Most meteorites that fall on Earth are fragments of broken-up asteroids which orbit the Sun mostly between the orbits of Mars and Jupiter. This is indicated by photography of meteorite fire balls as they enter Earth’s atmosphere, and extrapolation of the orbits which reach into the Asteroid Belt.

Photograph of the bolide resulting from the entry of the Lost City, Oklahoma, stone meteorite (ordinary chondrite) on January 3, 1970.

The Lost City, Oklahoma, stone meteorite, recovered based on the triangulation of the place of fall from the photographs of the associated bolide

Telescopically measured optical reflectance spectra of asteroids (points, with error bars) with the spectra of powders of various types of meteorites (solid lines). Conclusion: The meteorite spectra match the asteroid spectra and, hence, the meteorites come from asteroids

A = Comet Toit-Hartley; B = Asteroid 1982 DB

The Galileo spacecraft, on its way to Jupiter, was the first to obtain close-up pictures of asteroids

The complex orbit of the Galileo spacecraft

The Galileo spacecraft during construction

Launch of the Galileo spacecraft from the Shuttle

Galileo Spacecraft after launch from the Shuttle

The Galileo spacecraft imaged the asteroids Gaspra and Ida. Mathilde was imaged by the NEAR spacecraft.

Asteroids are highly irregular in shape, and some may have moons too! Ida and its moon Dactyl, imaged by the Galileo spacecraft Dactyl

Asteroid Mathilde, imaged by the NEAR Spacecraft on its way to Asteroid Eros

Asteroid Eros – Orbited by NEAR* spacecraft in 2000 * NEAR = Near Earth Asteroid Rendezvous mission

Landing site Asteroid Eros and landing site of NEAR Spacecraft

NEAR Spacecraft approaching Asteroid Eros

NEAR Spacecraft approaches the landing site on Asteroid Eros

Landing site of the NEAR Spacecraft on Asteroid Eros, February 12 th, 2001

Topography of Asteroid Eros, based on the NEAR mission

How many asteroids are there and where are they? Inclination of orbits of asteroids as a function of distance from the Sun (in AU) Number of asteroids as a function of their distance from the Sun (in AU). The Kirkwood Gaps are apparent, and the resonances with Jupiter are also indicated

Orbits of some major asteroids

Summary

Chondrites are meteorites from broken-up primitive, undifferentiated asteroids that never melted. Thus, the properties of their constituents [calcium-aluminum-rich inclusions (CAIs), chondrules, matrix, and metallic Fe,Ni] are today as they were when the Solar System formed Chondrite NWA 5028

Polished slice of an ordinary chondrite. White = metallic Fe,Ni; brown = silicates.

Handspecimen of an ordinary chondrite with a large chondrule

Thin section photomicrograph of an ordinary chondrite. Chondrules are ~ mm in diameter

Chondrule consisting of glass (pink) and olivine (Fe-Mg- silicate) crystals (white)

Barred chondrule consisting of parallel bars of olivine (Fe-Mg silicate) crystals (white) and silicate glass (pink-brown)

When a chondritic asteroid melts, it differentiates, i.e., the dense metallic Fe,Ni sinks and forms the core, and the rocky material forms the mantel (e.g., ureilite) and crust (e.g., basalt) of the asteroid. These rocks are collectively called achondrites, because they do not contain chondrules.

Minute diamonds (bright) in a ureilite. They formed from carbon by high-pressure shock when asteroids collided.

Basaltic meteorite (eucrite), most likely from the crust of the asteroid Vesta

Pallasite – possibly fragment from the core/mantle boundary of a broken-up asteroid. Shiny = metallic Fe,Ni; brown = olivine, an Fe-Mg-silicate.

When a chondritic asteroid melts, it differentiates, i.e., the dense metallic Fe,Ni sinks and forms the core, and the rocky material forms the mantel (e.g., ureilite) and crust (e.g., basalt) of the asteroid (just like in case of Earth).

Handspecimen of an iron meteorite – a fragment of the core of a broken-up asteroid

How many meteorites, and of what types? Early observations of “Falls” and “Finds”, excluding those from Antarctica and hot deserts.