Planetary astronomers generally categorize the Solar System as consisting of outer planets and inner planets. While many spacecraft have been sent to the inner planets-Mercury, Venus, and particularly Mars, far fewer have been sent to the outer planets-the gas giants Jupiter, Saturn, Uranus, and Neptune, and the small distant rock known as Pluto. In fact, only American spacecraft, with some European participation, have visited the outer planets. The reason is that these planets are so distant that they require spacecraft that can operate many years while travelling to reach them.

Starting in the late 1960s, scientists from the National Aeronautics and Space Administration (NASA) and engineers at California Institute of Technology's Jet Propulsion Laboratory (JPL), which operates under contract to NASA, began planning to send the first space probes past Mars to Jupiter and beyond. In 1971 they launched the first of these space probes, a craft named Pioneer 10, which became the first spacecraft to reach the outer planets, flying past Jupiter in December 1973. During its pass by Jupiter, it achieved enough velocity to become the first spacecraft to leave the solar system. Its twin, Pioneer 11, flew past Jupiter a year later, in December 1974. Both spacecraft returned colour photographs of the complex cloud structures of the Jovian atmosphere, measurements of the magnetic fields around the planets, and a few blurry images of several of Jupiter's larger moons. But in order to reach Jupiter at all, the two little space probes had to be so small and had to travel so fast that they could not enter orbit around the planet and sped on past it, making it extremely difficult for astronomers to obtain much information from the data.

Pioneer 11 left Jupiter behind to encounter Saturn five years later in September 1979. It provided new information on the ringed giant's magnetic and charged-particle environment. But the spacecraft's images of Saturn were less impressive than those of Jupiter, and it did not return images of complex cloud patterns like those found on Jupiter.

 
Image of Saturn collected by Pioneer 11 in September 1979.

NASA planetary astronomers got another chance beginning in January 1979, when a more capable spacecraft named Voyager 1 approached Jupiter. Only 17 hours before its closest approach, the spacecraft turned its cameras to the side for an 11-minute-long exposure. When the image came back, it startled virtually everyone on the project team by revealing that Jupiter had a thin ring, something that they thought impossible due to the planet's gravity.

Voyager 1 also took pictures of a number of Jupiter's moons. One of the moons it photographed was Europa. The blurry images of Europa revealed that it was crisscrossed with lines that the scientists could not explain. An even bigger surprise came as the spacecraft approached a second moon, Io, covered with splotchy patches of black, white, orange, and yellow sulphur. On March 8, as one of Voyager 1's navigators, Linda Morabito, was looking at an image of the edge of Io to evaluate the spacecraft's trajectory, she discovered a small volcanic plume rising from its surface. Soon other scientists found other volcanic eruptions, and Io proved to be the most geologically active body in the Solar System.

Voyager 2 reached Jupiter four months after its sister craft. It revealed significant changes in the weather patterns on the gas giant. It took better pictures of the volcanoes on Io and also higher quality images of Europa that showed its very smooth, intricately cracked ice surface. There were no well-defined impact craters like on most other moons, and the surface features implied that something strange was going on under Europa's surface.

Voyagers 1 and 2 then sped on toward Saturn. Voyager 1 reached Saturn in the fall of 1980. One of its more fascinating discoveries was that the winds at Saturn's equator reached speeds of 1,800 kilometres per hour, four times faster than on Jupiter. On October 6, a Voyager team member noted dark, spoke-like features extending radially across one of the planet's rings, which they could not explain. Voyager 1 also photographed Saturn's moons, including Titan, which has a thick atmosphere of its own, and Mimas, which has a giant impact crater that makes it look like the Death Star from the movie Star Wars. Voyager 2 encountered Saturn 10 months later and also returned vivid photographs. Both spacecraft discovered new moons around Jupiter and Saturn that are invisible from Earth.

 
A heliocentric (sun-centered) view of the Voyager trajectories.

Although their beautiful pictures were what made them famous, Voyagers 1 and 2 were also equipped with other scientific instruments for detecting magnetic fields, charged particles, and other invisible phenomena. While Voyager 1's pictures of Saturn's rings were extremely useful, scientists wanted to get a better idea of how thick the rings were, so they sent the spacecraft zooming up, past the rings. This allowed them to measure the spacecraft's transmissions as they travelled through the rings and thereby determine the rings' thickness. But this manoeuvre also meant that Voyager 1 left the ecliptic, or the plane that the planets travel in around the sun, and it could not continue on toward any other planets. Voyager 1's cameras were shut off in December 1980 as the spacecraft sped “up” out of the Solar System (i.e., “north” according to the Earth's orbit around the sun).

Voyager 2 continued even farther out in the Solar System. In November 1985 it started providing regular data on Uranus, a bluish ball that previously had appeared as only a tiny dot in the most powerful ground-based telescopes. It returned information about the planet's wispy rings and its moons, each radically different from the other. But the featureless Uranian atmosphere disappointed most of the scientists. In late August 1989, Voyager 2 swept through the Neptune system, providing a spectacular end to the spacecraft's career. Neptune was a deeper blue than Uranus and also had complex but subtle features in its atmosphere. Surprisingly, it also had a feature that scientists named the Great Dark Spot that looked similar to the Great Red Spot on Jupiter. (Both are massive hurricane-like storms.)

Voyager 1 and 2 rank as the most spectacularly successful robotic spacecraft ever built. They returned a tremendous amount of data about planets that were previously nothing more than smudges on photographic film. But they both sped past their targets. NASA scientists wanted more and continuous data that they knew could be achieved only by orbiting a spacecraft around the big planets.

Starting in the 1970s, JPL scientists began designing a new spacecraft that would actually orbit Jupiter. NASA named the probe Galileo, and it was scheduled for a 1986 launch aboard the Space Shuttle until the Challenger accident grounded it for several years. When it was finally launched, in October 1989, its delicate high-power communications antenna failed to properly unfurl, and the spacecraft could send data back to Earth at a much lower rate. This forced JPL scientists to completely revise the mission plan and dramatically scale back their experiments.

In December 1995 Galileo entered orbit around Jupiter. Five months before it arrived, a small atmospheric probe separated from Galileo and entered the Jovian atmosphere. After using a heat shield to brake, it popped open a parachute and descended through the clouds of Jupiter's upper atmosphere for 58 minutes until the pressure became so great that it squashed the tiny probe.

 
Double ridges, dark spots, and smooth icy plains on Europa, one of Jupiterďs moons. Photo taken by the Galileo spacecraft, November 1998.

Meanwhile, after Galileo entered orbit, the spacecraft began the first phase of its revised exploration plan, which focused upon Jupiter itself. The primary phase ended in December 1997 and a two-year Galileo Europa Mission (GEM) started as the spacecraft turned its attention to Jupiter's largest and most interesting moons, particularly Io and Europa. This mission was extended again into the 21st century. Successive flybys of Io and Europa revealed that Io's surface is constantly being remade by volcanic forces, and provided strong evidence that underneath Europa's cracked icy surface is a vast saltwater ocean, an important prerequisite for life. This conclusion has placed Europa near the top of the list of places that planetary scientists wish to study in much greater detail. In addition, because of the possibility of life on Europa, NASA scientists decided to crash Galileo into Jupiter to prevent it from possibly crashing on Europa and contaminating it with bacteria.

In 1997 NASA launched the Cassini spacecraft to Saturn atop a powerful Titan IV rocket. Cassini is a big spacecraft by current standards and was designed based upon some of the mistakes learned from Galileo. For instance, it has a large, rigid communications antenna rather than the extendable type that failed on Galileo. Cassini also includes a small European Space Agency probe named Huygens for descending into the moon Titan's atmosphere. Cassini is scheduled to reach Saturn in July 2004 and will orbit it for years, studying Saturn, its amazingly complex rings, and its moons.