On the morning of Tuesday June 19, members of the Japan Aerospace Exploration Agency (JAXA) viewed images beamed from deep space that triggered a flurry of responses.
“I see a Death Star.”
“We will strike it with the SCI!”
“The SCI should only be used peacefully!”
“May the Force be with you!”
They were gazing at the latest images of Ryugu, a 900-meter near-Earth asteroid that orbits the Sun between our planet and Mars. The images had been snapped by the camera aboard JAXA’s Hayabusa2 spacecraft from approximately 300 kilometers away from the asteroid.
Hayabusa2 is an asteroid sample-return mission led by JAXA’s Institute of Space and Astronautical Sciences (ISAS). The mission’s goal is to rendezvous with Ryugu, analyze its properties and perform a set of surface operations. Action on the surface involves deploying three small rovers and the European-designed lander known as MASCOT, as well as gathering three different samples of material to return to Earth. Prior to collecting the third sample, Hayabusa2 will release a two-kilogram copper projectile dubbed the SCI (Small Carry-on Impactor) to create an artificial crater on Ryugu’s surface so that subsurface material can be gathered. As of this writing, the probe has just arrived at Ryugu, where it will stay until the end of 2019, before dispatching its sample-filled capsule back to Earth at the end of 2020.
Ryugu is a carbonaceous asteroid, a variety that typically is rich in organic material and has undergone minimal changes since the birth of the solar system. This makes Ryugu kin to the meteorites that slammed down on the young Earth more than four billion years ago, perhaps delivering some of our temperate planet’s first water and organics—the raw feedstock for life. Hayabusa2’s science goal to explore this aspect of our origins is reflected in the name of its asteroid target. Chosen by public vote, “Ryugu” is the name of a mythical underwater palace from a Japanese folktale, in which a young fisherman is given a box that, once opened, transforms him into an old man. The samples returned to Earth in 2020 may similarly hold transformative secrets of our planet’s youth.
In the run-up to Hayabusa2’s arrival at Ryugu, the Hayabusa2 team began posting steadily closer and clearer images of Ryugu on the mission’s website. This stream of teasers culminated in a close-up image taken from just 20 kilometers above Ryugu on Tuesday, June 26. So far, the public response has echoed that of the joking team members; an eye-catchingly large crater makes Ryugu look worryingly like the Death Star from Star Wars. But assuming Ryugu is related to the formation of planets, not their destruction, what does its shape mean for the mission?
Analysis of the asteroid’s rotation has revealed good news for the Hayabusa2 team. Ryugu’s axis of rotation points nearly perpendicular to its path around the sun and to the perspective of Hayabusa2. This allows the spacecraft to get a global view of Ryugu as the asteroid spins. Since Hayabusa2 will not actually orbit around the asteroid, the team worried that if the spacecraft by chance found itself staring straight down at one of Ryugu’s poles, much of the rest of the asteroid’s surface could be permanently blocked from view. This would have greatly limited the choices for landing sites.
Interestingly, Ryugu is spinning “backwards”—that is, its direction of rotation is opposite from the rotation of the Earth and the direction of Ryugu’s orbit around the sun. This is known as a retrograde rotation and is a common outcome of collisions or other destructive forces. Such a violent history would be unsurprising for Ryugu. Small asteroids on near-Earth orbits are thought to originate in the asteroid belt, a diffuse band of space rocks orbiting the sun between Mars and Jupiter. Collisions between asteroids there can cause larger bodies to fragment, sending showers of smaller remains towards the Earth. As surface details of Ryugu have come into focus, the pockmarked surface is further indication that this asteroid has experienced many collisions during its existence.
Yuichi Tsuda, Hayabusa2’s project manager, noted that while the asteroid’s rugged surface features offer important clues about Ryugu’s history, they also pose challenges for landing. Similarly, the asteroid’s angular, almost octahedral shape could result in an uneven gravitational pull that might make landing more difficult.
“Globally, the asteroid had a shape like fluorite,” Tsuda noted in a recent article posted on the mission’s Web site. “This means we expect the direction of the gravitational force on the wide areas of the asteroid surface to not point directly down.”
Gravity is a result of an object’s mass. If that mass is evenly distributed, like in a sphere, the gravitational force points directly downwards towards the sphere’s center. This is true of the Earth, where gravity keeps us anchored to the surface. But Ryugu is not massive enough for its gravity to pull it into a ball. Instead, its mass is distributed unevenly in space, and so the direction of its gravitational pull will change across the surface. This is liable to make some locations much more dangerous for landing than others.
To find safe landing locations for collecting samples and releasing the MASCOT lander and the three rovers, the Hayabusa2 team plans to carefully map Ryugu’s gravity by constructing a three-dimensional model of the asteroid over the summer. The team will be aided in this task by researchers from NASA’s OSIRIS-REx mission, another sample-return mission that will arrive at a different asteroid, Bennu, later this year. The software tools being shared between the teams have previously been used for creating three-dimensional models for NASA missions to Ceres and Mercury. Employing this software for Ryugu will be particularly valuable for the OSIRIS-REx team’s preparations for their own encounter, because Ryugu’s shape is quite similar to that of Bennu.
Makoto Yoshikawa, Hayabusa2’s mission manager, admitted he was very surprised by this similarity between the two asteroids, as their other physical properties differ.
“These celestial bodies,” he says, referring to Bennu and another similar shaped asteroid, 2008 EV5, “are small and rotating fast compared to Ryugu.”
Ryugu’s diamond-like shape is due to the asteroid’s prominent equatorial ridge. Such landforms can arise from rapid rotation, in which centripetal force pushes material together to make a bulge. Yet, Ryugu’s rotation period is about 7.6 hours, roughly double that of Bennu. The similarities and differences of these two objects will be important guideposts for understanding asteroid formation and evolution.
Over the summer, Hayabusa2 will conduct remote observations of Ryugu and search for the best landing sites. Surface operations will begin around September or October, with the gathering of the first sample as well as the release of the MASCOT lander and one or two rovers. The SCI is due to be dropped (very peacefully) next March.
The team is publishing regular updates on its Web site, allowing everyone on Earth to follow the progress of exploring this small world for the first time.