On July 1, 2004, NASA's Cassini spacecraft arrived at Saturn orbit seven years after launch from Earth in 1997. Over ten years after arriving at the majestic gas giant, Cassini has profoundly transformed our understanding of the planet's dynamic atmosphere, rings and moons. Now the mission is entering the final stages of its prolific Saturnian odyssey and mission planners hope to use what fuel remains on board for a series of daring "proximal orbits" through the ring plane in 2016; a final phase called the "Cassini Grand Finale."
The science that Cassini has made since orbital insertion has been nothing short of mind blowing, so let's just take a small sample of key discoveries and stunning observations as featured on Discovery News from the past 12 months as Cassini sailed through its 10th year of Saturnian exploration and toward the history books.
The Hexagon etched into Saturn's upper atmosphere in its north pole is one of the most enduring mysteries of the solar system -- it was first observed by the Voyager probes in the early 1980's. Thought to be caused by high-altitude jet streams, the hexagon, as recently imaged by the Cassini mission, also seems to rotate with Saturn's inner core and global magnetic field. Although the underlying flow dynamics of this feature is not entirely understood.
There are few worlds that hold more promise than Saturn's largest moon Titan. The moon is known to have a thick atmosphere, large bodies of liquid methane and ethane, hydrocarbon-rich landscapes filled with rivers and valleys, and vast dune fields. One would be mistaken in thinking this moon is in fact an embryonic Earth, filled with organic potential. It may be frozen and unsuitable for life as we know it, but astrobiologists dream of having a follow-up mission to Titan after seeing the European Huygens lander (which hitched a ride to Saturn on board Cassini) float down and land on the moon in 2005.
To add to the familiarity of Titan with a young Earth, Cassini spotted waves in one of the large Titanian seas, whipped up by surface winds. Shown here at an oblique angle, the waves can be seen glinting in the sunlight.
Zooming in on Titan's vast dune fields during multiple flybys, Cassini has shown us even more Earth-like aeolian (wind-blown) features. In the case of Titan's black dunes, small grains of organic compounds have been blown into rippled 'seas' of their own.
With Cassini's high-resolution cameras, the mission has captured countless dreamy shots of the planet's 62 moons. Shown here is one of Discovery News readers' favorites: Saturn's moon Mimas looming ominously with the smaller moon Pandora passing behind. Mimas is also nicknamed the "Death Star Moon" in reference to Star Wars, with the moon's large Herschel Crater resembling the Death Star's planet-killing superweapon.
When it comes to organic chemistry, it's not all about Titan. The small icy moon of Enceladus isn't only famous for its impressive water-rich geysers that also have hints of life-giving chemistry, there's also the possibility that the moon has extensive sub-surface seas of liquid water. It is almost like a mini-Europa, Jupiter's second-largest moon, and scientists are keen to find out what lies beneath both moons' cracked crust.
In a surprise discovery this year, Cassini made the chance observation of something in one of Saturn's rings. Could this bright knot in the outer edge of the gas giant's A ring be the birth of a baby moon? Astronomers think it might, providing new clues as to the formation of natural satellites and proving that although our solar system is old, its planets still have active moon-formation processes persisting to this day.
Like Saturn's weird hexagon, the bright 'spokes' in Saturn's rings have puzzled astronomers for some time. Originally spotted by the Voyager flybys in the early 1980s, Cassini has been monitoring the phenomenon and revealed that they are not caused by gravitational interactions in or around Saturn. Instead, they are most likely generated by the interaction between dust in the planet's B ring and Saturn's global magnetic field that threads through the ring system.
In a breathtaking observation, the impact of one of Saturn's tiny moons on the planet's outermost ring was captured by Cassini. Prometheus, which is only 53 miles across, periodically careens through the F-ring, creating channels and streamers through the icy particles.
In one of the most iconic space photographs ever taken, in July, 2013, Cassini looked back at Earth from nearly 900 million miles away. Earth is just a dot of light and the moon is even smaller.
This image reminds us, like Voyager's famous "Pale Blue Dot" photo did in the 1990s, how tiny we are when compared with the immensity of space and Cassini continues to inspire the people on our tiny planet that we are capable of doing great things.