC).
Potentially Hazardous Object (PHO). A near-Earth object (NEO) with a diameter larger than 140 metres and an orbit that crosses Earth’s orbit. It could be a potentially hazardous asteroid (PHA) or, more rarely, a potentially hazardous comet (PHC).
Superbolide. A phenomenally bright meteor, reaching an apparent magnitude of −17. For comparison, about 100 times brighter than a full moon. The Chelyabinsk event was actually a superbolide.
Don’t forget Asteroid Day is on 30 June.