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The Hubble telescope: A giant peep for mankind

Yesterday, Nasa pledged to save Hubble from destruction - and boost its powers. Steve Connor celebrates the telescope that unlocked space's most distant secrets

Apicture is said to be worth a thousand words and that has certainly been true of the Hubble Space Telescope, which yesterday won a reprieve from imminent technical failure. Michael Griffin, Nasa's administrator, announced that a space shuttle will be sent to repair and refurbish the Hubble in 2008, a much-needed servicing mission that should keep the ailing optical telescope working until at least 2013.

Of the many images of planets, stars and galaxies gathered by Hubble over the years, the one early picture that came to exemplify its power was the one published in January 1996 - six years after the machine was launched. One scientist said at the time that it was the astronomical equivalent of finding the Dead Sea scrolls.

It was called the Hubble Deep Field, an image gathered over 10 days with the help of 342 separate camera exposures concentrated on the same tiny patch of sky. With the naked eye, the patch was about the size of a penny held 75 feet away and it was so devoid of any starlight that it was thought to provide a peep-hole to peer through our own galaxy, and any intervening galaxies, to see the very edge of the Universe.

It was a narrow, deep "core" - analogous to the geological cores drilled into the Earth's crust - through which astronomers hoped to see into the farthest reaches of space. The advantage of the Hubble is that it has the power to detect objects that are far too faint to be seen with even the most powerful ground telescopes.

Yet no one was quite prepared for what the Hubble Deep Field showed. When the digital images first emerged on scientists' screens at Space Telescope Science Institute in Baltimore, Maryland, they showed a bewildering assortment of galaxies - hundreds of them. Subsequent digital refinements showed that this tiny patch of sky - devoid of any light when viewed through most ground-based telescopes - held at least 3,000 galaxies, each composed of hundreds of millions of stars.

"The variety of galaxies we see is amazing. In time these Hubble data could turn out to be the double helix of galaxy formation," the institute's director, Robert Williams, said at the time of the announcement in 1996. "We are clearly seeing some of the galaxies as they were, more than 10 billion years ago, in the process of formation."

The Deep Field image came to represent what was at the very heart of the Hubble mission. Hubble was not just a sophisticated optical telescope perched in space, it was a time machine that allowed us to look back to the beginning of the Universe. It takes light only one second to travel 186,000 miles. Yet the light from the galaxies in the Deep Field had taken more than 10 billion years to reach Hubble.

Hubble let us dream about seeing the moments following the Big Bang when the first stars and galaxies condensed from newly created matter.

The telescope was named after the American astronomer Edwin Hubble, who in 1929 discovered that the galaxies of the Universe were moving rapidly away from us. It meant the Universe was expanding, as Albert Einstein had predicted with his General Theory of Relativity. Unfortunately, at the time Einstein did not believe his own prediction and so altered his equations to suit. (Later, when he heard of Hubble's observation, he described those alterations as the biggest blunder of his life.)

After delays, the Hubble telescope was finally launched in 1990. But a hugely embarrassing problem immediately emerged - its main mirror, honed to within a millionth of an inch, was the wrong shape. So Hubble had myopic vision, completely corrected in 1993 when a shuttle delivered a hi-tech "contact lens".

Now Hubble came into its own, and provided Nasa with possibly its best PR coup since the Moon landings. Images of galaxies, nebulae and stars were stunning - a testament to Hubble's prime position in space.

Ground-based telescopes have an inherent problem - atmospheric interference - which is why scientists have put bigger and bigger telescopes on higher and higher mountains. Indeed, a frequent quip of astronomers is that peering at the stars through Earth's hazy atmosphere is a bit like bird-watching from the bottom of a lake. Air turbulence and other interference from the gases and the water vapour of the atmosphere gives stars their endearing twinkle, but makes stars fuzzy when viewed through a telescope.

Hubble was a way of overcoming the problem. By putting the telescope in a high orbit far above the Earth's atmosphere, the light from distant stars and galaxies could be captured in all its virgin glory. It meant that the Hubble could now see objects that were about 30 times fainter than the faintest objects seen from Earth.

In 1995, Nasa released one of Hubble's most iconic images - the act of starbirth in the Eagle Nebula. Through swirling clouds and cosmic dust, young stars emerged like freshly forged diamonds in a night sky.

Within our Solar System, Hubble shed valuable insights into our own planetary neighbourhood. It checked out the dust storms on Mars in preparation for the safe landing of Nasa's Pathfinder mission with its roving vehicles. It took images of the intense fireballs that erupted on Jupiter when in 1994 giant fragments of the comet Shoemaker-Levy 9 slammed into it with the energy of many thousands of hydrogen bombs.

And only Hubble had the power to see details of Pluto, the most distant planet, as it marked its lonely orbit around the edges of the Solar System. Last year Hubble discovered that Pluto may have not one but possibly two or even three moons.

But one of the greatest scientific successes of Hubble was helping to solve one of the most difficult problems in cosmology - determining the rate at which the Universe is expanding. Edwin Hubble confirmed that galaxies were moving away from us, but he and subsequent generations of astronomers could not agree on how fast they were flying apart.

This is important because if we could work out the rate of the Universe's expansion it would be possible to extrapolate back to when the Big Bang occurred. In other words, we could then work out the age of the Universe.

Prior to Hubble's launch, estimates of the age of the Universe varied from between 10 billion to 30 billion years, with many scientists believing it would turn out to be 15 billion years. Hubble set out about helping to solve the problem by accurately measuring the distance to bright, young stars called cepheids, which can be used as "milepost" markers to calculate the distances to nearby galaxies. Once these distances were known, it was easier to make better calculations as to the speed with which they are moving away, and hence the expansion rate of the Universe.

Astronomers and cosmologists now estimate that the most likely age of the Universe is 13.8 billion years - plus or minus the odd million years. We can thank the Hubble for that too.