This Week On Galileo
February 15-21, 1999
Galileo has returned to operating in a normal mode after executing two turns last week to point the spacecraft's radio antenna back to Earth. This week's activity focuses on playback of science information acquired during Galileo's January 31, 1999 flyby of Europa. Playback is interrupted once this week to perform another spacecraft turn to keep Galileo's radio antenna pointed towards Earth.
Last week's spacecraft turns were required after on-board fault protection software detected that a previous spacecraft turn, attempted on January 31, was taking too long to complete. The software correctly placed the spacecraft in safe mode to await instructions from Earth, but prevented it from achieving the desired Earth-pointed attitude. All remaining encounter commands were cancelled when Galileo entered safe mode, preventing the spacecraft from making planned distant observations of Europa, Io and Jupiter.
Investigations into the anomaly suggest that it was caused by a minimum or inflection point in the sensitivity of two sensors used to detect the sun, which was being used as a guide for the spacecraft turn. This vulnerability of the sun sensors may be partially due to exposure to radiation at Jupiter, but is not expected to have any impact on the remainder of Galileo's mission.
Cruise operations were initiated last Thursday, February 11, and included standard maintenance on the spacecraft's propulsion system, and a standard gyroscope performance test. Processing and transmission to Earth of observations taken prior to safing was also initiated last Thursday. Through this week Galileo's playback schedule includes observations obtained by the near-infrared mapping spectrometer, the photopolarimeter radiometer, the spacecraft camera, and the suite of six fields and particles instruments.
The photopolarimeter radiometer returns four observations during this period, all containing polarimetry measurements. Two of these focus on Jupiter and will allow scientists to learn more about the vertical cloud structure of Jupiter's atmosphere, including particle shape and size. The other two observations focus on Europa and will provide information on the texture and composition of Europa's surface materials.
The near-infrared mapping spectrometer returns six observations--three of a hot spot and three of a region of Jupiter's Northern Temperate Belt. Each observation will provide measurements of the composition and thermal properties of these regions of Jupiter's atmosphere.
The camera returns five observations during this week. All contain Europa and were taken during the spacecraft's close flyby of the icy moon. The first observation contains the Tegid crater region and will be used to characterize the crater's shape and determine if it has a central dome feature similar to craters seen on other Galilean satellites. The second observation contains a region of mottled or blotchy-looking terrain in an attempt to determine if there is any relationship between this type of terrain and Europa's well known triple band features. In a regional observation, the camera captures two images. The first of these images will fill a gap in an already existing map of Europa, while the second will be used to determine whether or not Europa's rotation is synchronous. The last two observations of this week will provide information on the structure of Europa's north polar plains.
Finally, the fields and particles instruments begin the return of their observation of the plasma, dust, and magnetic and electric fields surrounding Europa. These data were recorded during the 50 minutes when Galileo was closest to Europa and will improve the understanding of the interaction between Europa and Jupiter's magnetosphere.