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Saturn is the most distant of the five planets known to ancient stargazers.
In 1610, Italian Galileo Galilei was the first astronomer to gaze at
Saturn through a telescope. To his surprise, he saw a pair of objects on
either side of the planet, which he later drew as "cup handles" attached to
the planet on each side. In 1659, Dutch astronomer Christiaan Huygens
announced that this was a ring encircling the planet. In 1675, Italian-born
astronomer Jean Dominique Cassini discovered a gap between what are
now called the A and B rings.
Like the other giant planets - Jupiter, Uranus, and Neptune - Saturn is a
gas giant made mostly of hydrogen and helium. Its volume is 755 times
greater than Earth's. Winds in the upper atmosphere reach 500 meters per
second in the equatorial region. (In contrast, the strongest hurricane-force
winds on Earth top out at about 110 meters per second.) These super-fast
winds, combined with heat rising from within the planet's interior, cause
the yellow and gold bands visible in its atmosphere.
Saturn with moons Tehtys (above) and Dione photographed by Voyager 1 in 1980.
Saturn's ring system is the most extensive and complex in our solar system;
it extends hundreds of thousands of kilometers from the planet. In fact,
Saturn and its rings would just fit in the distance between Earth and the
Moon. In the early 1980s, NASA's two Voyager spacecraft revealed that
Saturn's rings are made mostly of water ice, and they found "braided"
rings, ringlets, and "spokes" - dark features in the rings that seem to circle
the planet at a different rate from that of the surrounding ring material.
Some of the small moons orbit within the ring system as well. Material
in the rings ranges in size from a few micrometers to several tens of meters.
Saturn has at least 30 satellites. The largest, Titan, is a bit bigger than the
planet Mercury. Titan is shrouded in a thick nitrogen-rich atmosphere that
might be similar to what Earth's was like long ago. Further study of this
moon promises to reveal much about planetary formation and, perhaps,
about the early days of Earth as well.
In addition to Titan, Saturn has many smaller "icy" satellites. From
Enceladus, which shows evidence of surface changes, to Iapetus, with one
hemisphere darker than asphalt and the other as bright as snow, each of
Saturn's satellites is unique.
Saturn, the rings, and many of the satellites lie totally within Saturn's enormous
magnetosphere, the region of space in which the behavior of electrically
charged particles is influenced more by Saturn's magnetic field than by
the solar wind. Recent images by NASA's Hubble Space Telescope show that
Saturn's polar regions have aurorae similar to Earth's Northern and
Southern Lights. Aurorae occur when charged particles spiral into a planet's
atmosphere along magnetic field lines.
The next chapter in our knowledge of Saturn is already under way, as the
Cassini/Huygens spacecraft began its journey to Saturn in October 1997
and will arrive on July 1, 2004. The Huygens probe will descend through
Titan's atmosphere in late November 2004 to collect data on the atmosphere
and surface of the moon. Cassini will orbit Saturn more than 70 times during
a four-year study of the planet, its moons, rings, and magnetosphere.
Cassini/Huygens is a joint NASA/European Space Agency mission.
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