The . NASAās Cassini spacecraft has spent more than a decade orbiting Saturn, dipping in and out of its rings and peering at its moons. But in just one year, it will begin its grand finale, a daring series of manoeuvres that will bring views of Saturn like never before ā and end with the spacecraft plummeting to its death in the gas giantās atmosphere.
āItās going to be a tough day,ā says project manager of NASAās Jet Propulsion Laboratory in Pasadena, California, who presented details of Cassiniās last act in Vienna on 20 April at the .
The so-called programme will begin next year, following six months studying Saturnās outermost F ring, including 12 close fly-bys of Titan, Saturnās biggest moon, which sports rivers and lakes filled with liquid hydrocarbons.
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Cassiniās last act will be its most daredevil mission yet. From April 2017 to September, the probe will zoom 22 times between the planet and the innermost of Saturnās famous rings. āNo spacecraft has flown in that region before, so itās very exciting,ā said Spilker.
Enduring mysteries
The scientific payback will be enormous. Flying closer than ever before to the gas giantās surface ā some 64,000 kilometres from its centre ā Cassini will gather information previously unobtainable to help solve Saturnās enduring mysteries. These include such basic questions as whether it has a rocky core, how its weather works and the precise length of its day.
āWeāll have the opportunity to study Saturnās internal structure, measure its gravity and magnetic fields and hopefully measure Saturnās rotation rate for the first time,ā says Spilker.
At such close quarters, Cassiniās instruments can gather unprecedented information about the composition of the planetās ionosphere and atmosphere. The craft will also collect data on radiation belts girding the planet, and will be in a position to use gravity measurements to give the best estimate yet of the mass of the rings.
āI am especially interested in Cassiniās measurement of the mass of Saturnās rings,ā says at the Laboratory for Atmospheric and Space Physics in Boulder, Colorado. Knowing the mass will help determine whether the rings were recently created, or whether they are as old as the solar system ā Espositoās favoured idea. āIt will be a major scientific discovery either way, with implications for the formation of planets in proto-planetary discs.ā
And the images it will send back should be spectacular. āWe will get some of the highest resolution images ever obtained of the inner D ring,ā says Spilker. āWeāll get wonderful views with the sun high in the sky for the solstice.ā
Daredevil descent
In terms of derring-do, the only comparably hazardous mission was in 2008, when Cassini flew through enormous plumes of warm water 100 kilometres across gushing from the surface of Enceladus, one of Saturnās moons.
The risk was worth it, because the data collected from that and subsequent fly-bys demonstrated that Enceladus has a salty ocean containing organic material, energy and a hydrothermal system at the base of the ocean ā all ingredients necessary for life.
Ironically, that potential for life is part of why Cassini must die. After a decade of orbiting and two mission extensions, Cassini is almost out of fuel.
āWe are ending the mission by burning up in Saturnās atmosphere to protect two worlds, Enceladus and Titan, that might have oceans suitable for life,ā Spilker says. āNASA required that we dispose of Cassiniā¦ in a way that would protect Enceladus and Titan from a later impact.ā
Sentimental goodbye
The mission will end on 15 September, 2017. āEven in its final plunge, it will continuously download data until the last few minutes,ā says Spilker.
The team speaks of the end sentimentally. āAs Mephistopheles told Faust, āall that exists is worthy to perishā,ā Esposito says. āThis is a worthy ending for Cassini that allows us to take more risk to discover new results.ā
āThereās something romantic about the idea of Cassini sailing on forever, but itās trapped in Saturn orbit and canāt roam the cosmic void,ā says of Cornell University in Ithaca, New York. āThereās some solace in thinking that Cassiniās atoms will merge in Saturnās interior with atoms that have been there since the beginning.ā
āOn the day, weāll gather at the Jet Propulsion Laboratory with the Cassini family, and when the signal stops, there will be a moment of silence for the loss of a dear friend weāve known since 1990,ā says Spilker. āIt will be a sad day, but itās achieved so much over the course of many years, weāll say goodbye with a mixture of sadness and pride.ā
