Asteroid Bennu has been hanging out with Earth for over a million years

The near-Earth asteroid Bennu has been keeping our planet company for a long time. Bennu, new research suggests, has been in an orbit that brings it near to Earth for 1.75 million years.

The study published Monday in the journal Nature.

If this asteroid sounds familiar, it’s because the boulder-covered space rock was recently the target of a historic NASA mission. The OSIRIS-REx spacecraft briefly touched down on the surface of the asteroid on October 20 to collect a sample that will be returned to Earth in 2023.

This was NASA’s first time landing on and collecting a sample from an asteroid. The OSIRIS-REx mission stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer.

Beyond its landmark touchdown, the OSIRIS-REx mission has revealed a wealth of information and surprises about the asteroid since launching in 2016 and arriving at Bennu in 2018.

This research is the latest in a string of studies to be published using images from the suite of cameras on the spacecraft.

Bennu was once part of a much larger asteroid parent body in the main asteroid belt located between the orbits of Mars and Jupiter, scientists believe. When this large asteroid collided with another object, Bennu broke off and became its own asteroid. Then, it migrated to an orbit closer to Earth.

Bennu is about the height of the Empire State Building. What’s more, the asteroid has a one-in-2,700 chance of impacting Earth late next century. Currently, it’s about 200 million miles away from Earth.

But scientists weren’t quite sure when it arrived in the near-Earth space. They analyzed images of Bennu that show the varied craters across the asteroid’s surface.

Studying the size and depth of these craters helped the researchers to understand when the depressions were formed, and where the impacts that caused the craters occurred in the solar system.

Large craters up to 16 feet across spotted on the asteroid’s boulders likely formed when the asteroid was still in the asteroid belt. Likewise, small craters less than 19.6 inches are the result of more recent impacts by micrometeorites during the asteroid’s time in near-Earth space.

The researchers used the size and age of the craters to create a time line and determined Bennu arrived in its current orbit 1.75 million years ago.

“An asteroid’s history is determined in large part by its strength against collisions with other objects,” the authors wrote in the study.

“Our derived exposure age of Bennu’s boulders is substantially shorter than the expected total lifetime of this asteroid after catastrophic disruption of its parent. As the source region of (near-Earth asteroids) is the main asteroid belt, Bennu has spent most of its lifetime in a collisional environment different than the one at its current orbit.”

Full of surprises

There are more than a million known asteroids in the solar system, but Bennu had the right size, location and composition when scientists were determining where they wanted to send OSIRIS-REx.

And after a couple of years spent orbiting and imaging the asteroid, the mission’s team couldn’t be happier with their choice.

That doesn’t mean Bennu hasn’t presented challenges along the way, but those challenges shed a lot of light on the asteroid as well.

Scientists expected Bennu to have a surface similar to a sandy beach covered in fine grain material that would be easy to sample. But when the spacecraft arrived at Bennu in 2018, the researchers saw boulders the size of buildings on the surface, as well as mountains. In fact, there were very few spots not covered in boulders.

Those rocks turned out to be very porous and fragile — and covered in organic molecules and hydrated minerals. Bennu is a carbon-rich asteroid and its intriguing surface composition suggests that it could contain the building blocks of life once delivered to Earth by asteroid impacts.

Soon after arriving at Bennu, the spacecraft’s cameras captured images of material being ejected from the asteroid into space. Bennu is an active asteroid with a surface that kicks rocks, pebbles and fine material into space on a regular basis. The mission team was surprised to see these plumes.

“We discovered a new world that offered us much more than we hoped for,” said Patrick Michel, study coauthor and director of research at the French National Centre for Scientific Research in Paris, in an email.

“This is because its surface is so rich and diverse, geologically, and because many of our assumptions have been proven wrong, as usual when we go into the unknown. OSIRIS-REx shows us that asteroids are much more complex than simple stones in space!”

And the data, images and sample collected by OSIRIS-REx will only further the surprises and truths associated with this intriguing asteroid.

“By returning samples from primitive asteroids like Bennu and by analyzing them at the highest possible accuracy with instruments in our labs, we will contribute in the best way to two fundamental questions: how did our solar system evolve from a disk of gas and dust surrounding our young sun to (become) a planetary system, and how did life emerge on Earth?” Michel said.

“The adventure continues with its accompanying suspense. This is food for dreams and enlightenment, in a period when we all need it!”