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The Dark Asteroid Ryugu Finally Comes Into the Light

In 2018, the Japanese space agency’s Hayabusa2 probe visited the near-Earth asteroid Ryugu, which occasionally traverses our world’s orbit (but has yet to come dangerously close). It extracted a tiny fragment of that hurtling space rock and in December became the first spacecraft to deliver a piece of an asteroid to Earth , ahead of a NASA mission that will return a sample from a different asteroid in 2023.While the initial analysis from that precious sample likely won’t be available until early next year, scientists are now releasing findings from Hayabusa2’s onboard cameras and instruments. This new research, led by Deborah Domingue at the Planetary Science Institute in Tucson, Arizona, and Yasuhiro Yokota at Kochi University in Japan, sheds light on Ryugu’s complex structure, revealing it to be a dark, weathered pile of rubble tumbling in space, different from anything seen on the surface of the Earth. “I’m very hopeful our results will be helpful for the sample team,” Yokota says. Ryugu’s known as a C-type, or carbonaceous, asteroid, meaning its rocks and pebbles are packed with carbon molecules, contributing to its sooty coloration. It’s only a kilometer in size, less than half the width of Manhattan, and it travels in a nearly circular orbit around the sun, closer to Earth than either the asteroid belt or Mars. Scientists want to study it because its composition might tell us compelling things about the building blocks that formed the rocky inner planets in the early days of the solar system.When Hayabusa2 first arrived at the asteroid, scientists wanted to use its tools and mini rovers to collect samples, but they were surprised to see that one couldn’t simply scoop up some sand or dust, like you would on a beach. (Or on the moon or Mars .) Despite expectations based on telescope observations from afar and from a bread-loaf-sized rover called Mascot, it looked like Ryugu was somehow made of rocks of various sizes mashed together—but no dust.That made some scientists wonder if Ryugu simply didn’t have any. Since Ryugu is so small, its gravitational pull is far less than that of the moon. On the moon, jumping astronauts don’t launch themselves into space, but on Ryugu, “if you even took a step, you’d fly off the surface,” says Erica Jawin, a planetary geologist at the Smithsonian National Museum of Natural History in Washington, DC. “The asteroid has a micro-gravity on the surface which might not be large enough to hold fine-grain material.”In findings that will be published in the October issue of Planetary Science Journal, Domingue and Yokota showed that the dust isn’t missing, but it is elusive, coating surfaces instead of turning up in piles. They took images with Hayabusa2’s Optical Navigation Camera (ONC) and used its near-infrared spectrometer (NIRS3) to measure spectra, maps of light at a range of wavelengths. Their spectral analysis, tuned toward picking up the presence of tiny things, showed that at least some dust is indeed present. “Where did the dust go? Our study shows that it’s there. It’s ubiquitous,” says Domingue.But instead of being in a soft, sandy pile, that dust could be mixed in with coarser-grained sand or coating the bigger rocks and in their nooks and crannies. The rocks and boulders of Ryugu aren’t solid and hefty like those of Earth, says Michele Bannister, a planetary astronomer at University of Canterbury in New Zealand. They’re so rough, porous, and lightly held together that they could easily break up, producing the kinds of sand and dust Domingue and Yokota see. Tiny meteorites and cosmic radiation pockmarking the surface could also help to erode the rocks into smaller bits. But the mystery probably won’t be solved until researchers finish studying the contents of the sample capsule. After they retrieved it from the South Australian outback last December, scientists did see some dark grains inside the container. They hope Hayabusa2 successfully collected at least 0.1 gram of material from Ryugu, and perhaps much more, in that treasure box from space.