The Cassini spacecraft is sending back remarkable pictures of the largest planet in our solar system, Jupiter, and in two days will reach its closest point to the gas giant. On Saturday it will pass within 9.7 million kilometres of Jupiter as it uses a gravity boost to rush on to its final destination, Saturn.
Cassini is a co-operative project by the European Space Agency, the US National Aeronautics and Space Administration, the Jet Propulsion Laboratory in Pasadena and the Italian Space Agency.
Jupiter and its environs have been getting a lot of attention in recent months because of the new Cassini images but also because of Jupiter's resident satellite, Galileo, in place since 1995.
It has not just been about pretty pictures. Instruments on board Galileo provided evidence that Europa may be covered by a frozen ocean with liquid water underneath.
This was more than enough to excite astrobiologists who believe water may mean life, but then Galileo delivered data that suggest another Jovian moon, Ganymede, may also be covered with water.
Although the pictures being returned by Cassini are spectacular, the planet is really nothing more than a way-station for the spacecraft which is headed for Saturn. All going well, it should arrive during 2004 and begin an ambitious period of exploration which includes dispatching a piggy-backed probe for a visit to Saturn's moon, Titan.
There are actually two separate spacecraft, Cassini and Huygens, the former the orbiter and the latter a lander which will touch down on Titan. They were launched together from Cape Canaveral, Florida, in October 1997 and combined form one of the largest, heaviest and most complex interplanetary spacecraft ever built.
When fuelled up and ready to go they weighed a hefty 5.6 tonnes and stood 6.8 metres high. Only Russia's two Phobos spacecraft were heavier. The pair also had to take a circuitous route, making use of gravity boost fly-bys of the Earth and Jupiter to get them to the outer solar system.
Cassini will deliver unprecedented views of Saturn, and a range of experiments will study the planet, its striking rings and its host of moons. It is the Huygens probe, built by the European Space Agency, that will capture the public imagination, however.
Huygens is shaped like a giant soup bowl with a package of experiments and equipment inside. The bowl is a heat shield which will allow it to penetrate and then slow down in Titan's atmosphere. A parachute will eventually deploy, bringing Huygens's robotic laboratory safely to rest on the moon's surface.
It will have taken Huygens almost seven years to get to Titan, but its mission is planned to last no more than 153 minutes, and maybe 30 minutes more if things go well.
Its instruments will not go live until the descent is under way. These will study the moon's atmosphere, mostly nitrogen with some methane and argon.
The descent will last about 2-1/2 hours. The probe will hit the surface at seven metres per second (15 m.p.h.) but is designed to survive this shock. If it does it will continue to record information about Titan until its batteries run out 20 to 30 minutes after impact.
Throughout its descent Huygens will be broadcasting data back to Cassini orbiting overhead, which will record everything it receives. Once three hours have elapsed, Cassini will finally and permanently turn its back on Huygens, by then just a collection of space junk, to relay the precious data back to Earth.
The Voyager 1 spacecraft which flew past Jupiter in 1979 and past Saturn in 1980 is still alive and on its way to the "termination shock zone" which marks the start of the border between our solar system and interstellar space. Voyager 1 and its sister ship, Voyager 2, which follows behind, are both still working 23 years after launch and will be sending back shockzone data when this barrier is reached, possibly early next year.
Clearly they knew how to build satellites back in 1977 when Voyager 1 was launched. Its handlers in Pasadena, California, are confident that both probes will keep working at least until 2020. The two have been streaming back data continuously, and Voyager 1 is now more than 1,450 million kilometres away from Earth. It is travelling at a speed that would bring it from Los Angles to New York in under four minutes.
The shock zone is actually not a wall but a region of unknown width. It is the point where solar wind particles streaming away from the sun are counterbalanced by particles flowing through the galaxy called the interstellar wind. The shock zone begins this region and is where the solar wind first begins to back up, as further ahead it collides with the interstellar wind.
