The Big Question: Has the Hubble space telescope project been worthwhile?

Why are we asking this now?

Because four years after Nasa had decided to abandon it in space, a new mission has been approved that will see the Hubble Space Telescope given one final upgrade that should prolong its life by five years.

The iconic telescope has had its ups and downs since its launch in 1990, but when the extensive surgery is completed next August during Nasa's fifth Hubble visit, it will be more powerful than at any point in its lifetime. During a five-day spacewalk 350 miles above the Earth, a group of astronaut-mechanics will not only carry out running repairs to broken equipment, but they will also be installing two brand new instruments.

Will this really be the last trip to Hubble?

Yes. With the space shuttles coming out of commission at the end of the decade, there will be no means of carrying out the maintenance that Hubble needs to keep on going. Even if no mechanical faults occur, its power supply will probably die within five years, along with the telescope's gyroscopes, which keep it travelling along the right orbit around the Earth.

What work is being done to Hubble?

By the time Nasa's mechanics finish kitting out Hubble with its new gadgets, it will almost be a new telescope in all but name. The new WideField Camera 3 will allow Hubble to peer deeper into the recesses of space, hunting out new galaxies and helping scientists to build a clearer picture of what the universe was like shortly after its birth. The removal of the old camera will be one of the trickiest operations of the mission.

The other new device to be fitted will be a state-of-the-art spectrograph, which will be able to examine atoms drifting through intergalactic space. These atoms make up most of the material of the universe.

Repairs will also be carried out to the Advanced Camera for Surveys, which was severely damaged after short circuiting at the end of last year, and its old spectrograph will also be patched up. All of Hubble's gyroscopes will be changed and the power supply will be replaced to keep it going for another five years.

What has Hubble taught us?

Hubble has been an invaluable source of information, helping astronomers understand phenomena such as dark energy and quasars. It has even helped scientists to calculate the age of the universe. Thanks to Hubble, scientists now think that it is between 13 and 14 billion years old. Thousands of research papers have been written on the basis of information collected by Hubble, while all its data has been collected into a public archive for all to use. It has also brought us the deepest telescopic views of the universe ever produced, allowing scientists to look back in time at how the universe looked a few hundred million years after its birth.

What has gone wrong in the past?

The project got off to a sticky start soon after it was launched in 1990. It was given a grand build-up from Nasa and scientists across the world, who were expecting Hubble to bring about the greatest advances in telescope technology since the instrument was invented. However, after it was sent into space in April 1990, embarrassed scientists had to admit that its huge mirror, 94-inches across, had been polished to the wrong shape. Images coming back to earth were not the awe-inspiring pictures all had hoped for, but blurry and out of focus. A repair mission was sent in 1993 to fix the fault. Three further service missions have taken place since.

Has it been worth all the trouble?

It has certainly cost a lot to keep going. The trip next August will be the fourth mission to the telescope in the past decade. One estimate puts the total bill at $9bn (4.5bn), funds that some say could have been better spent elsewhere. But, according to Professor Martin Barstow, part of a European Space Agency group advising Nasa on the next service mission and head of Leicester University's physics and astronomy department, there is no doubt that the Hubble project as a whole has been a great success. "Since the early problems with its mirror, the project really hasn't looked back since," he says.

"Of course there have been ups and downs, but it has been invaluable in terms of the information it has given scientists to work with. And this latest service mission is much more than a last gasp. It will give Hubble more instruments than it has had at any other time."

Should it have been abandoned sooner?

Actually, Hubble has already had a reprieve. In 2003, the telescope's days looked numbered after the space shuttle Columbia disintergrated when returning to Earth, killing seven crew members. After the crash, Nasa's administrator, Sean O'Keefe, judged that any more shuttle trips to Hubble would be too dangerous. The subsequent public reaction was enormous, proving that the telescope had become a popular symbol of modern space exploration.

Hubble was given a second chance with the arrival of a new administrator, Michael Griffin. He immediately set up an in-depth assessment of the risks involved in flying to it. A two-day summit concluded that it was just as safe as missions to the space station while astronauts were also keen on a return to the telescope. When the fleet of shuttles was brought back into use last year, another Hubble mission was on the agenda again. The shuttle Atlantis will be used for the new mission next August, with the Endeavour craft readied for a rescue mission.

Why has Hubble become so popular?

The main reason for its fame has been the images it has sent back of deep space in all its mysterious glory. Those images of distant galaxies and stars have captured the public's imagination. And it has now been in space for 17 years, meaning that it has become an international symbol of space exploration. For scientists such as Professor Barstow, Hubble's new devices are even more exciting, and will ensure that Hubble's final days will also be its brightest.

So what will replace Hubble?

The next-generation space telescope is already being developed, and it has a primary mirror seven times the size of the one used on Hubble. As it orbits 1.5 million kilometres from Earth, the James Webb Space Telescope will be able to tell scientists even more about how stars and planets were first formed. It is due to be launched in 2013, the year Hubble is expected finally to run out of steam.

Has Hubble been worth all the work?

Yes...

* Having been in orbit for 17 years, it has become a symbol of space exploration in the modern age

* It has given us some of the most dramatic images of deep space ever produced, creating great public interest

* The new data it has delivered has formed the basis of more than 6,000 research articles

No...

* It had faults from the start and running repairs have cost Nasa billions of dollars

* Next summer's will be the fifth service mission. All of them so far have been extremely dangerous

* If all that money had been focused on a mission to Mars, we could have made it to the red planet by now

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