Today on Galileo
31 Jan 1999
(Source: Jet Propulsion Laboratory)
Today is a very busy day for the spacecraft as it gathers science information describing Jupiter, Europa and Ganymede. Galileo flies over Europa's surface at 6:20 pm PST [see Note 1] at an altitude of 1439 kilometers (894 miles). Just after 9:00 pm, the spacecraft will pass within 8.1 Jupiter radii (580,000 kilometers, 360,000 miles) of Jupiter's cloud tops. In addition, the spacecraft performs several turns to improve the view for some of the instruments' observations of Europa.
The day begins with an observation of Jupiter by the photopolarimeter radiometer. This measurement is one of several that will be used to complete a polarimetry map of Jupiter's atmosphere. These polarimetry measurements will provide the science community with information on the texture and composition of Jupiter's atmosphere. This observation is followed by a series of six performed by the near-infrared mapping spectrometer. In sets of two, the observations alternate between gathering measurements of a hot spot and a region of Jupiter's Northern Temperate Belt. The ultraviolet spectrometer follows these observations with a look at Europa's surface, again searching for clues as to how the surface has been affected by external phenomena.
Shortly after 8 am PST, the radio science team will begin careful tracking of the changes in the frequency of Galileo's radio signal. These changes are caused by Europa's gravitational pull on the spacecraft, and the resulting Doppler shift in Galileo's radio signals. The team will make these measurements for 20 hours, centered on the point of closest approach to Europa, and will use them to refine models of Europa's gravity field and internal structure.
Mid-morning, the spacecraft performs its first turn of the day. The purpose of the turn is to allow the spacecraft's camera and other remote sensing instruments a view of Europa that is unobstructed by the spacecraft's booms and sunshield. The turn is necessitated by the geometry of this Europa flyby. About 30 minutes after the turn is complete, the photopolarimeter radiometer takes advantage of the improvement in viewing geometry to take a look at Europa on a global scale, taking another set of polarimetric measurements. A few minutes later, the instrument looks back at Jupiter and takes a second set of polarimetry measurements. A couple of hours later, the photopolarimeter radiometer looks back at Europa to gather more polarimetry measurements at regional resolutions.
Shortly after the first photopolarimeter observation, the spacecraft camera takes the first of its 10 observations during close approach to Europa. The camera first looks at the Tegid crater region to characterize its shape and determine if it has a central dome feature similar to craters seen on other Galilean satellites. The camera then looks at a region of mottled or blotchy-looking terrain to determine if there is any relation between this type of terrain and well known triple bands. In a regional observation, the camera captures two images, the first of which will be used to fill in a gap in an already existing regional map of Europa. The second is used to obtain data to determine whether Europa is or is not in synchronous rotation, a question of importance in regard to the issue of tidal heating of that body. Another set of images taken by the camera will provide information on the structure of Europa's north polar plains, and an observation of Rhadamanthys Linea covers a region of the feature that may have been formed by cryo-volanic activity. The camera's next three observations capture data on Europa's Pwyll crater, a region of mottled terrain, and a region with specular or mirror-like appearance. Finally, the camera takes a series of images that will capture Europa's bright limb in a search for plumes. All of these observation are completed just prior to the spacecraft's closest approach to Europa.
Following closest approach, the near-infrared mapping spectrometer performs two observations. Both of these are designed to search for a particular spectral signal that comes from ice in a hexagonal crystal structure. This structure may indicate relatively recent cryo-volcanism on the surface of Europa. The photopolarimeter radiometer takes the observing stage next, but turns its attention to Ganymede and gathers a set of polarimetry measurements of the satellite. This data will be used to fill in gaps in data sets obtained on previous orbits.
Interleaved with these observations, for an hour surrounding the Europa flyby, the fields and particles instruments perform a high time resolution recording of measurements of the plasma, dust, and magnetic and electric fields surrounding Europa. These data will help scientists improve their understanding of the interaction between Europa and Jupiter's magnetosphere.
Following the photopolarimeter radiometer's observation of Ganymede, the spacecraft performs the second turn of the day. This turn brings the spacecraft back to a more normal near-Earth pointed attitude. Following completion of the turn, the photopolarimeter radiometer takes another set of polarimetry measurements of Ganymede, and brings to an end today's observing schedule.
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page:
Note 1. All times listed correspond to the Pacific Time zone (currently standard 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, this time is 46 1/2 minutes. Currently, Pacific Standard Time (PST) is 8 hours behind Greenwich Meridian Time (GMT).