The Cassini
spacecraft orbiting Saturn has finally spotted spokes cutting across the planet's
rings, a phenomenon astronomers have long hoped their plucky orbiter might find.
While
flying past the dark side of Saturn's B ring, Cassini's camera eye photographed
the spokes - which appear as radial markings - in a series of three images
taken over about 27 minutes. The find is a gem of sorts for mission imaging
scientists, who have been hunting for the ring spokes since Cassini arrived at
Saturn.
"We've been
on the lookout for them since February, 2004," said Carolyn Porco, Cassini imaging team leader at the Space Science
Institute in Boulder, CO, of the spokes in an e-mail interview. "Spokes
are one of those Saturn-system phenomena that we are keenly interested in
understanding."
Saturn's
odd ring spokes were photographed during NASA's Voyager mission, which
swung passed the planet in the 1980s, and later observed by astronomers using
the Hubble Space Telescope.
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NASA's Voyager spacecraft photographed spokes in Saturn's rings in the 1980s.
IMAGE: NASA/VOYAGER
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But spokes were
noticeably
absent when Cassini made its final approach toward Saturn in February 2004,
and are a prime target for astronomers because of their role and formation within the
planet's rings are not fully understood.
"These are
among the things we hope to learn," said Porco, who participated in the Voyager
mission as well. "[The spokes] are obviously related to a host of processes...and
may point to some important effects in understanding the magnetic field and the
planet's magnetosphere, and how these systems interact with the rings and atmosphere."
Porco and
her imaging team did not initially expect to observe ring spokes until about 2007,
when certain models predicted spoke formation and visibility.
"Well, in
some sense we should have expected, if the recent models are correct, to see
them on the dark side where the photoelectron abundance is low," Porco said of
the spokes. "So, I was surprised to see them. But once they showed up, I
realized we should have expected them there all along."
While the
images were released on Sept. 13, Cassini actually photographed the ring spokes
on Sept. 5, 2005, using clear filters and its wide-angle camera from a distance
of about 198,000 miles (318,000 kilometers) from Saturn. The spokes themselves
are fairly faint, and are about 60 miles (100 kilometers) wide and 2,200 miles
(3,500 kilometers) long, researchers said.
Unlike
Voyager or Hubble, Cassini is in a unique position to study ring spoke phenomena
at Saturn, Porco said.
"Remember,
Voyager was just a flyby, Cassini is in orbit," Porco said, adding that Cassini
is a vastly superior observation platform when compared to Voyager. "We have
the opportunity for monitoring them and their behavior, their comings and
goings, how they evolve, when they appear and disappear."
By
observing the spokes on Saturn's rings, Cassini rekindled fond memories of Voyager for Porco.
"It felt
like the old days, when we first saw the spokes," Porco said. "They are one
weird phenomena and it was a joy to see them again...especially since we hadn't
seen them yet and were eager to know why."
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