The pounds 2.2bn Cassini spacecraft is probably the last mission to the planet Saturn during our lifetimes and could revolutionise our knowledge about the second biggest planet in the solar system. But first it has to get there, says Charles Arthur, Science Editor.

There will be more crossed fingers than a church full of liars this morning as scientists around the world wait for a launch of the Cassini- Huygens mission to Saturn. The lift-off is set for about 10am British time from the launchpad in Florida, and British scientists will be eager to hear that it has been successful after a couple of disastrous failures in the past 18 months.

Cassini is a seven-year mission which will be probably the last during our lifetimes to the ringed planet Saturn. It will send a sophisticated robotic spacecraft, equipped with 12 scientific experiments, to orbit Saturn for four years, plus a European lander craft - the Huygens probe - which will crash land on to one of Saturn's moons, Titan, and examine its strange chemistry.

Saturn is the second-largest planet in the solar system after Jupiter, and is a gas giant, made up mostly of hydrogen and helium.

Its placid-looking, butterscotch-coloured face masks a windswept atmosphere where jet streams blow at 1,800 kilometres per hour and swirling storms of methane ice roil just beneath the cloud tops. Previous spacecraft passing by Saturn found a huge and complex magnetic environment, called a magnetosphere, where trapped protons and electrons interact with each other, the planet, rings and surfaces of the moons. Titan, the only moon in the solar system with its own atmosphere, was chosen for further investigation because remote chemical analysis of its atmosphere suggests that it contains many complex molecules of carbon and nitrogen. Though too cold to support life, it could hold clues to how the primitive Earth evolved into a life-bearing planet. It has an Earth-like, nitrogen-based atmosphere and a surface which probably consists of rocks interspersed with freezing lakes of ethane and methane, beneath a continuous drizzle of a sticky brown organic rain.

Ideal holiday destination? Perhaps not. But Cassini will offer fascinating insights into what makes Saturn so unusual. "We're trying to understand the origins of the solar system," said Professor Fred Teller of Oxford University. "And Saturn and Titan are places to do it."

While Huygens examines the atmosphere and surface of Titan (as long as it doesn't sink into a lake), the Cassini spacecraft will continue investigating the planet's rings and its magnetosphere, using equipment made by scientists at Imperial College. "This is the biggest spacecraft we have ever sent into space," said David Southwood, of the physics department. Imperial College is even set up to control Cassini and Huygens during the long trip to its destination. It will still be possible to carry out experiments and update computer programs while the spacecraft is en route.

Getting there is no picnic though. Heading directly out towards Saturn is unfeasible: the rocket could not carry enough fuel. Instead, Cassini will head inwards, towards Venus, and twice use the "slingshot" effect of its gravity to gather speed, before heading outwards past Earth in August 1999, and then on towards Saturn - arrival date, June 2004.

But some people have worried about Cassini's nuclear power source. Solar cells would be insufficient to power Cassini beyond Mars, as sunlight is too dispersed. So just like many earlier missions, Cassini will use plutonium dioxide - mixed into a ceramic matrix, like china - to generate heat and electricity.

Environmental groups have protested that if the rocket blows up on lift- off (as those with British experiments on board have recently done) then the radioactive fall-out would be deadly. But the US government, and a number of independent auditing groups, disagree. "The health risks to humans in the event of an accident are negligible," the American Astronomical Society's members declared on Friday.