The San Juan Capistrano Meteorite

SJC's Rock from Space

On March 15, 1973, at 10:00 PM, a meteorite smashed through the aluminum roof of a car port in San Juan Capistrano, California (USA). The object became known as the San Juan Capistrano meteorite. It made its way eventually into the hands of Dr. Robert Finkel at UCSD. Finkel and his colleagues performed research into its composition and history. This meteorite is the only one in California to be both seen falling and later found. The SJC meteorite is very important scientifically because scientists received it so soon after its fall. Research on it began before key radioactive elements had time to decay. To the average person, the SJC meteorite is worth no more than the average rock. Scientifically speaking, it is priceless.

What Are Meteorites?

Most people have seen "shooting stars." However, not everyone knows that the momentary flash of a shooting star is caused, not by a star, but by cosmic debris. Shooting stars are more formally called meteors. Meteors are pieces of rock or ice colliding with the Earth's atmosphere at high speed. As these pieces hurtle through the atmosphere, air friction heats their outsides white hot, vaporizing layers of material. Most meteors that you see are the size of a grain of sand. Because they are so small, they burn up in a fraction of a second, leaving only a streak of light in the sky. Occasionally, meteors are large enough that part of them survives the fall through the atmosphere. The object that hits the ground is called a meteorite.

Because meteorites have often been floating in space unchanged for billions of years, they provide important information about the early history of the solar system. Sometimes large meteoritic impacts with the Moon or a planet blast planetary material into space. Chunks of this planetary material sometimes collide with Earth and become meteorites themselves. There is strong evidence that a very few meteorites even come from Mars, providing our only rock samples from another planet.

Meteorite Classifications

Scientists roughly classify meteorites into stones, irons, and stony-irons. Stones make up about 94% of all meteorites. Stones are further subdivided into chondrites and achondrites. The SJC meteorite is a chondrite. Chondrites have a composition that closely resembles that of the photosphere of the sun. Scientists believe that many chondrites formed during the early stages of the solar system.

Cutting Diagram

This diagram shows how scientists sliced the SJC meteorite for research purposes:

What happened when the meteor came through the Earth's atmosphere?

Once in the atmosphere, air friction slowed the falling rock while heating it white hot. The heat of enrty ablated (burned off layers of) the meteorite. During this time of intense heating, thermal streesses may have cracked the rock and caused it to break apart. As the rock slowed down due to air friction, the ablation slowed and eventually stopped. By then the rock was falling nearly straight down.

Recovered pieces of the SJC meteorite. Note the dark "fusion crust" caused by the heat generated during the meteorite's fall. (scale in cm shown at bottom of picture)