Competing Asteroid Announcements Reveal Big News About Bennu and Ryugu

bennu asteroid

Competing announcements about the newly famous asteroids Bennu and Ryugu on Tuesday sparked a friendly rivalry for the attention of space lovers. In nearly back-to-back press conferences, scientists involved with missions to sample the asteroids revealed unexpected news about both projects.

These two asteroids have enjoyed much of the space spotlight in the past year. Bennu is a carbon-filled, water-kissed asteroid a little taller than the Empire State Building. It is the domain of NASA, whose craft OSIRIS-rEX reached the asteroid in December 2018 to collect samples of its pristine regolith. The 0.6-mile-wide Ryugu, meanwhile, was shot by a bullet fired by the Japanese spacecraft Hayabusa2 in February, part of JAXA’s mission to bring back samples as well.

Asteroids were formed alongside the planets in the early days of the solar system, around 4.5 billion years ago. That’s why everyone is so eager to bring home samples: Their chemical makeup can offer a glimpse of the chemicals that existed in those early days, perhaps even pointing to the origin of life. Tuesday’s back-to-back announcements about unexpected discoveries on both asteroids, however, provide a reminder of how little we know about these nearby space objects.

JAXA scientists announced that Ryugu is shaped like a spinning top made of rubble.
JAXA scientists announced that Ryugu is shaped like a spinning top made of rubble.

Ryugu

Prior to Tuesday, observers on Earth may have heard about the Hayabusa2 spacecraft shooting Ryugu with a bullet to collect rock fragments that will eventually return to Earth. During Tuesday’s conference, scientists from the Japan Aerospace Exploration Agency revealed the first big set of scientific findings from the mission since the bullet was shot. They announced that Ryugu is a low-density rubble pile shaped like a spinning top — which basically means it has a pronounced ridge around its equator. It’s composed of similarly-sized boulders, and has an extremely dark surface.

While there is ample evidence of hydroxyls on its surface, Ryugu appears to have had a lot less water contact than Bennu. This isn’t exactly what scientists expected to find, and Yuichi Tsuda, Ph.D., one of the JAXA scientists behind the mission, told reporters that what they did find on Ryugu’s surface posed logistical challenges to the Hayabusa2 mission — especially when it came to finding places to touch down and collect samples.

The yellow arrows in these images indicate large, flat boulders that litter Ryugu's surface.
The yellow arrows in these images indicate large, flat boulders that litter Ryugu's surface.

“The biggest challenge we have faced in terms of operational planning and engineering-wise is that Ryugu is covered by numerous boulders all over the globe, which is unsafe for the spacecraft landing,” he said. Because of that, the originally planned October 2018 landing was postponed. Scientists re-evaluated the asteroid in light of their newer observations, and Hayabusa2 made contact with Ryugu on February 22 — its first of several sampling excursions that involve firing a projectile into the asteroid to kick up material.

While the composition of rocks on the surface suggest that Ryugu was produced by larger parent bodies that heated up the asteroid and dehydrated it, Hayabusa2 will collect more samples to help fill out these details. Later this year, the spacecraft will create an artificial crater with a baseball-sized impactor that weighs 4.4 pounds — this will expose material from below the surface, which Haybusa2 will collect.

Haybusa2 will eventually depart Ryugu at the end of 2019, and we will have to wait until the samples return to Earth in late 2020 to confirm these initial measurements.

bennu space rock
Bennu the asteroid has been observed unexpectedly shooting out plumes of rocks into outer space.

Bennu

Scientists studying Bennu already knew it was packed with organic compounds, amino acids, and volatile chemicals like nitrogen, water, carbon dioxide, ammonia, hydrogen, methane, and sulfur dioxide — all of which are necessary ingredients for life to begin. Evidence of hydroxyls on its surface also suggested Bennu had once been in close contact with water — perhaps before it had broken off from a water-pooling parent body.

But in an announcement that OSIRIS-REx principal investigator Dante Lauretta, Ph.D. called “one of the biggest surprises of my scientific career,” it was revealed that Bennu isn’t just a pile of rubble held together by gravity, like Ryugu. Bennu is an “active asteroid,” which means it regularly spits out plumes of particles into outer space.

“Bennu has a continuous population of particles raining down on it,” said Lauretta in the teleconference on Tuesday.

The team is still trying to “make sense of what’s going on with this asteroid,” said Lauretta, explaining that they still don’t know the mechanism behind the plumes. What they do know is that some of the particles, which range in diameter from a few centimeters to tens of centimeters, get stuck in orbit around Bennu before raining back down onto its surface. Fortunately, a rapidly executed risk assessment showed that OSIRIS-REx is not in danger of getting hit by particles that Bennu may spit out.

bennu boulders
A view of Bennu's southern hemisphere, snapped on March 7. It's unexpectedly covered in huge boulders, like the 24-foot stone just left of center.

In addition to the plumes, the team also discovered that Bennu’s surface is unexpectedly covered in large boulders, which will make it slightly trickier for OSIRIS-REx to land and collect samples — a similar situation as the one Hayabusa2 encountered. They also noticed that Bennu’s rotation rate is increasing. They chalk up the acceleration to the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect, which posits that uneven heating and cooling of the rotating asteroid cause it to spin faster.

Lauretta, thrilled by all the unexpected discoveries made on Bennu, called this an “incredibly exciting time.”

What’s Next for Ryugu and Bennu?

Ryugu and Bennu are both part of the same near-Earth asteroid group known as the Apollos, comprising 10,485 space rocks that orbit the sun and occasionally cross paths with Earth and Venus.

When OSIRIS-rEX first set out for Bennu, many people were concerned that it was on a trajectory to hit Earth. That’s not completely false: Bennu is considered a near-Earth object that has a 1-in-2,700 chance of hitting our planet when it swings our way in the 22nd century.

Scary as that might seem, collisions between asteroids and planets may have been necessary for the elements of life to originally take root on our planet. “The reason that’s important is because we always talk about if meteorites, asteroids, comets could have delivered material to the earth, and so if we find that material is pretty widespread throughout the early solar system then yeah, they probably did deliver it to the early planet,” said Amy Simon, Ph.D., NASA’s deputy instrument scientist on the OSIRIS-REx project, in a previous interview with Inverse.

Until the samples come back, we’ll have to rely on data beamed back to Earth from both Hayabusa2 and OSIRIS-REx to learn what they can tell us about the early days of the solar system. It’ll happen sooner than you might think: We’ll have to wait until 2023 to know what lives in Bennu’s regolith, but Hayabusa2 is scheduled to come home by December 2020.

This story includes additional reporting by Peter Hess.