Moons
Moons

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Moons

7.1.7 Cassini–Huygens

Cassini–Huygens rates as an even more successful mission than Galileo. It was a joint project between NASA and ESA (the European Space Agency), launched in 1997 and achieving orbit about Saturn in July 2004, after flybys of Earth, Venus and Jupiter. Its landing probe, named Huygens, descended by parachute to the surface of Titan in January 2005. The orbiter Cassini plus lander Huygens had a total mass of 2500 kg, making it the largest unmanned mission to date.

Figure 5 Cassini image of Pandora, a 110 km long inner moonlet. Its orbit lies just outside Saturn’s narrow F-ring, whereas Pandora’s partner Prometheus has an orbit just inside the inner edge of the F-ring.

The Huygens lander transmitted data and images all the way down to the surface of Titan, and continued to do so for 90 minutes after touchdown until its batteries ran out. The Cassini orbital tour was even more complex than Galileo’s tour of the Jupiter system and has been extended several times, but will probably end in 2017 with a plunge into Saturn’s atmosphere. You should realise why that was done if you answered the question posed in the previous step – crashing it into Saturn while it is still under control means that it will never be able to accidentally contaminate a possible life-bearing moon, such as Enceladus.

Headline moon discoveries of the Cassini–Huygens mission include:

  • the first images sent back from the surface of any moon other than our Moon
  • discovery of methane lakes on Titan
  • discovery of jets of ice crystals erupted from cracks on Enceladus
  • the best views of inner moonlets ever achieved – in the case of Pandora (see image) and Prometheus, their gravitational/tidal interactions with the ring particles maintain the F-ring’s narrow shape
  • close-up views of Phoebe, an irregular outer moon that may be a captured icy asteroid
  • disturbances at the outer edge of Saturn’s ‘A-ring’ that may show a new (tiny) moon forming (announced in April 2014)
  • unexpectedly large libration shown by Mimas suggesting a strange interior (announced October 2014)
  • evidence for hydrothermal vents on the floor of Enceladus’s internal sea (announced March 2015)
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