This is an artist's concept of Kuiper Belt object 2003 UB313 (nicknamed Xena) and its satellite Gabrielle. NASA/ESA Hubble Space Telescope observed Xena with the Advanced Camera for Surveys on Dec. 9 and 10, 2005. The Hubble measurements of Xena yielded a diameter of 1,490 miles with an uncertainty of 60 miles (about 2400 kilometers with an uncertainty of 100 kilometers). Pluto's diameter is only 1,422 miles (or 2,288 kilometers).