galileo_gflyby_000521 NASAs aging Galileo orbiter swooped by the jovian moon Ganymede on Saturday in a trajectory set-up for joint observations of Jupiter the probe will make with fellow spacecraft Cassini this winter.
Galileo passed within 501 miles (808 kilometers) -- about the distance separating San Diego and San Francisco -- of Ganymede at 6:10 a.m. Eastern Daylight Time (10:10 GMT) on Saturday. However, radio signals indicating the flyby had occurred took 50 minutes to reach Earth, traveling across 558 million miles (898 million kilometers) of space.
"Its scary to be thinking G 28 on a mission that was supposed to go to 11."
With each pass close to Jupiter, Galileo risks system glitches caused by the planets intense radiation environment. Because of that, NASA will be uncertain how well the spacecraft fared during the flyby until late Sunday at the earliest. Early Saturday, NASA gave a positive report on the spacecraft, which had completed about half its scheduled observations.
"Everything looks fine," said Jim Erickson, the Galileo project manager at NASAs Jet Propulsion Laboratory (JPL).
NASA has dubbed the flyby Ganymede 28 (G 28), since it takes place on the 28th orbit of Jupiter since Galileo first arrived at the giant world in 1995 for what was supposed to have been a two-year main mission. NASA has since granted the mission two reprises.
"Its scary to be thinking G 28 on a mission that was supposed to go to 11," Erickson said.
'An inside man and an outside man'
The close passage was the fifth time Galileo has flown by Ganymede. The chunk of ice and rock, named for the cupbearer to the gods in ancient Greek mythology, is the solar systems largest moon, larger than the planets Mercury and Pluto.
Although Galileo is gathering valuable science during the leisurely two days of observations leading up to, during and following the flyby, NASA designed the maneuver as a set-up, Erickson said.
NASA's Galileo spacecraft flew by Ganymede, seen here in an image taken June 26, 1996, for the fifth time Saturday.
For most of the time Galileo has spent at Jupiter, the largest planet in the solar system has held it in a tight orbital embrace. Over the last five years, the spacecraft has rarely traveled outside of Jupiters magnetosphere, the region where the influence of its magnetic field holds sway.
Saturdays pass by Ganymede, however, will lengthen the period of Galileos orbit, stretching it farther out into space. A second flyby, scheduled for December 28, will complete the task.
"G 28 starts it and G 29 gets it out as far as we want to be," Erickson said.
Once NASA's Cassini probe begins to swing by Jupiter to gain momentum for its journey to Saturn, it will begin making joint observations with Galileo of the planets magnetic field and radiation environment and how the solar wind affects both.
For a portion of the observations, Cassini will remain outside Jupiters magnetosphere, while Galileo lurks within it.
"Well have an inside man and an outside man," Erickson said. The two will then flip-flop, thanks to Galileos new orbital path.
Following the joint observing phase of its mission, Galileo faces an uncertain future.
Low on propellant, the spacecraft is expected to last another year or two at Jupiter. How the mission will end remains uncertain, but the spacecraft may be sent crashing into Jupiter itself to avoid the possibility of hitting the icy moon Europa. That moon could be a possible abode for extraterrestrial life that NASA scientists would not want to risk contaminating with any earthly bugs stowed away on Galileo.
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