Today on Galileo
Wednesday, May 5 - Friday, May 7, 1999
Galileo's encounter activities continue to clock out for the next three days, ending on Friday, May 7 at 5 am PDT [see note 1]. Science data gathering continues through Noon PDT on Wednesday, at which time, playback of the data stored on the spacecraft's onboard tape recorder during the last few days is initiated. Playback is interrupted once, on Wednesday evening, to perform a standard gyroscope performance test, and then continues through the end of the week.
On Wednesday, the spacecraft flies past Callisto at 6:56 am PDT at a distance of 1315 kilometers (817 miles). This is the first in a series of four flybys that will slowly change Galileo's orbit around Jupiter enough to allow two flybys of Io later this year. Galileo's science instruments take advantage of the flyby to make some close observations of Callisto.
The spacecraft's Solid State imaging camera makes three observations at different spatial resolutions designed to gather statistics on the sizes and distribution of craters on Callisto's surface. These observations will allow scientists to estimate the age of Callisto's surface and gain a better understanding of the variations in the distribution of craters in different regions of the surface. Images of a feature believed to be a dark surface flow will also be taken, and a sample of dark terrain will be imaged through seven color filters to provide information on surface composition. Finally, the camera takes an image of the Bran crater, a young, single-ring crater that should be a good probe of Callisto's crust.
The Near-Infrared Mapping Spectrometer and Utraviolet Spectrometer also make observations of the Bran crater to provide information on the chemical composition and variations of the region. The two instruments continue to collaborate in a series of five observations. The first is designed to scan Callisto's bright limb in hopes of detecting and determining the composition of Callisto's tenuous atmosphere. In the remaining four observations, the two instruments look at different regions of Callisto's surface to obtain measurements of the chemical composition of the regions.
In addition, the Dust Detector will be making measurements of the enhanced number of dust particles near Callisto by using a combination of real-time and recorded data. Some of these dust particles are likely to be ejecta from impact events on Callisto's surface.
Playback is initiated soon after all remote sensing observations have been completed. First on the playback schedule is an observation taken by the Near-Infrared Mapping Spectrometer designed to provide information on the surface composition of the Prometheus volcano region. This is followed by observations of Europa taken while it was in Jupiter's shadow, performed by the spacecraft camera and the Near-Infrared Mapping Spectrometer. The observations are designed to detect atmospheric emissions, best seen in the dark, that could be indications of geologic surface activity. Finally, the Photopolarimeter Radiometer returns an observation of Jupiter's clouds in an attempt at finding small temperature variations within a given cloud band.
Beginning Friday morning and for nearly eight days, the six Fields and Particles instruments will collect real-time data and transmit them to Earth. This observation is the first opportunity to map the dusk-side portion of Jupiter's magnetosphere. As plasma is carried by magnetic field lines from the night side of Jupiter around to the day side, it becomes compressed. The compression is due to the solar wind, which "pushes" on Jupiter's magnetosphere, forcing the plasma into a smaller region of space as it rotates toward the day side. As it rotates toward dusk, the plasma can then expand and accelerate. Some of the plasma escapes outward, away from both Jupiter and the Sun. During the eight day observation, scientists hope to begin to understand where this plasma acceleration takes place, and to observe the detailed characteristics of the plasma and magnetic and electric fields in the region.
With this, another encounter for the Galileo Europa Mission comes to a close. Come back on Saturday for the return of This Week on Galileo and keep tuned as we make our way back to Io!
Note 1. All times listed correspond to the Pacific Time zone (currently daylight time) and spacecraft event time. Radio signals indicating that an event has occurred on the spacecraft reach the Earth 33 to 50 minutes later, depending on the time of year. Currently, Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian Time (GMT), and it takes radio signals 49 minutes to travel between the spacecraft and Earth.
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page.
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