Astronomy Picture of the Day
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October 10, 1995
Dione's Lagrange Moon Helene
is very unusual in that it circles
Saturn near the
orbit of a bigger moon:
Helene is situated in what is called a
"Lagrange point" of Dione - a place of stability created by Dione's
Helene to stray slightly from its orbit 1/6 ahead of Dione,
the larger moon's gravity would cause Helene to move back toward the
Lagrange point. Many massive orbital bodies have stable Lagrange points,
including the Earth and Moon. Helene was discovered from the ground by P.
Laques & J. Lecacheux in 1980. The photograph above was taken by Voyager 2
as it passed Saturn in 1981. NASA's
mission to Saturn is currently
scheduled for launch in October 1997.
Tomorrow's picture: LMC Star Clouds
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