One asteroid is dry as a bone. The other regularly spits out vapors like a comet. And yet they may be pieces of the same failed planetesimal that fell apart millions of years ago, sending the two space rocks on very different journeys.
Another thing unites this cosmic pair: a visit from humanity. Spacecraft from Japan's JAXA space agency and from NASA have reached the two asteroids and plan to bring back pieces of them to the Earth. In the meantime, both Ryugu and Bennu leave scientists with plenty of questions—and fewer answers.
Ryugu
First, there's the asteroid Ryugu. The Hayabusa2 craft has already touched down on the 0.6-mile world, but not before shooting the dang thing.
JAXA, the Japanese space agency that launched the craft, found out a few preliminary things about Ryugu. It's not particularly solid, resembling a "rubble pile" of accumulated rocks and dust. And it seems to have very little water, especially for a near-Earth object.
Sei-ichiro Watanbe, project scientist for Hayabusa2, says the water may have disappeared for one of two reasons. "The parent body is a weakly heated icy asteroid," he says, meaning there could be ice below the surface, "or a dehydrated asteroid by internal heating, impact, or solar heating." If there is water down below the surface, we'll find out next month when JAXA bombs the asteroid.
Ryugu is shaped a bit like a spinning top—and may have once spun much, much faster. The rocks on the surface—and likely below it—seem to be uniform in size. It's dark and carbon-rich.
Bennu
Then there's Bennu, where the OSIRIS-REx craft arrived in December. Bennu is about half the size of Ryugu and has a similar top-like shape. It also seems to be a rubble pile, more or less a collection of loose natural space junk. It's from the same group of asteroids as Ryugu—a bunch of carbon-rich, near-Earth asteroids called the Apollo group.
"The equatorial cross-section of Bennu is less circular (rather rectangular) than that of Ryugu, suggesting some difference in the rotational deformation," Watanbe says of the surface level differences between the two.
But Bennu is weird. Despite a similar shape and underlying composition to Ryugu, it's rich with water. And rather than slowing down over time, it seems to perpetually be speeding up—so much so that large pieces might eventually break off. "In about a million and a half years, we predict Bennu will be spinning at twice its current rate," Dante Lauretta, OSIRIS-REx principal investigator, said in a press conference Tuesday. Pressure from the Sun is a big culprit here.
Bennu belongs to a rare class called active asteroids, which are objects that look like an asteroid, but act a bit more like a comet. It releases gases. At times some active asteroids have a tail, though this activity is unconfirmed with Bennu. And pieces of Bennu get blown off the surface through this activity, sometimes falling back down, other times entering orbit around it, creating a series of microsatellites.
"It looks like Bennu has a continuous collection of particles raining down on the surface," Lauretta said.
All this is to say: whereas Ryugu is bone dry, Bennu is rife with water. And unlike Ryugu, it doesn't seem to have relative uniformity in rock size—perhaps owing to its active geology. This also means that OSIRIS-REx will have a tougher time setting down on Bennu than Hayabusa2 had on Ryugu.
"The asteroid is so rugged that there’s unlikely to be any 25 feet radius hazard free zone," Rich Burns, project manager of OSIRIS-REx, said in the press conference.
Weird Cousins
Despite these differences, though, it still seems like both asteroids came from the same body, an ancient theoretical planetesimal that created the Polana-Eulalia family of asteroids, introduced into scientific literature in 2013. They're named for two larger, dark main belt asteroids sharing similar characteristics that likely originated from one body. How fragments of those bodies ended up in near-Earth objects is likely to cause a lot of speculation in coming years.
Pieces of both asteroids will return to Earth for analysis, which could confirm whether or not they came from the same body and, if they did, why there are strange differences, whether in topography or composition. Then, we might unlock the secrets of both. For now, we're left with two strange pieces of what may or may not be the same puzzle—so similar, and yet so, so different.
