Box 2.1 Galileo Europa Mission (GEM)
Galileo was originally scheduled to end its exploration of the jovian system on December 7, 1997, but NASA and Congress approved the extension of the mission through the last day of 1999. The Galileo Europa Mission (GEM), as it is called, was designed as both a streamlined, low-cost extension to Galileo's exploration of the jovian system and as a precursor to future missions to Europa and lo. GEM encompasses 14 orbits of Galileo around Jupiter and is divided into three phases, each with its own tightly focused objectives; the Europa, Perijove Reduction, and lo campaigns.
Mission Phases and Major Science Objectives
Europa Campaign — A 1-year intensive study of Europa comprising eight consecutive close encounters. Europa's crust, atmosphere, and possible subsurface ocean are studied using imaging, gravity, and space physics data gathered by Galileo's full complement of remote-sensing and in situ instruments. The design of this phase of GEM allows for a number of unique imaging opportunities. These include high-resolution imaging and spectral observations (<50 m/pixel for images) and stereo imaging of selected topographic features and views of Europa's polar regions.
Careful tracking of Galileo during this phase of GEM yielded geophysical information, such as Europa's moment of inertia, that will allow refinement of knowledge on the interior configuration of the satellite. Magnetospheric data obtained during close flybys and other periods chosen to provide maximum spatial coverage will help to further refine understanding of Europa's ionosphere and possible internally produced magnetic fields, and the satellite's interaction with the jovian plasma disk.
Although data were not collected during one encounter. (E-13) because it occurred during solar conjunction, additional observations of Europa are planned for the final scheduled orbit (l-25) of the lo Campaign.
Perijove Reduction Campaign — A series of four encounters with Callisto designed to modify Galileo's orbit sufficiently to enable close flybys of lo. In addition to observations of Callisto, a major focus of this phase of GEM is observation to characterize the lo plasma torus, including studies of satellite/magnetosphere interactions.
lo Campaign — Close flyby of lo in October 1999 with the possibility of a second flyby 6 weeks later. The scientific focus of these flybys is high-resolution imaging as well as in situ observations of los' volcanic processes, atmosphere, and magnetospheric environment.
Such decoupling leads to non-synchronous rotation of the surface ice shell with respect to the interior and appears to be expressed in stress fractures of the surface as the shell changes shape.9 The non-synchronous rotation may arise because the balance between the torque exerted by Jupiter, tending to speed up Europa, and a resisting torque, associated with a slight departure of Europa's minimum-moment-of-inertia axis from the Jupiter line, cannot be permanently maintained if the material of the satellite can adjust in some way to bring the minimum-moment-of-inertia axis back into alignment with Jupiter. This may be the case on Europa if the ice shell is decoupled from the underlying rock. If so, the rate of non-synchronous rotation depends on the viscosity of the ice, which determines the readjustment time of the shell. The absence of any perceptible offsets in the positions of features imaged by both Voyager and Galileo implies that the current non-synchronous rotation period must be in excess of 10,000 years.10