Meteoroids, Meteors and Meteorites

It’s a dilemma universally acknowledged in Hollywood that the makers of asteroid and comet disaster movies must ignore the laws of physical science and make events look the way moviegoers expect from their own experience on Earth, otherwise they won’t believe it. And so asteroids and comets are called meteors, meteoroids and meteors are called meteorites, and meteorites are called…well, meteorites, actually.

The term meteor, in particular, is most often misleadingly used in the movies and as a result its misuse begets misuse – in real life too, including in the news media. So what is the difference between meteoroids, meteors and meteorites?

Before I start, if you were looking for clarity on the difference between asteroids and comets, read Asteroid vs Comet instead. Or if you came here wanting to read about movies about asteroids, comets, meteoroids, meteors and meteorites, read this post which aims to provide a list of every asteroid or comet impact disaster film and TV show ever made.

In a subsequent post, I discuss the real impact risks associated with near-Earth potentially hazardous objects and some of the simulated impact scenarios being run by NASA.

The official definition of meteoroid, meteor and meteorite

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 are, on the face of it, easy enough to understand:

Meteoroid. The solid part of a natural body passing through the atmosphere and the physical source that feeds a meteor. (There is more on meteoroids below, in particular the size debate.)

Meteor. The bright phenomenon (heat, light, ionization) that we see in the sky due to a meteoroid (or any other object) burning up in the atmosphere. There is still a meteoroid (or something) ‘inside’ the meteor, burning up. In meteor astronomy, a meteor is caused by a meteoroid irrespective of the size of the incoming object causing it (whether an asteroid or any other natural body).

Meteorite. The part of the meteoroid that survives the meteor stage and falls to Earth. 

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 space 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 spent end product.

There is a full glossary of terms at the end of this post.

Meteorites and meteor wrongs

In the movies, an asteroid or comet is commonly referred to as “the meteor” from the first moment it is observed. But it’s only a meteor when it’s in the meteor phase, not before and not after. A space rock isn’t a meteor in space because there is no atmosphere in which it can burn up.

Similarly in the movies, we hear fragments or debris of a comet or asteroid referred to as a “meteor cloud” (or swarm or storm) even though there can be no meteors at that stage.

But it’s not always the moviemakers’ fault. It’s a bit of a dilemma. Wthout the excitement of a huge asteroid ablaze in space, people might either be bored or presume that the budget didn’t stretch enough for special effects. People also want things to look the way they expect, which translates to what they see on Earth, even if the physics is wrong.

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 colonise 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 asteroid strike on the spacecraft, with quotes like: “There is no fair or unfair – to a meteorite, you get hit, you die.

Well, yes, that could also be true. There is, after all, is the apocryphal Legend of The Nakhla Dog that was (as is often told) struck and killed by a meteorite from Mars in 1911. But this tale might just as well be called Schrödinger’s Dog, seeing as no-one really seems to know the outcome of that particular meteoritic event, or will ever know without having seen it, because the story differs depending on who first told it, who can remember it and who now translates it.

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:

METEORS:
L (top): Chelyabinsk superbolide 2013 (YouTube)
L (bottom): Botswana bolide 2018 (B Swanepoel/V van Zyl)
R: Kamchatka meteor 2018 (Japan Met. Agency)

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) and all went 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, 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, the one 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 that 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 would not be alone, because the IAU – the very ones who concocted the definition of the meteoroid – agree. The quirk in their definition of size is in the third blob under “remarks to meteoroid” in their notes here.

METEORITES
L: Stony meteorite in the Libyan desert (Svend Buhl)
R: Iron meteorite on Mars (NASA/JPL-Caltech/MSSS)

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.

That is, unless it’s a micrometeorite or maybe an IDP. Or unless it’s a tektite or impactite or other non-alien by-product of a meteoritical event.

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 reduced a dog to ashes.

Meteorite finds are what you come across by chance, strolling through the desert, trekking across Antarctica, or remotely driving a rover across Mars, 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 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?

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.

Asteroid vs Comet discusses the difference between asteroids and comets.

For a nice long read about asteroid taxonomies, History of Asteroid Classification 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 Scent from Heaven.

If you want to watch some asteroid and comet disaster movies, you can find them all in Lights, Camera and Asteroid!

How high is the sky? Read The Edge of Space.


Glossary

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 or objects (NEAs or NEOs). 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 collection 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.