Mission Objectives Summary    ==========================      Voyager's primary objective was exploration of the two giant      planets, Jupiter and Saturn, their magnetospheres, and their      satellites.  Major emphasis was placed on studying the      satellites, many of which are planet-sized worlds, in as much      detail as possible.  The study of Titan, the only satellite in      the solar system known to have an extensive atmosphere, was      nearly as high a priority as studies of Saturn itself      [MORRISON1982].  After the successful Voyager 1 encounter with      Titan, it was decided to expand the Voyager objectives to      include at least Uranus; Uranus and Neptune could both be      reached by proper reprogramming of the Voyager 2 trajectory.      Comparative studies then could include the four largest planets      in the solar system.      Eleven investigations were approved for the Voyager mission.      Investigation names and Principal Investigators, or Team      Leaders in the cases of ISS and RSS, are shown in the table      below; the trailing 'S' stands for 'subsystem' in most      acronyms.        Investigation, P/I or T/L                            Acronym        ------------------------------------------------     -------        Imaging Science Investigation                          ISS                       B.A. Smith        Infrared Interferometer and Radiometer Investigation   IRIS                       R.A. Hanel (Jupiter - Uranus)                       B.J. Conrath (Neptune)        Photopolarimeter Investigation                         PPS                       C.F. Lillie (Voyager 1 Jupiter)                       C.W. Hord (Voyager 2 Jupiter)                       A.L. Lane (Saturn - Neptune)        Radio Science Investigation                            RSS                       V.R. Eshleman (Jupiter)                       G.L. Tyler (Saturn - Neptune)        Ultraviolet Spectrometer Investigation                 UVS                       A.L. Broadfoot        Magnetometer Investigation                             MAG                       N.F. Ness        Plasma Science Investigation                           PLS                       H.S. Bridge (Jupiter - Uranus)                       J.W. Belcher (Neptune)        Plasma Wave Investigation                              PWS                       F.L. Scarf (Jupiter - Uranus)                       D.A. Gurnett (Neptune)        Planetary Radio Astronomy Investigation                PRA                       J.W. Warwick        Low-Energy Charged Particle Investigation              LECP                       S.M. Krimigis        Cosmic Ray Investigation                               CRS                       R.E. Vogt (Jupiter - Saturn)                       E.C. Stone (Uranus - Neptune)      Broadly stated, the science goals of the mission were: high      resolution imaging of the gas planets and inference of      atmospheric dynamics; high resolution imaging of satellites and      inference of geologic processes; spectral measurements of      atmospheres and satellite surfaces, inference of compositions,      and inference of thermal properties and structure;      identification and study of aerosols and surface physical      structure using polarized light; occultation measurement of      atmospheric thermal, ionospheric charged particle, and ring      structure; and measurement of magnetic fields and particle      environments and inference of Sun-planet-satellite      interactions, magnetospheric structure, and mechanisms within      each planetary system for generating the observed fields.      Jupiter      -------        The largest planet in the solar system, Jupiter is composed        mainly of hydrogen and helium, with small amounts of methane,        ammonia, water vapor, traces of other compounds and a core of        melted rock and ice.  One of the objectives of Voyager was to        quantify the composition of the atmospheres of Jupiter and        the other giant planets.        Colorful latitudinal bands, atmospheric clouds, and storms        characterize Jupiter's dynamic atmosphere.  By taking a        series of images, Voyager could show the time variability of        the atmosphere.  The Great Red Spot was revealed as a complex        storm moving in a counterclockwise direction.  An array of        other smaller storms and eddies were found throughout the        banded clouds.        Jupiter is now known to possess 16 moons.  An objective of        the Voyager mission was to search for new moons and to obtain        high resolution quantitative measurements on those that had        been discovered earlier.  Active volcanism on the satellite        Io was easily the most surprising discovery at Jupiter.  It        was the first time active volcanoes had been seen on another        body in the solar system.  Together, the Voyagers observed        the eruption of nine volcanoes on Io, and there is evidence        that other eruptions occurred between the Voyager encounters.        Although interpretations vary, the cratered surfaces of the        terrestrial planets (and the Moon) are believed to contain        the record of small body populations in the inner solar        system from as far back as 4 billion years ago.  One of the        objectives of the Voyager mission was to obtain similar        cratering data from satellites in the outer solar system.        Impact craters on Io have been obliterated by that satellite's        volcanism.  Rather than craters, Europa was distinguished by        a large number of intersecting linear features with almost no        topographic relief.  There is a possibility that Europa is        internally active due to tidal heating at a level one-tenth        or less than that of Io and that the crust is very thin (less        than 30 kilometers).  Ganymede has two distinct types of        terrain -- cratered and grooved -- suggesting that its entire        icy crust has been under tension from global tectonic        processes.  Callisto has a very old, heavily cratered crust        showing remnant rings of enormous impact craters.  The        largest craters have apparently been erased by the flow of        the icy crust over geologic time.  Almost no topographic        relief is apparent in the ghost remnants of the immense        impact basins, identifiable only by their light color and the        surrounding subdued rings of concentric ridges.        Indirect evidence from Pioneer 10/11 suggested the presence        of a thin ring around Jupiter.  One of the objectives of the        Voyager mission was to search more systematically for such a        ring, and to quantify both the number-density and the size        distribution of particles within rings in the outer solar        system.  A faint, dusty ring of material was found around        Jupiter.  Its outer edge is 129,000 kilometers from the        center of the planet, and it extends inward about 30,000        kilometers.        Two new, small satellites, Adrastea and Metis, were found        orbiting just outside the ring.  A third new satellite,        Thebe, was discovered between the orbits of Amalthea and Io.        Jupiter's rings and moons exist within an intense radiation        belt of electrons and ions trapped in the planet's magnetic        field.  These particles and fields comprise the jovian        magnetosphere, or magnetic environment, which extends three        to seven million kilometers toward the Sun, and stretches in        a windsock shape at least as far as Saturn's orbit -- a        distance of 750 million kilometers (460 million miles).        As the magnetosphere rotates with Jupiter, it sweeps past Io        and strips away about 1,000 kilograms (one ton) of material        per second.  The material forms a torus, a doughnut-shaped        cloud of ions that glow in the ultraviolet.  The heavy ions        in the torus migrate outward, and their pressure inflates the        jovian magnetosphere to more than twice its expected size.        Some of the more energetic sulfur and oxygen ions fall along        the magnetic field into the planet's atmosphere, resulting in        auroras.      Saturn      ------        A major objective of the Voyager mission was to determine in        which ways the gas giants are the same and in which ways they        are different.  Saturn, like Jupiter, is mostly hydrogen and        helium.  Its hazy yellow hue has broad atmospheric banding        similar to (but much fainter than) that found on Jupiter.  It        also has a complex ring system, the details of which were        sketchy before Voyager, but which represented an important        objective in themselves.        It is thought that the rings formed from one or more moons        that were shattered by impacts of comets and meteoroids.  The        resulting material, ranging in size from dust to house-sized        particles, has accumulated in a broad plane in which both the        shape and density vary in ways which depend intricately on        gravitational interactions with satellites.  This is most        obviously demonstrated by the relationship between the F-ring        and two small moons that 'shepherd' the ring material.  The        variation in the separation of the moons from the ring may        explain the ring's kinked appearance.  Shepherding moons were        also found by Voyager 2 at Uranus.  Very diffuse rings and        'spokes' (neither detected from Earth) were also found by        Voyager.        Winds blow at extremely high speeds on Saturn -- up to 1,800        kilometers per hour.  Their primarily easterly direction        indicates that the winds are not confined to the top cloud        layer but must extend at least 2,000 kilometers downward into        the atmosphere.        Saturn has 18 known satellites ranging from Phoebe, a small        moon that travels in a retrograde orbit and is probably a        captured asteroid, to Titan, the planet-sized moon with        an atmosphere that had been detected from Earth before        Voyager.  A major objective of Voyager was to        investigate these satellites and, in particular, to learn a        great deal more about Titan.  Titan's surface temperature and        pressure were found to be 94 K and 1.6 atmospheres.        Photochemistry converts some atmospheric methane to other        organic molecules, such as ethane, that may accumulate in        lakes or oceans.  Other more complex hydrocarbons form the        haze particles that eventually fall to the surface, coating        it with a thick layer of organic matter.  The chemistry in        Titan's atmosphere may resemble that which occurred on Earth        before life evolved.        The most active surface of any moon seen in the Saturn system        was that of Enceladus.  The bright surface of this moon,        marked by faults and valleys, showed evidence of tectonically        induced change.  Voyager 1 found that the surface of Mimas is        dominated by a crater so large that the impact nearly broke        the satellite apart.        Saturn's magnetic field is weaker than Jupiter's, extending        only one or two million kilometers.  The axis of the field is        almost perfectly aligned with Saturn's rotation axis.      Uranus      ------        Uranus is distinguished by the fact that it is tipped on its        side.  This unusual orientation is thought to be the result        of a collision with a planet-sized body early in the solar        system's history.  Clues to this event, as well as more basic        data about this planet (which has polar regions exposed to        sunlight or hidden in darkness for long periods) were        important Voyager objectives.  At about the time of Voyager's        launch, observations from Earth showed that Uranus was        circled by rings -- not bright and wide, as was the case        for Saturn, but extremely narrow and very dark.        Voyager 2 found that one of the most striking influences of        the orientation of the rotation axis is its effect on the        tail of the magnetic field, which is itself tilted 60 degrees        from the planet's axis of rotation.  The magnetotail was        shown to be twisted by the planet's rotation into a long        corkscrew shape behind Uranus.        The existence of a magnetic field at Uranus was not known        until Voyager's arrival.  The intensity of the field is        roughly comparable to that of Earth's, though it varies much        more from point to point because of its large offset from the        center of the planet.  The peculiar orientation of the        magnetic field suggests that the field is generated at an        intermediate depth in the interior where the pressure is high        enough for water to become electrically conducting.        Radiation belts at Uranus were found to be similar in        intensity to those at Saturn.  The intensity of radiation        within the belts is such that irradiation would quickly        darken (within 100,000 years) any methane trapped in the icy        surfaces of the inner moons and ring particles.  This may        have contributed to the darkened surfaces of the moons and        ring particles, which have lower albedos than coal and are        almost uniform in color.        A high layer of haze was detected around the sunlit pole,        which also was found to radiate large amounts of ultraviolet        light, a phenomenon dubbed 'dayglow'.  Surprisingly, the        illuminated and dark poles, and most of the planet, show        nearly the same temperature at the cloud tops.        Voyager found 10 new moons, bringing the total number at        Uranus to 15.  Most of the new moons are small, with the        largest measuring about 150 kilometers in diameter.        The five large moons appear to be ice-rock conglomerates like        the satellites of Saturn.  Titania is marked by huge fault        systems and canyons indicating some degree of geologic        (probably tectonic) activity in its history.  Ariel has the        brightest and possibly youngest surface of all the Uranian        moons and also appears to have undergone geologic activity        that led to many fault valleys and what seem to be extensive        flows of icy material.  Little geologic activity has occurred        on Umbriel or Oberon, judging by their old and dark surfaces.        The moon Miranda, innermost of the five large moons, was        revealed to be one of the strangest bodies yet seen in the        solar system.  Detailed images from Voyager's flyby of the        moon showed huge fault canyons as deep as 20 kilometers,        terraced layers, and a mixture of old and young surfaces.        One theory holds that Miranda may be a reaggregation of        material from an earlier time when the moon was fractured by        a violent impact.        All nine rings discovered from Earth in the 1970's were        studied by the spacecraft and showed the Uranian rings to be        distinctly different from those at Jupiter and Saturn.  The        ring system may be relatively young and did not form at the        same time as Uranus.  Particles that make up the rings may be        remnants of a moon that was fractured by a high-velocity        impact or torn up by gravitational effects.      Neptune      -------        Less was known about Neptune than about Uranus at the        beginning of the Voyager mission.  Approximately the same        size as Uranus, Neptune was expected to be a twin except for        having a rotation axis more likely to be normal to the        ecliptic.  About five years before the Voyager 2 Neptune        encounter, evidence began accumulating that Neptune had        atmospheric structure and (possibly) rings.  The ring data        were very ambiguous; only exotic ring models (transient        rings, partial rings, polar rings, etc.) were consistent        with the observations from Earth.        Even though Neptune receives only three percent as much        sunlight as Jupiter, it is a dynamic planet and showed        several large, dark spots reminiscent of Jupiter's        hurricane-like storms.  The largest spot, dubbed the Great        Dark Spot, is about the size of Earth and is similar to the        Great Red Spot on Jupiter.  A small, irregularly shaped,        eastward-moving cloud was observed 'scooting' around Neptune        approximately once every 16 hours.        Long bright clouds, similar to cirrus clouds on Earth, were        seen high in Neptune's atmosphere.  At low northern        latitudes, Voyager captured images of cloud streaks casting        their shadows on cloud decks below.        The strongest winds on any planet were measured on Neptune.        Most of the winds blow westward, or opposite to the rotation        of the planet.  Near the Great Dark Spot, winds blow up to        2,000 kilometers an hour.        The magnetic field of Neptune, like that of Uranus, turned        out to be highly tilted -- 47 degrees from the rotation axis        and offset at least 0.55 radii (about 13,500 kilometers or        8,500 miles) from the physical center.  The extreme        orientation may be characteristic of flows in the interiors        of both Uranus and Neptune -- and not related, in the Uranus        case, to the planet's rotation axis tilt or to any possible        field reversals at either planet.  Voyager studies of radio        emissions caused by the magnetic field revealed the length        of a Neptunian day (16.11 hours).  The spacecraft also        detected auroras, though they are much weaker than those on        Earth and other planets.        Triton, the largest Neptunian moon, was shown to be not only        the most intriguing satellite of the system, but also one        of the most interesting in all the solar system.  Intricate        surface patterns suggest a remarkable geologic history,        while Voyager 2 images captured active geyser-like eruptions        spewing invisible nitrogen gas and dark dust particles        several kilometers into the tenuous atmosphere.  Triton's        relatively high density and retrograde orbit offer strong        evidence that it is not an original member of Neptune's        family but, rather, is a captured object.  If so, tidal        heating could have melted Triton in its originally eccentric        orbit, and the moon may have been liquid for as long as one        billion years after its capture by Neptune.        An extremely thin atmosphere extends about 800 kilometers        above Triton's surface.  Nitrogen ice particles may form thin        clouds a few kilometers above the surface.  The atmospheric        pressure at the surface is about 14 microbars, 1/70,000th the        surface pressure on Earth.  The surface temperature is about        38 K -- the coldest known temperature of any body in the        solar system.        The new moons found at Neptune by Voyager are all small and        remain close to Neptune's equatorial plane.        Searches for 'ring arcs,' or partial rings, showed that        Neptune's rings actually are complete, but are so diffuse and        the material in them so fine that they could not be fully        resolved from Earth.  The arcs are confined by the actions of        nearby satellites.  Particle sizes are smaller than at Uranus.      Interstellar Mission      --------------------        The Voyager spacecraft are continuing to return data about        interplanetary space and some of our stellar neighbors near        the edges of the Solar System.  Their fields, particles, and        waves instruments are studying the environment around them.        In May 1993, the plasma wave experiment began picking up radio        emissions that originate at the heliopause, the outer edge of        our solar system, where the interstellar medium restricts the        outward flow of the solar wind and confines it within a        magnetic bubble called the heliosphere.  By studying the        radio emissions, scientists now theorize the heliopause        exists some 90 to 120 astronomical units from the Sun.        The Voyagers have also become space-based ultraviolet        observatories and their unique location in the universe gives        astronomers the best vantage point they have ever had for        looking at celestial objects that emit ultraviolet radiation.        The cameras on the spacecraft have been turned off and the        ultraviolet instrument is the only experiment on the scan        platform that is still functioning.  Voyager scientists        expect to continue to receive data from the ultraviolet        spectrometers at least until the year 2000.  At that time,        there will not be enough electrical power for the heaters to        keep the ultraviolet instrument warm enough to operate.        Yet there are several other fields and particle instruments        that can continue to send back data as long as the spacecraft        can stay alive.  They include the cosmic ray subsystem, the        low-energy charge particle instrument, the magnetometer, the        plasma subsystem, the plasma wave subsystem and the planetary        radio astronomy instrument.