Dark asteroid Ryugu finally comes to light

Hayabusa2 also provided researchers with unique opportunities to observe the asteroid from multiple angles, including hard-to-get images taken in “opposition.” This involved maneuvering the refrigerator-sized spacecraft to take snapshots while the asteroid and the sun were on its opposite sides, an alignment that offers views of the asteroid with the sun’s rays reflected directly toward it. the camera, without producing shadows.

Thanks to the physics of optics, anything that has a rough surface that reflects light will appear slightly brighter when in opposition. This means that small, weak, and distant asteroids can really only be seen in opposition. In fact, they are so dark that from Earth we cannot see a “crescent phase” like the moon did. Domingue and Yokota discover that Ryugu is one of the darkest objects ever: reflecting only about 3.5% of sunlight, it’s darker than other types of asteroids and even darker than a lump. of coal.

But taking pictures up close and in opposition allowed researchers to get a detailed picture of the asteroid’s surface; it improved the way asteroid dust interacts with light, making it more clear that it is in fact there. Bannister says the opposing images are like looking at a grassy lawn when the sun is directly behind you, which allows you to see individual blades, as opposed to when the sunlight falls obliquely on the lawn, which produces a lot. of shadows. Comparing images of opposition to those taken in near opposition “tells you how spiky your lawn is, but from a distance anything can look completely smooth,” she says.

The mostly shadowless photos also allowed researchers to map Ryugu’s surface structure, at least on one side.

This exploration of Ryugu is part of a larger 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, which was recently visited by NASA’s OSIRIS-REx spacecraft. They are both C-type asteroids that have the shape of vertices, but with differently accentuated central ridges. The first Hayabusa mission encountered a more stony S-type asteroid. NASA’s planned Psyche mission will head next year to an M-type asteroid full of iron and other metals, and the agency’s Lucy spacecraft, which will launch in October, will head to the Trojan asteroids of type D to study the building blocks that formed the Jovian worlds.

The inhabitants of the main asteroid belt, a scattered conglomerate of space rocks that Jupiter never allowed to become a planet, have had stable orbits for billions of years, says Andy Rivkin, planetary astronomer at Johns University Hopkins of Baltimore. In contrast, near-Earth asteroids have more unstable orbits. “Something like Bennu and Ryugu ends up hitting a planet or the sun over millions of years, so they can’t be there for very long,” he says.

Ryugu likely formed when something collided with a much larger asteroid, shattering a bunch of rocky debris which then clustered together and headed on a different path. Meteorites, or pieces of asteroids and comets hitting Earth, may have similar origins, although C-type meteorites are not common, Rivkin says. Comparing Ryugu’s structure, terrain, and makeup to a variety of other larger asteroids, Yokota thinks it likely originated from a “parent body” called Eulalia, which is just as dark and carbon-rich, though. that other asteroids were not ruled. outside like his parents.

Near-Earth asteroid research has implications for scientists’ understanding of bodies that may one day collide with Earth. “We don’t know of any asteroids that are going to hit Earth,” Rivkin is quick to point out, but scientists at NASA and elsewhere are trying to keep an eye on every traceable asteroid, just in case one is heading in. our management with an arrival time in a few decades. Sometimes their trajectories can change subtly, potentially pointing them in a more dangerous direction (from the Terrans perspective). This could happen through the impacts of smaller objects or the so-called Yarkovsky effect, which is when sunlight hits an asteroid and re-emits as heat, giving it a little push.