The asteroid 1999 JU3, a target of the Asteroid Explorer "Hayabusa2," was named "Ryugu".
The Japan Aerospace Exploration Agency (JAXA) conducted a naming proposal campaign between July 22 and August 31, 2015. The result of the careful study of proposed names by the selection panel of pundits is as follows.

1. Selected name


2. Selection reasons

  • In the Japanese ancient story "Urashima Taro", the main character, Taro Urashima, brought back a casket from the Dragon's palace, or the "Ryugu" Castle, at the bottom of ocean. The Hayabusa2 will also bring back a capsule with samples, thus the theme of "bringing back a treasure" is common.
  • Rocks containing water are expected to exist on asteroid 1999 JU3. The name "Ryugu" also reminds us of water, as "Ryugu Castle" is under the ocean.
  • The name is not similar or identical with any other already existing names of planets or asteroids, and there were many entries for this name among suggested names that are related to mythology.
  • According to the naming rule stipulated by the International Asteroid Union (IAU), the name "is preferably from mythology" and the "Ryugu" fits that rule. Also, there is little concern of infringing the Trademark Law or any other third party trademarks.

3. Entries

Total number of entries was 7,336. Among them, 30 were for "Ryugu". Some similar ones are "Ryugujyo (meaning Ryugu Castle)" (5 entries), "RyuuGuu" (5), "Ryuguu" "Ryugujyo" and "Ryugujou" (one entry each).

4. Selection panel members

Yuichi Takayanagi Director, Tamarokuto Science Center

Jun-ichi Watanabe Vice-Director General, National Astronomical Observatory of Japan (NAOJ)

Yoshio Tsukio Professor Emeritus, the University of Tokyo

Yuichi Tsuda "Hayabusa2" Project Manager Associate Professor, Department of Space Flight Systems, ISAS/JAXA Makoto Yoshikawa "Hayabusa2" Mission Manager Associate Professor, Department of Spacecraft Engineering, ISAS/JAXA

5. Decision process after selection

The selected name was proposed to the Lincoln Near Earth Asteroid Research (LINEAR) Team of the United States, which had the right to suggest the name for 1999 JU3. The LINEAR Team then proposed the name to the IAU, which had the final naming right.
The naming of an asteroid usually takes about three months to review. The review process for the "Ryugu" this time was unprecedentedly quick, and the name was published on September 2015 issue of the Minor Planet Circular by Minor Planet Center, which manages the names of asteroids in the solar system.


Ryugu(1999 JU3) © JAXA