Quaoar - Hubble Space Telescope Measures

The Hubble Space Telescope has measured the diameter of a distant world more than half the size of Pluto.

Astronomers have dubbed it "Quaoar" (pronounced kwa-whar) after a Native American god. It lies a billion kilometres beyond Pluto and moves around the Sun every 288 years in a near-perfect circle. Until recently it was just a curious point of light. That's all astronomers could see when they discovered it last June 2002 using a ground-based telescope.

But now it's a world.

NASA's Hubble Space Telescope has measured Quaoar and found it to be 1300 km wide. That's about 400 km wider than the biggest main-belt asteroid (Ceres) and more than half the diameter of Pluto itself. Indeed, it's the largest object in the solar system seen since the discovery of Pluto 72 years ago.

Quaoar is greater in volume than all known asteroids combined. Researchers suspect it's made mostly of low-density ices mixed with rock, not unlike the makeup of a comet. If so, Quaoar's mass is probably only one-third that of the asteroid belt.

Michael Brown and Chadwick Trujillo of the California Institute of Technology, Pasadena, California, reported these findings at the 34th annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Birmingham, Alabama in Oct 2002.

Quaoar's size compared to that of Earth, Earth's moon and Pluto.

Earlier in 2002, Trujillo and Brown had used the Palomar 48-inch telescope to discover Quaoar as an 18.5-magnitude object creeping across the summer constellation Ophiuchus. Although Quaoar was relatively bright (by the feeble standards of such distant objects) its disk was too small for the Palomar telescope to resolve.

Brown followed-up their discovery using the Hubble Space Telescope. Hubble's new Advanced Camera for Surveys revealed the object's true angular size of 40 milliarc seconds, corresponding to a diameter of about 800 miles (1300 kilometres). Only Hubble has the sharpness needed to actually resolve the disk of such a distant world.