The planning, fabrication, and operation of the Cassini spacecraft, which was led by NASA, ESA, and the Italian space agency, the Agenzia Spaziale Italiana (ASI), was a truly international endeavor, representing the work of over 5,000 individuals hailing from a total of 17 countries.
When launched in October 1997 atop a Titan IVB/Centaur rocket, Cassini carried a suite of nine scientific instruments that would allow the probe to execute an ambitious four-year primary mission to explore the Saturnian system. As it forged the 2.2 billion mile journey across the solar system to rendezvous with Saturn and her moons, Cassini was not entirely without company.
Much like Rosetta carried Philae, Cassini was tasked with the safe passage of the Huygens lander, whose objective was to descend to the surface of Saturn's largest moon, Titan. The lander had been designed to harvest as much data as possible regarding the nature of the inhospitable moon during its two-hour descent period, and for a short time after setting down on the surface of the alien world.
However, before Cassini could drop off its passenger, the probe's handlers had to orchestrate a complex ballet across the Solar System in order to deliver the robotic explorers to their objective. The cosmic road trip saw Cassini undertake two flybys of Venus and one of Earth, before spending six months exploring Jupiter and her moons.
This pit stop afforded Cassini the perfect opportunity to test out its instruments before capitalizing on the gas giant's gravitational influence to slingshot itself onwards to its ultimate mission objective, Saturn.
Exploring a gas giant
Cassini finally arrived in orbit around Saturn on June 30, 2004. Huygens would have to wait a further five months to be cut loose. Upon finally descending to the surface of Titan on January 14, 2005, Huygens revealed it to have a surprisingly Earth-like terrain, albeit blemished with hydrocarbon lakes. A recent study based on data harvested by the robotic duo suggests that this inhospitable world may be capable of hosting non-water based life.
The Cassini probe has now been exploring Saturn for more than 12 years, and has revolutionized our knowledge of how gas giants tick. The probe has allowed us a greater understanding of the complex processes that drive the storms raging across the surface of the chaotic world including the vast, dark expanse of Saturn's south polar storm.
Cassini data suggests that Saturn's polar storms are generated and sustained by a multitude of smaller storms scattered across the surface of the gas giant, which work to divert air to the polar regions. A greater understanding of Saturn's storms could allow future astronomers to divine whether distant exoplanets host polar cyclones of their own simply by observing the overall storm intensity spread across a planet's surface.
Scientists at NASA are exploring the potential of using probes known as windbots to explore Saturn's upper cloud layer. The concept mission suggests using lightweight probes that would harvest the turbulence present in the upper atmosphere of a gas giant, and supply this energy to a propulsion system. This would allow the probe to harvest energy even while operating on the night side of a gas giant.
Many planets in our solar system boast weak ring systems of their own, yet none can even come close to comparing to the majesty of Saturn's adornment. Even through a weak telescope, they are striking to behold, and through the eyes of the Cassini spacecraft, they are simply breathtaking.
Observations made by the veteran probe have revolutionized our understanding of Saturn's rings. It is thought that the ring system was once much larger than it is today, and that much of the material contained in the early rings coalesced to form some of the gas giant's many moons.
In April 2013, the Cassini spacecraft was lucky enough to see this process in action as it observed the formation of a new moon. The spacecraft detected disturbances in the edge of Saturn's A ring, created by the weak gravitational influence of the newly formed satellite, which has since been named Peggy.
It is possible that, as Cassini moves closer to the A ring (here's a good primer on how Saturn's rings are named) prior to the end of its mission, the probe may be able to image Peggy directly, and that this newest addition may well be the final moon to be created by Saturn's depleted rings.
In February 2016 Cassini made history once more by becoming the first spacecraft to measure the weight of one of Saturn's rings. This was achieved by observing the light from distant stars as it passed through Saturn's B ring.
It was discovered that the B ring was much lighter than earlier estimates had suggested. Saturn's rings are thought to owe a proportion of their mass to dust from meteorites and other sources. Since the B ring weighs less than predicted, it is possible that it has not been around long enough to accrue the expected amount of dust. This led astronomers to hypothesize that the B ring may have formed a few hundred million years ago, rather than the few billion years that had previously been hypothesized.
The moons of Saturn
Saturn plays host to dozens of moons that display an astounding range of diversity. The orbital paths of a number of these satellites, known as shepherd moons, are thought to be responsible for creating visible divisions in Saturn's rings.
Others, such as Hyperion boast bizarre traits, such as a surface density so low that collisions with asteroids simply compress the surface, rather than impacting it, creating a sponge-like porous appearance.
Repeated close proximity passes between Cassini and a number of these satellites have allowed scientists to gain a greater understanding of the surprising levels of diversity on display in our solar system. One of Cassini's key targets for observation, the Saturnian moon Enceladus, represents one of the most likely breeding grounds for life outside of Earth's atmosphere.
In 2005, Cassini discovered the first evidence of icy plumes rising from "tiger stripe" vents marking Enceladus' south polar region. It has been hypothesized that the material thrown out in the geyser eruptions was drawn from a subsurface ocean with the potential to play host to microbial life.
Data collected by Cassini suggests that tidal forces generated by Saturn's gravitational pull are the driving force behind the eruptions.
In October 2015, Cassini made its final dive through Enceladus' plumes at a distance of only 30 mi (49 km) above the moon's south pole, harvesting data that would help scientists to analyze the composition of the satellite's subsurface ocean. NASA is so intrigued with Enceladus that a mission concept has received funding under the agency's NIAC program to examine the feasibility of using a Russian nesting doll-like spacecraft and lander to plumb the depths of one of the moon's tiger stripe vents.
Cassini's Grand Finale
These are but a few examples of how Cassini has advanced our understanding of the Saturnian system. The final stage of Cassini's mission will be known as the "Grand Finale," and will see the probe draw closer to Saturn and her rings than ever before, providing an unprecedented opportunity to gather data on the workings of the gas giant, while providing stunning high-resolution imagery.
Prior to the onset of the Grand Finale, Cassini will undertake a series of orbits in close proximity to Saturn's, F-ring. This phase of the endgame, which is set to begin on 30. Nov. 2016, will see Cassini undertake 20 week-long orbits that take the spacecraft to within 4,850 miles (7,800 km) of the centre of Saturn's F-ring.
"During the F-ring orbits we expect to see the rings, along with the small moons and other structures embedded in them, as never before," states Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory, Pasadena, California. "The last time we got this close to the rings was during arrival at Saturn in 2004, and we saw only their backlit side. Now we have dozens of opportunities to examine their structure at extremely high resolution on both sides."
Following the F-ring orbits, Cassini's mission team plans to use the gravitational influence of Titan to manipulate the probe's orbit to send the spacecraft between the gas giant's surface and its innermost ring.
Cassini is expected to dive between Saturn's uppermost cloud layer and closest ring, a gap of only 1,500 miles (2,400 km), a total of 22 times, beginning on April 27, 2017. The spacecraft's proximity to the planet and her rings during this Grand Finale will allow it to map Saturn's magnetic and gravitational fields to a precision never before achieved. Cassini will also provide new insights as to the composition of Saturn's atmosphere, and make detailed measurements regarding the weight of the gas giant's rings.
Cassini's incredible journey is set to come to an end on September 13, 2017, with the spacecraft plunging into Saturn's atmosphere. Unlike the Huygens lander, Cassini will never hit solid ground, instead burning up due to the friction created between the fast moving spacecraft and the dense layers of atmospheric cloud.
The aged spacecraft will provide one final gift as it plunges to its demise, transmitting atmospheric data right up until the signal is lost
Already, there is a countdown clock running in NASA JPL's mission control room keeping track of the days, hours and minutes left before Cassini's final embrace with Saturn, and we have a strong feeling that when it ticks to zero, there won't be a dry eye in the room.
Scroll down to view a time lapse video displaying four days on Saturn, courtesy of NASA.
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