Cassini gets a ringside seat to solve Saturn's ancient riddle

Ever since the 17th century, when they were spotted by a Dutch astronomer, Saturn's rings have delighted and perplexed stargazers in equal measure. Delighted, because of the spectacular appearance of the shining girdle of the solar system's second largest planet. Perplexed, because nobody has been able to establish what they are made of.

Ever since the 17th century, when they were spotted by a Dutch astronomer, Saturn's rings have delighted and perplexed stargazers in equal measure. Delighted, because of the spectacular appearance of the shining girdle of the solar system's second largest planet. Perplexed, because nobody has been able to establish what they are made of.

Today, we may be a little closer to finding the answer. After a seven-year, two billion-mile journey, the Cassini spacecraft is now in orbit around Saturn. And that journey, completed yesterday, represents the best chance yet of unravelling the mystery.

Over the next four years, the £1.6bn space probe will make 76 orbits of the planet and its 31 moons in the closest examination yet of the planet and its rings.

Cassini will examine their gravitational fields, determine their chemical composition, and measure any ultraviolet energy they give off. Scientists think the rings are mainly frozen water, but they could contain rock from the beginnings of the solar system.

From Earth, we can only see two prominent rings ­ called A and C. But there are many more; the spacecraft today sneaked through the F and G rings. Although they look continuous in the photos, astronomers reckon if you compressed them into a single body it would be no more than 60 miles across. That is because each ring is made of innumerable tiny particles, each orbiting independently. And though they are 150,000 miles across, they're no more than a mile thick ­ infinitesimal in astronomical terms.

The ring particles can't form into a new body, nor drift away; so they are held in a dance, "shepherded" by some of the dozens of moons that pass around and through the rings.

The rings' origin is unknown. They might have been there since the planet's formation, about six billion years ago, at the birth of the solar system. The ring systems must be replenished by some destructive process ­ probably the break-up, through tidal forces, of larger orbiting bodies, which are torn apart as the outside pieces try to move more quickly than those on the inside of the orbit.

"I can tell you it feels awfully good to be in orbit around the lord of the rings," said Charles Elachi, the director of the US space agency Nasa's Jet Propulsion Laboratory, and a Cassini radar team member, as the spacecraft allowed itself to be caught by Saturn's gravity.

Having passed just 12,500 miles above Saturn's clouds, the bus-sized lander fired its thrusters for 90 minutes. The firing finished just one minute early, to the relief of those watching live in California and at the European Space Agency's headquarters in Darmstadt, Germany. Without braking, the spacecraft would have sailed on into deep space.

"If you miss that operation, the mission is lost," said Gaele Winters, the ESA's director of operations. "Now it's right on target ­ it's perfect."

The ESA has put roughly £400m into the mission, which is the result of nearly 20 years' work by scientists in 17 countries, and the most ambitious uncrewed space mission in history.

Britain is playing a key role, providing half of the 12 instruments on board the Nasa orbiter, and two of the six instruments on the Huygens probe. The Huygens lander will detach on Christmas Eve and should land on the moon Titan three weeks later.