Voyage to Jupiter

by David Morrison; Jane Samz

The Scientific Capability of Galileo

The investigations of Jupiter and its system planned for the Galileo Project represented substantial advances over those carried out by Voyager. In part, this was the result of new spacecraft capabilities, particularly the atmospheric entry probe. It also represented increasing sophistication in scientific instrumentation over the seven-year interval between the selection of the payloads for the two missions.

The main emphasis in the study of Jupiter itself is on direct measurements with the Probe. For the first time it will be possible to examine directly the atmosphere of a giant planet. By measuring the temperature and pressure as it descends through the clouds, the Probe can determine the structure of the atmosphere with much higher precision than could ever be obtained from remote observations. The structure, in turn, provides information on dynamics—the circulation and heat balance of the Jovian atmosphere. In addition, the Probe can make direct measurements of the composition of the gases, with sensitivity in some cases to quantities as low as a few parts per billion. In addition to the elemental abundance, the amount of different isotopes can also be measured.

Direct studies of the clouds of Jupiter can be made from the Galileo Probe. With a device called a nephelometer (literally, cloud-meter), the sizes and compositions of individual aerosol particles will be determined. An infrared instrument will determine the temperatures of the cloud layers and measure the amounts of sunlight deposited in different regions of the atmosphere. Another instrument will search for lightning; it has the ability to detect both the flash of light and the radio static generated by each bolt.

Additional studies of the atmosphere, similar to those of Voyager, can be carried out from the Galileo Orbiter. Television pictures, ultraviolet and infrared spectra, and measurements of the polarization of reflected light will all be obtained with the same scan platform instruments that are used to study the surfaces of the satellites.

A full battery of fields and particles instruments is planned for the Galileo Orbiter. Many of these are direct descendants of Voyager instruments. In general, their capabilities have been improved, particularly their ability to determine the composition of charged particles. There is a steady progression from Pioneer to Voyager to Galileo: The early measurements were concentrated on particle energies, but more sophisticated instruments yield the composition of the ions and the details of their motion.

Many of the advances expected from Galileo in magnetospheric studies result from the Orbiter’s ability to explore many parts of the environment of Jupiter. The Pioneer and Voyager spacecraft made single cuts through the magnetosphere, and often it was difficult to distinguish temporal from spatial effects. Galileo will repeatedly swing around Jupiter, sampling conditions at many distances from the planet over a time span of two years or more. In addition, it is planned to adjust the orbit of Galileo to swing out into the magnetotail, the turbulent region of the magnetosphere that stretches “downwind” from Jupiter for hundreds of Jupiter radii. No flybys can reach the magnetotail; an orbiting spacecraft is required.

The Galilean satellites naturally will be a primary focus of Galileo science, particularly after Voyager. It is planned to have as many as a dozen individual encounters, most of them at much closer range than the Voyager flybys. To take advantage of these opportunities, the Galileo scan platform will carry two new remote sensing systems.

The Orbiter section of the Galileo spacecraft will carry both remote sensing and direct measuring instruments for the study of Jupiter, its satellites, and its magnetosphere. Several remote sensing instruments—an imagery system, a near infrared mapping spectrometer, an ultraviolet spectrometer, and a photopolarimeter/radiometer—will be mounted on a scan platform. The particles and fields instruments will be on a spinning section of the spacecraft. The Orbiter is expected to operate for at least two years around Jupiter, providing one close flyby of Io and several each of Europa, Ganymede, and Callisto. [P-20772]

Instead of the vidicon television camera on Voyager, Galileo imaging will be done with a new solid-state detector called a charged coupled device (CCD). The CCD has a wider spectral response and greater photometric accuracy. In addition, its increased sensitivity permits shorter exposures, so that even on very close flybys the pictures will not be blurred by spacecraft motion. Substantial coverage at a resolution of 100 meters should be possible, compared to Voyager’s best resolution of 1 kilometer for Io and 4 kilometers for Europa.

Galileo will be launched by the Space Shuttle, the central element of the new NASA Space Transportation System. Together with its upper stage launch rocket, Galileo will be placed into Earth’s orbit in the large Shuttle bay, about 20 meters long and 5 meters in diameter. After releasing Galileo, the Shuttle will be piloted back to a landing at Cape Canaveral, to be used again for many future flights. [5-78-23599]

The Space Shuttle and the Inertial Upper Stage of Galileo as they will appear in Earth orbit. The upper stage has been released from the Shuttle bay and is being prepared for launch to Jupiter.

To determine the composition of satellite surface materials, Galileo will also carry a near-infrared mapping spectrometer (NIMS). This instrument will obtain measurements over the visible and infrared spectra of areas as small as 10 kilometers across. With NIMS, it should be possible to investigate the composition of individual features as small as the volcanic calderas on Io or the ejecta blankets of Ganymede’s craters.