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The Most Cratered Object in the Asteroid Belt Looks Like a Golf Ball

Pallas’s odd orbit sends it crashing through the asteroid belt, colliding with other objects along the way

The asteroid Pallas, imaged by the European Southern Observatory's Very Large Telescope (European Southern Observatory / Vernazza et al., Nature Astronomy, 2020)
smithsonianmag.com

Astronomers just snapped the best images yet of Pallas, one of the solar system’s most infamous asteroids—and it seems the pictures illustrate the object’s remarkably violent past.

Boasting a width of about 318 miles—about 15 percent of the moon’s diameter—Pallas makes up a whopping 7 percent of the total mass of the asteroid belt. Researchers have known about this absolute cosmic unit, which whirls around the sun with a tiny entourage of smaller objects, for more than two centuries. But in spite of its size, the asteroid has proved difficult to study.

Now, with the help of the SPHERE instrument at the European Southern Observatory’s Very Large Telescope in Chile, a team of researchers has homed in on some of the beauty marks speckling the asteroid’s surface. Their findings, described in a study published this week in Nature Astronomy, reveal Pallas as the most cratered object in the asteroid belt—a title it’s almost certainly earned by bashing into some of its neighbors.

“This first detailed images of the Pallas suggest that the asteroid had a violent past,” study author Franck Marchis, a planetary scientist at MIT, says in a statement.

Though several factors may play into Pallas’ pronounced pockmarking, the researchers behind the project suspect the asteroid’s unusual orbit may be the biggest culprit. While most objects in the asteroid belt travel the same approximate course around the sun, Pallas is one of a few that goes rogue, zooming along a tilted track that brings it smashing through the rest of the belt at an awkward angle. By nature, collisions involving the avant-garde asteroid are far more calamitous than those that occur between objects moving in roughly the same direction—the difference between rear-ending a car on a suburban street and smashing headfirst into 16-wheeler careening over a highway median.

In other words, Pallus is often in harm’s way, and “experiences two or three times more collisions than Ceres or Vesta,” the two largest objects in the asteroid belt, study author Michaël Marsset, a planetary scientist at MIT, says in a statement. Though the researchers aren’t yet certain exactly how many craters freckle Pallas, an analysis of 11 images snapped by SPHERE showed that that the marks make up at least 10 percent of the asteroid’s surface. Pallas is so heavily pockmarked that the researchers behind the discovery are now calling it the “golf ball asteroid.”

And each of the impacts the asteroid experiences is particularly severe. Computer simulations also revealed that the impactors responsible for Pallus’ craters were traveling at speeds over 25,000 miles per hour—nearly twice as fast as is typical for asteroid belt collisions, reports Charles Q. Choi for Space.com.

At least 36 of the depressions spanned at least 18 miles in diameter, including one running 250 miles across—the likely battle scar left behind from the impact of an object up to 25 miles wide, reports Rafi Letzter for Live Science. The pair’s violent get-together could explain the asteroid’s band of followers, too: After smashing into Pallus some 1.7 billion years ago, the impactor might have shattered into fragments, which then drifted out into space and began to trail their leader.

“Because we are now able to see the surface of large asteroids in the main-belt, we have access to a fictive book on the history of our solar system,” Marsset says in a statement. “We are in the process of learning how to read it and each page is a surprise to us, including Pallas.”

About Katherine J. Wu
Katherine J. Wu

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University. Previously, she served as a Digital Editor at NOVA Next and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.

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