Today On Galileo - June 1, 1998
Observing activities become a bit more target diverse today and once again include Io, Jupiter and Europa. The spacecraft passes through closest approach to the remaining two Galilean satellites today, but no observing of these satellites is planned because these approaches are not as close as previous flybys. Early this morning, at 12:27 AM PDT*, the spacecraft passes within 1.2 million kilometers (0.8 million miles) of Callisto. At 2:51 PM PDT, it passes within 3.2 million kilometers (2 million miles) of Ganymede.
Today, the spacecraft completes all observations to be stored on its onboard tape recorder for later processing and transmission to Earth. As a result, Galileo begins processing and transmitting the first observations stored on the tape recorder. Also, in preparation for the end of the encounter period, flight team members transmit to the spacecraft the set of commands that will control its activities throughout the cruise period.
The morning starts out with an observation of Io performed by the spacecraft's camera. The observation is part of a global map that will be completed with data gathered later in the Galileo Europa Mission. The information that is acquired will also allow scientists to monitor changes in the volcanic regions of Pele, Marduk and Reiden. These regions are scheduled for further observation in October 1999. Later in the day, the camera takes another look at Io, but this time while eclipsed from the Sun by Jupiter. These images will be used to discover and monitor lava temperatures and barely visible interactions between volcanic plumes, Jupiter's magnetosphere and Io's atmosphere. Toward the end of the day, the camera takes a look at the Kanehekili plume region. This region was discovered to be active in May 1997 and the observation will allow scientists to determine whether any changes have occured since then.
Sprinkled throughout the day are observations of Europa and Jupiter performed by the near infrared mapping spectrometer and the ultraviolet spectrometer. The observations of Europa are performed 16, 20 and 22 hours after the close flyby of the moon. The distant mapping is performed with special emphasis given to the non-ice material of Europa's surface. A set of three observations of Jupiter is performed by the near infrared mapping spectromenter. The set is designed to search for variations in temperature and composition across Jupiter's different belts and cloud zones. A single ultraviolet spectrometer observation of Jupiter is performed to measure the changes in the amount of hydrogen in Jupiter's upper atmosphere. The observation is performed on Jupiter's dark side to study the changes in the absence of sunlight. Without sunlight, the changes are believed to be the result of the atmosphere's interaction with charged particles in combination with mixing with Jupiter's lower cloud levels.
Processing and transmission of science information stored on the spacecraft's onboard tape recorder starts just after 3 PM, Pacific Time. Two observations of Io, performed by the spacecraft camera, are on today's schedule. The first will contain information on the intensity of light reflected off Io's surface, which will be used to determine the size and age of sulfur dioxide grains. The second contains Io in Jupiter's shadow, and like the observations that were performed today, will provide information on the lava temperatures and interactions between volcanic plumes, the magnetosphere and Io's atmosphere.
In parallel with the processing and transmission activity, the ultraviolet spectrometer and extreme ultraviolet spectrometer take a quick look at the Io torus. The observation can be performed in parallel because the data the instruments gather are immediately packaged and transmitted to Earth and not stored on the spacecraft's tape recorder for later processing. This observation starts this evening and continues through most of tomorrow.
* All times of day listed correspond to the Pacific Time zone (currently daylight savings) and spacecraft event time. Radio signals indicating that an event has occured on the spacecraft reach the Earth 35 to 50 minutes later, depending on the time of year.