As the Probe approaches the atmosphere of Jupiter at the awesome speed of 26 kilometers per second, it will be traversing a region of space never before explored. An energetic particle detector will investigate the innermost magnetosphere before the entry begins. Then, within a period of just a few minutes, friction with the upper atmosphere must dissipate the Probe energy until it is falling gently in the Jovian air.
Jupiter, being the largest planet, presents the most challenging atmospheric entry mission ever undertaken by NASA. The design of the Galileo Probe calls for a massive heat shield to protect the instruments during the high-speed entry phase. After the Probe has slowed to subsonic velocities, a parachute will be deployed, and the heat shield, having done its job, will be dropped free.
| GALILEO ORBITER SCIENCE INVESTIGATIONS | ||
|---|---|---|
| Project Scientist: T. V. Johnson, JPL | ||
| Investigation | Principal Investigator | Primary Objectives |
| Solid state imaging | M. J. S. Belton, Kitt Peak Observatory (Team Leader) | Provide images of Jupiter’s atmosphere and its satellites; study atmospheric structure and dynamics on Jupiter; investigate the composition and geology of the satellite surfaces; study the active volcanic processes on Io. |
| Ultraviolet spectrometer | C. W. Hord, U. Colorado | Study composition and structure of the upper atmospheres of Jupiter and its satellites. |
| Near-infrared mapping spectrometer (NIMS) | R. W. Carlson, JPL | Provide spectral images and reflected sunlight spectra of Jupiter’s satellites, indicating the composition of their surfaces; measure reflected sunlight and thermal emission from Jupiter’s atmosphere to study composition, cloud structure, and temperature profiles; monitor hot spots on Io. |
| Photopolarimeter/radiometer | J. E. Hansen, NASA Goddard | Measure temperature profiles and energy balance of Jupiter’s atmosphere; measure Jupiter’s cloud characteristics and composition. |
| Magnetometer | M. G. Kivelson, UC Los Angeles | Measure magnetic fields and the ways they change near Jupiter and its satellites; measure variations caused by the satellites interacting with Jupiter’s field. |
| Plasma particles | L. A. Frank, U. Iowa | Provide information on low-energy particles and clouds of ionized gas in the magnetosphere. |
| Energetic particles | D. J. Williams, NOAA Space Environment Lab | Measure composition, distribution, and energy spectra of high-energy particles trapped in Jupiter’s magnetosphere. |
| Plasma waves | D. A. Gurnett, U. Iowa | Investigate waves generated inside Jupiter’s magnetosphere and waves radiated by possible lightning discharges in the atmosphere. |
| Dust detection | E. Grün, Max-Planck-Institut (Germany) | Determine size, speed, and charge of small particles such as micrometeorites near Jupiter and its satellites. |
| Celestial mechanics | J. D. Anderson, JPL (Team Leader) | Use the tracking data to measure the gravity fields of Jupiter and its satellites; search for gravity waves propagating through interstellar space. |
| Radio propagation | H. T. Howard, Stanford U. (Team Leader) | Use radio signals from the Orbiter and Probe to study the structure of the atmospheres and ionospheres of Jupiter and its satellites. |
| Interdisciplinary Scientists: | F. P. Fanale (JPL), P. J. Gierasch (Cornell U.), D. M. Hunten (U. Arizona), H. Masursky (U.S. Geological Survey), M. B. McElroy (Harvard U.), D. Morrison (U. Hawaii), G. S. Orton (JPL), T. Owen (SU New York), J. B. Pollack (NASA Ames), C. T. Russell (UC Los Angeles), C. Sagan (Cornell U.), F. L. Scarf (TRW), G. Schubert (UC Los Angeles), C. P. Sonett (U. Arizona), J. A. Van Allen (U. Iowa). | |
The Probe will spend nearly an hour descending from a pressure level of about 0.1 bar, where the heat shield is jettisoned, to a depth of 10-20 bars. During this time it will make most of its scientific measurements, relaying them back to Earth via the Probe carrier. Designers expect the Probe to sink through regions of ammonia clouds, ammonium hydrosulfide clouds, and ice and water clouds during this hour.
By the time it has descended below the water clouds, the increasing pressure will exceed the strength of some Probe components. Engineers expect the Probe to have completed its mission, exhausted its battery power, and been crushed by the atmospheric pressure before the 20-bar level is reached. Lifeless, it will then sink on into the thick, hot lower atmosphere of Jupiter.
During its two years in orbit, the Galileo Orbiter will carry out many investigations of the planet, the Galilean satellites, and the Jovian magnetosphere. Repeated close flybys of the satellites are used to modify and shape the orbit to provide additional flybys at an optimum viewing geometry. Initially, the orbit is a long loop that extends in the general direction of the sunset side of Jupiter. The orbit is then contracted, and the encounters with the satellites rotate it behind the planet for a long excursion into the magnetotail late in the tour.
Beyond Galileo
After Galileo, the future cannot be predicted. Perhaps there will no longer be a program of planetary exploration. But if humanity still has the vision to seek a future in the stars, there will surely be other Jupiter missions.
Perhaps the next mission will concentrate on Jupiter itself. Probes could be built to withstand pressures as high as several hundred bars, feeling their way deep into the murky depths of the planet. Or a hot-air balloon could be deployed from a probe to carry instruments for long-term studies of the atmosphere. A number of proposals have also been made for additional satellite missions, including orbiters or landers for Ganymede and Callisto. Or perhaps it will be desirable to land a vehicle on one of the satellites and collect a sample and return it to Earth for laboratory analysis.