LORD OF THE RINGS
Saturn casts a long shadow across its icy rings in a 2007 picture taken by NASA's Cassini-Huygens spacecraft—an unattainable perspective from Earth's line of sight. The picture is among Cassini's ten best, chosen by National Geographic News photo editors to mark the probe's anniversary.
Fifteen years ago this week, a Titan rocket launched the bus-size spacecraft from Cape Canaveral, Florida. In July 2004 Cassini finally entered Saturn orbit. Since then the probe has beamed back a steady stream of some 300,000 pictures and nearly 450 gigabytes of data about the ringed giant and its dozens of enigmatic moons.
Scientists used red, green, and blue filters to create the above natural-colour mosaic, taken 700,000 miles (over a million kilometres) above Saturn's northern hemisphere.
After completing its extended mission in 2017, Cassini will likely be commanded to plunge into the gas-giant planet to prevent contamination of its moons, including Titan and Enceladus.
Jupiter's volcanic moon Io spins against the backdrop of its parent planet in 2001. Cassini snapped the image as the craft swung past the gas giant en route to the Saturnian system.
While Io may appear to be floating close to Jupiter, the moon is about 220,000 miles (350,000 kilometres) away—about as far as Earth is from our moon.
Saturn's concentric rings—and the gaps in those rings—stand out in unprecedented detail in this 2008 natural-color picture taken by Cassini.
Discoveries of higher concentrations of ice in the outer rings are offering more clues to their origin and evolution.
From less than 39,000 miles (63,000 kilometres) away, Cassini snapped the first ever picture of Saturn's highly pitted moon Hyperion (pictured) in 2005.
Hyperion, with its porous appearance, is 1 of 34 known Saturn moons—only 18 of which had been spotted before Cassini arrived on the scene.
Saturn's rings cast a razor-thin shadow on the planet in a 2009 Cassini picture.
Despite being 1.3 million miles (2.1 million kilometres) from Saturn at the time, Cassini's cameras revealed never before seen structural details in the planet's gossamer rings, which are made up of billions of particles ranging in size from dust grains to house-size ice boulders.
A large storm (top right) roils in Saturn's upper atmosphere in a 2011 infrared composite image.
Cassini's long-term orbit of the ringed giant has offered insight into short-term weather patterns and previously unseen disturbances, including the giant storm, said Carolyn Porco, Cassini's imaging team leader, who is based at the Space Science Institute in Boulder, Colorado.
"In this case, we were present to record the state of the planet before, during, and after the rare, once-every-30-years eruption of a colossal and long-lived atmospheric storm—a circumstance guaranteed to yield unprecedented insights into the detailed workings of planetary atmospheres," added Porco, who processes nearly all the orbiter's images, mosaics, and movies.
IN LIVING COLOUR
While cruising by Saturn in February 2007, Cassini captured a series of haze-penetrating infrared images, which were combined to create this false-colour mosaic.
Various colours reflect the differing light levels around the planet, with blue indicating the brightest areas and reds the dimmest.
JUST HANGING OUT
Saturn's heavily cratered moon Mimas seems to hover over its home planet in a 2007 Cassini image.
Just visible on the right side of the moon is the 88-mile-long (140-kilometre-long) Herschel crater, the result of an ancient impact that nearly pulverised the satellite.
Smoggy, golden Titan, Saturn's largest moon, is set against the backdrop of the giant planet in May 2012.
In December 2004, just five months after arriving at the Saturnian system, Cassini jettisoned the Huygens probe, which arrived at Titan 22 days later. After orbiting the moon, Huygens landed in January 2005, sending scientists the most detailed images yet of Titan's surface.
"While this gorgeous image was being processed, I noticed that the colours in the Saturn atmosphere were noticeably changed from those seen upon our arrival in 2004," Porco said. "A decade's worth of monitoring Saturn can do that: reveal to us eager onlookers the subtle changes brought about by the gradual advance of the seasons.
"Our exploration of Titan ... has been like a Jules Verne adventure come true," she added.
TIGER STRIPES ON THE MOON
"Tiger stripes" on Saturn's moon Enceladus (pictured) are seen in an enhanced-colour mosaic created with Cassini images taken in 2005.
The bluish, grooved terrain appears to represent warmer regions filled with jets of ice. The jets suggest Enceladus has an under-ice sea of organic, salty water, which may make the moon a promising target for the search for life as we know it.
"Of all the remarkable discoveries we have made at Saturn over the last eight years, none has been more profound ... than the discovery of this Yellowstone of the solar system," Porco said.
"As planetary explorers, we could not have asked for more."
All images courtesy of SSI/NASA.
Header Image: A vast storm stretches across Saturn's northern reaches early last year. IMAGE COURTESY SSI/CALTECH/NASA