Built out of spares from other missions - an antenna from a spare Voyager craft, and a computer from the Galileo Jupiter probe - Magellan was constructed on a shoestring budget during the Eighties and launched on board the Space Shuttle Atlantis on 4 May 1989. This little probe was to be part of the most successful interplanetary mission Nasa had undertaken. It has sent back stunning images of Venus's surface that have changed the way we think about the origins of our own planet.
Venus was long thought to be Earth's 5twin planet. Long before Magellan was even a twinkling in Nasa's eyes, Earthbound observations through Victorian telescopes had shown that Venus was of similar size to Earth. The thick swirling carbon and oxygen atmosphere led early astronomers to suggest that the surface of Venus would be wet and swampy, and probably inhabited by creatures the size of dinosaurs.
This could not have been further from the truth. As technology advanced, and with the coming of the space age, any notions of life on Venus were shattered. The Russians successfully landed probes on the surface during the late Seventies and early Eighties. These landers revealed an ultra-corrosive atmosphere of sulphuric acid rain that never reaches the surface, where temperatures average 480C, gas mark 26 (gas mark 9 is a mere 240C). This temperature is hot enough to melt lead. The pressure is 90 times that on Earth, equivalent to being one kilometre below the sea. The choking carbon dioxide atmosphere on Venus had created a runaway greenhouse effect. The Russian landers were crushed and burnt within hours.
It had long been the dream of geologists to strip away the world's oceans and gain a glimpse of the whole of the planet's crust. The Russian images of Venus provided a tantalising peek at a world where this dream experiment would be possible.
Lifeless Venus has no oceans to conceal its tectonic terrain, therefore a lava structure would stay pristine until altered by another geological event. By studying Venus, geologists could at last obtain some idea of how the Earth's great mountain ranges would appear in the absence of erosion. The need to map Venus in its entirety was overwhelming.
In one Venusian day, equivalent to 243 Earth days, Magellan looked through the thick Venusian atmosphere using a powerful microwave radar. More than 80 per cent of the surface of Venus was found to be covered with lava flows, fractured plains and other volcanic features. But mapping by radar is not easy and planetary geologists have had to relearn their mapping skills to interpret these 'sound' pictures. The exercise is rather as if they were mapping like a bat. Rough landscapes such as mountains, and steep slopes that face the probe, are highly reflective, appearing bright in Magellan images, while flat smooth areas such as lava plains reflect less energy and appear darker.
The radar imaging was so successful that the amount of data returned by Magellan was more than all the previous US planetary missions put together. The images revealed a place where geology has gone wild. Incredible volcanoes towering 4 to 5km high had erupted vast quantities of lava on to the plains below. Hundreds of thousands of smaller volcanoes looking like upturned Roman shields and similar to those seen in Iceland peppered the rolling plains. Devana Chasma, a 1,000km rift valley that would stretch from London to Rome on Earth, cuts the crust where huge internal forces have torn it apart. Equally huge mountain chains have been thrown up - boasting one of the solar system's highest mountains, the 39,000ft Maxwell Mons, 10,000ft higher than Everest.
Magellan's diligent dish has beamed back enough data to keep planetary geologists busy for years. Its mission complete, and with no more funding to archive the data, Magellan will be hurled to its death on the fiery Venusian surface later today. But Nasa plans to squeeze every last byte of information out of its prize probe. The final transmission from Earth will instruct the satellite to descend slowly into the Venusian atmosphere, where it will eventually burn up - measuring how the drag of the atmosphere slows it down in the process.
In its wake lies a number of unanswered questions. As with most scientific endeavours, greater understanding has given rise to new, more intriguing questions. Despite the prolific evidence of vulcanism on Venus, scientists have yet to find a smoking volcano - or any other evidence of current activity. Professor Donald Turcotte, a geologist at Cornell University in New York state, thinks this is because Venus ejects its heat from the mantle every 500 million years in a catastrophic volcanic event that resurfaces the whole planet. It then lies dormant for another half a billion years.
With such a cacophony of volcanic features unveiled by Magellan, Professor Dan McKenzie, a geologist at Cambridge, suggests that Venus is continuously ejecting its heat. Professor McKenzie is an advocate of gradualism and sees tectonics on Venus as a much more continuous process, driven by rising plumes of heat that bulge the crust - similar to the process that created the volcanic islands of Hawaii on Earth.
Even after its death, Magellan's radar 'soundbytes' will challenge our thoughts about both Venus and Earth for decades to come.