Asteroid Bennu is essentially a ball of cosmic confetti — studies
The lightly-packed surface of a near-Earth asteroid is cool and a little scary.
NASA’s newest science discovery looks like a cliché scene in a bakery. In October 2020, a University of Arizona-led team commanded the OSIRIS-REx spacecraft to scoop the dirt of near-Earth asteroid Bennu. The maneuver produced a plume of particles that burst out like cosmic confectioner's sugar. The footage is spectacular.
Scientists are stunned by what they saw, and published two studies on Thursday — one in Science and the other in Science Advances — about the spacecraft’s sights. The mission, which launched in September 2016, is one of many thrilling projects to gather precious parcels of material from the rocks adrift in our Solar System.
The roughly half-pound (250 grams) of Bennu dirt won’t reach Earth for at least another year, but the spacecraft caught footage that advances the science of this asteroid far sooner than September 24, 2023, when the parcels will parachute down, land in Utah’s West Desert and finally get into scientists’ hands.
A view of OSIRIS-REx sampling pieces from asteroid Bennu’s surface.
In particular, the team looked at how the rocks behaved during the rendezvous, when OSIRIS-REx — short for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer — released a pulse of nitrogen gas to wiggle loose the specks and physically touched Bennu’s surface following a 10-cm-per-second approach.
Why it matters — The material began billowing after coming into contact with the spacecraft’s Touch-and-Go Sample Acquisition Mechanism (TAGSAM), shocking the team and causing them to rethink the physical processes happening on what they thought was a much sturdier asteroid.
If Bennu’s surface material is easy to toss, it bodes well for learning new things about the Solar System’s history. But this quality would also mean having to rethink how to deflect a dangerous asteroid on a collision course with Earth.
The images immediately before and after OSIRIS REx’s contact with Bennu. They show that in the approximately 1 second between the shots, the spacecraft’s sampler head disturbed an area nearly 3 feet across and tossed debris into the air.
When settled, the particles probably sit with large spaces in between, loosely held together by Bennu’s low gravity. These pieces are ancient. Bennu is a collection of crumbs from an earlier asteroid floating around the early Solar System roughly 4.5 billion years ago. Unlike the meteorites scientists get to study from Earth, the rocks on Bennu are in pristine condition. They haven’t been warped by the intense heat of our planet’s atmosphere, and the precious porous qualities of space rubble may be ripe with clues about how water reached ancient Earth — and other worlds — when they were planetary fledglings.
The rocks’ puzzling behavior on camera is “telling us that there’s this chunk of the Solar System that we really haven’t had a chance to play with yet,” Kevin Walsh, an astronomer at the Southwest Research Institute in Boulder, Colorado and principal investigator of one of the new studies, tells Inverse. Samples from Bennu will help astronomers describe the scenes of our cosmic neighborhood’s earliest chapters more accurately.
But then there’s a more ominous implication.
As OSIRIS-REx backs away from Bennu’s surface, the spacecraft glimpses debris lofted by the sampling event. The video is stitched images taken by OSIRIS-REx’s NavCam 2 camera.
What they found — “On Bennu, we are dealing with microgravity — literally five divided by 1 million times the gravity on Earth,” Dante Lauretta, principal investigator of the OSIRIS-REx mission and lead author of the second study, tells Inverse.
As Lauretta describes it, a gardener doesn’t think twice about dirt’s cohesion because Earth’s gravity is acting on it to hold the material together when their shovel is slicing into the ground and pulling up dirt.
Lauretta laughs when describing how wrong their initial predictions were of Bennu’s gravity.
“We thought cohesion,” the force holding the boulders together, “literally was going to be a million times stronger than what we measured during the sampling event,” Lauretta says. They thought that Bennu’s small grains would be the glue that holds the bigger stuff on the surface.
Asteroid Bennu.
But Walsh and Lauretta found very little fine grain. It’s not being produced — as Walsh describes, it's percolating down towards Bennu’s center like a cosmic game of Plinko.
So, the rocks are held together by incredibly small forces “about what the astronauts on the International Space Station experience,” Lauretta says. That’s why contact produced a massive crater. “We blew a 20-foot wide hole in the surface of the asteroid! The only way we could make such a huge crater,” Lauretta says, “is if there was no cohesion between the particles.”
What’s next — “This is important if you want to stop Bennu from hitting the Earth in the future,” Lauretta says.
“Let’s say you try to fire a nuclear weapon at it. [Bennu] is like a droplet of liquid; it’s just going to fly apart and then come back together probably,” he adds.
Bennu is unlikely to strike Earth — at least in the near future — but its rubble pile nature would make other future space endeavors like asteroid mining difficult. Lauretta says that most mining equipment would sink if it were designed to sit on Bennu’s surface.
For now we wait. OSIRIS-REx will return to Earth next year to deliver its pristine samples.
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