Planetary escorts

Published: Sunday 30 April 2000

Astronomers analysing images of Uranus from the Hubble Space Telescope have found two tiny moons orbiting the planet. The finding supports a theory that the moons -- one inside Uranus's main ring and one outside -- act as "shepherds", keeping the ring from dispersing. The moons, named Cordelia and Ophelia, were first spotted by the Voyager 2 spacecraft in January 1996 when it whizzed passed the planet. But scientists could not get a precise fix on their orbits because light from the main ring blotted out the moons.

Philip Nicholson of Cornell University in New York State and Richard French of Wellesley College, Massachusetts, however, analysed images of Uranus from a range of different telescopes. They saw that the ring pass in front of distant stars and spotted what looked like ripples produced by the moons. This prompted the theory that the moons were keeping the ring in line ( New Scientist , Vol 165, No 2229).

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