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On 11 September, at 19:04 (GMT) the Flagship-class unmanned robotic spacecraft Cassini will make its last, distant flyby of Titan. Dubbed the ‘goodbye kiss’ by its mission planners, the flyby will take it 119049 km from the surface of this moon of Saturn, sealing Cassini’s fate. It will cause the spacecraft to slow down slightly, lowering its orbit around Saturn, causing it to plunge into the atmosphere. This course of action was taken to ensure an unplanned impact with a pristine icy satellite, such as ocean-bearing Enceladus.

Cassini-Huygens was the orbiter-lander combination planned, built, launched and operated by the National Aeronautics and Space Administration (NASA), European Space Agency (ESA) and the Italian Space Agency (Agenzia Spaziale Italiana: ASI) and sent to the planet Saturn. Launched in 1997, the combination reached Saturn on 1 July 2004, after an interplanetary voyage including flybys of Earth, Venus and Jupiter. Huygens separated from the orbiter on 25 December, landing on Titan 14 January 2005. It was the first landing ever in the outer Solar System, and successfully returned data to Earth, using Cassini as a relay.

During its parachute descent, Huygens found features that looked remarkably like river systems and shore lines on earth – but rather than being made by water, they were made by liquid methane; the surface temperature is -180ºC. Touching down on a frozen surface littered with rounded pebbles, continuing to transmit to Cassini for 72 minutes before the ships lost contact. The stream of data returned from the surface provided a unique treasure trove of in situ measurements that is still being mined by scientists today.

Views of Titan from different altitudes

Running out of fuel, Cassini has now entered the final phase of its mission. It will perform a number of risky passes through the gaps between Saturn and its inner rings. The purpose of this phase is to maximise Cassini’s scientific outcome, before the spacecraft is intentionally destroyed.

In the course of its 13-year mission, Cassini made 127 close flybys of Titan, including radar-mapping its surface, finding numerous hydrocarbon lakes and seas, evidence for a global ocean of water beneath its thick crust and an atmosphere teeming with prebiotic chemicals. Titan’s atmosphere is thought to be similar to that on Earth, before life developed, and ESA therefore considers it to be a planet-sized laboratory. Studying it may help understand the chemical reactions that might have led to life on Earth. Cassini also watched Titan’s seasons change over time, including the development of a swirling vortex and clouds of methane rain that precipitate onto the surface.

Seasons on Titan: artist’s rendition