Over the next four years, varying trajectories will fly "Cassini" past most of Saturn's 31 moons, taking pictures and making scientific measurements. In December, it is scheduled to release the smaller European probe "Huygens" to land on the giant moon Titan.
Titan – which is bigger than the planet Mercury -- possesses a dense atmosphere that probably hides methane seas.
The probe is named after Christiaan Huygens, a Dutch astronomer who discovered Titan in 1655.
Scientists say a great chapter of planetary discovery is set to begin -- and they are both excited and nervous.
Stan Cawley is a professor at Britain's Leicester University and is involved with developing the magnetic field measurement devices on the "Cassini" orbiter.
"Certain nervousness, yes, because, of course, being able to be put into orbit depends on the rocket motor firing exactly the right time. But, of course, it has been thoroughly checked out and tested, so our fingers are crossed, but not too tightly crossed, I would say. We are quietly confident that everything will work well," Cawley said.
The first test of "Cassini" came earlier this month, on 11 June, during a fly-by of Saturn's most distant moon, Phoebe. "Cassini" relayed stunning pictures and measurements of its composition back to Earth.
Scientific measurements confirm that the moon is mainly made of ice, with a 500-meter blanket of black dust covering its surface. Phoebe could be an ancient asteroid from the outer reaches of our solar system, captured by Saturn's gravitational pull.
Cawley says the American "Pioneer 10" and the "Voyager 1" and "Voyager 2" spacecrafts that passed by Saturn some 20 years ago managed to reveal many strange phenomena that have defied easy explanations.
"Saturn -- never mind the rings and the moons -- is really a very peculiar planet," Cawley explains, noting that, like Uranus, it gives out more energy than it absorbs from the sun.
"Saturn is mostly made of hydrogen gas. That's why it would float in water if you could find a bathtub big enough. As you go further down, the gas becomes compressed. It turns into an almost metallic state, and thus the surface layers are a little bit hotter than you would expect simply from the input from the sun. It's believed to be due to the shrinkage of the planet," Cawley said.
Fred Taylor is a professor at Britain's Oxford University and a member of the team that built part of the infrared spectrometer on the orbiter, which will study the weather of both Saturn -- our solar system's largest planet -- and its moons.
"We've spent many years studying Jupiter from the 'Galileo' mission, and now we want to do the same thing with Saturn using 'Cassini.' And what we are interested in there is mainly composition and meteorology, trying to understand the dynamics of the atmosphere," Taylor said.
Taylor adds his team has spent nearly 15 years preparing the experiments for "Cassini." He says he is especially looking forward to studying Titan.
"It is such an unusual object, being a satellite with an atmosphere that is actually thicker than the Earth's. And it has the same main constituent as the Earth, as well -- nitrogen -- and that is the only other example we know of. The data we have on it at the moment is really pretty superficial. People still argue about whether there are liquid oceans on the surface or not, and whether there is rain, which would be methane rain rather than water rain," Taylor said.
Peter Challenor of the Southampton Oceanography Centre in Britain has been involved with the European Space Agency's "Huygens" lander, which might provide some answers.
"The lander has an instrument on it that, if it lands in an ocean on Titan, it's actually going to measure waves, and that's one of the things I do on Earth. So we are going to see if we can actually measure waves on a distant planet for the first time," Challenor said.
"Huygens" is scheduled to land on Titan in January. But the focus now is on 1 July. Doctor Michele Dougherty of London's Imperial College is the principal investigator behind "Cassini's" magnetometer instrument.
"The Saturn orbit insertion period is particularly critical for us, because that's when we get as close to the planet as we're ever going to get, and that's going to allow us to take measurements of the internal planetary field," Dougherty said.
A new chapter in the exploration of our solar system -- and perhaps of mankind's origins -- is about to begin.