A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized by the Earth's atmosphere.
While most meteoroids are rocky in composition, some are almost pure metal.
Intense meteorite showers can only occur after a very large collision between asteroids. Such events have been rare in the last billion years of the solar system's history.
Scientists say that meteorite showers are also rare because Jupiter's titanic gravitational pull diverts much of the space debris before it has a chance to venture toward Earth.
The phenomenon known as orbital resonance plays a key role in the "meteorite express" described in the new study. Resonance occurs when two orbiting bodies exert a gravitational influence on each other.
As Heck explained: "If a [meteoroid] goes into an important orbital resonance with Jupiter, for example the 3:1 resonance, then while Jupiter goes once around the sun, the [meteoroid] circles the sun three times."
"So the small object experiences periodically a gravitational tug from Jupiter, and its orbit gets changed quite fast," he said.
The efficient transfer of asteroid material into the inner solar systemthe basis of the so-called meteorite expressonly happens when an asteroid collision occurs close to a resonance, however.
"Once the fragments are injected into the resonance, their orbital shape is changed rather quickly until they become Earth-crossing," Heck said. This is what has happened in the distant past, scientists believe.
Heck and his colleagues studied meteorites that measured several centimeters in diameter. They were found in Thorsberg, a rock quarry in southern Sweden.
Around 500 million years ago the meteorites, of a type known as chondrites, fell into a shallow sea. There, they were covered with fine-grained marine sediments and well preserved.
A chemical analysis showed the meteorites were all of the same class, the so-called L chondrites class. Scientists believe that L chondrites come from a precursor to the Flora family of asteroids, a prominent group of asteroids circling the sun near the inner edge of the asteroid belt.
Most L chondrites experienced a major gas loss 500 million years ago, which is best explained by a violent collision in space.
"The transfer time [of these meteorites] to Earth is around a hundred thousand years, consistent with a long-lasting rain of meteorites after the destruction of an asteroid and the existence of at least one trajectory [in which] material was flung towards the inner solar system," Heck said.
The findings do not contradict present scientific thinking about the rate at which meteoroids can travel. But the short transfer times suggested by the new study are at the very low end of theoretical predictions.
"This analysis shows again what happened hundreds of millions of kilometers away can eventually make for a real bad day on Earth," said Bruce Betts, director of projects at the Planetary Society in Pasadena, California.
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