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Ryugu, pictured by a camera aboard Hayabusa 2 from a distance of about 22 kilometers.

JAXA, University of Tokyo & collaborators

Japan’s new asteroid probe reaches its target after 3.2-billion-kilometer journey

SAGAMIHARA, JAPAN—After 3.5 years traveling 3.2 billion kilometers through space, Japan’s Hayabusa2 spacecraft officially arrived at the asteroid it will land on later this year to pick up surface and subsurface soil and rock samples and—hopefully—return them to Earth for analysis. The findings are expected to shed light on the materials that existed in the early solar system and the formation and evolution of planets and their arrangement. They might provide evidence for the theory that asteroids and comets are one source of Earth’s water and its amino acids—the building blocks of life.

The ground crew in Japan confirmed this morning that Hayabusa2, launched in December 2014, reached its home position 20 kilometers away from Ryugu, an asteroid in orbit between Earth and Mars. With the spacecraft now at its target, “I’m just really happy,” Yuichi Tsuda, project manager for the Japan Aerospace Exploration Agency’s Institute for Space and Astronautical Science (ISAS) in Sagamihara, told a packed press conference on the ISAS campus this afternoon.

For the next 18 months, this second edition of Hayabusa, Japanese for peregrine falcon, will be maneuvering around the asteroid, while a suite of instruments map it; measure its mass, density, and gravity; determine its mineral and elemental composition; and scout out landing sites. The first of a series of touchdowns is scheduled for October. In addition to gathering surface soil samples, Hayabusa2 will release a German-French rover called MASCOT that will hop across the surface, using its four instruments to analyze soil samples in situ.

Next spring, Hayabusa2 will blast a crater into Ryugu using a 2-kilogram projectile with a hardened copper nose traveling at 2000 meters per second. (To avoid damage from scattering debris, the spacecraft will hide on the opposite side of the asteroid after releasing the projectile and use a camera to document the collision.) Images of the impact are expected to shed light on how craters are formed on heavenly bodies. Hayabusa2 will then return to the site of the blast to collect rock samples that have not been subjected to eons of space weathering, hopefully yielding insights into the material as it was during the formation of the solar system. The craft is expected to return its samples to Earth at the end of 2020.

Preliminary observations “are really thrilling,” says Seiichiro Watanabe, a project scientist at Nagoya University in Japan. The diamond-shape asteroid is about 900 meters across and rotates around its own axis every 7.5 hours or so, more slowly than other similarly sized asteroids for reasons that are not yet clear, Watanabe says. The surface is strewn with boulders larger than would be expected to have accumulated on an asteroid of its size, something that has triggered a debate among planetary scientists. One line of thinking is that Ryugu was originally part of a larger asteroid that broke up. But others contend the boulders could have landed over time or might have been hard masses incorporated into the asteroid at its formation and later exposed as softer material eroded away. “It’s a strange phenomenon,” Watanabe says.

This is the second ISAS mission to retrieve samples from an asteroid. The original Hayabusa, launched in 2003 to explore an asteroid named Itokawa, “has been a stunning success,” planetary scientist Erik Asphaug at the University of California, Santa Cruz, wrote in a commentary in Science in June 2006, despite engine failures, a daunting variety of mechanical malfunctions, fuel and solar power losses, and communications blackouts during a 7-year, 6-billion-kilometer odyssey. The return sample capsule, which landed in the Australian outback in June 2010, carried 1500 particles of asteroid dust, despite glitches with the sample collection mechanism. And even before Hayabusa touched down on Itokawa, it had returned enough data for scientists to publish a clutch of papers in Science, including one concluding that the asteroid was a loose agglomeration of rubble rather than a solid rock.

Hayabusa2 will provide complementary data, particularly because it is landing on a C-type, or carbonaceous, asteroid; Itokawa is an S-type, or silicaceous, asteroid. C-type asteroids populate the outer regions of the asteroid belt that stretches between the orbits of Mars and Jupiter, where the distance from the sun might have preserved water as well as amino acids. S-type asteroids are typically closer in where high temperatures would have burned off water. Additional data on C-type asteroids will come from NASA’s OSIRIS--REx spacecraft, now en route to asteroid Bennu, where it will collect samples and return them to Earth in 2023. The two teams will exchange some of their samples. “This has been an international collaboration from the beginning of development,” Tsuda says.