Kuiper Belt Object 2003 UB313
NASA's Hubble Space Telescope resolved Kuiper Belt object "Xena" for the first time and found that it is only just a little larger than Pluto.
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) for Xena. Pluto's diameter, as measured by Hubble, is 1,422 miles.
Located 10 billion miles away, but with a diameter that is a little more than half the width of the United States, Xena is 1.5 picture elements across in Hubble's view.
Xena is officially catalogued as 2003 UB313. Its orbital period is about 560 years, and the Kuiper Belt object is now very close to aphelion (the point on its orbit that is farthest from the Sun).