By Robin McKie From The Guardian UK

Last week, the US space probe Cassini survived an encounter of the very closest – and riskiest – kind. In response to signals that had been transmitted to it across several hundred millions of miles of space, it swooped through a tiny gap between Saturn and its rings, then skimmed over the planet’s upper atmosphere. Now the little spaceship is beaming back information about the giant, ringed world from the very closest of viewpoints.

The achievement was remarkable – for this fly-by represents just one of a host of triumphs for Cassini, though in this case its manoeuvre did have a particular significance. It will be the spacecraft’s swan song. Having brought a stunning new awareness of Saturn and its system of moons, including observations suggesting that at least two could harbour life, the US space agency Nasa is now preparing to bring an end to Cassini as the spacecraft’s reserves of fuel run out.

Once it has completed several more swings through Saturn’s complete system of rings, the craft will be sent spiralling into the planet’s atmosphere, where it will be destroyed. And its date with death has already been picked – 15 September later this year.

It will be the conclusion of one of the most successful planetary missions ever undertaken. As Carolyn Porco, the head of Cassini’s imaging team, put it: “We’ve lived a bold and daring adventure around the solar system’s most magnificent planet.”

Cassini was launched in 1997 and took seven years to reach Saturn, which – with its large family of moons – orbits the sun more than a billion kilometres from Earth. In early 2005, the spacecraft released a small European-built probe,Huygens, which landed successfully on Saturn’s huge moon Titan, a world with a dense atmosphere, and which is the only object in space other than Earth where there is clear evidence of bodies of surface liquid. Huygens revealed these to be made up of lakes of liquid ethane and methane, while sensors on the lander indicated that Titan had a crunchy surface with a texture like crème brûlée, according to one researcher.

Since then, observations by Cassini of Titan have indicated that the planet may also have a subsurface ocean, made mostly of water, and that this could provide a home for alien life. Titan’s hot core is probably keeping that underground ocean in a warm liquid state, while ethane, methane and other hydrocarbons on the surface could be filtering down through Titan’s crust into this hidden sea, creating a perfect incubator in which primitive life forms could evolve.

That on its own was enough to excite astrobiologists. But Cassini had other remarkable finds to make. As it swept past the different moons of Saturn, the spacecraft revealed that one, Enceladus – a relatively small rocky object – was spraying plumes of water into space from a region around its south pole. More than 100 geysers have since been pinpointed there.

In addition, scientists have made several other equally intriguing discoveries about Enceladus. It appears, like Titan, to have an underground ocean that is supplying its geysers, while other elements thought to be essential to the evolution of life – carbon, nitrogen, oxygen, hydrogen – have been observed in these plumes. As a result, many scientists believe that Enceladus, along with Titan and Jupiter’s icy moon Europa, offers the best prospects for finding primitive microbial life forms elsewhere in the solar system. As a result, engineers are considering new probes that could collect water molecules from these plumes for study on Earth, while others are planning missions that could drop probes on the hydrocarbon lakes of Titan.

Ironically, Cassini’s success in finding possible homes for extraterrestrial life was instrumental in deciding to end its life by plunging it into Saturn’s atmosphere. The robot spaceship is now running out of fuel and scientists are anxious that they do not lose control of the craft, which might otherwise crash on to Enceladus or Titan. Such an event would contaminate either moon with Earth microbes that might have survived on the spacecraft for the past two decades. So Nasa has decided to take the craft on a series of approaches to Saturn to allow it to return spectacular images of the planet and its rings before bringing it to its fiery end. Initiating this sequence was the crucial event of last week.

“I am delighted to report that Cassini shot through the gap just as we planned and has come out the other side in excellent shape. No spacecraft has ever been this close to Saturn before,” said Cassini project manager Earl Maize.

Cassini’s next dive through the gap is scheduled for 2 May, with several more being scheduled over the next few months. It remains to be seen what will be discovered, of course. However, if Cassini’s previous discoveries are anything to go by, we could be in for one last treat from a truly great scientific mission.

IMAGES:

Cassini diving through a plume of Saturn’s moon Enceladus. Photograph: Reuters

A still from the short film Cassini’s Grand Finale, with the spacecraft diving between Saturn and the planet’s innermost ring. Photograph: AP

For more on this story go to: https://www.theguardian.com/science/2017/apr/30/cassini-saturn-space-probe-fiery-finale-nasa