Page 312 - NS-2 Textbook

ASTRONOMY 307

A spectacular view of the shadowed side of the planet Saturn returned by Voyager 1.

completed its" grand tour" of the outer planets, Voyager 2 its surface. The data have given scientists a detailed idea
then followed its sister out of the solar system. Voyager 1 of the composition of the moon's atmosphere and its sur-
overtook Pioneer 10 in 1998 to become the most distant face features.
human-made object in space. It is now some 9 billion The Cassil1i orbiter will continue its mission over the
miles from Earth, with Voyager 2 some 7 billion miles dis- next three years, during which it will make repeated fly-
tant. Both are still retuming useful data, and they are ex- bys of Titan and many of Satum's other moons, gather-
pected to continue doing so until around 2020, when ing data and photographing them and the planet be-
their power supplies will be exhausted. neath. Many new discoveries about the planet and its
In 1997 an exploratory spacecraft called Cassini was nunlerous satellites are sure to result.
launched to proceed to Jupiter for a four-year investiga-
tion of the planet and its moons. A joint venture of
NASA, the European Space Agency (ESA), and the Ital- URANUS
ian Space Agency (ISA), Cassini is the largest and most
sophisticated spacecraft ever built to explore a planet. It Uranus, named for the Greek god of the heavens, was
was outfitted with a probe called HlIygt1ls designed by discovered by Sir William Herschel in 1781. It is located
ESA to study the atmosphere and surface of Sahrrn's almost 1.8 billion miles from the Sun. The methane,
largest moon Titan. ammonia, and hydrogen that make up the planet are
In July 2004 the Cassini-HlIygens spacecraft arrived at primarily in a solid state, due to the -300 degrees F
Sahrrn to start its mission. In December during its third temperature.
orbit around the planet it released the HlIygens probe for The planet has twenty-seven known small moons,
its 20-day journey to Titan, which it reached on 14 Janu- including five that may be fragments of others that broke
1
ary 2005. The probe successfully completed a 2 h-hotIr apart in the distant past. Uranus appears greenish when
descent to the surface, and once there, relnained opera- seen through the telescope, probably because of its at-
tional for another 70 minutes, all the while sending back mospheric methane. In March 1977 scientists at Cornell
much data about the moon's atmosphere and pictures of University and Arizona's Lowell Observatory observed