The twin Voyager spacecraft, over the course of a dozen years, drew back the curtain on nearly half of the solar system. From launch in 1977 through the spectacular parting shots of Neptune at the outer reaches of the solar system in 1989, this pair of spacecraft explored four planets – Jupiter, Saturn, Uranus and Neptune – as well as dozens of moons, and the rings and magnetic environments of those planetary systems.
The Voyagers were designed to take advantage of a rare geometric arrangement of the outer planets that occurs only once every 176 years. This configuration allows a single spacecraft to swing by all four gas giants without the need for large onboard propulsion systems; the flyby of each planet both accelerates the spacecraft and bends its flight path. Without these gravity assists, the flight time to Neptune would have been 30 years.
The second of the two Voyager spacecraft, Voyager 2, was launched first, on 20 August 1977. It was followed on 5 September 1977 by Voyager 1, which was put on a faster, shorter trajectory to Jupiter. Both launches took place from the Cape Canaveral Air Force Station in Florida.
Eighteen months after launch, Voyager 1 reached Jupiter, 650 million kilometers away. The spacecraft made its closest approach on 5 March 1979, while Voyager 2 followed on 9 July of the same year. Images streamed back from the pair of spacecraft showing the complex, swirling turbulence of Jupiter’s atmosphere in exquisite detail. A giant storm, three times the size of Earth, raged in Jupiter’s upper atmosphere, surrounded by rippling currents that rotated about it. Voyager 1 found nine active volcanoes erupting on Io, the innermost of Jupiter’s four major moons. Four months later, Voyager 2 found that eight of the nine volcanoes were still active. A thin, dusty ring was also discovered around Jupiter, forcing revision of theories about origins and mechanics of planetary ring systems.
At Saturn, both Voyagers took high-resolution images to help determine ring composition and dynamics. The Voyager 1 encounter took place in November 1980 and the Voyager 2 encounter was in August 1981. Voyager 1 was targeted to fly close to Saturn’s largest moon Titan. This resulted in a south polar passage of Saturn, which redirected the spacecraft northward of the ecliptic.
Voyager 2 continued on to Uranus where ten new moons were discovered in the Uranus system. The planet’s magnetic field was found to be significantly offset from the planet’s axis of rotation.
In August 1989, Voyager 2 flew past Neptune. Because Neptune receives so little sunlight, many scientists had expected to see a placid, featureless planet. Instead, Voyager showed a dynamic atmosphere with winds blowing westward, opposite the direction of rotation, at speeds faster than the winds of any other planet. Neptune revealed its Great Dark Spot, a storm system that resembled Jupiter’s Great Red Spot, and a smaller, eastwardly moving cloud, called ‘scooter’, which went around the planet about every 16 hours. The blue planet was circled by diffuse, dusty rings; six new moons were discovered.
Voyager 2 passed over the north polar region on Neptune, using the planet’s gravity to redirect the trajectory for a final encounter – with Neptune’s largest moon Triton. It then departed the solar system southward of the ecliptic.
At about the same time as Voyager 2 was encountering Neptune, Voyager 1, continuing its journey to the edge of the solar system on the north side of the ecliptic, turned its cameras back to look at the planets and take one last parting shot. Voyager 1’s ‘family portrait’ illustrates the vastness of the solar system and the huge expanses of emptiness within which the outer planets lie.
Both Voyagers are now headed for the outer boundary of the solar system, where the Sun becomes just one of many contributors to the interstellar environment. That edge is thought to be somewhere between 8 billion and 23 billion kilometers from the Sun. Engineers are optimistic that the Voyagers will still be transmitting data when that boundary is encountered sometime in the first quarter of the twenty-first century.
The spacecraft were assembled at and the mission was managed by the Jet Propulsion Laboratory, Pasadena, CA. Early parts of the mission have been described in more detail by [MORRISON1982].
The launch vehicle for Voyager 1 was a Titan/Centaur. The first stage Titan was powered by both solid and liquid fuel engines. The Centaur stage, 20 meters long and 3 meters in diameter burned a fuel combination of liquid hydrogen and liquid oxygen. The Titan boosted the Voyager Centaur combination into low Earth orbit, and the Centaur plus a small solid fuel rocket provided the energy for Voyager 1 to escape Earth orbit.
Spacecraft Id : VG1 Mission Phase Start Time : 1977-09-05 Mission Phase Stop Time : 1977-09-05 Spacecraft Operations Type : LAUNCH
During the period between Launch and Jupiter Encounter, Voyager 1 probed the interplanetary medium and conducted tests and calibrations of its systems.
Spacecraft Id : VG1 Mission Phase Start Time : 1977-09-05 Mission Phase Stop Time : 1979-01-06 Spacecraft Operations Type : CRUISE
The Voyager 1 flyby of Jupiter took place on 5 March 1979 at 12:04:36 UTC with the spacecraft closest approach only 348890 kilometers from the center of Jupiter. Among the highlights of the encounter were the discovery of a faint ring and one new satellite. Satellite encounter information is given below; ‘UNK’ denotes ‘unknown’ at time of this writing. The Voyager 1 Jupiter encounter is described in more detail by [STONE&LANE1979A;].
Satellite Satellite Radial Closest Approach Name Dimensions Distance Date Distance (km) (km) (1979) (km) --------- ----------- ---------- ------ --------- Metis 40 128,000 UNK UNK Adrastea 24x20x14 129,000 UNK UNK Amalthea 270x166x150 181,300 5 Mar 420,200 Thebe 110x90 222,000 UNK UNK Io 3630 422,000 5 Mar 20,570 Europa 3138 661,000 5 Mar 733,760 Ganymede 5262 1,070,000 5 Mar 114,710 Callisto 4800 1,883,000 6 Mar 126,400 Leda 16 11,094,000 UNK UNK Himalia 186 11,480,000 UNK UNK Lysithia 36 11,720,000 UNK UNK Elara 76 11,737,000 UNK UNK Ananke 30 21,200,000 UNK UNK Carme 40 22,600,000 UNK UNK Pasiphae 50 23,500,000 UNK UNK Sinope 36 23,700,000 UNK UNK Spacecraft Id : VG1 Target Name : JUPITER Mission Phase Start Time : 1979-01-06 Mission Phase Stop Time : 1979-04-13 Spacecraft Operations Type : FLYBY
During the period between Jupiter Encounter and Saturn Encounter, Voyager 1 probed the interplanetary medium, observed selected celestial targets, and conducted tests and calibrations of its systems. Mission planners used the 16 months to develop and test activity sequences which would be used during the Saturn Encounter.
Spacecraft Id : VG1 Mission Phase Start Time : 1979-04-13 Mission Phase Stop Time : 1980-08-22 Spacecraft Operations Type : CRUISE
The Voyager 1 flyby of Saturn took place on 12 November 1980 at 23:46 UTC with the spacecraft closest approach only 184300 kilometers from the center of Saturn. Among the highlights of the encounter were the separate encounter with Titan, discovery of intricate patterns within the ring system, and observation of variations among the many moons of Saturn. Closest approaches to some of the satellites were on the dates and at the distances shown below. ‘UNK’ denotes ‘unknown’ at the time of this writing. The encounter is described in more detail by [STONE&MINER1981;].
Satellite Satellite Radial Closest Approach Name Dimensions Distance Date Distance (km) (km) (1980) (km) --------- ----------- ---------- ------ ---------- Pan 10 133,583 12 Nov UNK Atlas 40x20 137,670 UNK 219,000 Prometheus 140x100x80 139,353 UNK 300,000 Pandora 110x90x80 141,700 12 Nov 270,000 Epimetheus 140x120x100 151,472 13 Nov 121,000 Janus 220x200x160 151,422 12 Nov 297,000 Mimas 392 185,520 12 Nov 88,440 Enceladus 520 238,020 12 Nov 202,040 Tethys 1060 294,660 12 Nov 415,670 Telesto 34x28x26 294,660 12 Nov 233,000 Calypso 34x22x22 294,660 13 Nov 432,000 Dione 1120 377,400 12 Nov 161,520 Helene 36x32x30 377,400 13 Nov 237,000 Rhea 1530 527,040 12 Nov 73,980 Titan 5150 1,221,860 12 Nov 6,490 Hyperion 410x260x220 1,481,000 13 Nov 880,440 Iapetus 1460 3,560,830 14 Nov 2,470,000 Phoebe 220 12,952,000 UNK 13,500,000 Spacecraft Id : VG1 Target Name : SATURN Mission Phase Start Time : 1980-08-22 Mission Phase Stop Time : 1980-12-14 Spacecraft Operations Type : FLYBY
After conclusion of the Saturn Encounter, Voyager 1 left the ecliptic at an angle of about 30 degrees. Its scan platform instruments were turned off, but some of the remaining instruments (primarily fields and particles) continued to monitor the environment in the outer solar system as the spacecraft traveled outward toward the heliopause.
Spacecraft Id : VG1 Mission Phase Start Time : 1980-12-14 Mission Phase Stop Time : UNK Spacecraft Operations Type : CRUISE
The launch vehicle for Voyager 2 was a Titan/Centaur. The first stage Titan was powered by both solid and liquid fuel engines. The Centaur stage, 20 meters long and 3 meters in diameter burned a fuel combination of liquid hydrogen and liquid oxygen. The Titan boosted the Voyager Centaur combination into low Earth orbit, and the Centaur plus a small solid fuel rocket provided the energy for Voyager 2 to escape Earth orbit.
Spacecraft Id : VG2 Mission Phase Start Time : 1977-08-20 Mission Phase Stop Time : 1977-08-20 Spacecraft Operations Type : LAUNCH
During the period between Launch and Jupiter Encounter, Voyager 2 probed the interplanetary medium and conducted tests and calibrations of its systems.
Spacecraft Id : VG2 Mission Phase Start Time : 1977-08-20 Mission Phase Stop Time : 1979-04-25 Spacecraft Operations Type : CRUISE
The Voyager 2 flyby of Jupiter took place on 9 July 1979 at 22:29 UTC. This was 18 weeks after the Voyager 1 Jupiter Encounter and was at a closest approach distance of 721670 kilometers from the center of Jupiter. The Voyager 2 trajectory was chosen to complement that of Voyager 1, including a much closer approach to Europa, probing southern latitudes in Jupiter’s atmosphere, and an extensive investigation of Jupiter’s magnetotail. Satellite encounter information is given below; ‘UNK’ denotes ‘unknown’ at time of this writing. The Voyager 2 Jupiter encounter is described in more detail by [STONE&LANE1979B;].
Satellite Satellite Radial Closest Approach Name Dimensions Distance Date Distance (km) (km) (1979) (km) --------- ----------- ---------- ------ --------- Metis 40 128,000 UNK UNK Adrastea 24x20x14 129,000 UNK UNK Amalthea 270x166x150 181,300 9 Jul 558,370 Thebe 110x90 222,000 UNK UNK Io 3630 422,000 9 Jul 1,129,900 Europa 3138 661,000 9 Jul 205,720 Ganymede 5262 1,070,000 9 Jul 62,130 Callisto 4800 1,883,000 8 Jul 214,930 Leda 16 11,094,000 UNK UNK Himalia 186 11,480,000 UNK UNK Lysithia 36 11,720,000 UNK UNK Elara 76 11,737,000 UNK UNK Ananke 30 21,200,000 UNK UNK Carme 40 22,600,000 UNK UNK Pasiphae 50 23,500,000 UNK UNK Sinope 36 23,700,000 UNK UNK Spacecraft Id : VG2 Target Name : JUPITER Mission Phase Start Time : 1979-04-25 Mission Phase Stop Time : 1979-08-05 Spacecraft Operations Type : FLYBY
During the period between Jupiter Encounter and Saturn Encounter, Voyager 2 probed the interplanetary medium, observed selected celestial targets, and conducted tests and calibrations of its systems. Mission planners used the 22 months to develop and test activity sequences which would be used during the Saturn Encounter.
Spacecraft Id : VG2 Mission Phase Start Time : 1979-08-05 Mission Phase Stop Time : 1981-06-05 Spacecraft Operations Type : CRUISE
The Voyager 2 closest approach to Saturn was on 26 August 1981 at 03:24 UTC and at a distance of 161000 km from the center of Saturn. The trajectory was chosen so that the spacecraft could obtain a gravitational assist from Saturn and continue on to Uranus; the timing was selected to provide better views of several satellites than had been obtained from Voyager 1. Design of science sequences was influenced by Voyager 1 results. Satellite encounters were on the dates and at the closest approach distances shown below; ‘UNK’ denotes ‘unknown’ at the time of this writing. The scan platform seized temporarily 110 minutes after Saturn closest approach, causing the central computer to disable further commands and resulting in loss of some data. When commanded again three days later (at low rate), it moved as instructed. A gyroscope calibration error between closest approach and five hours later also caused loss of data. Scan platform activities ended on 5 September 1981. This encounter is described in more detail by [STONE&MINER1982;].
Satellite Satellite Radial Closest Approach Name Dimensions Distance Date Distance (km) (km) (1981) (km) --------- ----------- ---------- ------ ---------- Pan 10 133,583 26 Aug UNK Atlas 40x20 137,670 26 Aug 287,000 Prometheus 140x100x80 139,353 26 Aug 247,000 Pandora 110x90x80 141,700 26 Aug 107,000 Epimetheus 140x120x100 151,472 26 Aug 147,000 Janus 220x200x160 151,422 26 Aug 223,000 Mimas 392 185,520 26 Aug 309,930 Enceladus 520 238,020 26 Aug 87,010 Tethys 1060 294,660 25 Aug 93,010 Telesto 34x28x26 294,660 26 Aug 270,000 Calypso 34x22x22 294,660 26 Aug 151,590 Dione 1120 377,400 26 Aug 502,310 Helene 36x32x30 377,400 25 Aug 314,090 Rhea 1530 527,040 26 Aug 645,260 Titan 5150 1,221,860 24 Aug 666,190 Hyperion 410x260x220 1,481,000 24 Aug 431,370 Iapetus 1460 3,560,830 22 Aug 908,680 Phoebe 220 12,952,000 4 Sep 2,075,640 Spacecraft Id : VG2 Target Name : SATURN Mission Phase Start Time : 1981-06-05 Mission Phase Stop Time : 1981-09-25 Spacecraft Operations Type : FLYBY
During the period between Saturn Encounter and Uranus Encounter, Voyager 2 probed the interplanetary medium, observed selected celestial targets, and conducted tests and calibrations of its systems. Mission planners used the 49 months to develop and test activity sequences which would be used during the Uranus Encounter. Considerable attention was paid to the scan platform capabilities, following its seizure during the Saturn Encounter. Full scan platform operation was restored before the end of 1981.
Spacecraft Id : VG2 Mission Phase Start Time : 1981-09-25 Mission Phase Stop Time : 1985-11-04 Spacecraft Operations Type : CRUISE
The Voyager 2 closest approach to Uranus was on 24 January 1986 at 17:59 UTC at a distance of 107000 km from the center of Uranus. The trajectory was chosen so that the spacecraft could obtain a gravitational assist from Uranus and continue on to Neptune; NASA permission for the Neptune Encounter was granted during the approach to Uranus. The timing of the Uranus closest approach was selected to provide a close approach to Miranda and to allow capture of radio occultation data at the DSN tracking station in Australia (southern declination of Uranus meant that Australia was preferred for DSN tracking). Radio occultation data were also collected using the 64-m antenna at Parkes in Australia. Satellite encounters were on the dates and at the closest approach distances shown below. ‘UNK’ denotes ‘unknown’ at the time of this writing. Satellite images were improved by implementation of image motion compensation on the spacecraft. Reed-Solomon encoding was used for the first time; real-time imaging data rates were reduced by almost 70 percent. Ground antennas were arrayed to increase receiving aperture. This encounter is described in more detail by [STONE&MINER1986;].
Satellite Satellite Radial Closest Approach Name Dimensions Distance Date Distance (km) (km) (1986) (km) --------- ----------- ---------- ------ ---------- Cordelia 26 49,800 UNK UNK Ophelia 30 53,800 UNK UNK Bianca 42 59,200 UNK UNK Juliet 62 61,800 UNK UNK Desdemona 54 62,700 UNK UNK Rosalind 84 64,400 UNK UNK Portia 108 66,100 UNK UNK Cressida 54 69,900 UNK UNK Belinda 66 75,300 UNK UNK Puck 154 86,000 UNK UNK Miranda 472 129,900 24 Jan 29,000 Ariel 1,158 190,900 24 Jan 127,000 Umbriel 1,172 265,969 24 Jan 325,000 Titania 1,580 436,300 24 Jan 365,200 Oberon 1,524 583,400 24 Jan 470,600 Spacecraft Id : VG2 Target Name : URANUS Mission Phase Start Time : 1985-11-04 Mission Phase Stop Time : 1986-02-25 Spacecraft Operations Type : FLYBY
During the period between Uranus Encounter and Neptune Encounter, Voyager 2 probed the interplanetary medium, observed selected celestial targets, and conducted tests and calibrations of its systems. Mission planners used the 39 months to develop and test activity sequences which would be used during the Neptune Encounter.
The DSN used this time to add a 34-m tracking antenna at the Madrid complex, to increase the diameter of their 64-m antennas to 70 meters, and to make the 70-m systems more efficient. A special microwave link was installed to permit the Parkes radio telescope to be arrayed with the Canberra DSN antenna in Australia.
Spacecraft Id : VG2 Mission Phase Start Time : 1986-02-25 Mission Phase Stop Time : 1989-06-05 Spacecraft Operations Type : CRUISE
The Voyager 2 closest approach to Neptune was on 25 August 1989 at 03:56 UTC at a distance of 29240 km from the center of Neptune. The trajectory and timing were chosen so that the spacecraft could obtain a gravitational assist from Neptune and continue on for an encounter with Neptune’s large satellite Triton about five hours later (closest approach at 09:10 UTC). The timing was also selected so that radio occultation data would be collected at the DSN tracking station in Australia (southern declination of Neptune meant that Australia was preferred for DSN tracking). Radio occultation data were again collected with the Parkes antenna and with a new 64-m antenna at Usuda in Japan. Satellite encounters were on the dates and at the closest approach distances shown below. ‘UNK’ denotes ‘unknown’ at the time of this writing. Data rates were increased over those at Uranus by including the Very Large Array (VLA) in New Mexico for receiving and by taking advantage of DSN upgrades made over the previous three years. This encounter is described in more detail by [STONE&MINER1989;].
Satellite Satellite Radial Closest Approach Name Dimensions Distance Date Distance (km) (km) (1989) (km) --------- ----------- ---------- ------ ---------- Naiad 54 48,000 25 Aug UNK Thalassa 80 50,000 25 Aug UNK Despina 180 52,500 25 Aug UNK Galatea 150 62,000 25 Aug UNK Larissa 190 73,600 25 Aug 60,180 Proteus 400 117,600 25 Aug 97,860 Triton 2,700 354,760 25 Aug 39,790 Nereid 340 5,509 090 25 Aug 4,652,880 Spacecraft Id : VG2 Target Name : NEPTUNE Mission Phase Start Time : 1989-06-05 Mission Phase Stop Time : 1989-10-02 Spacecraft Operations Type : FLYBY
After conclusion of the Neptune Encounter, Voyager 2 left the ecliptic at an angle of about -30 degrees. Its scan platform instruments were turned off, but some of the remaining instruments (primarily fields and particles) continued to monitor the environment in the outer solar system as the spacecraft traveled outward toward the heliopause. During the Shoemaker-Levy 9 impact with Jupiter in July 1994, the ultraviolet spectrometer was trained on Jupiter and radio signals were recorded; but no emissions from the impact were detected.
Spacecraft Id : VG2 Mission Phase Start Time : 1989-10-02 Mission Phase Stop Time : UNK Spacecraft Operations Type : CRUISE "
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.
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.
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 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.
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.
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. “