162173 Ryugu, provisional designation 1999 JU3, is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately 1 kilometer (0.6 mi) in diameter and is a dark object of the rare spectral type Cg, with qualities of both a C-type asteroid and a G-type asteroid. In June 2018, a spacecraft, Hayabusa2, arrived at the asteroid. After making measurements and taking samples, Hayabusa2 left Ryugu for Earth in November 2019.
Monochrome view of Ryugu[a]
|Discovery site||Lincoln Lab's ETS|
|Discovery date||10 May 1999|
|MPC designation||(162173) Ryugu|
|Apollo · NEO · PHA |
|Orbital characteristics |
|Epoch 12 December 2011 (JD 2455907.5)|
|Uncertainty parameter 0|
|Observation arc||30.32 yr (11,075 d)|
|1.30 yr (474 d)|
|0° 45m 34.56s / day|
|Earth MOID||0.0006 AU (0.2337 LD)|
Equatorial surface gravity
|SMASS = Cg  · C |
Discovery and nameEdit
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 (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 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 95,400 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 kilometers, (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 and confirmed its retrograde rotation. 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 about 450 million tons.
As of August 2019, there are 13 surface features that are named by the IAU. The three landing sites are not officially confirmed but are referred by specific names to in media by JAXA. The theme of features on Ryugu is "children's stories." Ryugu was the first object to introduce the feature type known as the Saxum, referring to the large boulders found on Ryugu's surface.
|Kibidango||Kibi dango featured in Momotaro|
A dorsum is a ridge. There is a single dorsum on Ryugu.
A fossa is a ditch-like feature.
A saxum is a large boulder. Ryugu is the first astronomical object with them being named. Two boulders have been named "Styx" and "Small Styx" unofficially by the JAXA team, it is unknown if these names will be submitted for IAU approval. Both names refer to the River Styx. 
|Catafo Saxum||Catafo, from Cajun folktales |
|Ejima Saxum||Ejima, the location where Urashima Taro rescued the turtle |
JAXA has given informal names to the specific landing and collection sites.
|Alice's Wonderland||Alice in Wonderland||MASCOT landing site|
|Tritonis||Lake Tritonis||MINERVA-II1 landing site, initially referred to as "Trinitas," as of February 2019 this has been rectified.|
|Tamatebako||Tamatebako||Site of first sample collection|
|Uchide-no-Kozuchi||Uchide no kozuchi||Site of second sample collection|
The Japan Aerospace Exploration Agency (JAXA) spacecraft Hayabusa2 was launched in December 2014 and successfully arrived at the asteroid on 27 June 2018. It is planned to return material from the asteroid to Earth by December 2020.
The Hayabusa2 mission includes four rovers with various scientific instruments. The rovers are named HIBOU (aka Rover-1A), OWL (aka Rover-1B), MASCOT and Rover-2 (aka MINERVA-II-2). On 21 September 2018, the first two of these rovers, HIBOU and OWL (together the MINERVA-II-1 rovers) which hop around the surface of the asteroid, were released from Hayabusa2. This marks the first time a mission has completed a successful landing on a fast-moving asteroid body.
On 3 October 2018, the German-French Mobile Asteroid Surface Scout (MASCOT) lander successfully arrived on Ryugu, 10 days after the MINERVA rovers landed. Its mission was short-lived, as was planned (the device had no way to charge its batteries, instead relying by design on pre-charged batteries), with only 16 hours available from its batteries.
Hayabusa2 touched down briefly on February 22, 2019 on Ryugu, fired a small tantalum projectile into the surface to collect the cloud of surface debris within the sampling horn, and then it moved back to its holding position. The second sampling was from the sub-surface, and it involved firing a large copper projectile from 500 m altitude to expose pristine material, and after several weeks, it touched down in 11 July 2019 to sample the sub-surface material, using its sampler horn and tantalum bullet.
The last rover, Rover-2 or MINERVA-II-2, failed before release from the Hayabusa2 orbiter. It was deployed anyway on 2 October 2019 in orbit around Ryugu to perform gravitational measurements. It impacted the asteroid a few days after release.
On 13 November 2019, commands were sent to Hayabusa2 to leave Ryugu and begin its journey back to Earth.
- Photograph of the full disc of 162173 Ryugu by the Optical Navigation Camera – Telescopic (ONC-T) instrument aboard the Hayabusa2 spacecraft. The photograph was taken on 26 June 2018, at a distance of 20 kilometres (12 miles) from the asteroid's surface.
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The brave young man who defeated a giant
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|Wikimedia Commons has media related to (162173) Ryugu.|
- 162173 Ryugu at NeoDyS-2, Near Earth Objects—Dynamic Site
- 162173 Ryugu at the JPL Small-Body Database