Cassini Gets Ready to Run Rings Around Saturn

Saturn’s rings acquire a new adornment tonight as the Cassini-Huygens spacecraft prepares to begin its four-year orbit around the sixth planet. n At 10:36 p.m. ET, the spacecraft will fire a braking rocket for 96 minutes and knife into a gap between two of Saturn’s seven main rings. The craft’s largest antenna will be turned to act as a shield against small hazardous objects.

”This is not going to look like Star Wars. This (gap) is quite civilized,” says Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Telescopes have determined that little dust exists in the gap between the targeted rings.

Radio waves sent to Earth during and after the passage will provide data on the rings’ composition, thought to be a mixture of ice and dust. The rocket burn is scheduled to be completed at 12:12 a.m. ET Thursday.

At a NASA briefing Tuesday, mission scientists were upbeat.

Signals take about 83 minutes to reach Saturn from Earth, making direct control of the spacecraft impractical. Final instructions were sent to the craft on Sunday, leaving little for controllers to do until the passage besides ”chewing my nails,” says JPL’s Robert Mitchell, who heads the mission team.

”We expect to see this going very, very smoothly,” says Julie Webster, JPL’s spacecraft operations manager.

The mission’s highlight is expected in January, when the Huygens probe parachutes into the dense clouds swathing Titan, a moon larger than Mercury and Pluto. Images of Titan captured by Cassini and released Tuesday hint at methane oceans, possibly offering a soft landing for the short-lived Huygens probe, which otherwise will crash into the surface.

No probes have visited Saturn since Voyager 2′s flyby in 1981. With its 18 scientific instruments, Cassini will seek data on Saturn’s rings, moons, atmosphere and magnetic field. The $3.27 billion mission, which was launched in 1997, is a partnership of NASA, the European Space Agency and the Italian Space Agency.

Already, Cassini has delivered intriguing findings. Information from a flyby of the moon Phoebe reveals it to have been a stray object from a comet belt beyond Neptune before being captured by Saturn.

Natural radio signals that Cassini picked up suggest that Saturn’s day lasts 10 hours, 45 minutes and 45 seconds. That’s about six minutes longer than radio measurements taken by the Voyager spacecraft, a puzzle that researchers hope to solve during Cassini’s sojourn.