A Meteor in the Saturnian Sky
Late one night in January 2005, I stood, freezing, in a car park on an industrial estate in Darmstadt, outside the European Space Operations Centre. The sky was beautifully clear, allowing the smattering of amateur astronomers present to point their telescopes at Saturn. A quick glance through a relatively modest instrument shows the orange disk of the planet, its system of rings and, visible as a point of light to one side, its largest moon, Titan.
The European Space Agency's Huygens probe had just touched down on Titan. The first spacecraft to make a soft landing on a body in the outer solar system, it had revealed a world of rivers and lakes, filled not with water but with liquid methane and complex hydrocarbons.
Huygens's mothership, a much larger spacecraft called Cassini, has been travelling the Saturnian system ever since. Cassini, which launched in 1997, is the last of an old breed of mission, with a lineage that stretches back to the Pioneers and Voyagers of the 1970s. Equipped with a complex suite of instruments capable of working individually or in concert, it has collected enough data to keep scientists busy for decades.
A few days ago, the final navigational commands were sent to the spacecraft, and Cassini is now on a course that will end with its plunging into Saturn's atmosphere on 15 September. It will finish its voyage of discovery as a meteor in the Saturnian sky, burning up in the outer layers of the planet's thick envelope. With its demise, we will have turned our attention away from this fascinating place; neither Nasa nor the European Space Agency, joint proprietors of the Cassini-Huygens mission, have any plans to return to Saturn until 2040 at the earliest.
The end of the mission was imminent, whatever the team on the ground decided. Cassini is running out of fuel, and so the spacecraft's operators have been (somewhat gleefully) embracing risk. For the last few months it has been diving between the planet and its rings, a set of unprecedented manoeuvres designed to give close views of Saturn's clouds and allow astronomers to weigh the rings, a measurement which seems to indicate their relative youth.
Cassini's immolation is necessary partly because of discoveries it made earlier in the mission. Flying past the moon Enceladus, only a few hundred miles in diameter, it discovered plumes of water. These fountains have a source which is believed to be a large, and possibly long-lived, ocean under the moon's icy surface. As Cassini flew back through the plumes, organic chemicals including formaldehyde were found, along with abundant hydrogen gas. Taken together, these indicate that Enceladus, despite its small size, has most of what's needed for life.
Whether there are Enceladean squid swimming around under the ice, or merely bacteria, these discoveries make the moon astrobiologically interesting. Cassini's fiery end is designed to prevent even the smallest chance of a collision with either Enceladus or Titan, to avoid the tiny but non-zero possibility of inoculating alien biospheres with hardy Terran life that might have hitched a ride and somehow survived this long.
A successful end to Cassini's mission should be celebrated, and that's the tone Nasa are taking; the events of 15 September are described as a 'grand finale' and reporters have been asked not to use the word 'death' to describe what is merely a 'cessation of operations'. But some astronomers and planetary scientists aren't taking it so well. Twitter has been awash with pleas for more time, and @CassiniNooo - 'a bot which is sad about Cassini's mission ending' and which simply screams into the ether every few hours - has nearly a thousand followers, including some very distinguished names.
It will be a strange day for the mission control team, too. The engineers who have been keeping their spacecraft safe and operational for the last decade or more will watch, helpless, as the signal disappears. I hope some of them will take a chance to do on Friday what the Huygens team did on that cold evening in Germany - look up at Saturn, visible low in the evening sky after dusk, and wonder at the ingenuity of the team that showed us this marvellous system of worlds.