An artist's concept of Kuiper Belt object 2003 UB313 (nicknamed Xena) and its satellite Gabrielle. - stock illustration

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 December 9 and 10, 2005, yielded a diameter of 1,490 miles (with an uncertainty of 60 miles) for Xena. Pluto's diameter, as measured by Hubble, is 1,422 miles. Xena is the large object at the bottom of this artist's concept. A portion of its surface is lit by the Sun, located in the upper left corner of the image. Xena's companion, Gabrielle, is located just above and to the left of Xena.