The Hubble Space Telescope will next week be celebrating its 10th anniversary, having reached the mid-point of its orbital lifespan. In that time, it has observed 14,000 celestial targets, gathered 3.5 terabytes of data - enough to fill about 2bn paperback books - and spawned 2,651 scientific papers. Despite Hubble's undoubted success with the public, who for the first time have seen astronomical wonders in awe-inspiring detail, some astronomers are questioning whether the space telescope has lived up to scientific expectations.
The Hubble telescope has undoubtedly made essential contributions to our knowledge of the age and evolution of the universe. But has it passed the test of a truly great astronomical observatory? To some specialists in the field, that test is to have discovered something unexpected and extraordinary. When it was launched into orbit, one astronomer said that the most exciting thing Hubble could do is to find the unexpected. Ten years on, Hubble has sharpened and extended our view of the cosmos, and in doing so has improved our understanding of what we know. However, it has not actually added to it.
Was the telescope oversold? Funding for science projects involving billions of dollars means that politicians, and sometimes the media, are fed a diet of unrealistic expectations. Has the Hubble become a supreme example of political science, with its steady stream of visually stunning images providing a convenient counterpoint to the other more worldy woes of the American National Aeronautics and Space Adminisration (Nasa)?
In the late Seventies, when astronomers were lobbying the US government for money to build the Hubble, one Senator said: "They want us to give them more billions for a telescope to solve the mysteries of the universe. But we've just given them a lot of money to build an array of radio telescopes in New Mexico called the Very Large Array. They said that would solve the mysteries of the universe. Do they need two?"
To some extent the nature of astronomical discovery has changed and predicting the unexpected is even more of a chance affair than it once was. In the past our progress in understanding the universe has been driven by improvements in technology. As telescopes became bigger and better, so our gaze out into the universe became deeper, with us inevitably seeing new things.
A century ago astronomers in the United States started building giant reflecting telescopes the like of which had never been seen before. One of them, the 100-inch Mount Wilson reflector, made one of the greatest discoveries ever made in science; that the universe was expanding. It meant that the universe had an origin in a cataclysmic explosion we call the Big Bang.
Larger telescopes followed. In 1948, the mighty 200-inch reflector on Mount Palomar was built which enabled astronomers in the Sixties to discover quasars, the super luminous, super dense black-hole cores of distant galaxies.
By then many astronomers yearned for a telescope that was free of the interference caused by the Earth's murky and turbulent atmosphere. They wanted a new telescope with a clear view of the Cosmos - a telescope in space.
Now they have had it for 10 years, what has it done? It has witnessed the dust storms on Mars, the atmospheres of distant giant gas planets, the sites of stellar births, along with galaxies, black holes and the edge of the universe. Hubble has, without doubt, produced a series of remarkable pictures.
One of Hubble's most extraordinarily haunting images is of the so-called Eagle nebula, a vast cloud of gas and dust out of which stars are being formed. Hubble saw three towering columns of gas containing bright points, each of which was larger than our own solar system.
Another stunning image is called the Hubble Deep Field, a "bore-hole" through outer space showing galaxies being formed close to the very start of time itself. Hubble produced this image by staring at what was a blank patch of sky (no bigger than a postage stamp in the night sky when viewed with the naked eye) for hundreds of hours. What for other telescopes seemed an empty void became, through Hubble's single eye, emblazoned with thousands of young galaxies. The resulting image has become an icon of Hubble's extraordinary powers of perception. It was the first visual glimpse of the dawn of the universe.
In 1994, Hubble displayed its prowess, witnessing the impact of the shattered remnants of Comet Shoemaker-Levy 9 as these great balls of cometary ice and dust crashed into Jupiter with the force of a million H-bombs. The pictures of a dark blemish spreading over Jupiter's surface are unique.
In scientific terms, one of its most spectacular observations was of the giant cluster of galaxies called Abell 2218, a collection of objects whose gravity distorts the light from more distant objects. This so-called "gravitational lens" effect creates remarkable arcs of smeared light. Studying them yields important clues about the size and evolution of the universe.
But if Hubble was built for any one thing, it was to answer the question of how old the universe is. Before Hubble was launched, astronomers could not measure the size of the universe to better than a factor of two. One of the biggest challenges for measuring the universe is to measure very remote distances.
Hubble did that by looking for stellar signposts called Cepheids. These are stars that vary in brightness in a way that depends upon their total light output. They can be used to measure distances. We now know the age and size of the universe to an accuracy of 10 per cent. It could be argued that Hubble has achieved what it set out to do.
After 10 years, the trauma of Hubble's misshapen main mirror - which had to be corrected by Shuttle astronauts - is now history. At the time it threatened to kill the entire mission. Now it seems like a passing inconvenience, Hubble sitting at the epicentre of modern astronomy. Whenever an object is discovered using another kind of telescope, they turn to the Hubble to see what it looks like. It has become astronomy's default instrument.
For many, this is the golden age of astronomy. We have sent space probes to most of the planets and landed a robot explorer on Mars. We have discovered planets circling other stars, measured the age of the universe and have even begun to discern its possible fate. Hubble has contributed in no small measure to this golden age. Who knows what is waiting to be discovered out there in the next decade. Perhaps the Hubble Space Telescope will, after all, find the unexpected.
Dr David Whitehouse is science editor of BBC News OnlineReuse content