4 September 2017
Stuart Gary
On September 15 NASA’s Cassini spacecraft will finally end its historic mission to the ringed world of Saturn and its many moons. During its 13 years of ground-breaking observations, Cassini has provided enough data to fill more than 4000 scientific papers.
Cassini is now undertaking its final series of 22 so-called ‘grand finale’ orbits, designed to bring it ever closer to its ultimate demise. These orbits take the 2150-kilogram, mini-bus sized probe through the gap between Saturn’s inner rings and the planet’s swirling cloud tops.
Cassini’s final orbit on September 9 will send it through the outermost fringes of Saturn’s atmosphere, passing just 1680 kilometres above the clouds.
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Two days later, Cassini will make a final 119,049-kilometre flyby of Saturn’s largest moon, Titan. The gravitational perturbations caused by the Titan flyby will slow the spacecraft down and re-position it for a final descent into Saturn’s atmosphere.
Cassini will take its final images of Saturn on September 14, sending back its last look at the moons Titan and Enceladus, as well as the hexagon-shaped vortex around the planet’s north pole and weird “propeller pattern” features in the rings.
The spacecraft will then turn its antenna towards Earth, and begin a communications link that will continue until the end of the mission.
Cassini will also use these final hours to undertake detailed, high-resolution observations of Saturn’s auroras and temperatures, before being configured for entry into Saturn’s thick atmosphere.
Finally, in the early hours of September 15, Cassini’s suicidal death plunge will send it into Saturn at a speed of more than 120,000 kilometres per hour.
With its 12 science instruments running, Cassini will continue transmitting near real time data back to Earth as it travels deeper into the hydrogen, methane and ammonia atmosphere of the gas giant.
Cassini’s final act begins at 04:37 in the morning U.S. Pacific Daylight Time – 21:37 Australian Eastern Standard Time, 11:37 in the morning Greenwich Mean Time – as the spacecraft starts a 5-minute roll to position its ion and neutral mass spectrometer for optimal sampling of the atmosphere and for transmitting the data back to Earth.
Cassini project scientist Linda Spilker from NASA’s Jet Propulsion Laboratory in Pasadena, California, says directly sampling the atmosphere’s composition could provide new insights into Saturn’s formation and evolution.
Cassini will enter Saturn’s atmosphere at 04:53 in the morning U.S. Pacific Daylight Time – 21:53 Australian Eastern Standard Time, 11:53 in the morning Greenwich Mean Time.
The spacecraft’s thrusters will begin firing at 10% of their capacity to maintain directional stability, allowing Cassini’s high-gain antenna to remain pointed towards Earth, so data can be transmitted to NASA’s Deep Space Communications Network.
Just a minute or so later, Cassini’s thrusters will be firing at full capacity as Saturn’s atmospheric forces overwhelm the thrusters’ ability to maintain control of the spacecraft’s orientation.
As that happens, Cassini’s high-gain antenna will lose signal lock with Earth, cutting all communications, bringing Cassini’s historic mission of exploration to an end.
Mission managers expect that to occur about 1510 kilometres above Saturn’s cloud tops as the planet’s thick crushing atmosphere destroys the spacecraft, ripping it to pieces, and turning the blackened debris into fiery meteors.
Cassini’s final dive will end an historic mission which has provided new insights into the worlds of the outer solar system, including close up views of three-dimensional structures towering above Saturn’s rings, and a giant Saturnian storm which circled the planet for most of a year.
Even during these grand finale orbits, Cassini’s science mission continues.
The final 22 orbits are venturing into a previously completely unexplored region of Saturn – allowing Cassini to provide unprecedented observations of the planet’s rings and its magnetic and gravitational fields.
The observations have already shown scientists that Saturn’s spectacular rings have less mass than expected.
That suggests they’re far younger than previously thought – possibly just 100 million years old – the result of a moonlet being ripped apart by Saturn’s gravity, or a collision between moonlets.
Scientists hypothesised that if the ring system was more massive, then it would be older, because its combined gravity would have held it together, preventing it from being gradually eroded over time through meteoroid collisions.
Because Saturn’s rings are a relatively recent addition, it means we’re seeing Saturn at a unique time in its evolution.
Cassini was launched on October 15, 1997, aboard a Titan 4B Centaur rocket from the Cape Canaveral Air Force Base in Florida on a 7-year journey to the Saturnian system.
Controversy surrounded the launch, with protesters concerned over Cassini’s plutonium-238 powered generators, needed to power the spacecraft’s systems so far from the Sun.
There were initial fears of radioactive fallout if the launch failed, or if the Earth gravity assist flyby two years later went wrong.
Those concerns quickly faded as Cassini continued its journey without incident.
As well as the Earth flyby, Cassini also used gravity assists from Venus and Jupiter to slingshot it to Saturn.
Cassini achieved Saturn orbit insertion on July 1, 2004.
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Six months later on December 25, 2004, Cassini deployed its European Space Agency Huygens lander, which descended through the thick atmosphere of the mysterious Saturnian moon Titan – long considered to be a primordial version of the Earth.
Huygens touched down on Titan’s strange alien surface on January 14, 2005. It was the first landing on a world in the outer solar system. The surface of Titan was described as being like cold wet sand.
Titan was found to be the only place in the solar system — other than Earth — where it rains, forming rivers, which flow into lakes and seas.
Recent observations have also discovered complex pre-biotic chemicals form in Titan’s atmosphere and rain down on to its surface.
Of course, the surface of Titan is so cold that water is frozen solid, forming much of the moon’s bed rock. Instead of water, the liquids on Titan are hydrocarbons: methane and ethane.
But Cassini did find oceans of liquid water below the frozen ice crust of another Saturnian moon, Enceladus. Plumes of water were found jetting out from geysers dotted across the spectacular tiger-stripe formations at the Enceladan south pole.
Readings by Cassini indicate that, like the Jovian ice moon Europa, Enceladus has a global subsurface liquid water ocean beneath its frozen crust.
And minerals detected in those plumes of water indicate the likely presence of hydrothermal vents on the subterranean sea floors of Enceladus.
On Earth, sea floor hydrothermal vents, thousands of metres below the surface, host ecosystems rich in life. And there’s a growing view among biologists that life on Earth may even have begun in the nutrient-rich chemical soups around these vents.
That raises the possibility of similar processes taking place around the hydrothermal vents of Enceladus and Europa.
If evidence of past or present life is ever found on Mars, it could be the result of meteorite contamination from Earth, since the red planet and ours have constantly swapped rocks over the 4.6-billion-year history of the solar system.
However, if life is found to exist in the distant frozen worlds of Enceladus or Europa, then it would have most likely arisen independently from that on Earth. And that prospect would raise the possibility of life being common throughout the universe.
It’s consideration for potential alien life that led to NASA’s decision to end Cassini’s mission with a suicidal plunge into Saturn’s atmosphere. The administration always aims to limit possible Earth microbial contamination of other worlds by as much as possible.
With Cassini’s fuel supplies running low, NASA wanted to ensure the probe doesn’t crash onto any of the Saturnian moons that could possibly harbor life. Burning up the spacecraft in Saturn’s atmosphere will limit the contamination any surviving hitchhiking Earth microbes could cause.