The next stage of planetary exploration clearly required orbiters. Pioneer Venus went into orbit around the planet in 1978 and mapped the surface using radar to a resolution of about 100km. Eleven years later we had the orbiter. The Magellan planetary probe was released from the cargo bay of the space shuttle Atlantis in May 1989 and rocketed off to Venus. After breaking hard it started to orbit Venus on 10 August 1990. Two years later the Magellan craft had been round the planet 5500 times and its radar system had mapped over 98% of the surface. Features down to 120m across could be resolved and heights could be measured to an accuracy of 10m. Continents and plains were revealed in detail, as were the multiple lava-flows, and the apparently random scattering of volcanic and impact craters. Interestingly the present surface seemed to have an age that was only about 15% that of the planet.

Space shuttle Atlantis was also used to launch the Galileo probe to Jupiter in October 1989. Gravity assists from Venus (in February 1990) and Earth (in December 1990 and December 1992) increased the size of the orbit, and the spacecraft eventually rendezvoused with Jupiter in December 1995. On its way through the Asteroid Belt, Galileo flew past Gaspra and Ida, taking superb colour pictures. As it was getting close to Jupiter Galileo released a small instrumented probe. With both parachute and heat-shield braking, this probe descended through 200km of the jovian clouds, transmitting about 70 minutes of data as the ambient atmospheric pressure changed from 0.08 to 30 times that of the Earth’s ground level pressure. As often happens with space missions, there were many surprises. The atmosphere was denser than expected and it was also less rich in helium and neon. The main Galileo spacecraft started its Jupiter investigation on a highly eccentric 230 day orbit. Successive passes of Jupiter slightly altered this orbit and led it on a series of swing-by manoeuvres, shaped like flower-petals, that took it on low altitude flybys of the major satellites Io, Europa, Ganymede and Callisto. Much time was spent investigating the high-pressure anti-cyclonic Great Red Spot in Jupiter’s southern hemisphere cloud band, the tenuous Jovian ring system and the Io plasma torus. It was clear that the sulphurous volcanic landscape of Io had changed considerably in the 17 years that separated the Voyager and Galileo missions. Visiting planets once is clearly not good enough. Many features need to be continuously monitored.

The success of the Galileo mission paved the way for the funding of an orbiter of the next planet from the Sun. The Cassini spacecraft was launched on top of a Titan 4 Centaur rocket on 15 October 1997, finally reaching Saturn in July 2004. During the nominal mission of 4 years it will orbit Saturn 63 times and will fly above Titan, the big (Mercury-sized) moon, on 33 occasions. There will also be close flybys of Iapetus, Enceladus, Dione and Rhea and more distant observations of Tethys, Mimas and Hyperion. One of the highlights of the mission will be the Huygens probe descent to the surface of Titan. Titan appears from afar like a rather uniform ball of orange fog. Voyager 1 data indicated that it has a very dense atmosphere of about 80% nitrogen, 6% methane plus hydrogen and other. It is thought that this atmosphere might be similar to the original atmosphere of Earth, the atmosphere that was subsequently oxygenated by life-forms. Temperature, pressure, density, composition, winds and aerosol content will be measured as the probe parachutes down to the surface. Images and spectra will be obtained. It is hoped that the Huygens probe will continue to function for several minutes after landing and the onboard instrumentation is designed to measure the characteristics of the surface in detail. It is not known, however, whether the probe will hit methane icy glaciers, solid rock mountains or a liquid hydrocarbon-methane oceans when it touches down. Images taken by the Hubble space telescope of Titan through one of the atmospheric ‘windows’ in the near infra-red part of the spectrum indicate that the surface reflectivity varies by about 10% as one moves from bright parts, that might by clean ice surfaces, to dark parts, that might by hydrocarbon seas. Hopefully data from the Huygens probe will solve this mystery.