This Week on Galileo - Jul 28 - Aug 3, 1997
This week on Galileo is again a mixed bag of activities. The spacecraft's trajectory is absolutely littered with occultations of Earth by both Jupiter and the Galilean satellites. These provide radio scientists with many opportunities, eight total, to study and characterize the atmospheres, thick and tenuous alike, of the Jupiter system. Diligently continuing in the background is the playback of data from Galileo's previous passage through the Jupiter system. No high time-resolution observations of Jupiter's magnetosphere are planned this week. However, the fields and particles instruments continue to collect low rate data as a part of their second mini-tour of the mission.
For new comers to This Week On Galileo, Earth occultations occur when the spacecraft passes behind another celestial body as seen from Earth. As the spacecraft disappears behind the body and as it reappears on the body's opposite side, the body's atmosphere will bend and change the characteristics of Galieo's radio signal. Radio scientists can measure these changes and learn about the structure of the atmosphere that the radio signal has traveled through. The data will tell them how the temperature, pressure and density of the atmosphere changes as a function of the altitude from the body's "surface" (Jupiter does not have a surface that we know of, but the altitude is typically measured from the point in the atmosphere where the pressure is 1 bar or the same pressure as on the surface of the Earth). In addition, by noting the time at which the radio signal is lost and the time at which it is regained, the scientist can also provide a measurement of the body's diameter.
This week the spacecraft's path takes it through eight different Earth occultations, one by Jupiter, two by Ganymede and FIVE by Io. The occultation by Jupiter will last 25 hours while the satellite occultations are in matter of minutes. The Ganymede occultations occur on Tuesday and Saturday. The Jupiter occultation occurs Friday into Saturday. The Io occultations occur on each of Tuesday through Friday and Sunday, the longest being on Wednesday and lasting eight minutes.
Playback plans for this week are also varied in nature. Observations of Callisto's Valhalla multi-ringed region are returned by SSI (Solid State Imaging camera) and NIMS (Near Infrared Mapping Spectrometer). These observations hope to provide clues as to the origin of this interesting surface feature. Was it created by some phenomena internal to Callisto? Or was it created by an meteor impact? Does the material that is present look like it came from Callisto or from an external source? Do the rings look like fault lines or do they look like the fallout from an impact?
Four Jupiter observations are returned this week by SSI. Two of these are of the Great Red Spot and two of a plume head region. Recall that these Jupiter atmosphere observations are typically coordinated amongst all of Galileo's remote sensing instruments. SSI's contribution to these coordinated observations is the measurement of wind velocities, cloud heights and cloud particle properties. And you thought an image was just an image!
Completing the playback schedule for this week is a photometric and polarimetric map of Callisto returned by PPR (Photopolarimeter Radiometer), observations of Thebe, Adrastea and Metis returned by SSI and a global map of Ganymede returned by NIMS.