Today On Galileo - March 29, 1998
The spacecraft continues to make its way through the Jupiter system for the 14th time since June 1996. Science information has been collected on 11 of the previous 13 orbits, 11 of which occurred during the Galileo primary mission. Today's observation schedule is heavily focused on Jupiter's moon Europa as the spacecraft flies within 1,645 kilometers (1022 miles) of its surface at 5:21 am, Pacific Time. Later in the day, at 4:09 pm, the spacecraft will pass the point of closest approach to Ganymede at a distance from the surface of 918,000 kilometers (571,000 miles).
Many different areas of scientific interest on Europa are covered by today's observing activities. The radio science team, for 20 hours surrounding the point of closest approach, measures changes in Galileo's radio frequency due to Europa's gravitational pull on the spacecraft. By using the Doppler effect, the radio science team will be able to use these measurements to refine the map of the gravity field produced by Europa. Also tied to closest approach, the fields and particles instruments will perform a high time-resolution observation, for just under an hour, of Jupiter's magnetic and electric field environment in the region of space near Europa. This will add to the scientific knowledge of the interaction between Europa and the magnetosphere.
The photopolarimeter radiometer performs three observations of Europa today. Together they are designed to determine the temperature variation across Europa's surface and how it relates to different surface ages, how the surface might have been put together, or the different materials on the surface. Two of these observations are performed at better resolutions than was possible during Galileo's primary mission. The resolution of the third is comparable. Three other global scale observations are performed during the day -- one by the spacecraft's camera, or solid-state imaging (SSI) subsystem, and two by the Near Infrared Mapping Spectrometer (NIMS).
Among the specific regions of Europa that Galileo looks at today, we start with a region characterized by rifts or crevasses in Europa's surface observed by NIMS together with the Ultraviolet Spectrometer (UVS). This is followed by a couple of observations of the Mannann'an crater region performed by the SSI instrument. The pair of images is designed to provide stereo coverage of this region. A region of dark spots is also observed twice by the camera. These two images will also result in stereo coverage of this region, which also lies within regional data obtained during its orbit in November 1997. In addition, a transition from a dark spot region to a region of pull-apart wedge shapes is observed by NIMS and is accompanied by the UVS instrument.
Later in the day, a region of triple-bands is observed by all three instruments (SSI, NIMS, UVS). Remember that triple-bands are believed to be formed when Europa's surface cracks, material upwells from below the surface and spills to both sides of the central crack. Coverage of this region is planned to be obtained during the the Europa orbit in February 1999. A transition region between bright plains, pull-apart wedges and dark material is also observed by all three instruments.
A high-resolution picture of the Tyre Macula region is obtained by the SSI instrument. This region was observed at a lower resolution in the primary mission's Ganymede orbit in April 1997, and an observation is planned at even higher resolution during the Europa encounter this coming May. The SSI instrument also performs a photometry observation of the surface of Europa. These photometric measurements will tell us how intensely light is reflected from the surface and provide more information on its makeup.
Two non-Europa observations are performed today. The first is a global color image of Ganymede performed by the SSI instrument. This image will provide data on the radius, shape, color, and photometry of Ganymede as well as the mobility of frost on its surface. Also observed by SSI is Io while eclipsed from the sun by Jupiter. These eclipse images are considered one of the best ways to discover new lava flows, monitor lava temperatures, and study the interaction between volcanic plumes, Io's atmosphere and Jupiter's magnetosphere.