NASA's Osiris-Rex spacecraft has beenfor several weeks now, and its latest batch of data shows a tiny world that's not quite what scientists expected.
Analysis of the surface shows Bennu is between 100 million and a billion years old, making it a significantly more senior celestial body than predicted. It may have formed out of the debris from some ancient cosmic collision involving a larger object in the main asteroid belt eons ago.
But the biggest and most confounding discovery from the first-ever close-up views of Bennu is that it's kind of a mess. Based on earlier observations from telescopes, scientists had expected the asteroid to have some relatively wide, smooth spots ideal for swiping a sample to bring back to Earth. Instead, its surface looks to be strewn with all kinds of large boulders, including at least one the size of a supermarket at 58 meters (190 feet) in diameter that could be an asteroid in its own right.
"Bennu does not contain the extensive patches of fine-grained regolith according to which we designed the mission," the Osiris-Rex team said in a research letter published in the journal Nature on Tuesday.
Measurements from the Osiris-Rex cameras confirm previous estimations that Bennu is one of the darkest known objects in the solar system, which is another factor that's not exactly helpful for navigation. As if that weren't enough to worry about, there seem to be some floating particles around the asteroid to watch out for.
The particles seem to be ejected from the surface of the asteroid itself, something that surprised scientists.
"The discovery of plumes is one of the biggest surprises of my scientific career," said Dante Lauretta, Osiris-Rex principal investigator, in a statement.
During a call with media Tuesday, Osiris-Rex project manager Rich Burns said that analysis performed after the particles were first spotted helped determine that they didn't pose an immediate threat to the spacecraft.
The team will still move forward and select two possible sample sites in areas that are relatively free of any hazards to the spacecraft, though it doesn't look like the sites will be the nice, open parking lot-size spaces it'd originally planned for.
"That task looks more challenging than we expected," the team said in the letter. "Regardless of the final site selected, the requirements for guidance, navigation and control accuracy need to be tightened."
Ingredients for life?
Overall, the emerging picture of Bennu is of a large, black asteroid littered with boulders of all sizes, pocked with craters and sprinkled with a layer of dust and other small particles.
But analyzing what lies within the small world reveals the presence of some exciting things that scientists were hoping to find.
Another new paper examines the mineral composition of Bennu and finds a number of elements that make up the building blocks of life, including molecules of water and its components. A popular theory holds that water and other key molecules that make Earth habitable may have been originally delivered to the surface of our planet by primordial asteroid and comet impacts.
"Osiris-Rex spectroscopic observations ... show that the pristine sample that will be returned from Bennu has the potential to inform our understanding of water in the early Solar System and its origins on Earth," said Victoria Hamilton and other co-authors from the Osiris-Rex science team in a paper published Tuesday in Nature Astronomy.
Rather than being one big hunk of solid monolithic rock, Bennu's interior also appears to be made up of a literal pile of rubble that clumped together to form a larger object.
A different study takes a look at Bennu's shape, which is often described as similar to a spinning top. It finds the asteroid has a volume that is about one-sixth that of the asteroid Ryugu, currently being visited by.
Bennu and Ryugu are both classified as Apollo-type asteroids, which are a type of near-Earth asteroid that hang out between the sun and the orbit of Mars. Both space rocks have elliptical orbits that cross the orbital paths of both Earth and Mars.
In a separate series of papers published Tuesday in the journal Science, the Hayabusa2 team confirms that Ryugu also has a spinning-top shape formed from similarly dark cosmic rubble.
"Thanks to the parallel missions of Hayabusa2 and Osiris-Rex, we can finally address the question of how these two asteroids came to be," said professor Seiji Sugita of the University of Tokyo, a co-author of one of the papers, in a release. "That Bennu and Ryugu may be siblings yet exhibit some strikingly different traits implies there must be many exciting and mysterious astronomical processes we have yet to explore."
The most striking difference so far between the two asteroids is that Ryugu is far drier. While this may seem to be a disappointing finding at first, Sugita says the discoveries from Ryugu and Bennu, including the subtle contrasts between the two, could actually have implications for finding life beyond Earth.
"There are uncountably many solar systems out there and the search for life beyond ours needs direction," Sugita said. "Our findings can refine models that could help limit which kinds of solar systems the search for life should target."
But for now, both missions are focused on bringing samples home. Hayabusa2 has already collected one sample and has plans to gather another, while Osiris-Rex prepares for its own sample swipe in 2020.
Update, 11:20 a.m. PT: Added new images and Osiris-Rex team comments from a NASA press call.