A far out moon called Phoebe

A satellite the size of a 30-seater bus is close to achieving a long-held ambition of scientists - to put the first probe into…

A satellite the size of a 30-seater bus is close to achieving a long-held ambition of scientists - to put the first probe into orbit around Saturn, writes Dick Ahlstrom

All eyes will be on Phoebe tomorrow, not the character from Friends but a distant moon that orbits Saturn. The biggest satellite ever flown sails past Phoebe tomorrow and all going well will drop into orbit around Saturn itself by July 1st.

The Phoebe fly-by is the latest stage in a hugely ambitious attempt by European and US space scientists to send a satellite into orbit around Saturn for the first time. The Cassini-Huygens satellite mission has been under way since its launch on October 15th, 1997, but it all comes down to July 1st and whether the spacecraft pair can perform as planned.

For the past seven years Cassini-Huygens has been hurtling through space on its way towards a dramatic rendezvous with the ringed planet. Once circling Saturn, the Cassini orbiter will begin to beam back a wealth of data using a cluster of experiments.

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The smaller Huygens probe will then detach from Cassini to begin a far more captivating adventure that has space scientists on tenterhooks, a landing on Titan, Saturn's largest moon.

"The most important aspect of the Cassini-Huygens project is the Huygens probe which is going to land on Titan," says Kevin Nolan, a planetary scientist and lecturer in physics in the school of applied science, Institute of Technology Tallaght.

Huygens will travel separately to Titan and then parachute through the planet-sized moon's thick nitrogen rich atmosphere. Its journey could take up to two and a half hours, during which Huygens will take pictures, sample the chemical haze that surrounds Titan and take other measurements. It is hoped Huygens will also survive a landing and continue to radio back information to its sister ship Cassini.

Titan is bigger than Mercury with an atmosphere denser than earth's. Astronomers believe its chemical make-up may mirror that which existed on earth billions of years ago, says Nolan.

"Titan is essentially a frozen out earth from the early days of the solar system," explains Nolan. "It will tell us about the origins of the solar system and give us information about life away from earth."

All this hinges on whether Cassini-Huygens can insert itself safely into orbit around Saturn on July 1st. Its handlers will send a signal to initiate a 90-minute engine burn to slow the pair and allow Saturn's gravity to grab hold.

Cassini-Huygens will approach the planet from below, heading through Saturn's complex rings via a gap before flying up and over the planet just 20,000 km from the surface. The manoeuvre should bring the seven-tonne satellite - about the size of a 30-seater bus - into orbit to begin 27 scientific investigations to help our understanding of this gas giant.

Not since 1981 has a satellite been as close, when the famous Pioneer 11 and Voyagers 1 and 2 swept by. None went into orbit however and unlike Cassini-Huygens didn't get a chance to stick around and do continuous analysis of Saturn's atmosphere, magnetic field, rings and moons.

Cassini-Huygens is a "very audacious plan" developed by the European Space Agency, NASA and the Italian Space Agency, states Prof Paul Callanan, an astronomer in the department of physics, University College Cork. "It is an incredibly ambitious mission."

It will tell us about Saturn and Titan but also about the potential for life beyond earth. Astronomers so far have identified 100 large planets orbiting distant stars, so called "extrasolar planets", he says.

"These are unlikely places for life but they may have moons and places like Titan where conditions do exist for life. By investigating Titan and its atmosphere, we might get some idea of the characteristics of other moons orbiting other planets," he adds.

Cassini-Huygens will travel 3.2bn km (2bn miles) to reach Saturn. Crucial to the success of the mission once in orbit will be Cassini's radio transmitters. They output 20 watts, about the same power needed to operate a light bulb in your fridge. By the time this signal reaches earth an hour later it only amounts to 0.0000000000000001 watt for capture by the Deep Space Network antennas on earth.

Huygens will depart for its fateful descent to Titan's surface about six months after the pair go into orbit.