In classical times, the only Outer Planets known to humankind were Jupiter and Saturn, and then only as a bright lights that moved across the sky. This changed when Galileo turned his telescope to the heavens in the early 17th Century. He discovered the Jupiter was a world with colored bands, a red spot, and—most astonishingly—four moons. Galileo also discovered the first indication of Saturn’s rings, though he thought that Saturn was either three distinct bodies or sometimes an oval shape. In the middle of the 17th Century, Christiaan Huygens correctly concluded that Saturn was surrounded by a ring. Huygens also discovered Titan, a moon of Saturn. Huygens thought that the ring was solid. However, in the 19th Century James Clerk Maxwell concluded that the ring was made up of tiny particles, a conclusion that was later confirmed by spectrographic studies.

Uranus, hitherto an unknown planet, was discovered by William Herschel in 1781. Herschel also discovered Uranus’ moons, Titania and Oberon, in 1787. Neptune was discovered by Johann Gotttfried Galle of the Berlin Observatory and Louis d'Arrest, an astronomy student, through mathematical predictions made by Urbain Jean Joseph Le Verrier in 1846. William Lassell discovered Neptune’s moon Triton in the same year. Lassell has also disovered Hyperion, a moon of Saturn, and Ariel and Umbriel, moons of Uranus.

The last outer planet to be discovered was Pluto by Clyde Tombaugh in 1930. Unlike the other outer planets, which are gas giants, Pluto is thought to be a solid, icy world orbiting the sun at the very edge of the Solar System.

Until the 1970s, studies of the outer planets and their moons could only be accomplished with telescopes. That changed, starting in 1972 with the launch of Pioneer 10.

Pioneer 10

Pioneer 10 was launched on March 3rd, 1972 and passed by Jupiter on December 3rd, 1973 at a distance of 200,000 kilometers. Pioneer 10 carried Fifteen experiments were carried to study the interplanetary and planetary magnetic fields; solar wind parameters; cosmic rays; transition region of the heliosphere; neutral hydrogen abundance; distribution, size, mass, flux, and velocity of dust particles; Jovian aurorae; Jovian radio waves; atmosphere of Jupiter and some of its satellites, particularly Io; and to photograph Jupiter and its satellites. Further scientific information was obtained from the tracking and occultation data.

Pioneer 11

Pioneer 11 was launched on April 6th, 1973, passed by Jupiter on December 4th, 1974 at a distance of 34,000 kilometers and, after using Jupiter’s gravity field to alter its velocity and trajectory, passed by Saturn on September 1, 1979 at a distance of 21,000 kilometers. Pioneer 10 carried the same experiments as Pioneer 11 to study Jupiter and Saturn as well as a low-sensitivity fluxgate magnetometer.

Voyager

The two Voyager spacecraft were originally designed to perform close-up studies of the atmospheres, magnetospheres, rings, and satellites of Jupiter and Saturn. However, following its planned encounter with Saturn, Voyager 2's planetary mission was extended, and it was placed on a trajectory to allow flybys of Uranus and Neptune. Between them, Voyager 1 and 2 made numerous discoveries, including new moons about several of the planets, Uranus' unique magnetic field, and the presence of volcanic activity on Io. Following their final planetary encounters, the vehicles began the Voyager Interstellar Mission, which will measure interstellar fields, particles, and waves to the outer limits of the Sun's sphere of influence, and possibly beyond. Both spacecraft will eventually depart our solar system and will travel towards other star systems. Each vehicle carries a gold phonograph record called "Sounds of Earth", bearing messages, sounds, and pictures from our planet as greetings to any species who recovers the spacecraft.

Voyager 1

Voyager 1 was launched on September 5th, 1977, flew by Jupiter on March 5th, 1979 and flew by Saturn on November 12, 1980. Voyager 1 is now passing the outer edges of the Solar System and is still transmitting data.

Voyager 2

Voyager 2 was launched on August 20th, 1977, flew by Jupiter on November 9th, 1979, flew by Saturn on August 26th, 1981, flew by Uranus on January 24th, 1986, and flew by Neptune on August 29th, 1989. Voyager 2 is now passing the outer edges of the Solar System and is still transmitting data.

Galileo

Galileo was launched from the space shuttle on October 18, 1989. After a interplanetary cruise that involved multiple flybys of Venus and Earth to use those planets’ gravity fields as a velocity boost and two flybys of asteroids (Gaspra, October, 1991) and (Ida, August, 1993) it entered Jupiter orbit on December 7th, 1995.

Galileo returned images of the fragments of Comet Shoemaker-Levy as it plunged into the Jovian atmosphere. Upon approaching Jupiter, Galileo released a probe that plunged into the Jovian atmosphere on December 7th, 1995 and returned data for several hours. The probe discovered that Jupiter has thunder storms, caused by the circulation of water in the upper atmosphere, many times the size of those on Earth.

Galileo spent eight years in orbit around Jupiter and, despite the failure of a high gain antenna, returned an enormous amount of data. Among its discoveries were the existence of volcanoes on the moon Io, an ocean beneath a layer of ice on Europa, a magnetic field around Ganymede, and another possible, subsurface ocean on Callisto.

Galileo ended its mission on September 21st, 2003 and was ordered to crash into Jupiter’s atmosphere to avoid possible contamination of Europa, considered a prime candidate as an abode of extraterrestrial life.

Cassini

Cassini, a joint project of NASA, the European Space Agency, and the Italian Space Agency, was launched on October 15th, 1997 and entered Saturn orbit after a long cruise that included gravity boosts from Venus and Earth on July 1, 2004. Cassini will spend the next several years studying Saturn, its rings, and its satellites. It has already made passes of the moons Titan, Iapetus, and Enceladus.

On December 25th, 2004 Cassini release a probe named Huygens which descended into Titan’s atmosphere twenty one days later and touched down safely on its surface. For over two hours, Huygens returned data from the surface of Titan, including haunting images of an alien landscape with rivers and lakes of liquid methane, and hills made of water ice, shrouded in a hydrocarbon rich atmosphere.

The Future

NASA has approved two further robotic probes to the Outer Planets. The first will be the Pluto New Horizons probe, to be launched in January, 2006. If all goes well it will fly by Pluto, the only planet so far never to have been explored by a robotic mission, around 2015, after a gravity assist at Jupiter. The second is called Juno, which will launch by 2010 and will enter a polar orbit around Jupiter five years later.

Further into the future, NASA hopes to build a launch the Jupiter Icy Moons Orbiter (JIMO), a huge, nuclear powered space craft that is envisioned to orbit and study several of the moons of Jupiter in turn. The mission was at first designed to showcase NASA’s Prometheus nuclear power and propulsion technology, but has been postponed indefinitely.

Will human beings ever venture to the Outer Planets, as once imagined in the film, 2001: A Space Odyssey? Almost certainly they will, eventually. But tremendous technological problems must be solved first. Even with nuclear propulsion, trip times to the Outer Planets would be measured in years. Either the lives and health of human explorers must be maintained over that time, or new, faster propulsion techniques must be developed. And some sort of active shielding against radiation must be built, perhaps an electromagnetic field infused with plasma to simulate Earth’s magnetosphere. It is certain, though, that given the human desire to see unknown places with their own eyes, these challenges and other will, sooner or later, be overcome and the great, human adventure in space that began over forty years ago will continue to the edge of the Solar System.