162173 Ryugu

162173 Ryugu (provisional designation 1999 JU₃) is a near-Earth object and also a potentially hazardous asteroid of the Apollo group. It measures approximately 900 metres (3,000 ft) in diameter and is a dark object of the rare spectral type Cb, with qualities of both a C-type asteroid and a B-type asteroid. In June 2018, the Japanese spacecraft Hayabusa2 arrived at the asteroid. After making measurements and taking samples, Hayabusa2 left Ryugu for Earth in November 2019 and returned the sample capsule to Earth on 5 December 2020. The samples showed the presence of organic compounds, such as uracil (one of the four components in RNA) and vitamin B3.

Ryugu was discovered on 10 May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Lab's ETS near Socorro, New Mexico, in the United States. It was given the provisional designation 1999 JU₃. The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015 (M.P.C. 95804). The name refers to Ryūgū-jō (Dragon Palace), a magical underwater palace in a Japanese folktale. In the story, the fisherman Urashima Tarō travels to the palace on the back of a turtle, and when he returns, he carries with him a mysterious box, much like Hayabusa2 returning with samples.

Ryugu formed as part of an asteroid family, belonging either to Eulalia or Polana. Those asteroid families are most likely fragments of past asteroid collisions. The large number of boulders on the surface supports a catastrophic disruption of the parent body. The parent body of Ryugu probably experienced dehydration due to internal heating and must have formed in an environment without a strong magnetic field. After this catastrophic disruption, part of the surface was reshaped again by the high speed rotation of the asteroid forming the equatorial ridge (Ryujin Dorsum), through internal failure and/or mass wasting. The geologically distinct western area ('Western bulge') is probably the result of asymmetrical internal failure. It is hoped that surface samples will help to reveal more of the geological history of the asteroid.

Ryugu is hypothesized to be an extinct comet.

Ryugu orbits the Sun at a distance of 0.96–1.41 AU once every 16 months (474 days; semi-major axis of 1.19 AU). Its orbit has an eccentricity of 0.19 and an inclination of 6° with respect to the ecliptic. It has a minimum orbital intersection distance with Earth of 95,443.442 km (0.000638 AU), equivalent to 0.23 lunar distances.

Early analysis in 2012 by Thomas G. Müller et al. used data from a number of observatories, and suggested that the asteroid was "almost spherical", a fact that hinders precise conclusions, with retrograde rotation, an effective diameter of 0.85–0.88 km (0.528 miles) and a geometric albedo of 0.044 to 0.050. They estimated that the grain sizes of its surface materials are between 1 and 10 mm.

Initial images taken by the Hayabusa2 spacecraft on approach at a distance of 700 km (430 mi) were released on 14 June 2018. They revealed a diamond-shaped body 1 km in diameter and confirmed its retrograde motion. Between 17 and 18 June 2018, Hayabusa2 went from 330 to 240 km (210 to 150 mi) from Ryugu and captured a series of additional images from the closer approach. Astronomer Brian May created stereoscopic images from data collected a few days later. After a few months of exploration, JAXA scientists concluded that Ryugu is actually a rubble pile with about 50% of its volume being empty space.

The acceleration due to gravity at the equator has been evaluated at about 0.11 mm/s², rising to 0.15 mm/s² at the poles. The mass of Ryugu is estimated at 450 million tonnes. The asteroid has a volume of 0.377 ± 0.005 km³ and a bulk density of 1.19 ± 0.03 g/cm³ based on the shape model.