Most asteroids come from a few ancient planetesimals

Eighty-five percent of inner main belt asteroids come from five or six ancient objects.

Most asteroids in the belt between Mars and Jupiter as well as meteorites on Earth are the remnants of five or six planetesimals that broke apart in the ancient solar system, according to a new study published in the journal Nature Astronomy.

The asteroid belt is home to about 400,000 asteroids, with those posing the greatest possible threat to Earth located mostly in the inner main belt.

Understanding the origin and evolution of asteroids is important to protecting the Earth from potential impactors, noted study lead author Stanley Dermott of the University of Florida.

“These large bodies whiz by the Earth, so of course we’re very concerned about how many of these there are and what types of materials are in them. If ever one of these comes toward the Earth, and we want to deflect it, we need to know what its nature is.”

In earlier studies, scientists found that many asteroids have very similar compositions and orbits, which they labeled “asteroid families.” Each family likely originated from one larger body broken apart in a long ago impact.

Prior to this study, scientists believed just 44 percent of inner main belt asteroids to belong to five asteroid families.

Dermott and his research team studied asteroids never assigned to any families, specifically focusing on whether their orbits are circular or eccentric along with their tilt or inclination to the Sun’s equator. Their findings indicated 85 percent of inner main belt asteroids come from just five asteroid families. The remaining 15 percent either come from these same families or come from “ghost families,” most of whose asteroids have since been destroyed.

Meteorites on Earth, all of which are somewhat different from one another, also came from the same initial, ancient objects. “We’re saying these meteorites generally come from a small number of objects that were fairly large, hundreds of kilometers in diameter or more,” Dermott added.

Asteroids in the middle and outer main belt likely come from a different, but equally small number of larger parent bodies.

As a next step, Dermott plans to research the process by which asteroids leave the main belt and travel inward to become Near-Earth Objects.

A better understanding of ancient solar system bodies will also help scientists better understand the processes that shaped Earth and provide insight as to where to look for Earth-like exoplanets, he said.


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