CHAPTER 10. Return to Jupiter

RETURN TO JUPITER

A Successor to Voyager

The spectacular discoveries of the Voyagers did not exhaust our interest in the Jovian system. Both the giant planet and its system of satellites will almost certainly play a central role in any future program of solar system exploration and research. Thus, even as the two Voyager spacecraft directed their attention further outward toward Saturn, NASA had begun development of the next Jupiter mission, named Galileo.

Galileo is an ambitious, multiple-vehicle planetary mission. It has two major interlocking elements: a probe to be placed in the atmosphere of Jupiter and an orbiter to explore Jupiter, its satellites, and its magnetosphere. By using individual satellite flybys to alter its orbit, the Galileo spacecraft can carry out a satellite “tour” consisting of flybys of the Galilean satellites at different geometries and a deep penetration into the magnetosphere in the unexplored region of space behind Jupiter.

Both an atmospheric probe for the planet and a long-lived orbiter to study the satellites and magnetosphere are logical successors to Voyager. In 1974, three years before Voyager launch, the Space Science Board of the National Academy of Sciences was already emphasizing the scientific advantages of both of these approaches. In suggesting goals for 1975-1985, the Board wrote, “We recommend that a significant effort in the NASA planetary program over the next decade be devoted toward the outer solar system. Jupiter is the primary object of outer solar system exploration.” Looking at specific mission goals, the Board recommended that “the primary objectives in the exploration of Jupiter and its satellites for the period 1975-1985 in order of importance are (1) determination of the chemical composition and physical state of its atmosphere, (2) the chemical composition and physical state of the satellites, and (3) the topology and behavior of the magnetic field and the energetic particle fluxes. In order to carry out this program, it will be necessary to utilize orbiting spacecraft and probe-delivering spacecraft.”

In the same period NASA carried out studies of two possible orbiter and probe missions. Working through the Ames Research Center, a scientific panel chaired by James Van Allen explored the adaptation of the Pioneer 10 and 11 spinning spacecraft to carry a probe to Jupiter and to carry out an orbiter mission emphasizing magnetospheric studies. William B. Hubbard of the University of Arizona chaired a JPL-based panel investigating the use of a Mariner-class fully stabilized spacecraft similar to Voyager to carry out a satellite-oriented orbiter mission. In 1976 these concepts were combined in a study, again chaired by Dr. Van Allen, of a Voyager-type orbiter with probe-carrying capability. This mission concept was given the name JOP, for Jupiter Orbiter Probe, and lead responsibility was assigned by NASA to JPL, with Ames carrying out the design of the probe.

In 1977, as Voyager activity was building toward autumn launch, a struggle was underway in Washington to obtain approval for the new Jupiter orbiter and probe mission. Budgeting authority was requested in the President’s Fiscal Year 1978 budget, but only after extensive testimony and several Congressional votes was the mission approved. The official new start for JOP, soon to be renamed Galileo, was set for July 1, 1977, and the scientific investigators and their instruments were selected in August.

At JPL, many members of the Voyager Team made a smooth transition to the Galileo Project. Much of the knowledge that had gone into the design of the Voyager spacecraft and its subsystems was now incorporated into Galileo. Similarly, at Ames the knowledge gained from the design of the Pioneer Venus probes, which were launched to Venus in 1978, a year after Voyager launch, was applied to design of a Jupiter probe. Among the individuals who brought their Voyager experience to Galileo were John Casani, who left the position of Voyager Project Manager to become Galileo Project Manager, and Torrence Johnson of the Voyager Imaging Team, who became Galileo Project Scientist.

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.

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.

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 R_J 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.

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 H. B. Neimann, NASA Measure the spectrometer Goddard composition of the gases in Jupiter’s atmosphere and the variations at different levels in the atmosphere. Helium abundance U. von Zahn, Bonn U. Measure with high interferometer (Germany) 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 L. J. Lanzerotti, Measure lightning emission Bell Labs flashes in Jupiter’s atmosphere, from the light and radio transmissions from those flashes. Energetic particles H. M. Fischer, U. Measure energetic Kiel (Germany) 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 R_J 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 R_J, 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 Provide images of Peak Observatory Jupiter’s (Team Leader) 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. Study composition and Colorado structure of the upper atmospheres of Jupiter and its satellites. Near-infrared mapping R. W. Carlson, JPL Provide spectral spectrometer (NIMS) 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 Measure temperature Goddard profiles and energy balance of Jupiter’s atmosphere; measure Jupiter’s cloud characteristics and composition. Magnetometer M. G. Kivelson, UC Measure magnetic Los Angeles 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 Measure composition, Space Environment distribution, and Lab 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, Determine size, Max-Planck-Institut speed, and charge (Germany) of small particles such as micrometeorites near Jupiter and its satellites. Celestial mechanics J. D. Anderson, JPL Use the tracking data (Team Leader) to measure the gravity fields of Jupiter and its satellites; search for gravity waves propagating through interstellar space. Radio propagation H. T. Howard, Use radio signals Stanford U. (Team from the Orbiter Leader) and Probe to study the structure of the atmospheres and ionospheres of Jupiter and its satellites. Interdisciplinary F. P. Fanale (JPL), P. J. Gierasch (Cornell Scientists: 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.

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.

Whatever the future holds, it is clear that the Pioneer and Voyager missions blazed the path to Jupiter and beyond. The little Pioneers proved that it could be done, and the Voyagers expanded their vision, exploring and discovering new worlds more remarkable and exciting than anyone could have imagined.

APPENDIX A PICTORIAL MAPS OF THE GALILEAN SATELLITES

These maps were prepared for the Voyager Imaging Team by the U.S. Geological Survey in cooperation with the Jet Propulsion Laboratory, California Institute of Technology and the National Aeronautics and Space Administration. Copies are available from Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202, and Branch of Distribution, U.S. Geological Survey, Box 25286, Federal Center, Denver, CO 80225.

Preliminary Pictorial Map of Callisto

Atlas of Callisto 1:25,000,000 Topographic Series Jc 25M 2RMN, 1979 I-1239

This map was compiled from Voyager 1 and 2 pictures of Callisto. Placement of features is based on predicted spacecraft trajectory data and is highly approximate. The linkage between Voyager 1 and Voyager 2 pictures is particularly tenuous. Placement errors as large as 10° are probably common throughout the map, and a few may be even larger. Feature names were approved by the International Astronomical Union in 1979. Airbrush representation is by P. M. Bridges.

Jc 25M 2RMN: Abbreviation for (Jupiter) Callisto, 1:25,000,000 series, second edition, shaded relief map, R, with surface markings, M, and feature names, N.

Scale 1:13 980 000 at 56° latitude Polar stereographic projection

Scale 1:25 000 000 at 0° latitude Mercator projection

Preliminary Pictorial Map of Ganymede

Atlas of Ganymede 1:25,000,000 Topographic Series Jg 25M 2RMN, 1979 I-1242

This map was compiled from Voyager 1 and 2 pictures of Ganymede. Placement of features is based on predicted spacecraft trajectory data and is highly approximate. The linkage between Voyager 1 and Voyager 2 pictures is particularly tenuous. Placement errors as large as 10° are probably common throughout the map, and a few may be even larger. A large unresolved discrepancy exists in the area bounded by the -45° and -55° parallels between 120° and 180° longitude. Relative placement of features is distorted in that area. Feature names were approved by the International Astronomical Union in 1979. Airbrush representation is by J. L. Inge.

Jg 25M 2RMN: Abbreviation for (Jupiter) Ganymede, 1:25,000,000 series, second edition, shaded relief map, R, with surface markings, M, and feature names, N.

Scale 1:13 980 000 at 56° latitude Polar stereographic projection

Scale 1:25 000 000 at 0° latitude Mercator projection

Preliminary Pictorial Map of Europa

Atlas of Europa 1:25,000,000 Topographic Series Je 25M 2RMN, 1979 I-1241

This map was compiled from Voyager 1 and 2 pictures of Europa. Placement of features is based on predicted spacecraft trajectory data and is highly approximate. Feature names were approved by the International Astronomical Union in 1979. Airbrush representation is by J. L. Inge.

Je 25M 2RMN: Abbreviation for (Jupiter) Europa, 1:25,000,000 series, second edition, shaded relief map, R, with surface markings, M, and feature names, N.

Scale 1:13 980 000 at 56° latitude Polar stereographic projection

Scale 1:25 000 000 at 0° latitude Mercator projection

Preliminary Pictorial Map of Io

Atlas of Io 1:25,000,000 Topographic Series Ji 25M 2RMN, 1979 I-1240

This map was compiled from Voyager 1 and 2 pictures of Io. Placement of features is based on preliminary control information provided by M. E. Davies of the Rand Corporation, Santa Monica, California, and is probably accurate within 50 to 100 km. Feature names were approved by the International Astronomical Union in 1979. Airbrush representation is by P. M. Bridges.

Ji 25M 2RMN: Abbreviation for (Jupiter) Io, 1:25,000,000 series, second edition, shaded relief map, R, with surface markings, M, and feature names, N.

Scale 1:13 980 000 at 56° latitude Polar stereographic projection

Scale 1:25 000 000 at 0° latitude Mercator projection

APPENDIX B VOYAGER SCIENCE TEAMS

Imaging Science

Bradford A. Smith, University of Arizona, Team Leader Geoffrey A. Briggs, NASA Headquarters A. F. Cook, Smithsonian Institution G. E. Danielson, Jr., California Institute of Technology Merton Davies, Rand Corp. G. E. Hunt, University College, London Tobias Owen, State University of New York Carl Sagan, Cornell University Lawrence Soderblom, U.S. Geological Survey V. E. Suomi, University of Wisconsin Harold Masursky, U.S. Geological Survey

Radio Science

Von R. Eshleman, Stanford University, Team Leader J. D. Anderson, Jet Propulsion Laboratory T. A. Croft, Stanford Research Institute Gunnar Lindal, Jet Propulsion Laboratory G. S. Levy, Jet Propulsion Laboratory G. L. Tyler, Stanford University G. E. Wood, Jet Propulsion Laboratory

Plasma Wave

Frederick L. Scarf, TRW Systems, Principal Investigator D. A. Gurnett, University of Iowa

Infrared Spectroscopy and Radiometry

Rudolph A. Hanel, Goddard Space Flight Center, Principal Investigator B. J. Conrath, Goddard Space Flight Center P. Gierasch, Cornell University V. Kunde, Goddard Space Flight Center P. D. Lowman, Goddard Space Flight Center W. Maguire, Goddard Space Flight Center J. Pearl, Goddard Space Flight Center J. Pirraglia, Goddard Space Flight Center R. Samuelson, Goddard Space Flight Center Cyril Ponnamperuma, University of Maryland D. Gautier, Meudon, France S. Kuman, University of Southern California

Ultraviolet Spectroscopy

A. Lyle Broadfoot, Kitt Peak National Observatory, Principal Investigator J. L. Bertaux, Service d’Aeronomie du CNRS, France J. Blamont, Service d’Aeronomie du CNRS, France T. M. Donahue, University of Michigan R. M. Goody, Harvard University A. Dalgarno, Harvard College Observatory Michael B. McElroy, Harvard University J. C. McConnell, York University, Canada H. W. Moos, Johns Hopkins University M. J. S. Belton, Kitt Peak National Observatory D. F. Strobel, Naval Research Laboratory Sushil Atreya, University of Michigan William R. Sandel, University of Southern California Donald Shemanski, University of Southern California

Photopolarimetry

Charles W. Hord, University of Colorado, Acting Principal Investigator D. L. Coffeen, Goddard Institute for Space Studies J. E. Hansen, Goddard Institute for Space Studies K. Pang, Science Applications Inc.

Planetary Radio Astronomy

James W. Warwick, University of Colorado, Principal Investigator Anthony Riddle, Radiophysics, Inc. Jeffrey Pearce, Radiophysics, Inc. J. K. Alexander, Goddard Space Flight Center A. Boischot, Observatoire de Paris, France W. E. Brown, Jet Propulsion Laboratory T. D. Carr, University of Florida Samuel Gulkis, Jet Propulsion Laboratory F. T. Haddock, University of Michigan C. C. Harvey, Observatoire de Paris, France Y. LeBlanc, Observatoire de Paris, France R. G. Peltzer, University of Colorado R. J. Phillips, Jet Propulsion Laboratory D. H. Staelin, Massachusetts Institute of Technology

Magnetic Fields

Norman F. Ness, Goddard Space Flight Center, Principal Investigator Mario H. Acuna, Goddard Space Flight Center K. W. Behannon, Goddard Space Flight Center L. F. Burlaga, Goddard Space Flight Center R. P. Lepping, Goddard Space Flight Center F. M. Neubauer, Technische Universitat, F.R.G.

Plasma Science

Herbert S. Bridge, Massachusetts Institute of Technology, Principal Investigator J. W. Belcher, Massachusetts Institute of Technology J. H. Binsack, Massachusetts Institute of Technology A. J. Lazarus, Massachusetts Institute of Technology S. Olbert, Massachusetts Institute of Technology V. M. Vasyliunas, Max Planck Institute, F.R.G. L. F. Burlaga, Goddard Space Flight Center R. E. Hartle, Goddard Space Flight Center K. W. Ogilvie, Goddard Space Flight Center G. L. Siscoe, University of California, Los Angeles A. J. Hundhausen, High Altitude Observatory

Low-Energy Charged Particles

S. M. Krimigis, Johns Hopkins University, Principal Investigator T. P. Armstrong, University of Kansas W. I. Axford, Max Planck Institute, F.R.G. C. O. Bostrom, Johns Hopkins University C. Y. Fan, University of Arizona G. Gloeckler, University of Maryland L. J. Lanzerotti, Bell Telephone Laboratories

Cosmic Ray

R. E. Vogt, California Institute of Technology, Principal Investigator J. R. Jokipii, University of Arizona E. C. Stone, California Institute of Technology F. B. McDonald, Goddard Space Flight Center B. J. Teegarden, Goddard Space Flight Center James H. Trainor, Goddard Space Flight Center W. R. Webber, University of New Hampshire