Today On Galileo - May 31, 1998
The level of activity increases dramatically today, the second day of the Europa-Orbit 15 encounter period. The bulk of today's activity is geared toward the acquisition of science information to study Europa. The spacecraft flies over Europa's surface at a distance of 2516 kilometers (1564 miles) at 2:13 PM (PDT*).
The Europa flyby is preceded by passage through the point of closest approach to Io for this orbit, which occurs at 1:59 PM (PDT) at a distance of 313,000 kilometers (194,000 miles). In the evening, the spacecraft passes through the point of closest approach to Jupiter. Jupiter closest approach occurs at 7:35 PM and a distance of 633,000 kilometers (393,000 miles).
The radio science team spends the day measuring changes in Galileo's radio frequency as the spacecraft flies past Europa. Using the Doppler effect, the change in frequency can be related to change in the speed of the spacecraft and Europa's gravitational pull on the spacecraft. These measurements are taken almost every orbit and are used to improve the map of the gravity field produced by Europa.
Today's remote sensing activity on Europa involves all four remote sensing instruments. The spacecraft camera focuses on a region containing the largest known massif on Europa. A massif is a block of crust that is surrounded by faults and has been displaced or moved without having broken apart. The region under observation is named Cilix. Two images of this region are obtained to allow the construction of a stereo image. An observation of the same region is also taken by the near infrared spectrometer. The near infrared spectrometer data will provide scientists with information on what types of materials are found in this region. The camera also looks at an unusually rugged region containing pits and mounds with a very prominent ridge. The region is located east of the Tyre Macula impact crater and will also be imaged twice to allow for the construction of a stereo image. Both the near infrared spectrometer and the ultraviolet spectrometer also look at this region in order to provide composition information.
Observations of Europa at global and regional scales are sprinkled throughout the day. The camera looks at a previously unexplored region of mottled, or blotchy, terrain and also takes photometric measurements (light intensity that provides clues for material identification) on a global scale. The near infrared mapping spectrometer looks for surface composition information, also on a global scale, and is accompanied on the search by the ultraviolet spectrometer.
Surface temperatures and thermal characteristics are examined by the photopolarimeter radiometer instrument. This data is expected to help determine how compact the surface on Europa is, how old it is, what makes it up and how it got the way it is. Both high and low resolution observations are planned.
Last, but not least, for about an hour surrounding the Europa flyby, the fields and particles instruments perform a high time resolution (several hundreds of bits per second) recording of measurements of the dust, magnetic and electric field environment. These data will help scientists improve their understanding of the interaction between Europa and its surroundings.
A few observations of Io are scattered throughout the day. Two of them are performed by the near infrared mapping spectrometer in conjunction with the ultraviolet spectrometer. A third is performed by the ultraviolet spectrometer alone. The joint observations are the second highest resolution observations of Io planned for the Europa orbits of the Galileo Europa Mission. The Pele, Marduk and Reiden volcanic regions are targeted for observation. These same regions are planned for observation again in October 1999. The remaining ultraviolet spectrometer observation is planned to provide data that will complement information gathered during Galileo's primary mission. The data should provide information that will allow scientists to better understand the density of Io's atmosphere and how it varies with location on the moon and the passage of time. Scientists also hope to uncover relationships to volcanic activity and sulfur frost sublimation.
* 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.