How asteroid 1999 UX18 became Jupiter XVII

The story of a faint dot that turned out to be circling a planet rather than the Sun.

Published 2026-10-23

Survey telescopes record thousands of moving dots. Most are asteroids. In late 1999, one of them was not.^[1]

The observations

NASA dates the discovery to 19 October 1999, made with the 36-inch (0.9 m) telescope on Kitt Peak by the University of Arizona’s Spacewatch program.^[2] The object was followed from 6 October to 4 November 1999.^[1]

An asteroid name first

Its motion looked like an asteroid’s, so it was catalogued as 1999 UX18.^[1]

The orbit gives it away

When the Minor Planet Center at the Smithsonian Astrophysical Observatory computed its orbit, the path only made sense around Jupiter.^[2] Tim Spahr established this on 18 July 2000, and the object became S/1999 J 1.^[1]

The credited team

NASA credits Jim V. Scotti, Timothy B. Spahr, Robert S. McMillan, Jeffrey A. Larsen, Joe Montani, Arianna E. Gleason and Tom Gehrels.^[2]

A name and a number

In October 2002 it was named Callirrhoe and numbered Jupiter XVII.^[1]

Numbered citations link to the sources list on the Callirrhoe overview page.