Artist's Concept of Kuiper Belt Object 2003 UB313 (Annotated)
This is an artist's concept of Kuiper Belt object 2003 UB313 (nicknamed "Xena") and its satellite Gabrielle. NASA's 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). Pluto's diameter is only 1,422 miles.