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.[11] After making measurements and taking samples, Hayabusa2 left Ryugu for Earth in November 2019.[12]

162173 Ryugu
162173 Ryugu.jpg
Monochrome view of Ryugu[a]
Discovery [1]
Discovered byLINEAR
Discovery siteLincoln Lab's ETS
Discovery date10 May 1999
MPC designation(162173) Ryugu
Named after
("Dragon palace")
1999 JU3
Apollo · NEO · PHA[1][2]
Orbital characteristics[2]
Epoch 12 December 2011 (JD 2455907.5)
Uncertainty parameter 0
Observation arc30.32 yr (11,075 d)
Aphelion1.4159 AU
Perihelion0.9633 AU
1.1896 AU
1.30 yr (474 d)
0° 45m 34.56s / day
Earth MOID0.0006 AU (0.2337 LD)
Physical characteristics
Mean diameter
0.865±0.015 km[3]
0.87 km[4]
0.90±0.14 km[5]
0.92±0.12 km[6]
0.980±0.029 km[7]
1.13±0.03 km[8]
Mass4.5×1011 kg[9]
Equatorial surface gravity
1/80,000 g[9]
7.627±0.007 h[7][10]
SMASS = Cg[2] · C[3]
18.69±0.07 (R)[4]


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.[1] The asteroid was officially named "Ryugu" by the Minor Planet Center on 28 September 2015 (M.P.C. 95804).[13] 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.[1][14]


Orbit between Earth and Mars


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.[2] It has a minimum orbital intersection distance with Earth of 95,400 km (0.000638 au), equivalent to 0.23 lunar distances.[2]


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.[3]

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.[15] 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.[16] Astronomer Brian May created stereoscopic images from data collected a few days later.[17] 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.[18]

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.[19]

Surface featuresEdit

As of August 2019, there are 13 surface features that are named by the IAU.[20][21] 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.


Feature Named after
Brabo Silvius Brabo[22]
Cendrillon Cendrillon
Kibidango Kibi dango featured in Momotaro
Kintaro Kintaro
Kolobok Kolobok
Momotaro Momotaro
Urashima Urashima Taro


A dorsum is a ridge. There is a single dorsum on Ryugu.

Feature Named after
Ryujin Dorsum Ryujin


A fossa is a ditch-like feature.

Feature Named after
Horai Fossa Penglai
Tokoyo Fossa Tokoyo


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. [23]

Feature Named after
Catafo Saxum Catafo, from Cajun folktales [24]
Ejima Saxum Ejima, the location where Urashima Taro rescued the turtle [25]
Otohime Saxum Otohime

Landing sitesEdit

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 missionEdit

Animation of Hayabusa2's orbit from 3 December 2014 to 29 December 2019
  Hayabusa2   162173 Ryugu   Earth   Sun

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.[26]

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.[27] This marks the first time a mission has completed a successful landing on a fast-moving asteroid body.[28]

On 3 October 2018, the German-French Mobile Asteroid Surface Scout (MASCOT) lander successfully arrived on Ryugu, 10 days after the MINERVA rovers landed.[29] 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.[30] 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.[31]

The origin of 162173 Ryugu may be either 495 Eulalia or 142 Polana[32]
   Sun ·    Earth ·    162173 Ryugu ·    142 Polana ·    495 Eulalia

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.[12]

See alsoEdit



  1. ^ 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.


  1. ^ a b c d e f "162173 Ryugu (1999 JU3)". Minor Planet Center. Retrieved 30 October 2018.
  2. ^ a b c d e f "JPL Small-Body Database Browser: 162173 Ryugu (1999 JU3)" (2016-08-09 last obs.). Jet Propulsion Laboratory. Retrieved 30 October 2018.
  3. ^ a b c d e Müller, T. G.; Durech, J.; Ishiguro, M.; Mueller, M.; Krühler, T.; Yang, H.; et al. (March 2017). "Hayabusa-2 mission target asteroid 162173 Ryugu (1999 JU3): Searching for the object's spin-axis orientation". Astronomy and Astrophysics. 599: 25. arXiv:1611.05625. Bibcode:2017A&A...599A.103M. doi:10.1051/0004-6361/201629134.
  4. ^ a b c Kim, Myung-Jin; Choi, Young-Jun; Moon, Hong-Kyu; Ishiguro, Masateru; Mottola, Stefano; Kaplan, Murat; et al. (February 2013). "Optical observations of NEA 162173 (1999 JU3) during the 2011-2012 apparition". Astronomy and Astrophysics. 550: 4. arXiv:1302.4542. Bibcode:2013A&A...550L..11K. doi:10.1051/0004-6361/201220673.
  5. ^ a b Campins, H.; Emery, J. P.; Kelley, M.; Fernández, Y.; Licandro, J.; Delbó, M.; et al. (August 2009). "Spitzer observations of spacecraft target 162173 (1999 JU3)". Astronomy and Astrophysics. 503 (2): L17–L20. arXiv:0908.0796. Bibcode:2009A&A...503L..17C. doi:10.1051/0004-6361/200912374.
  6. ^ a b c Hasegawa, S.; Müller, T. G.; Kawakami, K.; Kasuga, T.; Wada, T.; Ita, Y.; et al. (December 2008). "Albedo, Size, and Surface Characteristics of Hayabusa-2 Sample-Return Target 162173 1999 JU3 from AKARI and Subaru Observations". Publications of the Astronomical Society of Japan. 60 (SP2): S399––S405. Bibcode:2008PASJ...60S.399H. doi:10.1093/pasj/60.sp2.S399.
  7. ^ a b c d Abe, M.; Kawakami, K.; Hasegawa, S.; Kuroda, D.; Yoshikawa, M.; Kasuga, T.; et al. (March 2008). "Ground-based Observational Campaign for Asteroid 162173 1999 JU3" (PDF). Lunar and Planetary Science (1391): 1594. Bibcode:2008LPI....39.1594A. Retrieved 30 October 2018.
  8. ^ a b Yu, Liang-Liang; Ji, Jiang-Hui; Wang, Su (July 2014). "Investigation of Thermal Inertia and Surface Properties for Near-earth Asteroid (162173) 1999 JU3". Chinese Astronomy and Astrophysics. 38 (3): 317–329. arXiv:1805.05244. Bibcode:2014ChA&A..38..317L. doi:10.1016/j.chinastron.2014.07.008.
  9. ^ a b Clark, Stephen (6 September 2018). "Hayabusa 2 team sets dates for asteroid landings – Spaceflight Now". spaceflightnow.com. Retrieved 7 September 2018.
  10. ^ "LCDB Data for (162173) Ryugu". Asteroid Lightcurve Database (LCDB). Retrieved 30 October 2018.
  11. ^ Chang, Kenneth; Stirone, Shannon (19 March 2019). "The Asteroid Was Shooting Rocks Into Space. 'Were We Safe in Orbit?' - NASA's Osiris-Rex and Japan's Hayabusa2 spacecraft reached the space rocks they are surveying last year, and scientists from both teams announced early findings on Tuesday (03/19/2019)". The New York Times. Retrieved 21 March 2019.
  12. ^ a b https://spaceflightnow.com/2019/11/13/japanese-sample-return-craft-departs-asteroid-heads-for-earth/
  13. ^ "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 30 October 2018.
  14. ^ "Name Selection of Asteroid 1999 JU3 Target of the Asteroid Explorer "Hayabusa2"" (Press release). JAXA. 5 October 2015. Retrieved 30 October 2018.
  15. ^ "From a distance of about 700km, Ryugu's rotation was observed". JAXA. 16 June 2016. Retrieved 30 October 2018.
  16. ^ Plait, Phil (20 June 2018). "Asteroid Ryugu Starts to Come Into Focus". SyFy Wire. Retrieved 30 October 2018.
  17. ^ Bartels, Meghan (10 July 2018). "Queen's Brian May Will Rock You with This Stereo Image of Asteroid Ryugu". Space.com. Retrieved 24 December 2018.
  18. ^ Hayabusa-2: Asteroid mission exploring a 'rubble pile'. Paul Rincon, BBC News. 19 March 2019.
  19. ^ Operation status for the asteroid explorer, Hayabusa2, in the vicinity of Ryugu (PDF), JAXA, 5 September 2018, retrieved 30 October 2018
  20. ^ http://www.planetary.org/blogs/jason-davis/hayabusa2-updates-sample-collection.html
  21. ^ http://www.hayabusa2.jaxa.jp/en/topics/20190121e_Nomenclature/
  22. ^ "Jan. 21, 2019. What's new". JAXA Hayabusa2 project (in Japanese). Retrieved 7 September 2019. The brave young man who defeated a giant
  23. ^ "July 8, 2019. What's new". JAXA Hayabusa2 project (in Japanese). Retrieved 7 September 2019.
  24. ^ http://www.hayabusa2.jaxa.jp/en/topics/20190121e_Nomenclature/
  25. ^ http://www.hayabusa2.jaxa.jp/en/topics/20190121e_Nomenclature/
  26. ^ "Current status of the asteroid explorer, Hayabusa2, leading up to arrival at asteroid Ryugu in 2018" (PDF). JAXA. 14 June 2016. Retrieved 30 October 2018.
  27. ^ Wall, Mike (21 September 2018). "Japanese Probe Deploys Tiny Hopping Robots Toward Big Asteroid Ryugu". space.com. Retrieved 30 October 2018.
  28. ^ Yoshimitsu, Tetsuo; Kubota, Takashi; Tsuda, Yuichi; Yoshikawa, Makoto (23 September 2015). "MINERVA-II1: Successful image capture, landing on Ryugu and hop!". JAXA Hayabusa2 Project. JAXA. Retrieved 30 October 2018.
  29. ^ "Touchdown! Japan space probe lands new robot on asteroid". phys.org. 3 October 2018. Retrieved 30 October 2018.
  30. ^ Touchdown: Japan probe Hayabusa2 lands on distant asteroid. Kyoko Hasegawa, PhysOrg. February 22, 2019.
  31. ^ "Hayabusa2 successfully collects 1st-ever subsurface samples: JAXA". Kyodo News. 11 July 2019. Retrieved 15 July 2019.
  32. ^ S. Sugita; et al. (19 March 2019). "The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes". Science. 364 (6437): eaaw0422. doi:10.1126/science.aaw0422. PMID 30890587.


External linksEdit