Asteroid Ryugu and the Hayabusa2 Mission

Abstract

The carbonaceous type (C-type) asteroid Ryugu is a near-Earth object measuring ~1 km in equatorial diameter. C-type asteroids of this size are seldom found in the near-Earth region, making Ryugu an invaluable target for a sample return mission. Studying Ryugu offers insights into the Solar System formation and the transportation of volatile components from the asteroid belt to the early Earth. The Hayabusa2 spacecraft, developed by the Japan Aerospace Explosion Agency (JAXA), was launched on an H-IIA rocket in December 2014. It reached Ryugu in June 2018, and for 17 months, it closely observed the asteroid using optical and thermal imagers, a near-infrared spectrometer, and a laser altimeter. The spacecraft deployed three small rovers and a lander onto Ryugu surface, allowing for in-depth imaging and measurements. Furthermore, Hayabusa2 executed two precise touchdowns on different regions of the asteroid for sampling and initiated an impact experiment that created an artificial crater on Ryugu. During the second touchdown, subsurface materials ejected from the artificial crater were collected. Hayabusa2 departed from Ryugu in November 2019 and returned a capsule containing Ryugu samples to Earth in December 2020. Having successfully completed its sample return mission, Hayabusa2 is now en route to its next objective: a rendezvous with a small, rapidly rotating asteroid in July 2031. Ryugu is a rubble-pile asteroid, formed through the re-accumulation of fragments of a disrupted parent asteroid in the inner main asteroid belt. Its distinct spinning-top shape was likely molded by landslides, triggered by rapid rotation about ten million years after its formation. Chemically, Ryugu’s surface material closely resembles that of CI (Ivuna-type) carbonaceous chondrites, known for their primitive compositions. The high porosity of Ryugu particles hints at a past presence of ice. Moreover, the plentiful carbonates, combined with the limited presence of high-temperature inclusions larger than 30 μm, suggest that Ryugu’s parent body originated in the outer Solar System, likely beyond the Saturn orbit. Within a few million years following the formation of the Solar System, gravitational interactions with giant planets may have scattered this parent body to the inner main asteroid belt. The decay heat from the short-lived radionuclide, 26Al, then facilitated aqueous alteration of the parent body and led to the genesis of diverse organic compounds. Many low-albedo asteroids in the main belt share spectra similarities with Ryugu. This implies that the structural water in phyllosilicates and organic matter could have been transported to the early Earth through dynamical and collisional evolution of these objects.