The Ordinary chondrites (sometimes called the O chondrites) are a class of stony chondritic meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s. They are by far the most numerous group and comprise about 87% of all finds. Hence, they have been dubbed "ordinary".
It is suspected that they are not representative of typical asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
parent bodies, but rather of a select few which are advantageously placed to send impact fragments to Earth-crossing orbits. Such positions are e.g.
near Kirkwood gap
A Kirkwood gap is a gap or dip in the distribution of main-belt asteroids with semi-major axis , as seen in the histogram below...
s and/or secular resonance
A secular resonance is a type of orbital resonance.Secular resonances occur when the precession of two orbits is synchronised . A small body in secular resonance with a much larger one will precess at the same rate as the large body...
s in the main asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
. In fact, only the one rather insignificant asteroid 3628 Božněmcová
3628 Božněmcová is a main belt asteroid with an orbital period of 1478.2672540 days .The asteroid was discovered on November 25, 1979.-References:* * * See also: List of asteroids...
has been identified to have a spectrum close to the ordinary chondrites.
A probable parent body of the H chondrite
The H type ordinary chondrites are the most common type of meteorite, accounting for approximately 40% of all those catalogued, 46% of the ordinary chondrites, and 44% of the chondrites....
s (comprising about 46% of the ordinary chondrites) is 6 Hebe
6 Hebe is a large main-belt asteroid, containing around half a percent of the mass of the belt. Its apparently high bulk density , however, means that by volume it does not rank among the top twenty asteroids...
, but its spectrum is dissimilar due to what is likely a metal impact melt component.
It is likely that the ordinary chondrites comprise a detailed sample of but a few select asteroids which happen to have been in the right place at the right time to send many fragments toward Earth at the present moment in solar system history. On the other hand, observations of 243 Ida
243 Ida is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Johann Palisa and named after a nymph from Greek mythology. Later telescopic observations categorized Ida as an S-type asteroid, the most numerous type in the inner asteroid belt. On 28...
by the Galileo spacecraft found weathering of Ida's surface, and the reflection spectra of freshly exposed parts of the surface resembled that of OC meteorites, while the older regions matched the spectra of common S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
The ordinary chondrites comprise three mineralogically and chemically distinct groupings. They differ in the amount of total iron, of iron metal and iron oxide in the silicates.
- The H chondriteH chondriteThe H type ordinary chondrites are the most common type of meteorite, accounting for approximately 40% of all those catalogued, 46% of the ordinary chondrites, and 44% of the chondrites....
s have the Highest total iron, high metal, but lower iron oxide (Fa) in the silicates;
- The L chondriteL chondriteThe L type ordinary chondrites are the second most common type of meteorite, accounting for approximately 35% of all those catalogued, and 40% of the ordinary chondrites....
s have Lower total iron, lower metal, but higher iron oxide (Fa) in the silicates;
- The LL chondriteLL chondriteThe LL chondrites are a class of stony meteorites, the least abundant group of the ordinary chondrites, accounting for about 10–11% of observed ordinary-chondrite falls and 8–9% of all meteorite falls ....
s have Low total iron and Low metal, but the highest iron oxide content (Fa) in the silicates.