Japanese probe's asteroid Ryugu encounter hints at space rock's dynamic history

A Japanese spacecraft trekking across the solar system has yet to deliver its precious cargo of space rocks back to Earth, but its data is already giving scientists a preview of a near-Earth asteroid's dynamic history.

The spacecraft is Hayabusa2, which arrived at an asteroid called Ryugu in the summer of 2018 and spent 16 months orbiting the asteroid. During its extended visit, Hayabusa2 dropped three rovers onto the asteroid and scooped some samples from the space rock's surface. 

The probe is already on its way back to Earth to deliver the samples to eager scientists affiliated with the Japan Aerospace Exploration Agency (JAXA) mission. But while the shipment won't arrive until December, scientists are getting a head start by studying data gathered during Hayabusa2's operations at the asteroid's surface, and they have realized that Ryugu has had quite a dynamic history.

Related: Japan's Hayabusa2 asteroid Ryugu sample-return mission in pictures

Right now, Ryugu is orbiting the sun at a distance somewhere between the orbits of Earth and Mars. But the new research suggests that at some point in the past, the asteroid darted awfully close to the sun and dramatically warmed, permanently changing Ryugu's appearance.

The research began with the February 2019 sampling maneuver, which Hayabusa2 carefully filmed to tell personnel at JAXA how the operation went. That footage was very detailed, and once it arrived, scientists eagerly watched it — and soon they spotted something strange.

"The thruster jet lifted up rocks and particles of Ryugu's surface, and the surface was greatly altered," Tomokatsu Morota, a planetary scientist at the University of Tokyo and lead author on the new research, told Space.com in an email. "It is interesting that the asteroid's surface, which was not expected to change significantly on geological time scales, can easily be changed by a spacecraft landing."

Some of the disturbed material consisted of large rocks, but most of it was tiny dust particles, which spread out up to 16 feet (5 meters) away from the sampling site. And as the scientists looked even more closely at the footage, they realized something else about the surface disturbance: the particles that were affected by the landing were also quite dark.

In fact, the particles seemed to match one of the two types of materials scientists had seen from aerial surveys of Ryugu, a material that looks a bit reddish to scientific instruments. Curiously, that material seemed to form stripes against bluer material found at the asteroid's poles and midline.

Hayabusa2's scientists were careful to direct the spacecraft to collect rock from a location where it should have captured samples of both materials, so once the souvenirs arrive the team should be able to learn much more about them.

Hayabusa2 casts a shadow on the surface of the asteroid Ryugu during a landing maneuver.  (Image credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST)

But in the meantime, the team has come up with a theory about Ryugu's past, based on combining the strange movement of rocks and dust with the apparent stripiness of the asteroid's surface. Most of Ryugu, the scientists believe, is made of a rubble-like aggregate of the bluer material. But at some point in Ryugu's past, the asteroid darted too close to the sun and the abrupt warming converted the outer layer of rock to the redder material.

Ryugu retreated to its current, more distant orbit, and over time, a barrage of other objects have collided with the asteroid, breaking some of its rocks into progressively smaller pieces. At the same time, a phenomenon called mass wasting pulled some material from the midline of Ryugu out toward its poles.

The scientists believe that this story would match both the stripy appearance of bands still covered by redder material and bands stripped to the bluer interior, and the tendency for finer material on Ryugu to be redder.

Now, it's a waiting game as scientists sit tight until Ryugu samples arrive at the end of the year for them to analyze up close. They hope the results will help them understand the messy early days of the solar system.

"We're interested to see how such molecules change chemically by solar heating," Morota said. "This is important for understanding the chemical evolutions of organic molecules, which could have been transported to the early Earth."

The research is described in a paper published today (May 7) in the journal Science.

Email Meghan Bartels at mbartels@space.com or follow her @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.

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Meghan Bartels
Senior Writer

Meghan is a senior writer at Space.com and has more than five years' experience as a science journalist based in New York City. She joined Space.com in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.