Investigations of Ryugu have been going on in tandem with OSIRIS-REx’s explorations of the dark, diamond-shaped Bennu. At the start of these visits, the two bodies were thought to have distinct origins, especially since Bennu was found to be far more water-rich than Ryugu. But recent simulations have shown how the two similar-looking asteroids could have been birthed from the same energetic collision roughly a billion years ago. Ryugu’s drier composition also favors the idea that it once sojourned close to the Sun.
Infrared images of the boulders on both asteroids have revealed another unexpected outcome — these rocks are highly porous.
“My guess is they would float like pumice and if you touch them they would just disintegrate,” says planetary scientist Harold Levison of the Southwest Research Institute in Boulder, Colorado. The discovery helps explain processes in the primordial disk, when dust grains first began sticking together into larger agglomerations. The foamy stones were probably created by slow churning, arising like clumps in a bag of flour, Levison says. Though not entirely shocking, the finding indicates that the initial bodies in the solar system probably formed in a low-energy environment.
A recent slew of papers from the OSIRIS-REx team have shown that Bennu is covered in carbon-rich material and contains rocks probably too weak to survive a fiery plummet through Earth’s atmosphere. If so, samples gathered by the spacecraft will be unique and not already present in meteorite collections on our planet. The rocks also reveal Bennu to be a Frankenstein body, with at least two separate boulder populations that probably originated from geologically distinct regions of its parent body.
But the biggest surprises are yet to come, when Hayabusa2 and OSIRIS-REx’s samples make it back to laboratories on Earth. Researchers will then be able to perform much more rigorous and detailed analyses of both asteroids’ components than can be done by spacecraft. OSIRIS-REx principal investigator Dante Lauretta says he is particularly interested in studying the small bodies’ organic material.
Terrestrial meteorites, which break off asteroids or other bodies and fall to our planet, are almost immediately contaminated by native organisms. Even for examples that are collected quickly, like the famous Murchison meteorite that crashed into Australia in 1969, scientists lack important contextual information about precisely where they came from. Because of the extensive mapping of Ryugu and Bennu, researchers will have a significant leg up in drawing conclusions about solar system formation and life’s origins from their extractions.
Future surprises
Scientists are also looking forward to two forthcoming NASA missions — designated Lucy and Psyche — that will provide data bookending the start and finish of planetary evolution. Lucy is set to launch next year, eventually to tour past at least six asteroids that share Jupiter’s orbit around the Sun. Known as Trojans, these small traveling companions are so diverse in their properties that scientists think they probably have separate origins.
Levison, the mission’s principal investigator, hopes to find evidence for a theory he helped develop called the Nice model. It posits that early in the solar system’s history, gravitational instabilities caused Jupiter and Saturn to move inward over the course of hundreds of thousands or millions of years. This process snagged the Trojans from various places in the primordial disk and knocked the rest of the planets into their present orbits.
In 2022, NASA plans to send the Psyche probe toward an asteroid named 16 Psyche. That small body is thought to be composed of between 30 percent and 60 percent metal, a type of asteroid that no spacecraft has ever seen up close. Scientists are still scratching their heads over how the asteroid formed, with some hypothesizing that it is the exposed core of a former planet that was ground down via a series of cataclysmic wallops. Psyche the spacecraft will investigate the little world, potentially providing important data on the later stages of planetary creation in our solar system.
Our understanding of small bodies will continue to deepen in the foreseeable future, with the European Space Agency recently selecting the Comet Interceptor mission for launch in 2028. The probe will be parked near the Earth as it waits to examine an ancient long-period comet — perhaps from the distant Oort cloud, a collection of the farthest and least changed icy bodies in the solar system.