Tiny robot finds an asteroid that’s freakishly free of dust

Ryugu's rubble-pile surface, taken by MASCOT shortly before it hit and started bouncing.
Enlarge / Ryugu's rubble-pile surface, taken by MASCOT shortly before it hit and started bouncing.

For the last few months, Japan's Hayabusa2 spacecraft has been engaged in various acts of interplanetary aggression, shooting the asteroid Ryugu in order to blast free material for a return to Earth. But Hayabusa2's visit has also featured various less violent activities, as its imaging and characterization of Ryugu has given us a new picture of the body, which is thought to act as a time capsule for material that formed at the earliest stages of our Solar System.

As part of these studies, Hayabusa2 dropped off a French-German robot that was meant to hop across the asteroid's surface in order to sample some of its rocks. Despite landing upside-down, the robot eventually hopped into the right orientation, and a paper describing what it found was published in Thursday's edition of Science.

Hopping, but not like a bunny

If you're like me, then the image of a small robot hopping across the surface of an asteroid brought something adorable and possibly anthropomorphic to mind. You may get rid of those images immediately. MASCOT, the Mobile Asteroid Surface Scout, is a rectangular box. Its ability to hop is provided by an internal weighted device. By rapidly rotating this weight, the robot could generate enough velocity to overcome Ryugu's tiny gravitational field and launch the box to new locations.

This hopping capability turned out to be critical to the mission. After Hayabusa2 dropped MASCOT off 40 meters above the surface of the asteroid, the robot bounced for 17 meters before settling down in an awkward position. An attempt to orient the probe didn't exactly go well: "An up-raising maneuver at this first measurement position left the lander upside down, with most instruments aimed toward the sky." Imaging from that position managed to confirm that Jupiter, Saturn, and the star σ Sagittarii do exist.

So, the appropriate commands were sent, and the box did another hop. This time, it landed in the right orientation to do some observing. In addition to its weighted arm for hopping, MASCOT had a camera, a magnetometer, a radiometer, and infrared spectrometer. These gave it the ability to get some sense of the materials around it and associate their properties with specific rocks.

A tale of two rocks

The area around MASCOT had two different rock types. The first was darker and rough, with a surface appearance the researchers describe as "cauliflower-like." The second was brighter and had smoother surfaces and more angular shapes. These two categories of materials are consistent with the idea that Ryugu's "rubble pile" appearance is the product of a collision between two bodies with different properties, with the debris slowly re-aggregating under the body's weak gravity. (It's also consistent with an alternative view that Ryugu is made from a small fraction of the remains of a single body that had formed different geological layers.)

The Hyabusa2 team suspects that the features of the two different materials are the product of the heating/cooling cycles that take place as the rocks are exposed first to sunlight then to the darkness of space. In some cases, this leads to the shattering of the rock, producing the smoother surfaces seen among some of the materials. In others, the rock slowly disintegrates, losing its internal structure and producing the other rocks observed by MASCOT.

The weird thing about that, however, is that crumbling and shattering rocks typically produce dust and sand-like materials. Yet there were no signs of any of this. "MASCam observed no deposits of fine material during the descent," the researchers note, before going on to say that "We expect dust to be formed continuously on the surface of Ryugu through exposure to the space environment." They assume the smaller grains are either lost to space or end up working their way into the interior of the rubble. But the rocks that could be seen by MASCOT were all tens of centimeters across or larger (some were tens of meters).

Ready for my closeup

In any case, closeups of several of the rocks viewed by Ryugu showed that they were composites of many small inclusions embedded into larger matrixes of material. This is exactly the sort of things scientists were hoping to see. That's because the first rocks formed as our Solar System started to condense were thought to be composites, with the inclusions representing the materials with high melting points that condensed first. If similar materials end up in the cache returned to Earth by Hayabusa2, then they can help inform our ideas about Solar System formation.

In any case, MASCOT's work on the asteroid ended in less than a day, as it only had the battery capacity for about 17 hours of operation. But there will undoubtedly be additional studies that come out of all the data it grabbed during the short time it spent operational.

Science, 2019. DOI: 10.1126/science.aaw8627  (About DOIs).