NASA’s most successful planetary orbiter made a final dive to destruction today, after 13 years exploring the giant ringed planet Saturn.
The Cassini mission, a joint project with the European Space Agency, has given the world a treasure trove of fantastic photos and useful scientific data.
But at 11.55 UT today, it entered Saturn’s yellow cloud tops at a speed of more than 70,000 mph. For a brief time it sent back precious information about its surroundings. Then as the “air” got denser it burned up like a meteor.
Cassini’s final signal did not reach Earth until nearly an hour and a half later, because it took them 86 minutes to travel at the speed of light across the Solar System. They were picked up by NASA’s Deep Space Network antennas at Canberra, Australia.
NASA mission controllers took the decision to destroy Cassini, rather than leave it in eternal orbit, to guard Saturn’s moons against contamination from a possible future collision. They were particularly concerned to protect Enceladus, which has an underground ocean of warm water and so could harbour a form of aquatic alien life.
Cassini, which is about the size of a school bus, was launched in October 1997 from Florida, and spent seven years travelling more than 2 billion miles across space to reach Saturn. No sooner had it arrived, than it scored its first triumph, soft-landing the UK-led Huygens probe on the surface of Saturn’s biggest moon, Titan, in January 2005.
Titan is the only moon with a dense atmosphere. The two spacecraft revealed that it also has seas, rivers and similar features like Earth – but containing liquid methane rather than water.
Related: Is there life on Titan?
Other major discoveries included the icy geysers spouting water from cracks in Enceladus’ crust, and massive hurricanes at Saturn’s north and south poles, plus a peculiar hexagon formation in the northern hemisphere. Another raging storm,or a kind that erupts every 30 years on Saturn, was observed to surround the entire planet from 2010 to 2011.
Cassini found the answer to something that had puzzled astronomers since before the space age – why another moon, Iapetus, varies hugely in brightness. It is because this moon really does have a dark side, formed of deposits of organic compounds and other minerals. Cassini also imaged a mysterious ridge that runs right around Iapetus’s equator.
Saturn’s famous ring system, which measures 282,000 km from one edge to the other, but is typically only 10km thick, is made up of countless particles of ice and dust. Cassini found them to be highly dynamic, with propellor formations, controlled by the pull of Saturn’s inner, small moons.
The beginning of the end came back in April when Cassini began a series of 22 weekly dives towards Saturn, flying between the rings of ice and dust and the planet itself. Then on Monday, the 11th, a flight past Titan sent it on a course straight towards Saturn and to a certain end.
One of the leading figures behind the Cassini mission has been Dr Carolyn Porco, a Senior Research Scientist at the Space Science Institute in Boulder, Colorado. Before the demise of the probe was planned, when I was reporting for Sen.com, I asked her what should follow the Cassini mission.
She told me: “We need to return to Enceladus with a mission that is properly equipped to find out if, in fact, there is liquid rising from the sea beneath to the surface and forming the jets we see in our images, and whether or not anything biotic, or even pre-biotic, has arisen within it.
“This to me is the most pressing question that has come from all our planetary investigations: to find out if life has arisen independently beyond Earth. We stand the best chance of asking those questions and arriving at the purest answers by going to Enceladus and taking a close, astrobiologically focused look.”
Related: Skymania’s guide to Saturn
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