After 13 years in orbit about Saturn, and making some of the most important discoveries in our Solar System, the Cassini spacecraft mission comes to an end this week with a final close encounter with Saturn itself.
Launched in 1997, the two-tonne Cassini spacecraft traveled for 7 years and covered over 3.5 billion kilometres at a speed of over 16 kilometres per second to reach Saturn in June 2004. Once there, it established an orbital trajectory enabling measurements of Saturn's rings, its 62 moons, the magnetic field and composition the Saturn's atmosphere. It also deployed a probe called the Huygens Lander, which successfully touched down on the surface of Titan, Saturn's largest moon and sent back analytical data for over an hour -no mean feat when the surface temperature is lower than minus 200 degrees C.
Standout discoveries made by Cassini, which has cost about $3.26 billion over its 20 year lifetime, include documenting 7 new - previously-unseen - moons orbiting the ringed planet, water, ammonia and other organic molecules in the atmosphere of Saturn, and the remarkable water jets issuing periodically from the south pole of the moon Enceladus. Analysis of these salty plumes, which extend kilometres into space above the moon's surface and even contribute to Saturn's ring structure, points towards a warm liquid ocean inside Enceladus, and therefore the potential for life.
Now, Cassini is running out of fuel. Fearful that this could cause mission operators to lose control of the probe leading to a crash-landing and contamination of one of the pristine moons, mission scientists have instead elected to put Cassini on collision course with Saturn so it can be crushed harmlessly as it descends into the planet's atmosphere.
This final plunge occurs on 15th September, but preparations were a made for the manoeuvre back in April when, with its remaining fuel, Cassini embarked on a pattern of 22 orbits taking it inside the ring system and sending it skimming across Saturn's cloud-tops at an altitude of about 2000 kilometres.
This ultimate close encounter, during which Cassini will continue to transmit back data right up to the end, has enabled scientists to make measurements that were judged to be far too risky to conduct earlier on during the project's operation. They'll be studying Saturn's rings, clouds gravity and magnetic fields close up to discover more about how the planet is arranged internally. This may also shed some light on how fast Saturn is rotating, which still isn't understood, and why the top and bottom halves of the planet turn at slightly different rates.
Cassini, we bid you a fond farewell!