The past 4.5 billion years has been an incredibly lonely time for asteroid 101955 Bennu. A gigantic impact in the early days of the solar system shattered an ancient cosmic rock, ejecting dust and debris into the void. Gravity forced the pile of debris to clot together and, since then, it has wandered on its own like Bennu, the spinning top-shaped space rock. For billions of years, it has drifted around the sun between Earth and Mars, intact and unaccompanied.
Until NASA’s Osiris-rex spacecraft accepted it into orbit on December 3, 2018.
After a 27-month journey from Earth, NASA’s asteroid-chasing spacecraft approached Bennu for a closer look. Bennu finally had company. The spacecraft is part of an ambitious plan to bring Bennu pieces back to Earth, the first time a NASA mission has attempted such a feat.
Ever since he arrived at the asteroid, Osiris-rex has been busy taking measurements and increasing Bennu̵
On Thursday, a series of new studies, published in the journals Science and Science Advances, shed light on these questions, revealing more about Bennu’s rocky surface. Additionally, Osiris-rex allowed a detailed examination of the “Nightingale” crater, target of Osiris-rex’s daring heist set for October 20.
“As a set, these documents help us fill in more about Bennu’s story 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 seem boring, a dull gray rock that spins into infinity. But it’s actually a message in a bottle. Adrift on the cosmic seas for eons, it holds secrets and clues to the formation and evolution of the solar system enclosed in its rocky exterior.
Bennu is, unflatteringly, described as a “pile of rubble”. It is as wide as the Empire State Building. From a distance it looked smooth, but as Osiris-rex came closer, the truth became clearer. “When we got there, we found that the surface was covered with boulders,” says Kaplan.
Officially called “Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer”, Osiris-rex has been circling Bennu, using the asteroid’s weak gravity to bypass it, for nearly two years. During that time, a series of instruments capable of seeing in visible light, infrared and X-rays were aimed at its surface. Taken together, they allow scientists to obtain a clear view of the asteroid and determine the types of elements and minerals incorporated. in the rocks on the surface.
In a study, published in Science Advances, the types of boulders on Bennu’s surface are clarified more clearly. Using the thermal and infrared imagers on Osiris-rex, the scientists determined that Bennu is likely composed of two types of boulders with similar mineral content but different strength properties. Researchers suggest that the “fainter” boulders on Bennu would likely burn in the atmosphere, which is why we haven’t found meteorites with the exact same properties.
Comparing some of the properties to meteorites – pieces of space rock found on Earth – helps reveal details about Bennu that are impossible only in orbit, says Kaplan. “If we can link Bennu to a specific meteorite or series of meteorites, we will be able to unlock a lot of new information,” he notes. If the faintest boulders are collected during Osiris-rex sample collection, we may have access to material not currently in Earth’s meteorite collections.
Red against blue
One of the most interesting findings is the detection of carbon material on the Bennu surface and around the Nightingale crater. Based on observations from Earth, Bennu was expected to have a high carbon content, but it is Osiris-rex’s observations that confirmed the previous hypotheses.
In two studies, both published in Science, the researchers were able to detect a widespread carbon-containing material on Bennu’s surface and a series of luminous veins with the telltale signs of the carbonate minerals. Veins tell us about water in the very early solar system, according to Kaplan.
“The water probably flowed through these veins, depositing the carbonates,” he says. Since 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 suggests that the fluid system was large, perhaps kilometers in size.”
Another study also sheds light on this parent asteroid by examining the variations in color and reflectance of Bennu’s surface. The images obtained by Osiris-rex can be processed to highlight Bennu’s surface in red or blue, which the researchers use to determine the amount of reflected sunlight.
Entirely exposed to space without atmospheric protection, Bennu is hammered by micrometeorites and the solar wind. Over time, this time the surface. But Bennu’s weathering suggests that something on the asteroid is different from what we see on other cosmic bodies.
“On the surface of the moon and in many asteroids, we have observed that space weather conditions darken and redden surfaces,” explains Daniella DellaGiustina, scientist responsible for processing the images 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 weather.”
The researchers found that Bennu’s surface is “highly different” in these wavelengths, suggesting a chaotic collision between his parent body and another object. That collision likely spewed material from the depths of the parent body where various geological processes were taking place, throwing them into the void. Eventually, gravity brought them into the pattern we see today on Bennu’s surface and is the reason there is no discernible pattern in the composition of the carbon on its surface.
This bodes well for Osiris-rex’s next milestone.
Across the Nightingale floor
The biggest challenge for Osiris-rex is yet to come: he must pickpocket Bennu, using commands sent by humans on Earth to the spacecraft, over 200 million miles away. On October 20, Osiris-rex will begin his descent, approaching Nightingale Crater for his shot.
“We spent most of the mission looking for a safe place for the spacecraft to land,” notes Kaplan. Team engineers have identified Nightingale as one of the few sites where boulders aren’t as abundant and there’s plenty of fine material to collect. The results of the Science and Science Advances studies provide guidance for what researchers should expect.
Its Touch-and-Go (also known as TAGSAM) sample acquisition mechanism, a robotic arm with a giant Roomba-sized head attached to one end, will briefly make contact with the surface. It will release a rapid burst of nitrogen gas, kicking up dust and debris, which it will capture and store in a capsule. NASA hopes to recover about 60 grams of Bennu by storing them in a capsule that Osiris-rex will send home.
In 2023, the capsule is expected to return to Earth, where scientists will be able to examine pristine material stolen from space rock.
The site bodes well for the research team as it will help answer other questions about the asteroid’s composition that cannot be answered during Osiris-rex’s orbit. DellaGiustina says the 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 the results with another sample return mission to a similar asteroid, known as Ryugu. The Japanese Space Agency will return a sample from Ryugu to Earth on December 6.
As researchers study the returned material closely, in the lab, we will begin to learn a little more about our place in the cosmos and how different the solar system was 4.5 billion years ago. The “pile of rubble” transformed into a message in a bottle will have been smashed, its secrets revealed. Usually, we only seem to fixate on asteroids when we have 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 what it is today. It’s not just dull gray rocks.
“They have complex surfaces that have been imprinted by physical processes that took place in the early solar system,” DellaGiustina says. “The more we can learn about them, the easier it is to understand our history among the stars and planets.”