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 the Japan Aerospace Exploration Agency, 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.
Hayabusa2 also provided the researchers with unique opportunities to observe the asteroid at multiple angles, including hard-to-get images taken at “opposition.” This involved maneuvering the fridge-sized spacecraft to catch snapshots while the asteroid and sun were on opposite sides of it, an alignment that provides views of the asteroid with the sun’s rays reflected directly back toward the camera, without producing any shadows.
Thanks to the physics of optics, anything with a rough surface that reflects light will seem slightly brighter when it’s in opposition. This means that small, faint, and distant asteroids can really only be seen at opposition. In fact, they’re so dark that from Earth we can’t see a “crescent phase,” like the moon has. Domingue and Yokota find that Ryugu is one of the darkest objects ever seen: Reflecting only about 3.5 percent of sunlight, it’s darker than other kinds of asteroids and darker even than a lump of coal.
But taking photos up close and at opposition allowed the researchers to get a detailed image of the asteroid’s surface; it enhanced the way the asteroid’s dust interacts with light, making it clearer that it is in fact there. Bannister says opposition images are like looking at a grassy lawn when the sun is directly behind you, allowing you to see individual blades, as opposed to when sunlight falls obliquely on the lawn, which produces lots of shadows. Comparing opposition images to those taken at near-opposition “tells you how bristly your lawn is, but from a distance, it can all appear completely smooth,” she says.
The mostly shadow-free photos also enabled the researchers to map Ryugu’s surface structure, at least on one side.
This exploration of Ryugu is part of a broader effort to investigate many types of asteroids to learn more about their shapes, contents, and origins. Ryugu is similar to another near-Earth asteroid, called Bennu, that was recently visited by NASA’s OSIRIS-REx spacecraft. They’re both C-type asteroids that are shaped like tops, though with differently accentuated central ridges. The first Hayabusa mission rendezvoused with a more stony, S-type asteroid. NASA’s planned Psyche mission will next year voyage toward an M-type asteroid full of iron and other metals, and the agency’s Lucy craft, which launches this October, will head towards the D-type Trojan asteroids to study the building blocks that formed the Jovian worlds.
The residents of the main asteroid belt, a scattered conglomeration of space rocks Jupiter never allowed to become a planet, have had stable orbits over billions of years, says Andy Rivkin, a planetary astronomer at Johns Hopkins University in Baltimore. In contrast, near-Earth asteroids have wonkier orbits. “Something like Bennu and Ryugu eventually hits a planet or the sun over millions of years, so they can’t have been there very long,” he says.
Ryugu likely formed when something collided with a much larger asteroid, breaking off a bunch of rocky debris that later glommed together and headed on a different trajectory. Meteorites, or chunks of asteroids and comets that hit the Earth, can have similar origins, though C-type meteorites aren’t common, Rivkin says. Upon comparing Ryugu’s structure, terrain, and composition to a variety of other, larger asteroids, Yokota believes that it probably originated from a “parent body” called Eulalia, which is similarly dark and rich in carbon, though other asteroids haven’t been ruled out as its parents.
Research on near-Earth asteroids has implications for scientists’ understanding of bodies that might one day collide with the Earth. “We know of no asteroids that are going to hit the Earth,” Rivkin is quick to point out, but scientists at NASA and elsewhere try to monitor every trackable asteroid, just in case one turns out to be heading in our direction with an arrival time within a couple decades. Occasionally their trajectories can subtly shift, potentially pointing them in a more hazardous direction (from Earthlings’ perspective). This could happen thanks to impacts by smaller objects or to something known as the Yarkovsky effect, which is when sunlight hits an asteroid and gets reradiated as heat, giving it a tiny thrust.
NASA and the European Space Agency are currently studying how to deflect an asteroid on a collision course. In November, NASA will launch the Double Asteroid Redirection Test (or DART), a mission Rivkin’s involved in that will try bumping an asteroid onto a new path by slamming a small probe into it. DART’s target asteroid is a bit different from the rubble pile that is Ryugu, but the same physics should apply to any asteroid: “We expect to kick off some debris, but the momentum will still get transferred,” Rivkin says.
Would-be asteroid miners are surely paying attention, too. Deep-space missions might one day depend on astronauts extracting water for consumption or fuel from them. Domingue and her colleagues used data from Hayabusa2’s onboard spectrometer to find evidence of water molecules on Ryugu, within its dust grains, pebbles, and rocks. “If they are mining for water, there is some present, but there are probably some objects that would have more than Ryugu would,” Domingue says. (Psyche’s destination asteroid lacks water, though it does have iron, nickel, and possibly more lucrative metals like platinum.)
While Yokota and Domingue aren’t directly involved in the study of the little piece of Ryugu, which has already begun, they’re looking forward to what their colleagues learn from it. “I suspect that the sample will have many surprises for us. It’s just exciting!” Domingue says.