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February
19, 2009: Something is about to happen on Saturn
that's so pretty, even Hubble will pause to take a look.
"On
Feb. 24th, there's going to be a quadruple transit of Saturn's
moons," says Keith Noll of the Hubble Space Telescope
Science Institute. "Titan, Mimas, Dione and Enceladus
will pass directly in front of Saturn and we'll see their
silhouettes crossing Saturn's cloudtops—all four at the same
time."
Hubble
won't be the only one looking. Amateur astronomers will be
able to see it, too. The timing favors observers along the
Pacific coast of North America, Alaska, Hawaii, Australia
and east Asia.
On
Feb. 8th, astrophotographer Christopher Go of the Philippines
got a preview when Titan transited Saturn all by itself. He
recorded this movie using an 11-inch telescope:
![](images/quadrupletransit/Christopher-Go1_strip.gif)
Above:
Titan transits Saturn on Feb. 8, 2009. Credit: Christopher
Go of Cebu City, the Philippines.
"I
woke up at one o'clock in the morning to photograph Titan's
passage across the disk of Saturn," says Go. "The
sky was overcast, but I was fortunate to see the end of the
transit through a break in the clouds. The emergence of Titan
was really stunning because it gave the moon a 3D appearance!"
Transits
like these are rare. "They only happen every 14 to 15
years when the orbits of Saturn's moons are nearly edge-on
to Earth," says Noll. In
1995-96, the last time the geometry was right, Hubble photographed
two (Titan and Tethys) and three (Mimas, Enceladus, Dione)
moons transiting Saturn. This will be the first time the great
telescope captures four.
The
event begins on Tuesday morning, Feb. 24th at 10:54 UT (2:54
a.m. PST) when Titan's circular shadow falls across Saturn's
cloudtops. About forty minutes later, the ruddy disk of Titan
itself moves over the clouds.
"Titan
is so big, you can see it just by looking through the eyepiece
of a small telescope—no special camera is required,"
says Go.
One
by one, the smaller moons Mimas, Dione and Enceladus will
follow Titan. At 14:24 UT, all four satellites and their shadows
will simultaneously dot Saturn's disk: animation.
"To
photograph the smaller moons, you'll need a mid-sized backyard
telescope equipped with a good CCD camera," notes Go.
![](images/quadrupletransit/hubble1_strip.jpg)
Above:
A Hubble photo of Titan and Tethys transiting Saturn in 1995.
Credit: E. Karkoschka (University of Arizona) and G. Bacon
(STScI)
Hubble's
observations are part of the Hubble Heritage Project, a 10-year
outreach effort aimed at producing images of exceptional beauty
for the general public. "Only 0.5% of Hubble's observing
time is devoted to Heritage work," says Noll, one of
the project's leaders, "so we're picky about our targets."
He thinks the quadruple transit could rank among the best
planet-shots in Hubble's archive.
The
images could yield hard science, too.
"The
transit of Titan will be of particular interest," says
Noll. "Researchers plan to use Saturn as a backlight
to probe the size and transparency of the giant moon's atmosphere."
Hubble will also capture a rare view of the rings almost edge-on,
a point of view that can reveal ring-warps, undiscovered satellites,
and new information about the reflectivity of ring particles.
"Hard
science can be beautiful."
Stay
tuned to Science@NASA for snapshots.
Editor's
note: To find Saturn on Feb. 24th, look southwest
before sunrise. The planet is easy to see shining like a golden
first-magnitude star in the constellation Leo: sky
map. By cosmic coincidence, Feb. 24th is also the date
Comet
Lulin makes its closest approach to Earth--and the comet
is right beside Saturn! Using a small telescope you can catch
a comet, a ringed planet and a quadruple transit; it's a nice
way to begin the day.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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