Washington, Apr 26 (IANS): Giant asteroids, similar or larger than the one believed to have killed the dinosaurs, hit Earth billions of years ago with more frequency than previously thought, US space agency NASA said Wednesday.

To cause the dinosaur extinction, the killer asteroid that impacted Earth 65 million years ago would have been almost 10 km in diameter.

By studying ancient rocks in Australia and using computer models, researchers estimate that approximately 70 asteroids the same size or larger impacted Earth 1.8 to 3.8 billion years ago. During the same period, approximately four similarly-sized objects hit the moon, Xinhua reported.

Evidence for these impacts on Earth comes from thin rock layers that contain debris of nearly spherical, sand-sized droplets called spherules.

These millimeter-scale clues were formerly molten droplets ejected into space within the huge plumes created by mega-impacts on Earth.

The hardened droplets then fell back to Earth, creating thin but widespread sedimentary layers known as spherule beds.

The findings were published Wednesday in the journal Nature.

"The beds speak to an intense period of bombardment of Earth," said William Bottke, principal investigator of the impact study team at the Southwest Research Institute in Colorado. "Their source long has been a mystery."

The team's findings support the theory Jupiter, Saturn, Uranus and Neptune formed in different orbits nearly 4.5 billion years ago, migrating to their current orbits about four billion years ago from the interplay of gravitational forces in the solar system.

This event triggered a solar system-wide bombardment of comets and asteroids called the "Late Heavy Bombardment".