An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

This is the last week before the August 4 start of Galileo's next encounter with the volcanic satellite Io. As playback of data from the May flyby of Callisto winds down, the final observations to be returned come from the Near Infrared Mapping Spectrometer (NIMS) and the Solid State Imaging camera (SSI). NIMS data concentrates on Jupiter atmospheric observations, including a global map of the giant planet. NIMS takes detailed looks at some persistent hot spots in the turbulent clouds and at the region trailing the Great Red Spot. SSI will be returning global color pictures of Ganymede, Jupiter's largest satellite.

While the Flight Team makes final preparations for next week's Io flyby, the spacecraft undertakes a few last housekeeping tasks to get ready. On Thursday, routine maintenance of the on-board tape recorder is performed.

On Friday, playback is stopped, and the final targeting orbit trim maneuver is executed. This engine burn could last as long as six hours, and ensures that Galileo reaches its scheduled rendezvous with Io at the correct time and place. Six hours may seem like a long time to run the engine, but remember that Galileo is like a large gyroscope, spinning at a stately 3 revolutions per minute. In order to nudge the path of the 1300 kilogram (2870 pound) spacecraft in a particular direction, a set of small 10 Newton thrusters (about 2.2 pounds of thrust each) are fired for less than one second per pulse on each revolution. Galileo has twelve such thrusters, some pointing forward, some backward, and some to the sides. The choice of which thrusters to fire and when to fire them determines what direction the spacecraft moves. They can also be used to turn the spacecraft in place, pointing its antenna in a new direction, with no change to its orbital path about Jupiter.

Typically, final targeting maneuvers such as this one change the spacecraft velocity by a few tenths of a meter per second. Compare this to the 7.1 kilometers per second speed of Galileo as it flies by Io. These are truly gentle nudges in the grand scheme of things!

After the maneuver, the tape is repositioned to the correct starting place to begin recording the next set of data from the upcoming Io flyby. It's about to get busy again!