In Greek mythology, a race of giants, called the Titans, first ruled the world. The ancient Titans of the solar system are the outer planets: Jupiter, Saturn, Uranus, and Neptune. The inner planets, Mercury, Venus, Earth and Mars, huddling close to the warm Sun, are pebbles by comparison. Stretching from 500 million to 3 billion miles from the Sun, these monsters are as remote as they are mysterious, dwelling so far from the Sun that water instantly freezes to solid ice. These so-called gas giants all have deep swirling atmospheres made up mostly of primordial elements. They do not have solid surfaces.

In the 1970s and 1980s, NASA's Pioneer 10 and 11, and Voyager 1 and 2, first made the long-distance trek to the outer solar system. They gave humankind dazzling close-up photos of these remarkably complex worlds. In the 1990s along came the Hubble Space Telescope to pick up where these interplanetary pathfinders left off. Annually, Hubble monitors changes in the colorful swirling atmosphere of Jupiter, seasonal storms coming and going on Saturn and Uranus, and a wandering dark spot that plays peek-a-boo on Neptune. As the solar system's weatherman, Hubble's ultra-sharp monitoring of these magnificent giants keeps giving astronomers insights into an ever-changing kaleidoscope of weather on other worlds.

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