Asteroid Bennu is flinging rocks into space

Asteroid 101955 Bennu, which NASA’s OSIRIS-REx sample-return mission has been orbiting since 31 December 2018, keeps throwing curveballs at scientists.

First, the 500-metre asteroid proved to be a giant rubble pile, with a prominent equatorial ridge – much like asteroid 162173 Ryugu, which is being investigated by a similar Japanese mission. Then, it proved to be studded with so many boulders that NASA will have to work hard to find a safe place for the spacecraft to touch down to collect a sample. 

And now it turns out that Bennu is periodically blasting particles as big as basketballs from its surface at speeds ranging from a few centimetres to three metres per second – a finding that the mission’s principal investigator, Dante Lauretta, of the University of Arizona, Tucson, called one of the biggest surprises of his entire scientific career. 

The mission tallied at least 11 such events in the five weeks from 11 January to 18 February. 

Three of these have been “substantial”, says Lauretta, with dozens or perhaps hundreds of particles being ejected in single bursts.

That means Bennu is part of a rare class of asteroids known as “active” – so rare that only a dozen or so have been found among approximately 800,000 asteroids thus far recorded. 

“We did not expect to see this activity,” Lauretta says.

One of the team’s first reactions was to ask if these outbursts posed a threat to the spacecraft, which at one time was orbiting less than a mile above the asteroid’s surface. 

“You can imagine that we were modestly concerned about particles being ejected by the asteroid while we were in orbit,” says the mission’s project manager, Rich Burns of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US. 

But within a day calculations using methods developed to determine the risks from space debris to Earth-orbiting spacecraft showed that there was a very low likelihood of the spacecraft being hit. 

At the moment, Lauretta says, it’s too early to figure out why the particles are being ejected. 

In fact, says the project’s flight navigator, Coralie Adam of KinetX Space Navigation and Flight Dynamics in Simi Valley, California, at the moment, a major challenge is to compute these particles’ trajectories and trace them back to their points of origin.{%recommended 8710%}

Following them as they continue to move will also important, because their motions offer scientists an unexpected tool for understanding Bennu’s interior.

Many of the particles are simply being shot off into interplanetary space. Others are falling back to the ground, but some are ejected at just the right velocities to go into orbits around Bennu that could persist for months or years. 

That makes them “natural gravity probes” the motions of which can be used to map tiny variations in the asteroid’s gravity field, allowing scientists to model its internal structure in unexpected detail, Lauretta says.

Also, he adds, there is a “reasonable probability” that Bennu’s activity may have ejected enough particles into interplanetary space to create a meteor shower here on Earth, each time we pass near its orbit. 

If so, that would happen in late September every year, and would be best seen from the southern hemisphere. Already, scientists are preparing to monitor the southern skies for such a shower, hoping that its intensity will help determine whether Bennu’s activity has continued for a long time, or is simply a short-term phenomenon that just happens to be occurring now.

OSIRIS-REx’s primary mission, however, is sample return. And, scientifically, the asteroid is looking like a well-selected target. 

The craft’s images have shown, for example, that its surface varies widely in brightness, suggesting that it’s made up of multiple different materials, many of which, hopefully, will be in the mix of sand and gravel that is eventually scooped up for return to Earth. 

“It’s really exciting because it means we’re going to have a wide range of material to study,” Lauretta says.

Also, spectroscopic analysis has shown that Bennu contains minerals that must have formed in the presence of large amounts of warm, liquid water, presumably in the interior of a now-shattered parent body. That makes Bennu exactly the type of asteroid scientists want a sample from, in order to help understand the role they played in bringing water to the early Earth.

To get a sample, however, OSIRIS-REx is going to have to navigate a field of boulders. 

“Our latest count is over 200 boulders over 10 metres in diameter,” Lauretta explains.

Originally, notes Burns, the plan was to find a hazard-free target area at least 50 metres across, where the spacecraft could safely hover while its robotic arm reaches out to scoop up dirt. 

But although smooth, flat areas do exist, none appear to be remotely close to the size needed.

The task still looks feasible, Burns says, because in the flight crew has learned to fly the spacecraft with unexpected precision. 

“Our navigation performance has been exquisite,” he says. 

“We feel confident that our team is up to the task of tagging a much smaller area than was previously envisioned.

“We call it bulls-eye tag.” he adds. 

Previously, he notes, the goal was akin to throwing a dart and simply hoping to hit the dartboard. “Now we’re going to try to hit the bulls-eye.”