Rocks – Classes

“Today, thanks to the pioneering asteroid survey Spacewatch and similar projects, our planetary system appears as a humming hive populated with countless asteroids circling the sun like a swarm of bees.” — Space Daily

Classifications of Space Rocks
1 AU = 1 Astronomical Unit = 93 million miles = 149 million kilometers
Inclination = degrees from the elliptic plane of the solar system, like that swarm of bees.

  • cross Earth’s orbit with a period greater than one year.
  • Mars crossing rocks. For example, Eros, see Mission NEAR Shoemaker, human’s first machine to orbit an asteroid.
Trojan minor planets
  • orbit the sun ~ 5.1-5.2 AU.
  • orbit the Sun between Jupiter and Neptune, typically 5 to 30 astronomical units (AU) away.
  • orbits are very elliptical and cross the orbits of Jupiter and Neptune
  • related to Comets but never produce a coma and tail
  • Plot of the Inner Solar System
  • orbit the sun at a distance of at least 38 AU

Meteorite Classifications

Mineralogically, meteorites consist of varying amounts of nickel-iron alloys, silicates, sulfides, and several other minor phases. Classification is then made on the basis of the ratio of metal to silicate present in the various compositions. No two meteorites are completely alike, and specific compositional and structural features give a particular meteorite its unique identity. 


Rare, (est. only 5%) characterized by the presence of two nickel-iron alloy metals: kamacite and taenite, combined with minor amounts of non-metallic phases and sulfide minerals, form three basic subdivisions of irons. Depending upon the percentage of nickel to iron, these subdivisions are classified as:
     hexahedrites (4-6% Ni)
     octahedrites (6-12% Ni)
     ataxites (12+% Ni)
Octahedrites, which are the most common type of iron meteorite, exhibit a unique structural feature called the Widmanstatten pattern when etched with a weak acid. This unique crystal pattern is the result of the combination of the two nickel-iron minerals
kamacite and taenite being present in approximately equal amounts.

Stony Irons

Achondrite Meteorites –Millbillillie

Consist of almost equal amounts of nickel-iron alloy and silicate minerals. Although all stony-irons may not be genetically related or have similar composition, they are combined into one group and divided into two subgroups for convenient classification. The Pallasite group is characterized by olivine crystals surrounded by a nickel-iron structure which forms a continuous enclosing network around the silicate portion. Mesosiderites, on the other hand, consist mainly of plagioclase and pyroxene silicates in the form of heterogeneous aggregates intermixed with the metal alloy. No distinct separation between the metal and silicate phases is readily apparent as it is with the Pallasites.


100.3 grams of
Allende Meteorite

The most abundant of the three meteorite groups and come closest to resembling earth rocks in their appearance and composition. The major portion of these meteorites consists of the silicate minerals olivine, pyroxene, and plagioclase feldspars. Metallic nickel-iron occurs in varying percentages and is accompanied by an iron-sulfide mineral. Aside from
being the most abundant meteorite type, stony meteorites have the greatest variety in composition, color, and structure. One particular structural feature called chondrules divides the group into two main subgroups:
     Chrondrites, those with chondrules
     Achondrites, those without chondrules
Many scientists believe that these small, rounded, nearly spherical chondrules may represent the most primitive material in the solar system.

What is a meteorite. Planetary Science Foundation
The Meteorite Market