Mark Showalter of Stanford University and Jack Lissauer of NASA's Ames Research Center studied images of Uranus taken last month by Hubble's Advanced Camera for Surveys (ACS). The two moons--temporarily dubbed S/2003 U 1 and S/2003 U 2--are extremely faint and eluded detection by the Voyager 2 spacecraft during its flyby of the planet in 1986. "It's a testament to how much our Earth-based instruments have improved in 20-plus years that we can now see such faint objects 2.8 billion kilometers away," Showalter says. Indeed, the two moons are 40 million times dimmer than Uranus is and the researchers had to overexpose the images in order to pinpoint their locations. S/2003 U 1 is the larger of the pair, measuring 16 kilometers across, whereas S/2003 U 2 is just 12 kilometers wide. The discovery brings the total number of moons in the Uranian system to 24, placing it third behind Jupiter's 38 officially-named moons and Saturn's 30.
Both S/2003 U 1 and S/2003 U 2 circle Uranus in less than a day, with paths that bring them close to the planet's bigger, better-known moons Puck, Miranda and Belinda. "The inner swarm of 13 satellites is unlike any other system of planetary moons," Lissauer remarks. "The larger moons must be gravitationally perturbing the smaller moons. The region is so crowded that these moons could be gravitationally unstable. So, we are trying to understand how the moons can coexist with each other."