The Cassini spacecraft, launched from the Earth in October 1997, is on its way back for a brief return visit. It is due to slingshot past our planet on August 18th, picking up speed as it departs on its way to Saturn.
Cassini is a joint effort by the US National Aeronautics and Space Administration, the European Space Agency and the Italian Space Agency. It carries with it the detachable Huygens probe, developed by ESA and designed to parachute on to the surface of Saturn's moon, Titan.
Saturn is 1,430 million kilometres away, 10 times farther from the sun than the Earth. There is no launcher powerful enough to lift the 5.65-tonne Cassini directly to the ringed giant. Instead, its controllers use the free energy available by using gravity to boost the spacecraft's speed.
It first flew past Venus in April 1998 and then again late last month on June 24th, 617 days into its mission. It shoots past us in August and then it is on to Jupiter for a final flyby in December 2000. The giant planet's gravity will bend Cassini's flight path to put it on course for arrival into orbit around Saturn on July 1st, 2004.
Once there it will spend four years studying Saturn's magnetic and radiation environment, its moons and complex ring system. Huygens will drop down on to the surface of Titan to carry out a range of experiments. Titan has many Earth-like characteristics, including an atmosphere full of nitrogen. It may also have lakes or seas of ethane and methane.
Cassini, named after the noted 17th-century Italian-French astronomer, has not been idle since launch. While many of its experiments are "parked" during the long transit flights, they can be switched on during close planetary encounters, as they were last month when it passed Venus.