IAU Commission F1 (Meteors, Meteorites, and Interplanetary Dust) officially approved some terms and definitions in meteor astronomy last year. This is a revision of the terms and definitions that were approved in 1961. Meteor astronomy knowledge has grown by leaps and bounds since then.
- A solid natural object of a size roughly1 between 30 micrometers and 1 meter moving in, or coming from, interplanetary space
- The light and associated physical phenomena (heat, shock, ionization) which results from the high speed entry of a solid object from space into a gaseous atmosphere
- Any natural solid object that survived the meteor phase in a gaseous atmosphere without being completely vaporized
1“Roughly”, because the 1 meter size limit is not a physical boundary; it is set by agreement. There is a continuous population of bodies both smaller and larger than 1 meter. Bodies larger than 1 meter tend to be dominated by asteroidal debris, rather than debris from comets. “Roughly”, also because the 30 micrometer size limit is not a physical boundary; it is set by agreement. There is a continuous population of bodies both smaller and larger than 30 micrometers. Bodies smaller than 30 micrometers, however, tend to radiate heat away well and not vaporize during an atmospheric entry.
“Small dust particles do not give rise to the meteor phenomenon when they enter planetary atmospheres. Being heated below the melting point, they sediment to the ground more or less unaffected.”
“When collected in the atmosphere, they are called interplanetary dust particles (IDPs). When in interplanetary space, they are simply called dust particles. The term micrometeoroid is discouraged.”
Looking at the definition for meteorite above, what about meteoroids that reach the surface of a world with little or no atmosphere, such as the Moon? The IAU Commission has a less-than-satisfying answer (to this writer, at least).
“Foreign objects on the surfaces of atmosphereless bodies are not called meteorites (i.e. there is no meteorite without a meteor). They can be called impact debris.”
What’s the harm in calling any meteoroid that reaches the surface of a planetary body (planet, moon, asteroid, etc.) a meteorite? To me, “impact debris” implies material pre-existing on the planetary body that is excavated by an impact event.
“In the context of meteor observations, any object causing a meteor can be termed a meteoroid, irrespective of size.”
“A meteoroid in the atmosphere becomes a meteorite after the ablation stops and the object continues on dark flight to the ground.”
“A meteorite smaller than 1 millimeter can be called a micrometeorite. Micrometeorites do not have the typical structure of a fresh meteorite—unaffected interior and fusion crust.”
“Meteor stream is a group of meteoroids which have similar orbits and a common origin. Meteor shower is a group of meteors produced by meteoroids of the same meteoroid stream.”
- Dust (interplanetary)
- Finely divided solid matter, with particle sizes in general smaller than meteoroids, moving in, or coming from, interplanetary space.
- “Dust in the solar system is observed e.g. as the zodiacal dust cloud, including zodiacal dust bands, and cometary dust tails. In such contexts the term ‘dust’ is not reserved for solid matter smaller than about 30 micron; the zodiacal dust cloud and cometary dust trails contain larger particles that can also be called meteoroids.”
- For consistency with the rest of the document, micron in the above paragraph should be micrometers.
- Meteoric smoke
- Solid matter that has condensed in a gaseous atmosphere from material vaporized during the meteor phase.
“The size of meteoric smoke particles (MSPs) is typically in the sub-100 nm range.”
“Meteors can occur on any planet or moon having a sufficiently dense atmosphere.”
“A meteor brighter than absolute visual magnitude (distance of 100 km) -4 is also termed a bolide or a fireball.”
The fireball definition makes sense, but it was always my understanding that a bolide is accompanied (later) by audible sound and is thus much rarer.
“A meteor brighter than absolute visual magnitude -17 is also called a superbolide.”
“Meteor train is light or ionization left along the trajectory of the meteor after the meteor has passed.”
“Small (typically micron-size) non-vaporized remnants of ablating meteoroids can be called meteoritic dust. They can be observed e.g. as dust trails in the atmosphere after the passage of a bolide.”
Again, for consistency with the rest of the document, micron-size in the above paragraph should be micrometer-size.
“The radiation phenomenon accompanying a direct meteoroid hit of the surface of a body without an atmosphere is not called a meteor but an impact flash.”
Koschny, D., & Borovička, J. 2017, WGN, The Journal of the IMO, 45,5