Ensign Chekhov suggests that they hide the Enterprise within the Saturnian system. In a memorable scene, Enterprise decelerates instantaneously from Warp 4 to impulse power and then, using its thrusters, emerges dramatically from the thick orange atmosphere of Saturn’s moon Titan. You feel like cheering – not only because the movie is ‘Trek’ at its best but because the script writers have got their planetary science right. Titan’s atmosphere is indeed like that, an ochre-colored soup of chemicals that is thought to resemble the atmosphere of the early Earth.
This year marks the sixth anniversary of the arrival of the Cassini-Huygens mission in orbit around Saturn and the fifth anniversary of the most distant landing on another heavenly body - the touchdown on Titan by the Huygens probe on January 14th 2005.
The Cassini-Huygens mission has its origins as far back as 1982, when the European Science Foundation got together with the US National Academy of Sciences to think about ways they could collaborate on space science. At that time the possibility of a joint mission to Saturn was suggested. The mission to Saturn was at least partly driven by the continuing interest in the largest moon of the Saturnian system – Titan - which had been previously visited by Pioneer 11 and both Voyager 1 and 2.
The Cassini-Huygens mission – at a cost of a little over three billion dollars the largest and most complex robotic spacecraft to the outer solar system ever devised - launched from Cape Canaveral on October 15, 1997. Measuring almost seven metres high and four meters wide, in unmanned spacecraft terms it was the marvel of its age. Onboard instrumentation included a synthetic aperture radar for mapping the surfaces of Saturn’s moons, an imaging system with both a wide and a narrow angle camera together with dedicated infrared and ultraviolet instruments capable of capturing high resolution images in the visible, infrared and ultraviolet parts of the electromagnetic spectrum, a magnetometer, and a mass spectrometer for analyzing the chemistry of the Saturnian system.
Cassini-Huygens arrived in orbit around Saturn on July 1, 2004, having already discovered three new moons as it approached the ringed system. On Christmas Day 2004 the Huygens probe separated from the Cassini orbiter and began to drift towards Titan.
On January 14, 2005 Huygens began its two-and-a-half hour descent through Titan’s turgid atmosphere. It was then that disaster struck. The Huygens probe had been designed to transmit data back to Earth via two channels known as Channel A and Channel B. John Zarnecki, principal investigator for the Huygens Science Surface Package (SSP), says having two channels not only provided redundancy, but also doubled the rate at which the probe could collect data. During the probe’s descent, however, they discovered that one of the channels didn’t work.
“It did not affect (the SSP) one jot - we got all our data on the one channel,” says Zarnecki. However, other experiments which had been relying on the dual-channel capability lost half of their data. For technical reasons the Doppler Wind Experiment was fully allocated to the “bad” channel, so all that data was lost. However, the scientists were able to reconstruct most of this lost data by other means.
The Huygens probe showed that the surface of Titan has a diverse topography of, hills, mountains, rivers, lakes and seas. Zarnecki points out that the latter three are the product of the most prosaic of phenomena – rain. The only difference is that on Earth these features are made by the action of flowing water, whereas on Titan the equivalent features are made by the action of flowing liquid methane. It is this contrast that intrigues Zarnecki – the similarity of features caused by a similar process, yet with different raw materials.
Carl Murray of Queen Mary, University of London echoes Zarnecki’s enthusiasm for the diversity of Titan, but also points out that quality is not limited to just one moon of Saturn.
“I think that the diversity of objects seen up close by Cassini and Huygens has been crucial,” says Murray. “The observations of Titan have been amazing, but the fact that we have had close flybys of all of Saturn's major
In a recent paper in the journal Science, Jeff Cuzzi of NASA Ames and colleagues summarized the state of knowledge about Saturn’s rings yielded by Cassini. They pointed out that the mission has revealed a far more dynamic and varied ring system than many expected. The particles that make up the rings range in size from a few centimeters to many meters. In addition, the interaction of two competing forces - gravitational and tidal attraction - cause the component particles of the A and B rings (Saturn’s two main rings) to form elongated clumps known as self-gravity wakes as they orbit the planet.
Some of Saturn’s moons also exert an influence on the structure of the rings, causing a scalloped pattern that rotates as each moon does. These ripples, known as spiral density wakes, tend to push these shepherd moons outward, away from the rings.
“We had suspected from Voyager images and ground-based data that there was something unusual about Enceladus,” says Murray. “The fact that parts of its surface were crater-less and that it was embedded in the diffuse E ring were clues. However, nobody really expected that we would see such an active body producing E ring material right in front of our eyes.”
For Zarnecki the most impressive achievement of the Cassini orbiter has been the discovery that Enceladus might have a liquid water reservoir at its south pole.
Carolyn Porco is the director of CICLOPS, Cassini’s imaging system. Not incidentally, she was also a science advisor to the Star Trek production team who made sure they depicted the Saturnian system correctly.
Porco points out that it is not just the possibility of water on Enceladus that is important, but also the fact that Cassini detected organics and excess heat there. The combination of these things is immensely exciting because they raise the possibility that life may have a toehold on this fantastically distant moon.
Read part 2 of this story next week