1. How did the Cassini probe get to Saturn and why did it take so long?
Pioneer 11 and the Voyager probes took the direct route, reaching Saturn in three years, but Cassini took seven. That is because it was too heavy to send express. The launch system lacked the power to heave six tonnes all the way to the outer solar system.
So Cassini went the long way. It first headed closer to the Sun, passing Venus twice, and then flew back past Earth and finally Jupiter. At each encounter with a planet, Cassini stole a little orbital energy to make up for its lack of rocket power.
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2. What is so special about Saturn?
With its beautiful retinue of rings and 40-odd moons, Saturn may be the most complex and fascinating member of our solar system. It is the second largest planet, after Jupiter, but holds several records of its own. It is the least dense planet 鈥� it would float in water 鈥� and also the most squashed, being 120,000 km across at the equator but only 108,000km from pole to pole. Its rapid rotation creates this flattened shape.
Saturn鈥檚 signature rings provide astronomers with a unique model of the disc of ice and gas that gave birth to all of the planets. And among its moons are Enceladus 鈥� blindingly white and spewing water vapour from its south pole 鈥� the mysteriously black and white Iapetus, and the gigantic moon Titan, the only satellite in the solar system with an atmosphere.
3. Why is Saturn made mostly of gas?
Because most of its raw material was gas. Hydrogen and helium are the most abundant substances in the solar system. Earth is not a gas giant because these low weight elements are difficult to hang on to. Our young Sun would have heated them up, making it impossible for Earth鈥檚 gravity to retain them. Out in the cold, where Saturn formed, there was no problem in keeping a thick blanket of gas.
But exactly how Saturn and the other giant planets formed is unknown. It may have started as a chunky nucleus of rock and ice, which then gathered gas about itself or, more controversially, it may have formed by the sudden collapse of a gassy cloud. Cassini鈥檚 measurements may help settle the debate.
4. Why does Saturn have such striking rings?
All the giant planets have rings, but most are thin and faint. Saturn鈥檚 are spectacular 鈥� broad, bright, multicoloured and made up of thousands of ringlets like the grooves on an old vinyl record. They consist of loose, icy debris, ranging from microscopic dust, to boulders bigger than a house.
So where did all this rubble come from? Astronomers have several theories. Either Saturn once had an inner moon which was pulverised by an asteroid, or a comet strayed too close and Saturn鈥檚 gravity tore it to pieces, or perhaps the rings had an even more complicated origin. Cassini could settle the question. It could also tell us how old the rings are, and how long they will last.
5. Why is Saturn鈥檚 moon Titan the only moon in the solar system with an atmosphere?
Some other moons have faint traces of gas above their surfaces, but Titan is the only one with a proper atmosphere. It is several times鈥� denser than Earth鈥檚, and made mostly of nitrogen and methane.
It is thought that all large bodies in the solar system once had an atmosphere, but in most cases it soon leaked away into space. Titan, however, has relatively powerful gravity, helping it to hold onto its gas. It is also much cooler than many other moons, such as Earth鈥檚 moon where the higher temperatures closer to the Sun would have rapidly evaporated an ancient atmosphere.
But one mystery remains: two moons of Jupiter 鈥� Ganymede and Callisto 鈥� lost their atmospheres, even though they are as large and almost as cool as Titan. It鈥檚 possible that the high rate of asteroid and comet impacts stripped them away.
6. What鈥檚 Titan鈥檚 weather like?
Chilly and overcast, with light winds and a slight risk of methane downpours.
Temperatures at the surface are around 94 Kelvin, or (-179掳C), cold enough to let methane condense and to freeze ice into a rock-hard state. The sky is stained orange by a thick layer of photochemical smog high in the atmosphere.
Fierce gales blow at super-hurricane speeds of up to 100 metres per second, while at ground level the wind speed is only about 1 m per second 鈥� a 鈥渓ight air鈥� on the Beaufort scale. There are few clouds, but clear evidence of erosion implies that liquids sometimes flow on the surface, so scientists believe that there are occasional methane rainstorms on Titan.
7. Is there life on Titan?
It鈥檚 still a possibility, but the chances are becoming more remote.
Microbes might shelter in a subsurface ocean, if it exists. Or there could even be life on the surface employing truly exotic biochemistry to function in the sub-arctic temperatures.
Exobiologists have suggested that bugs on Titan might leave clear chemical traces in the atmosphere, perhaps distorting the ratio of certain chemical isotopes, or eating up most of the hydrogen near ground level. But no such traces have been seen by Huygens鈥� chemical analyser.
8. How did Saturn鈥檚 walnut-shell-shaped moon, Iapetus, get its giant ridge?
The short answer is that no one has a clue. The ridge around Iapetus is perhaps the greatest out-and-out surprise that Cassini has sprung so far.
It girdles the 1400-kilometre-wide moon, running at least half way around the equator, and rising up to 20 km from the plain. There is nothing like this ridge on any of the scores of known planets and moons, and there is so far no adequate theory to explain it.
Whatever the explanation turns out to be, it may be connected with the strange black material coating one side of the moon, or with the slightly wonky overall shape of Iapetus.
9. What will happen to Cassini on completion of its mission in 2008?
June 2008 marks the end of Cassini鈥檚 鈥減rimary mission鈥� 鈥� the pre-planned four-year tour of the Saturn system. But engineers are confident the spacecraft will keep going for several more years. 杏吧原创s will doubtless want to revisit some of the moons, perhaps to get close-ups of any particularly fascinating features, and even if most of Cassini鈥檚 fuel has been used, it will be able to adjust its trajectory slowly, using Titan鈥檚 gravity, to reach most parts of the system.
Eventually, of course, the fuel tanks will be nearly empty. The team will then have four options. They could put Cassini in a long-term stable orbit, taking measurements for years to come. Or they could let it go out in a blaze of glory and fly it straight into Saturn, just as the Galileo spacecraft plunged into Jupiter in 2003, collecting fascinating data as it falls. Or it could crash-land on Titan, although that might risk polluting the moon with nuclear material from the spacecraft鈥檚 fuel cells.
And the last option? 鈥淲e could send it flying through the main rings of Saturn until we smack into something big,鈥� says Cassini mission planner David Seal. If the spacecraft survives even one ring crossing, it could get some amazing pictures and make some remarkable discoveries.
