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Io Dangles Near Jupiter
By SPACE.com Staff
posted: 12:26 pm ET
21 December 2000
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*** Like an ornament hanging in space, Jupiter's moon Io is seen suspended -- and dwarfed -- by the giant gas planet in the latest image from the Cassini spacecraft. Meanwhile, the craft suspended science operations Dec. 20 because of a system problem. 
Update: 22 December - Engineers at NASA's Jet Propulsion Laboratory resumed use of three electrically powered reaction wheels late Thursday, Dec. 21, because of encouraging test results. They speculated that a small bit of material,perhaps a magnet in one of the motors on the maneuvering devices, worked its way to a position that caused friction in the motor.
Cassini continues on course to make its closest approach to Jupiter -- about 6 million miles (9.7 million kilometers) -- on Dec. 30. A gravity boost from the planet will hurl the craft toward an encounter with Saturn in July 2004. As it nears Jupiter, Cassini has been returning some of the most detailed images ever of the giant planet. The shadow of Io, one of Jupiter's four largest moons, is seen on Jupiter's cloud tops. Io, along with Europa, Ganymede and Callisto, are called the Galilean moons after Galileo Galilei, who discovered them in 1610.The image was taken Dec. 12 and released Wednesday. Cassini problems The Cassini flight team has been testing one of the craft's maneuvering systems to try and figure out what caused the craft to switch automatically to a second system Dec. 17. Cassini uses a trio of electrically driven "reaction wheels" for turning itself in different directions to point science instruments. These wheels affect which direction the craft is facing, but not its trajectory through space. One of the wheels required more than the normal amount of force to turn it last Saturday, so Cassini automatically switched to a maneuvering system that uses small thrusters that burn a substance called hydrazine. Science operations were put on hold to conserve the hydrazine, which is needed for the later mission at Saturn."We are responding cautiously while we test the systems," said Cassini Program Manager Bob Mitchell at NASA's Jet Propulsion Laboratory. "This might turn out to have no long-term consequences, but we want to better understand what happened before we proceed with using the wheels more." More images of Jupiter are expected to be released in coming days, mission managers said.
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