Spacecraft makes ‘unexpected’ discoveries about asteroid Bennu

The spacecraft captured this view of Bennu ejecting particles from its surface on January 19. Full credit: NASA/Goddard/University of Arizona/Lockheed Martin

NASA’s OSIRIS-REx spacecraft has only been orbiting the asteroid Bennu for a few months but mission scientists have already learned new details about the near-Earth asteroid that is 70 million miles away, according to a NASA announcement.

The greatest surprise of the mission happened only a few days in and an unexpected observation occurred signaling activity on Bennu, the researchers said.

The mission science team detected particle plumes ejecting off of the surface on January 6, followed by additional plumes over the last two months. That makes Bennu an active asteroid that is regularly ejecting material into space, which is rare. This is the first time scientists have had close-up observations of particle plumes erupting from an asteroid’s surface, the agency said.

A suite of seven studies published Tuesday in the journals Nature, Nature Astronomy, Nature Geoscience and Nature Communications detail the spacecraft’s observations so far.

The particles range from centimeters to 10s of centimeters, and some of them are slow-moving while others are quickly making their way through interstellar space. Some fall back onto the surface, while others go into orbit around it, like small satellites. The researchers don’t currently understand why this is happening, but it poses no risk to the spacecraft.

“The discovery of plumes is one of the biggest surprises of my scientific career,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started.”

OSIRIS-REx’s instruments have confirmed that hydrated minerals, including magnetite, are abundant and widespread on the asteroid. The asteroid is full of valuable materials that may even contain clues about how life began. Bennu is essentially a leftover from the formation of our solar system billions of years ago, although some of the minerals inside it could be even older.

Asteroids could even serve as fuel stations for robotic and human missions if we can unlock the hydrogen and oxygen inside them, NASA said.

It’s also older than expected, between 100 million and 1 billion years old and likely originated in the main asteroid belt. Bennu probably broke off of a larger asteroid in the belt between Mars and Jupiter a couple billion years ago. This knocked it through space until an orbit close to Earth locked it in place.

The spacecraft also detected numerous larger boulders on the surface, with more than 200 boulders that are over 32 feet in diameter. The asteroid’s surface is like a time capsule with different regions representing different eras, from the remains of Bennu’s parent asteroid to recent activity.

The researchers were expecting a smooth surface with few large boulders due to Earth-based observations. Instead, it’s rough and crowded with boulders. This means that the sample collection part of the mission will have to be adjusted to make sure that OSIRIS-REx can touch down and collect a sample.

“Throughout OSIRIS-REx’s operations near Bennu, our spacecraft and operations team have demonstrated that we can achieve system performance that beats design requirements,” said Rich Burns, the project manager of OSIRIS-REx. “Bennu has issued us a challenge to deal with its rugged terrain, and we are confident that OSIRIS-REx is up to the task.”

Bennu has a shape comparable to a spinning top and it’s a “rubble pile” asteroid, a grouping of rocks held together by gravity rather than a single object.

Early results from Japan’s Hayabusa 2 space mission orbiting the asteroid Ryugu also released on Tuesday. Both Bennu and Ryugu are extremely dark, spinning top-shaped asteroids that are covered in large boulders, but the latest findings show that Ryugu is a lot drier.

“The first three months of OSIRIS-REx’s up-close investigation of Bennu have reminded us what discovery is all about — surprises, quick thinking and flexibility,” said Lori Glaze, acting director of the Planetary Science Division. “We study asteroids like Bennu to learn about the origin of the solar system. OSIRIS-REx’s sample will help us answer some of the biggest questions about where we come from.”

The mission

The OSIRIS-REx mission and the asteroid Bennu got to meet face to face on December 3. OSIRIS-REx has been orbiting the asteroid since December 31 and Bennu is the smallest body to ever be orbited by a spacecraft. Bennu is just a little bit wider than the height of the Empire State Building, according to NASA.

The mission — which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer — is NASA’s first asteroid sample return mission. It launched in September 2016 and will spend two years up close and personal with Bennu.

Over the next year, OSIRIS-REx will survey the asteroid using five scientific instruments on board the spacecraft. These instruments will help it determine a safe location from which to collect a small sample from Bennu’s surface that will be returned to Earth in September 2023.

The sample from Bennu, a near-Earth asteroid, could help scientists understand not only more about asteroids that could impact Earth but about how planets formed and life began.

The asteroid could pass close to Earth, closer than the moon, in 2135, with even closer approaches possible in 2175 and 2195. A direct hit is unlikely, but the data gathered during this mission can help determine the best ways to deflect near-Earth asteroids.

Another recent study published in Geophysical Research Letters also found that Bennu is spinning faster over time. Bennu moves through space at 63,000 miles per hour and spins, completing a full rotation every 4.3 hours. The new study determined that the rotation speed increases by one second per century, shortening by one second every 100 years.

But over millions of years, that could cause the asteroid to lose pieces or blow apart. Knowing this broadens the understanding of how asteroids evolve, as well as the threat they pose to Earth.

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