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.
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.