From the Project Manager

A Tremendous Bounty of Science Captured in Primary Mission

With only one orbit and satellite encounter remaining in its eleven-orbit, ten satellite encounter primary orbital mission, the Galileo Orbiter now joins the Galileo Atmospheric Entry Probe as an outstanding success. Recall that the Orbiter superbly delivered the Probe to Jupiter and received the Probe data in 1995 and later relayed the Probe data to Earth. But upon completion of the Probe relay in Spring 1996, the entire orbital mission still lay ahead. The success of the orbital mission is particularly stunning considering that it was achieved by one of the most remarkable inflight failure workarounds ever. Following extraordinary, but unsuccessful efforts to deploy the High- Gain Antenna (HGA), the Project extensively reprogrammed the spacecraft computers to provide onboard compression, editing, and improved coding while the DSN built new ground receivers and arraying capability. These integrated efforts produced an effective downlink information rate of one kbps over the Orbiter's Low-Gain Antenna. This has been achieved with a received power density at Earth 40 db (10,000 times) less than the HGA was to have provided!

An absolutely tremendous bounty of science has been obtained for which Galileo is acclaimed worldwide. Galileo is also widely acclaimed for its stunning recovery from the HGA deployment failure-particularly for the use of new concepts and technologies that have broad application for future deep space missions. Project Galileo's performance has spectacularly demonstrated the success of this remarkable recovery.

All eleven Galileo Orbiter science instruments and the two radio science investigations have produced excellent results. In addition to the spectacular images and other remote sensing, the six fields and particles instruments gathered extensive data on Jupiter's magnetosphere, including its interaction with the satellites. And the key objective of measuring the deep magnetotail has been accomplished.

The most significant finding of Galileo to date is the strong evidence in the high resolution images of Europa that liquid water may indeed presently exist under the ice crust. Images at 50-meter resolution show icebergs in a frozen sea. These icebergs (also, perhaps more descriptively, called ice rafts) are displaced from one another in a manner that clearly indicates they have broken apart and been moved and rotated into these positions as this particular surface formed. With no significant slopes or winds, it is logical that ocean currents produced the displacements. The paucity of craters indicates this particular surface is geologically very young, perhaps just tens of millions years old. Therefore, given the age of the system at 4.6 billion years, if liquid water existed there so recently, it is inconceivable that it has all frozen out in the "instant" of geologic time since the imaged surface was formed.

The Probe did the detailed in-situ measurements of Jupiter's atmosphere in its entry and descent region. The Orbiter also has a very significant role in the atmosphere investigation, namely, the global characterization of the atmosphere by remote sensing and radio occultation experiments. Correlation of the remote and in situ measurements is a key feature of the Galileo Mission. For example, a major finding by the Orbiter's Near-Infrared Mapping Spectrometer (NIMS) is that water vapor abundance varies by orders of magnitude from "dry" to "wet" regions in Jupiter's atmosphere. This is consistent with the dry Probe entry region while other regions can be factors above solar relative abundance. Images of Jupiter show clear evidence of thunderstorm-like activity.

There have been many exciting findings for the Jupiter satellites. Distant Io imaging was performed in every encounter sequence to do volcano monitoring. Large changes in volcanic activity are seen since the Voyager imaging in 1979. At least 19 hot volcanic areas have been located by Galileo. Silicate volcanism is implied. There is evidence of some very high lava temperatures suggesting extreme chemistry. Fewer plumes are visible than in 1979; perhaps there are more pure gas "stealth" geysers. Io's plumes may be the source of high-speed dust streams seen by both Ulysses and Galileo. Six precisely designed distant Io occultations have provided the first global picture of Io's complex ionosphere. Io gravity field measurements during the Arrival Day close flyby indicate a large iron/iron-sulfide core about half Io's radius.

In addition to the compelling evidence of liquid water, Galileo has found complex systems of faults and ridges covering Europa's surface. Galileo images show the first evidence for volcanic ice flows. Radio occultation measurements have discovered an ionosphere at Europa. Gravity measurements and other observations indicate Europa has three structural phases: a large metallic core, a rock mantle, and an outer shell of liquid and solid (ice) water perhaps 150-km thick.

Measurements during the Ganymede encounters resulted in the discovery that Ganymede has a magnetosphere-the first moon in the solar system determined to have a magnetic field. Gravity field measurements show Ganymede to be differentiated. The preferred model has three phases: a metallic core, rock mantle, and large icy outer shell. Galileo also discovered a tenuous hydrogen atmosphere at Ganymede and a large outflow of protons; also that ozone in Ganymede's surface ice is concentrated at high latitudes.

The encounters with Callisto also produced big surprises. Callisto has extensive blanketing by an apparently powder-like debris covering small craters. The origin and make-up of this material is a mystery under investigation. Gravity measurements show Callisto to be an essentially homogeneous body unlike the other big Jupiter satellites. Galileo's fields and particles measurements show Callisto has no significant magnetic field.

Exciting new measurements of the great plasma torus along Io's orbit and its plasma disk have been obtained. The disk was observed to extend as a thin sheet to radial distances of 100 Jupiter radii and beyond. The Orbiter's in-situ instruments detected global changes in the geometry of this rapidly rotating, undulating plasma disk. The geometry of the disk was observed to quickly-within one day or so-change in thickness and to exhibit distortions in its geometry. The reasons for this remarkable behavior are probably due to variations of the injections of plasmas from Io's volcanic activity and/or the action of a variable solar wind. The magnetic signatures which suggest the merging of magnetic field lines in the plasma sheet providing the release of large amounts of energy have been tentatively identified. Such processes are responsible for the magnetic substorms which cause intense auroral light activity at Earth. From remote Earth-based images of the polar regions of Jupiter, it is now known that Jupiter also exhibits great increases in auroral light intensities, which are suggestive of global changes in the magnetosphere of Jupiter. In order to produce these dramatic displays of light, it is necessary to force the flow of charged particles into Jupiter's atmosphere. Particle detectors onboard the Galileo Orbiter have detected these beams flowing along the magnetic field in the direction of Jupiter's atmosphere. And there is every reason to believe that the mysteries of the aurora will be unraveled with the in situ measurements of fields and particles in combination with the remote imaging of the auroras with the remote-sensing visible and near-infrared cameras onboard the Galileo Orbiter and those at Earth.

Measurements of the charged-particle environment in the Jovian magnetosphere show that there have been great changes since the Voyager epoch of 1979. There are very puzzling decreases in the hot plasma densities and changes in the composition of the ions during this current "Galileo epoch." Is it possible that all of these dynamical effects are due in large part tothe variable volcanic activity of Io? The answers to all of these above mysteries are anticipated to be found in Galileo data.

Galileo has, indeed, provided a tremendous bounty of science. Although not all of the original objectives could be met with the severely reduced bit rate, the great majority of the original objectives and many new objectives have been achieved. I believe that the scientific bounty from the now nearly completed primary mission with its many discoveries to date, and undoubtedly many more in the continuing data analysis, will easily exceed the original expectations. Recall there were no Europa or Io encounters in the original objectives!

Our upcoming encore-the two-year extended mission called the Galileo Europa Mission (GEM)-promises the most stunning Galileo images and other observations of Europa and Io yet. Stay tuned.v

--Bill O'Neil
Project Manager

• Educational Outreach Corner
  Galileo Outreach: Bringing Jupiter to Earth and across the USA
• It's the Real Thing! Local Students Support Galileo at Table Mountain