Galileo Mission Design
The Galileo Orbiter and Probe are to be launched with NASA’s new Space Shuttle and Inertial Upper Stage. To carry the maximum possible payload to Jupiter, a close flyby of Mars is planned en route. The gravitational field of Mars will give a boost to Galileo, just as that of Jupiter was used by Voyager to swing on to Saturn.
The exact launch date and trajectory for Galileo have not yet been specified, but if all goes well, the Orbiter spacecraft will approach Jupiter from the dawn side of the planet sometime in the mid-1980s. It will not be moving as fast as Voyager, since it must be placed into orbit around Jupiter rather than flashing past on its way to the outer solar system. On its initial trajectory, Galileo will probably come within 5 RJ of Jupiter, slightly closer than Voyager 1. At this time it will fire its rocket engines (supplied by the Federal Republic of Germany in a cooperative program with NASA) to shed excess speed and let itself be captured by Jupiter’s gravity. The first pass will also be the time for a close flyby of Io.
The most critical period of the Galileo flight will be the Probe entry at Jupiter. The Probe must strike the atmosphere at precisely the correct angle and speed to be slowed down without being destroyed. At a pressure level of about 0.1 bar the rapid deceleration period ends and the heat shield is released. A parachute is deployed to slow the descent further, and the Probe then has a period of nearly an hour to study the atmosphere and clouds of Jupiter. The Probe mission ends when its batteries run down or when it is crushed by the pressure of the Jovian atmosphere near the 20-bar level, whichever comes first. [SL78-545(3)]
| GALILEO PROBE SCIENCE INVESTIGATIONS | ||
|---|---|---|
| Probe Scientist: L. Collin, NASA Ames | ||
| Investigation | Principal Investigator | Primary Objectives |
| Atmospheric structure | A. Seiff, NASA Ames | Measure temperature, density, pressure, and molecular weight to determine the structure of Jupiter’s atmosphere. |
| Neutral mass spectrometer | H. B. Neimann, NASA Goddard | Measure the composition of the gases in Jupiter’s atmosphere and the variations at different levels in the atmosphere. |
| Helium abundance interferometer | U. von Zahn, Bonn U. (Germany) | Measure with high accuracy the ratio of hydrogen to helium in Jupiter’s atmosphere. |
| Nephelometer | B. Ragent, NASA Ames | Determine the sizes of cloud particles and the location of cloud layers in Jupiter’s atmosphere. |
| Net flux radiometer | R. W. Boese, NASA Ames | Measure energy being radiated from Jupiter and the Sun, at different levels in Jupiter’s atmosphere. |
| Lightning and radio emission | L. J. Lanzerotti, Bell Labs | Measure lightning flashes in Jupiter’s atmosphere, from the light and radio transmissions from those flashes. |
| Energetic particles | H. M. Fischer, U. Kiel (Germany) | Measure energetic electrons and protons in the inner regions of the Jovian radiation belts and determine their spatial distributions. |
Because of the intense radiation environment, the Galileo Orbiter will not be able to spend much time in the inner magnetosphere, near the orbit of Io. To do so would risk damage to the spacecraft electronics and a premature end to the mission. Additional thruster firing during the first orbit can be used to raise the periapse to 10 RJ or greater. No more close passes by Io will be possible, but studies of this satellite can be made on each subsequent orbit with imaging resolutions of about 10 kilometers, sufficient to see details of the volcanic eruptions and monitor volcano-associated changes in the surface.
At each subsequent orbit, Galileo will be programmed for a close flyby of one of the other satellites. Several passes each of Callisto, Ganymede, and Europa should be possible. The satellite tour does not need to be fully planned in advance; by adjusting the spacecraft trajectory with small bursts of the thruster motors, navigation engineers can modify the orbit to permit adaptation to scientific needs. As the Orbiter mission progresses, the spacecraft will also sample many parts of the magnetosphere, including one long excursion, at least 150 RJ, into the magnetotail.
The total duration of the Orbiter mission is planned to be at least 20 months. Additions to the basic mission are possible if the spacecraft remains healthy and fuel reserves are adequate. In contrast, the Galileo Probe mission lasts only a few hours.
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.