The sixth planet from the Sun, Saturn, is the second largest planet in our solar system. Its intricate ring system has fascinated astronomers for centuries. The rings are made of ice and rock particles--some as big as houses--that were probably pieces of comets or asteroids that broke up before they reached the planet.

At least 30 moons orbit Saturn. The largest, Titan, exceeds the size of the planets Mercury or Pluto and wraps itself in a dense, nitrogen-rich atmosphere reminiscent of the early Earth's atmosphere.

Overview

In the 1960s, mission designers recognized that a unique opportunity was going to present itself more than a decade later. Starting in the late 1970s, the giant gaseous outer planets -- Jupiter, Saturn, Uranus and Neptune -- would line up in such a way that single spacecraft might hop from one to the next, using the gravity of each one to keep speeding it on its way. Taking advantage of this alignment -- which occurs only once every 175 years -- NASA approved the Voyager Project, designed to send twin spacecraft to the outer solar system.

Voyager 2 was launched first from Cape Canaveral, Florida, on August 20, 1977; Voyager 1 was launched on a faster, shorter trajectory on September 5, 1977. Both spacecraft were delivered to space aboard Titan-Centaur expendable rockets. Voyager 1 made its closest approach to Jupiter on March 5, 1979, and Voyager 2 followed with its closest approach occurring on July 9, 1979. The first spacecraft flew within 277,500 kilometers (172,000 miles) of the planet's cloud tops, and Voyager 2 came within 650,500 kilometers (403,000 miles).

The Voyager 1 and 2 Saturn flybys occurred nine months apart, with the closest approaches falling on November 12 and August 25, 1981. Voyager 1 flew within 124,000 kilometers (77,000 miles) of the cloud tops, while Voyager 2 came within 100,800 kilometers (62,600 miles).

Voyager 1's flight path at Saturn bent it up and away from the ecliptic, the plane in which most planets orbit the Sun. Voyager 2, meanwhile, continued on for two more planetary encounters. Voyager 2 flew by Uranus on January 24, 1986, coming within 81,500 kilometers (50,600 miles) of the planet's cloud tops. Voyager 2 made a final flyby of Neptune on August 25, 1989, passing within 5,000 kilometers (3,000 miles). At the time, the planet was the most distant member of the solar system from the Sun. (Pluto once again became most distant in 1999.)

Following their planet flybys, both Voyagers are heading out of the solar system. Flight controllers believe both spacecraft will continue to operate and send back valuable data until at least the year 2020. On February 17, 1998, Voyager 1 passed the Pioneer 10 spacecraft to become the most distant human-made object in space.

Overview

The Cassini mission to Saturn is the most ambitious effort in planetary space exploration ever mounted. A joint endeavor of NASA, the European Space Agency (ESA) and the Italian Space Agency (known as ASI for its acronym in Italian), Cassini is sending a sophisticated robotic spacecraft to orbit the ringed planet and study the Saturnian system in detail over a four-year period.

Onboard Cassini is a scientific probe called Huygens that will be released from the main spacecraft to parachute through the atmosphere to the surface of Saturn's largest and most interesting moon, Titan, which is shrouded by an opaque atmosphere. Titan's atmosphere includes organic compounds leading scientists to believe that the moon may be like a frozen vault of conditions similar to those on Earth before life began. The Cassini orbiter will also use imaging radar to map Titan's surface.