Today On Galileo - March 28, 1998
The second science encounter of the Galileo Europa Mission, the follow on to Galileo's primary mission, started today when encounter-related computer commands began executing at 5 am, Pacific Time. For the next three days, science instruments onboard the Galileo spacecraft will gather more information on the Jupiter system and its intriguing moons. The information will be stored on the onboard tape recorder for later playback. Once again, the bulk of the science observations will focus on Jupiter's moon Europa, but Jupiter, Jupiter's magnetic and electric field environment, and each of the other three Galilean moons: Io, Ganymede and Callisto, are also observed.
This encounter is being performed without the gyroscopes. That's because of the recent discovery of further degradation of the anomalous behavior of the spacecraft's attitude control subsystem. During data collection by the remote sensing instruments (camera, spectrometers and radiometer), the gyroscopes are used to improve the stability of instrument pointing. They detect small wobbling of the spacecraft and compensate to keep the instruments steady. Without the gyros, no motion compensation is available. This is expected to result in a small degradation in the quality of the data that is gathered.
As the spacecraft flies through the Jupiter system, it passes points of closest approach to each of the bodies of the system. Today, the spacecraft passes within 250,000 kilometers (155,000 miles) of Io and 632,000 kilometers (393,000 miles) of Jupiter at 8:48 pm and 11:59 pm, respectively, both in Pacific Time.
The fields and particles instruments kick off the science data gathering activities by initiating a low-rate survey (information is collected at a rate of only a few bits per second) of Jupiter's magnetic and electric field environment, also known as the magnetosphere. Because of the low rate, this data can be packaged and transmitted to Earth almost immediately. This type of observation is said to be performed in 'real-time'. The survey is performed continuously for the duration of the encounter and the information obtained will provide scientists with a context for data gathered at a higher time resolution (hundreds of bits per second) later in the encounter. The information will also add to the existing record of orbit-to-orbit of activity levels within the inner, most active, portions of the magnetosphere.
The first remote sensing observation of the encounter is performed by the Ultraviolet Spectrometer (UVS). This observation of Europa will provide data on Europa's atmosphere. Also obtained on previous orbits, this information allows scientists to monitor changes in the characteristics of the atmosphere from orbit to orbit. Large changes in these characteristics could be indicative of geologic activity on the surface of Europa. A small observation of Jupiter is performed next by the Near Infrared Mapping Spectrometer (NIMS). The information gathered by this observation is designed to study long term changes in the composition and temperature of Jupiter's atmosphere.
As the spacecraft approaches its point of closest approach to Io for this orbit, the science instruments turn their attention to this fiery moon. The UVS instrument performs an atmospheric monitoring observation similar to the one performed earlier on Europa. This is followed by a series of color pictures taken by the camera of Io's north and south pole regions. These pictures will improve the detail of these regions by providing resolutions of 3 kilometers (1.8 miles) per picture element. The best color resolution obtained during Galileo's primary mission was 10 kilometers (6.2 miles) per picture element. Monochrome images as good as 2.5 kilometers (1.5 miles) per picture element were obtained in the primary mission, but the color in these new images will be critical to identifying surface materials. The pictures will also be useful to scientists as they plan observations for the return to Io at the end of the Galileo Europa Mission.
The photopolarimeter radiometer (PPR) contributes further to the Io data set for this orbit with an observation designed to characterize the different temperatures on Io's surface. This is followed by a joint observation of Io's surface performed by the NIMS instrument and the UVS instrument. The observation is designed to keep track of any changes due to volcanic activity.