The dark asteroid Ryugu is finally coming to light

Hayabusa2 also provided researchers with unique opportunities to observe the asteroid from multiple angles, including hard-to-reach images taken in “opposition.” This involved maneuvering the spacecraft the size of a refrigerator to take pictures while the asteroid and the sun were on opposite sides, an arrangement that provided a view of the asteroid with the sun’s rays reflected directly at the camera without creating shadows.

Thanks to the physics of the optics, anything with a rough surface that reflects light will look a little 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 we cannot see the “crescent phase” from the Earth as 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 is darker than other types of asteroids and even darker than a lump of coal.

But taking close-up and opposition photos allowed the researchers to get a detailed image of the asteroid’s surface; he improved the way the asteroid’s dust interacts with light, making it clearer that it was actually there. Bannister says the images of the opposition are like looking at grass when the sun is right behind you, allowing you to see individual blades, unlike when sunlight falls obliquely on the grass, producing many shadows. Comparing images of the opposition with those taken in near-opposition “tells you how rough your lawn is, but from a distance everything may look perfectly smooth,” she said.

Shadowless images also allowed researchers to map the structure of Ryugu’s surface, on at least one side.

This Ryugu study is part of a broader effort to study many types of asteroids to learn more about their shapes, content and origins. Ryugu is similar to another near-Earth asteroid called Bennu, which was recently visited by NASA’s OSIRIS-REx spacecraft. Both are C-type asteroids that are shaped like vertices, albeit with differently pronounced central ridges. The first Hayabusa mission met with a more rocky asteroid type S. NASA’s planned mission “Psyche” next year will travel to an asteroid type M, full of iron and other metals, and the ship of the agency “Lucy”, which launched in October this year, will focus on Trojan asteroids of type D to study the building blocks that formed the Jovian worlds.

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

Ryugu probably formed when something collided with a much larger asteroid, breaking up a pile of rock fragments that later came together and headed in a different trajectory. Meteorites or pieces of asteroids and comets that hit Earth may have similar origins, although C-type meteorites are not common, Rivkin said. Comparing the structure, terrain, and composition of Ryugu to a number of other, larger asteroids, Yokota suggests that it probably originated from a “parent body” called Eulalia, which is just as dark and carbon-rich, although other asteroids are not controlled as parents.

Research on near-Earth asteroids has implications for scientists’ understanding of bodies that could one day collide with Earth. “We don’t know of any asteroids to hit Earth,” Rivkin was quick to point out, but scientists at NASA and elsewhere are trying to observe every traceable asteroid, just in case someone heads in our direction with an arrival time within a few decades. Sometimes their trajectories can shift slightly, potentially directing them in a more dangerous direction (from the point of view of earthlings). This can happen due to the impact of smaller objects or something known as the Yarkovsky effect, which is when sunlight hits an asteroid and radiates as heat, giving it a small thrust.