This is an artist's concept of Kuiper Belt object 2003 UB313 (nicknamed Xena) and its satellite Gabrielle.

Ground-based observations suggested that Xena was about 30 percent greater in diameter than Pluto, but Hubble's Advanced Camera for Surveys images, taken on Dec. 9 and 10, 2005, yielded a diameter of 1,490 miles with an uncertainty of 60 miles (about 2400 kilometres with an uncertainty of 100 kilometres) for Xena. Pluto's diameter, as measured by Hubble, is 1,422 miles.

Credit: NASA, ESA, and A. Schaller (for STScI)