After a 3½-year journey, Japan's Hayabusa 2 spacecraft has arrived at its home for the next 18 months: the small, oddly-shaped asteroid Ryugu.
We got a peek at a new worldlet in the inner solar system this week, as the Japanese Aerospace Exploration Agency's (JAXA's) Hayabusa 2 returned its first good look at the half-mile-wide asteroid 162173 Ryugu.
Hayabusa 2's view of Ryugu has expanded this past week, zooming in on a world that looks like a multifaceted die straight out of Dungeons and Dragons. There's a bifurcated pole, an as-yet unnamed singular large crater crossing its equatorial bulge, and plenty of boulders strewn about its surface.
JAXA officials announced the spacecraft's arrival at Ryugu on June 27th at 00:35 Universal Time (June 26th at 8:35 p.m. EDT). Hayabusa 2 approached Ryugu via a slow corkscrew-like path over this past week, slowly scanning the region surrounding the asteroid for any potential moonlet companions that might have posed a hazard. With a diameter of only 1 kilometer (0.6 mile) along its longest axis, Ryugu's sphere of gravitational dominance extends only 90 km from its surface. Hayabusa 2 won't orbit Ryugu so much as keep pace with it as the two orbit the Sun. The spacecraft is currently hovering 20 km away.
Welcome to Ryugu
The space rock was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) survey on the night of May 10, 1999, and provisionally designated 1999 JU3 before receiving a number and being renamed Ryugu (“Dragon's Palace” from a Japanese folktale) in late 2015.
Ryugu is an Apollo-group asteroid, dubbed "potentially hazardous" because of its potential to pass as close to Earth as 95,400 km, about a fifth of the Moon's distance. However, it won't be hazardous any time soon — its closest approach to Earth this century, in December 2076, will bring it about four times the Earth-Moon distance.
During its approach this month, the spacecraft turned its thermal-infrared imager toward Ryugu and confirmed the 8-hour rotation rate found during previous Earth observations. Ryugu orbits the Sun once every 1.3 years with an orbital inclination of 5.9° relative the ecliptic plane.
“The shape of Ryugu is now revealed," says project manager Yuichi Tsuda in a recent JAXA press release. "This form of Ryugu is scientifically surprising and poses a few engineering challenges.” In particular, Tsuda notes that the asteroid's rotation axis is perpendicular to that of its orbital. "This increases the degrees of freedom for landing and rover decent operations,” he explains.
The mission team will also need to thoroughly analyze Ryugu's weak gravitational field before the spacecraft descends, as there are probably areas were the gravitational pull isn't directly toward the asteroid's center.
Launched atop an H-IIA rocket from the Tanegashima Space Center in Japan on December 3, 2014, Hayabusa 2 builds on the lessons learned from its predecessor, which visited asteroid 25143 Itokawa in 2005 and returned samples to Earth on June 13, 2010. Plucky Hayabusa 1 was plagued with difficulties, including a record-breaking solar flare that damaged its solar cells en route to Itokawa, failed reaction wheels, and a failure of its sample-collection system. However, Hayabusa 1 still managed to return some dust particles from Itokawa to Earth that it had stirred up on landing.
What's in Store
Now, exploration of Ryugu begins. Hayabusa 2 will continue to approach the asteroid through late July, bringing its altitude down to 5 km and mapping its surface for its climactic approach. August will bring the spacecraft even closer, to a range of just 1 km, enabling it to map out Ryugu's feeble gravitational field and see just how well this rock is put together internally.
Then, in September or October of this year, Hayabusa 2 will begin its final approach, ultimately touching down on the asteroid and performing the first of three sampling operations. This process will be done in stages: First, the Small Carry-on Impactor, a copper projectile encased in a charge shaped to focus its explosive energy, will be fired at the asteroid. The projectile will create a crater, clearing off the asteroid's space-weathered exterior and ensuring a fresh sample. DCAM3, a miniaturized deployable camera, will separate from the spacecraft and witness the impact at close range. The spacecraft will then descend over the impact site and fire a 5-gram tantalum bullet into the asteroid, putting its sampling horn in contact with the surface to collect the resulting debris.
Hayabusa 2 also carries four small hitchhikers, each of which will make the descent to Ryugu's surface starting from about 197 feet (60 meters) up.
The MINERVA II lander will deliver three of the robotic explorers. It's based on the lander that flew with Hayabusa 1 but didn't land successfully on the asteroid. (Its name is a "backronym" for MIcro-Nano Experimental Robot Vehicle for the Asteroid.) MINERVA II will deploy two small, solar-powered rovers, dubbed 1A and 1B, that were built at the University of Aizu and are equipped with thermometers and stereo and wide-angle cameras. The rovers will hop several times across the asteroid's surface to make measurements.
The MINERVA II container will also dispatch ROVER 2. Built by Tohoku University in northern Japan, it's a prism-shaped hopper that will use optical and ultraviolet LEDs to look for lingering dust suspended over the surface of the asteroid.
Meanwhile, the Mobile Asteroid Surface Scout (MASCOT), built by the French Space Agency in partnership with the German Aerospace Center, will make its own investigations. This lander is equipped with a magnetometer, infrared spectrometer, and a radiometer identical to the one currently headed to Mars aboard NASA's InSight lander.
“The project team is fascinated by the appearance of Ryugu and morale is rising at the prospect of this challenge,” Tsuda says in a JAXA press release. “Together with all of you, we have become the first eyewitnesses to see asteroid Ryugu. I feel this is an amazing honor as we proceed with mission operations.”
Hayabusa 2 will depart Ryugu late next year, in November or December, to deliver its capsule sample return to Earth by December 2020.
A new era of asteroid exploration is just beginning this summer, as NASA's OSIRIS-REx is set to reach asteroid 101955 Bennu this August. Further afield, watch for the Lucy and Psyche missions headed toward their destinations in 2021 and 2023, respectively.
Studying an Earth-grazing asteroid could prove handy, should we ever have to move one away from a collision course with Earth. It will be exciting to watch the drama unfold over the coming months, as Hayabusa 2 and its robotic crew descend toward destiny.