162173 Ryugu
162173 Ryugu, provisional designation , is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately 1 kilometer 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, a spacecraft, Hayabusa2, arrived at the asteroid. After making measurements and taking samples, Hayabusa2 left Ryugu for Earth in November 2019.
History
Discovery and name
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. The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015. The name refers to Ryūgū, 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.Geological history
Possible surface samples will help to reveal the geological history of the asteroid. There exists already a rough picture of the geological past of Ryugu, even before the samples arrived.Ryugu formed as part of an asteroid family, belonging either to Eulalia or Polana. Those asteroid families are 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 likely 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. Only the western bulge remained as an older structure.
Characteristics
Orbit
Ryugu orbits the Sun at a distance of 0.96–1.41 au once every 16 months. 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, equivalent to 0.23 lunar distances.Physical
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, 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 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 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. 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.
Shape
Ryugu has a round shape with an equatorial ridge, called Ryujin Dorsum. Ryugu is a spinning top-shape asteroid similar to Bennu. The ridge is shaped by strong centrifugal forces. The western side has a different shape compared to the rest of the asteroid. The western side, also called the western bulge has a smooth surface with a sharp equatorial ridge. The models showed that subsurface material is structural intact and relaxed in the western bulge, while other regions are more sensitive to structural failure. The eastern and western side of Ryugu are bordered by the Tokoyo and Horai Fossae. The structural differences are due to structural changes in the history of the asteroids. Landslides and internal alternations reshaped the asteroid during a phase of high speed rotation. The western bulge is the region that was not affected by these reshaping forces.Surface
Observations from Hayabusa 2 showed that the surface of Ryugu is very young and has an age of 8.9 ± 2.5 Million years based on the data collected from the artificial crater that was created with an explosive by Hayabusa 2.The surface of Ryugu is porous and contains no or very little dust. The measurements with the radiometer on board of MASCOT, which is called MARA, showed a low thermal conductivity of the boulders. This was an in situ measurement of the high porosity of the boulder material. This result showed that most meteorites originating from C-type asteroids are too fragile to survive the entry into earths atmosphere. The images from the camera of MASCOT, which is called MASCam, showed that surface of Ryugu contains two different almost black types of rock with little internal cohesion, but no dust was detected. One type of rocky material on the surface is brighter with a smooth surface and sharp edges. The other type of rock is dark with a cauliflower-like, crumbly surface. The dark type of rock has a dark matrix with small, bright, spectrally different inclusions. The inclusions appear similar to CI chondrites. An unanticipated side effect from the Hyabusa 2 thrusters revealed a coating of dark, fine-grained red material.
Craters
Ryugu has 77 craters on the surface. Ryugu shows variations of crater density that cannot be explained by randomness of cratering. There are more craters at lower latitudes and fewer at higher latitudes, and fewer craters in the western bulge than in the region around the meridian. This variation is seen as evidence of a complicated geologic history of Ryugu. The surface has one artificial crater, which was intentionally formed by the Small Carry-on Impactor, which was deployed by Hayabusa 2. SCI fired a 2 kg copper mass onto the surface of Ryugu on 2019-04-05. The artificial crater showed a darker sub-surface material. It created an ejecta of 1 cm thickness and excavated material from up to 1 meter depth.Boulders
Ryugu contains 4400 boulders with a size larger than 5 meters. Ryugu has more large boulders per surface area than Itokawa or Bennu, about one boulder larger than 20 meters per 50 km2. The boulders resemble laboratory impact fragments. The high number of boulders is explained with a catastrophic disruption of Ryugu's larger parent body. The largest boulder, called Otohime has a size of ~160 × 120 × 70 m and is too large to be explained with an ejected boulder from a crater.Magnetic field
No magnetic field was detected near Ryugu on a global or local scale. This measurement is based on the magnetometer on board of MASCOT, which is called MasMag. This shows that Ryugu and the bodies it was created from did not generate a magnetic field and that they were not generated in an environment with a strong magnetic field. This result cannot be generalized for C-type asteroids, because the surface of Ryugu was recreated in a catastrophic disruption.Surface features
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 to by specific names 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.Craters
Dorsa
A dorsum is a ridge. There is a single dorsum on Ryugu.Fossae
A fossa is a ditch-like feature.| Feature | Named after |
| Horai Fossa | Penglai |
| Tokoyo Fossa | Tokoyo |
Saxa
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.| Feature | Named after |
| Catafo Saxum | Catafo, from Cajun folktales |
| Ejima Saxum | Ejima, the location where Urashima Taro rescued the turtle |
| Otohime Saxum | Otohime |
Landing sites
JAXA has given informal names to the specific landing and collection sites.| Feature | Named after | Notes |
| 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 |
''Hayabusa2'' mission
The Japan Aerospace Exploration Agency 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, OWL, MASCOT and Rover-2. On 21 September 2018, the first two of these rovers, HIBOU and OWL 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 lander successfully arrived on Ryugu, 10 days after the MINERVA rovers landed. Its mission was short-lived, as was planned, 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 on 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.