The past 4.5 billion years have been an incredibly lonely period for the asteroid 101955 Bennu. A gigantic impact in the early days of the solar system smashed an ancient cosmic rock to pieces, ejecting dust and debris into the void. Gravity forced the rubble pile to clot together and, ever since, it’s been wandering alone as Bennu, the space rock shaped like a spinning top. For billions of years, it’s drifted around the sun between Earth and Mars, untouched and unaccompanied.
Until NASA’s Osiris-rex spacecraft greeted it in orbit on Dec. 3, 2018.
After a 27-month journey from Earth, NASA’s asteroid-chasing spacecraft sidled up to Bennu for a closer look. Bennu finally had company. The spacecraft is part of an ambitious plan to return pieces of Bennu to Earth, the first time a NASA mission has attempted such a feat.
Since arriving at the asteroid, Osiris-rex has been busy taking measurements and sizing Bennu up. It performed close flybys to get a high-resolution look at the surface and caught the asteroid unexpectedly spewing debris into space in late 2019. Its five instruments have been gathering data, mapping Bennu’s surface and slowly piecing together the asteroid’s story. Where did it come from? What is it made of? Will it collide with the Earth? (That last one isn’t likely, but Bennu is expected to pass close-by next century.)
On Thursday, a suite of new studies, published in the journals Science and Science Advances, shed light on these questions, revealing more about Bennu’s boulder-riddled surface. In addition, Osiris-rex has allowed for a detailed examination of “Nightingale” crater, the target of Osiris-rex’s daring heist set for Oct. 20.
“As a set, these papers help us to fill in more about Bennu’s history and allow us to anticipate what will be returned in the sample,” says Hannah Kaplan, a space scientist at NASA’s Goddard Space Flight Center.
And the collection of studies helps answer even bigger questions about the early solar system. Bennu may appear boring, a dull gray space rock spinning through infinity. But it’s actually a message in a bottle. Adrift on the cosmic seas for eons, it contains secrets and clues about the solar system’s formation and evolution locked within its rocky exterior.
Bennu is, unflatteringly, described as a “rubble pile.” It’s about as wide as the Empire State Building is tall. From a distance, it looked smooth — but as Osiris-rex approached, the truth became clearer. “When we got there, we found the surface was covered in boulders,” explains Kaplan.
Officially named “Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer,” Osiris-rex has been circling Bennu, using the asteroid’s feeble gravity to pass around it, for almost two years. In that time, it has pointed an array of instruments at its surface that can see in visible light, infrared and X-rays. In totality, they allow scientists to get a clear visual of the asteroid and determine the types of elements and minerals embedded within rocks on the surface.
In a study, published in Science Advances, the types of boulders on Bennu’s surface are elucidated more clearly. Using the thermal and infrared imagers on Osiris-rex, scientists determined Bennu is likely composed of two types of boulders with similar mineral content but different strength properties. The researchers suggest the “weaker” boulders on Bennu would likely burn up in the atmosphere and that’s why we haven’t found meteorites with the exact same properties.
Comparing some of the properties to meteorites — chunks of space rock found on Earth — helps reveal details about Bennu that are impossible just in orbit, Kaplan says. “If we can connect Bennu to a specific meteorite or set of meteorites, then we are able to unlock a lot of new information,” she notes. If the weaker boulders are scooped up during Osiris-rex’s sample collection, we may have access to material not currently in Earth’s meteorite collections.
Red vs. blue
One of the most exciting findings is the detection of carbon material on Bennu’s surface and around Nightingale crater. Based on observations from Earth, Bennu was expected to have high carbon content but it’s Osiris-rex’s observations that have confirmed prior assumptions.
In two studies, both published in Science, researchers were able to detect widespread carbon-bearing material across Bennu’s surface and a series of bright veins with the tell-tale signs of carbonate minerals. The veins tell us about water in the very early solar system, according to Kaplan.
“Water likely flowed through these veins, depositing the carbonates,” she says. Because Bennu is believed to be part of a larger asteroid that roamed the solar system in its early days, this helps paint a picture of the body and the water system it contained. “The size of the veins suggest that the fluid system was large, possibly kilometers in size.”
Another study also sheds light on this parent asteroid by examining the variations in color and reflectance of Bennu’s surface. Images obtained by Osiris-rex can be processed to highlight Bennu’s surface in red or blue, which researchers use to determine how much sunlight is being reflected.
Entirely exposed to space with no atmospheric protection, Bennu is pounded by micrometeorites and the solar wind. Over time, this weathers the surface. But Bennu’s weathering suggests something about the asteroid is different from what we see on other cosmic bodies.
“On the moon’s surface and many asteroids, we have observed that space weathering darkens and reddens surfaces,” explains Daniella DellaGiustina, image processing lead scientist on Osiris-rex. “On Bennu, however, the opposite is true — we see that over time Bennu has become brighter and bluer in response to space weathering.”
The researchers find the surface of Bennu is “highly diverse” in these wavelengths, which hints at a chaotic collision between its parent body and another object. That collision likely spewed up material from deep within the parent body where different geological processes were occurring, throwing them out into the void. Eventually, gravity pulled them into the configuration we see on Bennu’s surface today and is the reason there is no discernible pattern in the carbon composition on its surface.
That bodes well for Osiris-rex’s next milestone.
Across the Nightingale floor
The biggest challenge for Osiris-rex is yet to come: It must pickpocket Bennu, using commands sent from humans on Earth to the spacecraft, over 200 million miles away. On Oct. 20, Osiris-rex will begin its descent, pulling in close to Nightingale crater for its heist.
“We’ve spent much of the mission searching for a safe spot for the spacecraft to touch down,” notes Kaplan. The team’s engineers have identified Nightingale as one of the few sites where boulders are not as plentiful and there is an abundance of fine material to collect. Results from the Science and Science Advances studies provide a guide for what the researchers should expect.
Its Touch-And-Go Sample Acquisition Mechanism (aka TAGSAM), a robotic arm with a giant Roomba-size head attached to one end, will briefly contact the surface. It’ll release a quick burst of nitrogen gas, kicking up dust and debris, which it’ll capture and store in a capsule. NASA hopes to retrieve around 60 grams of Bennu, storing it in a capsule which Osiris-rex will post home.
In 2023, the capsule is expected to return to Earth, where scientists will be able to examine the pristine material stolen from the space rock.
The site bodes well for the research team because it’ll help answer more questions about the asteroid’s composition that can’t be answered during Osiris-rex’s orbit. DellaGiustina says researchers “will be able to test many of the hypotheses we have established using data from the Osiris-rex spacecraft.” The team will also be able to compare and contrast their findings with another sample-return mission to a similar asteroid, known as Ryugu. The Japanese Space Agency will be returning a sample from Ryugu to Earth on Dec. 6.
As researchers study the returned material up close, in the lab, we’ll start to learn a little more about our place in the cosmos and how different the solar system was 4.5 billion years ago. The “rubble pile” turned message in a bottle will have been smashed open, its secrets unveiled. Usually, we seem to fixate on asteroids only when we’ve been tricked into thinking they might collide with Earth by alarmist headlines. But Bennu — and Ryugu — are teaching us exactly how the solar system grew to be what it is today. They aren’t just dull, gray rocks.
“They have complex surfaces that have been imprinted by physical processes that took place in the early solar system,” says DellaGiustina. “The more we can learn about them, the easier it is to understand our own history among the stars and planets.”