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Overview of the Outer Planets

The first trips to the outer planets were the Pioneer 10 and 11 missions, launched in March 1972, and April 1973, respectively. Pioneer 10 went past Jupiter in December 1973, enroute to becoming the first man-made object to leave the solar system. (After 25 years of continuing service, communication with this spacecraft was finally terminated at the end of March, 1997, as Pioneer 10 reached a distance of more than 11 billion kilometers [6.8 billion miles] from Earth.) Its principal finding was to measure a magnetic field about 2,000 times stronger than Earth's and to detect intense radiation belts and a Jovian ionosphere. It sent back excellent close-up, black and white images of the planet's surface from its TV camera. Here is one of the classic (historic) images, taken through a blue filter on the Photopolarimeter lens, showing a full face view of Jupiter, with its conspicuous dark ellipse known as the Great Red Spot (an atmospheric cyclone). The small black spot to the right is the shadow cast by one of Jupiter's moons, Io. At the time, Pioneer 10 was about 2,500,000 km on the Earth side of the planet.

B/W Pioneer 10 full-face image of Jupiter showing it's Great Red Spot, December 1973.

Pioneer 11 passed Jupiter in December 1974, enroute to a flyby of Saturn in September of 1979. This spacecraft traversed Saturn's rings without being destroyed (as some predicted) by their widely-spaced particles. Pioneer 11 also found evidence that Jupiter's magnetosphere extends beyond Saturn's orbit in the direction away from the Sun.

We come now to what is indisputably the most outstanding planetary space missions ever sent forth by mankind: the Voyager 1 and 2 probes to the outer planets. At a cost of under one billion dollars¾ a true bargain even then¾ two remarkable spacecraft visited all of the giant planets, Jupiter, Saturn, Uranus, and Neptune. They started their grand tours in 1977 and are still going strong, as these vehicles leave the solar system. They imaged all four planets and 48 satellites, some of which they discovered. A thorough summary of the Voyager missions is offered on the Jet Propulsion Lab's (JPL) Voyager Home Page (

One impetus for the missions was the favorable alignment of the giant planets that occurs once every 175 years, such that initial trajectories plus corrections might allow at least one spacecraft to fly past Jupiter, all the way to Neptune. With each planetary flyby, the spacecraft received a gravitational "kick" to speed them along, so as to favorably alter their courses and shorten their travel time significantly (without this boost, the trip to Neptune, using the initial launch velocities, would have taken 30 years).

Voyager 2 launched from Earth on August 20, 1977, and arrived at Jupiter on July 9, 1979. Voyager 1 took off on September 5, 1977, reaching Jupiter on March 5, 1979. Even though leaving later, Voyager 1’s shorter path reduced its trip time. Both spacecraft then went by Saturn, Voyager 1 during November 1980 and Voyager 2 in August, 1981, receiving another gravity boost. At their Saturn encounters,the Voyagers had met the initial requirement of five years of operation, but Voyager 1's chosen trajectory (past Saturn’s moon, Titan) did not favor it continuing to Uranus. So JPL officials allowed it to depart upward (towards celestial north) out of the ecliptic plane. Voyager 2's "all systems go" status qualified it, with appropriate course corrections, to head to Uranus, encountering it on January 24, 1986. Further course adjustments sent it on to Neptune, with closest approach on August 25, 1989. At each new rendezvous, the spacecraft performed superbly, providing detailed data on each giant planet. Both spacecraft have now left the planetary region of the solar system.

An artist's rendition of the identical Voyager spacecraft, traveling through space, appears below. The scientific payload appears at its upper left and also on the large boom extending left to right.

Artists drawing of Voyager 1.

The Voyagers carried instruments designed to support 10 experiments. In addition to TV cameras, others included UV and IR sensors, and cosmic-ray, plasma and charged particle detectors, and magnetometers (see the Voyager Home Page for a schematic drawing of the spacecraft with its deployed instruments; click on each for a functional description). Because of their extreme distance from the Sun, they could not use solar panels for power, but, instead, they use radioisotope thermoelectric generators (RTGs) that convert heat from decaying plutonium into electricity. Because of this wise power choice, both spacecraft, even today, beam radio signals full of new science data from well beyond 100 A.U. (1 A.U. is the distance from the Sun to Earth: 150 million kilometers [93,000,000 miles]), having passed the heliopause, the outermost boundary of measurable solar wind.

As an introduction to the superb images sent back from the Voyagers, consider the next two views, each showing a segment of its planet, along with two visible moons. At the top is Jupiter, seen by Voyager 2, on February 3, 1979, from a distance of 20,000,000 km (12,400,000 mi), with its moons in the foreground: Io on the left and Europa to its right. At the bottom is an image from Voyager 1 that displays part of the Saturnian surface, its magnificent rings (innermost = A, then the Cassini Gap, then B, the Encke Division, and C and the outer rings), and the moons Dione and Tethys.


Color Voyager 2 image of Jupiter, February 3 1979. Color Voyager 1 image of Saturn, showing it's rings and several of it's moons.

19-52: What are the three dots (glitches?) in the Saturn picture? ANSWER

The giant planets are just that: vastly larger, near-spherical bodies, dominated by atmospheres that comprise the bulk of their volumes. Orbiting at 5.2 A.U., Jupiter is the largest, with an equatorial diameter of 142,984 km (88,850 mi, measured from the center to an outer level, where we estimate the atmospheric pressure is 1.0 atm.) and a mass of 1.9 x 1027 kg (4.19 x 1027 lbs) (greater than twice that of all other planets combined, and 318 times greater than Earth’s). Moving further from the sun, the planets are progressively smaller, as follows: Saturn (9.54 A.U.) - 120,530 km (74,897 mi); Uranus (19.22 A.U.) - 51,116 km (31,763 mi); Neptune (30.1 A.U.) - 49,528 km (30,777 mi). They experience polar flattening of about 2% to 10%. Each planet rotates very rapidly for its size. They all have rotational periods less than one Earth day. They consist mainly of hydrogen and helium (for Jupiter and Saturn, the ratio of H/He is about 9:1 by number of atoms and 4:1 by atomic weight); minor amounts of methane and ammonia; and traces of nitrogen and water. Jupiter and Saturn are massive enough to convert the inner layers of hydrogen into a metallic liquid state. For Jupiter, this region is about 40,000 km (24,856 mi)-thick. It is less than 15,000 km (9,321 mi)-thick for Saturn. For both planets, the region grades outward into liquid molecular hydrogen (Jupiter’s outer region is about 18,000 km [11,185 mi]-thick), and then comparatively thin atmospheres. Metallic hydrogen is absent in Uranus and Neptune, so, their interiors are mostly molecular hydrogen. Based on density calculations, all four planet's may have a rocky (silicate) core, subjected to high pressures and temperatures (Jupiter: about 20,000° K). The radius of Jupiter's core is about 5% and Saturn's about 10% of the planetary radius. The cores of Uranus and Neptune extend to about 1/2 their planetary radius.

Thermal energy released from compression is largely responsible for the internal heat that makes each planet warmer at the surface (reverse thermal gradient) than expected from solar heating alone. For Jupiter, heat from the interior provides 70% more thermal energy than received from the Sun. This energy provides the fluid dynamics that drive the atmospheres into intense motions in belts or zones, roughly parallel to their equators, with wind speeds far in excess of those on Earth. Thermal conditions at great depth cause electron stripping and other mechanisms involving conductive motions that produce magnetic fields around each planet. The fields of Jupiter and Saturn are about 4,000 and 1,000 times, respectively, that of Earth’s magnetic field. The fields of Uranus and Neptune are much weaker. However, the magnetospheres of Uranus and Neptune have unusual shapes and distributions, with the magnetic pole axis of each strongly inclined to the rotational axis. Uranus's spin axis tilts 98° , so it almost lies in the solar ecliptic plane.

Prior to the Voyagers’ visits, we only knew Saturn had rings of material moving in near-circular paths at varying distances beyond the planet's (gas-pressure-defined) surface. But, as the spacecraft imaged the three other planets, we found positive visual evidence of generally more subdued rings at each one, so that we now consider rings the norm for this type of gaseous body. The rings consist of minute particles of ice and rock. At Saturn, some are as large as a small house, and concentrate in orbits only a few kilometers thick. Within the rings, the particles are widely separated. The origin of such rings is still uncertain, but opinion favors disruption of earlier satellites. However the rings may have collected debris that eroded from existing satellites, or that the planet captured from interplanetary space, or from "left overs" during the planetary accretion.

The satellites of the giant planets fall into two principal types: larger ones that adopt spherical shapes, when their constituent materials are sufficiently liquefied to bring about gravitational equilibrium, resulting in an equipotential surface. Smaller ones retain irregular shapes, which suggests that melting has not significantly altered them, or that they are fragments from collisions that broke their parent body(s) apart. The planets may have captured some from asteroid-like belts or from passing comets and other space debris. Most of the satellites consist of mixtures of rock and ice (water, carbon dioxide, or other frozen gases). Ice is often the dominant material at the surface. The surfaces of many satellites show varying, often heavy, cratering that may be preserved states from soon after they formed. Some satellites have unusual, patchwork-like multityped terrains that suggest they were reassembled from pieces torn apart from a collision. Indications of later, partial melting are present in a few. One satellite has an atmosphere; several may contain liquids at or below their surfaces; and at least one has ongoing volcanic activity. All in all, the satellites have a lot in common, yet each retains its own distinctive "character".

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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