The Planetary Grand Tour was an idea devised in the 1960's, to send unmanned probes into space during a period when the planets aligned for easy access.The Grand Tour would occur in the late 70's and comprised four launches scheduled; two in 1976 and two more in 1977. This plan was crucial to NASA as the alignment of the planets would not occur again for 176 years.
During this time, the Mariner program was launched. Ten probes launched into space, beginning in 1962, with the intention of obtaining more information from Mars, Venus and Mercury. Seven probes were successful; three probes were lost to space.
By 1972, NASA budget cuts forced them to abandon the original plan. Instead, two probes originally destined for the Mariner program, added elements of the Grand Tour and were launched in 1977 to study Jupiter and Saturn.
The Voyager 2 launched August 20th, 1977. It was identical to the Voyager 1, but followed a slower trajectory, so the gravity assist would push it along to Uranus and Neptune, completing the original Planetary Grand Tour. The Voyager II is the probe usually referred to when the "Grand Tour" is discussed because the Voyager II is considered the most productive, successfully visiting four planets and their moons, for a fraction of the cost spent on later probes.
It arrived closer to Jupiter on July 9th, 1979, discovering several rings and volcanic activity on Io, one of Jupiter's moons. Until this point, the only active volcanoes ever seen were on Earth.
Together with sister probe, the Voyager 1, they observed nine eruptions.
On reaching Saturn, August 26th, 1981, the Voyager 2 passed behind the planet, measuring the depth and temperature. At it's highest and lowest points, the temperature ranged from 203 degrees Celsius to 130 degrees Celsius.
Just after leaving Saturn, the camera platform locked up, and plans to continue to Uranus may have been suspended. Luckily, the platform created more of the lubricant that had emptied, and the Voyager 2 was given the go-ahead.
On reaching Uranus on January 24th, 1986, the probe discovered ten unknown moons, a similar radiation belt to Jupiter and ultraviolet light radiating from the pole called "day glow."
Miranda, one of the new moons was found covered with craters caused by fault canyons. No one knows what has caused the faults, but many theorize it was damaged early in its history by a violent impact.
The Voyager 2 also discovered the actual mass of Neptune, when passing by in August of 1989. Knowing the right mass allowed the orbits of both Uranus and Neptune to become calculated accurately.
Both the Voyagers 1 & 2 were equipped with a gold plated audio visual disc, proposed by Carl Sagan called the "Sounds of the Earth." In case aliens should ever intercept the probes, it has greetings in over 60 languages, scientific information on Earth and humans and sounds of classical, jazz and rock music. It also includes messages from President Jimmy Carter and other world leaders. As a bonus, in case the aliens are in need of some relaxation, sounds of nature such as thunderstorms and the ocean were added to the recording.
As of 2006, the Voyager 2 became the last probe needed to have completed a visit of all 8 planets.
Eight? But what about Pluto?
The Voyager II was unable to approach Pluto, the former 9th planet. In 2006, as areas around Pluto, including the Kuiper Belt, had been further explored leading to the discovery of Eris, a mass similar or even larger, than Pluto. The General Assembly of the International Astronomical Union voted Pluto out of the planet club. Both Pluto and Eris were downgraded to "dwarf" planets, prompting people across Earth to commiserate with the fallen planet.
"It's OK Pluto; I'm not a planet either. "
The Voyager 2, as of September, 2008 was 8.077 billion miles away from the sun, and will continue transmitting until 2025.