Page last updated at 18:03 GMT, Friday, 1 May 2009 19:03 UK

The space giant is coming

By Andrew Luck-Baker
BBC Radio 4, The New Galileos

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Animation of JWST unfolding in orbit

The most distant cosmic explosion ever recorded would have made a fascinating target for the James Webb Space Telescope (JWST), according to scientists now building the successor to the Hubble Space Telescope.

The cataclysmic detonation reported this week is the most far-flung object in the Universe yet seen.

BBC R4: THE NEW GALILEOS
Mirror model (Nasa)

It was a class of celestial object known as a gamma-ray blast. At a distance of 13 billion light-years, the blast was so remote that today's telescopes were stretched to their limits in revealing much information about it.

Nasa's James Webb is designed with the purpose of imaging and studying this realm of the cosmos - and beyond - in extraordinary detail.

It is scheduled for launch in 2013.

To see so far, the observatory will be the space agency's largest and most technically challenging telescope mission to date.

Its primary mirror is 6.5m (21ft) across - close to three times wider than Hubble's.

Major deployment

The huge reflector will sit behind an even more expansive sun shield, the area of a tennis court.

This structure will shield the observatory from radiation from the Sun and the Earth.

Ariane 5 (Esa)
JWST will be folded to fit inside the top of an Ariane 5 rocket

It will keep the scope at a temperature of minus 250C in its deep space orbit, 1.5 million km away.

The mind-numbing temperature is necessary because the James Webb is to image the Universe with infra-red vision.

Heat is infra-red light, so any extraneous warmth from the Sun or even the Earth would spoil the telescope's images and measurements of galaxies and exploding stars in the deep cosmos.

The observatory's five-layer sunscreen will reduce the amount of the Sun's energy reaching it by a factor of a million.

The great size of everything on the James Webb calls for the most complicated space deployment ever attempted by Nasa and its partners on the mission, the European and Canadian space agencies.

'Like a chrysalis'

The whole spacecraft has to be folded up for its launch so that it can fit into the nose of Europe's Ariane 5 rocket.

Then, as members of the James Webb team describe in The New Galileos programme on BBC Radio 4, the observatory starts to unfurl like a butterfly emerging from its chrysalis as the journey into space begins.

JWST sunshield (Nasa)
The sunscreen is made of layers of very thin material

During three tense days, commands beamed from mission control will instruct two wide booms to swing out. From these, the giant aluminium and silicon-coated sunshield membranes will inch out into position.

Then, the main mirror can be deployed. It has a segmented structure - a mosaic of exquisitely polished hexagonal sections of the lightweight metal beryllium.

In the style of a drop-leaf table, two sides of the mirror are folded back on hinges for launch. Small motors will raise the two sections into position to re-make the 6.5m light-collecting dish.

After several months of testing and allowing the James Webb to chill to its working temperature, the observations will begin.

When astronomers are able to see the most distant objects in the Universe, they can also access the deep history of the cosmos.

Herschel (BBC)
Hubble views some near-infrared wavelengths. James Webb will seek longer wavelengths; and Europe's soon-to-launch Herschel telescope will go longer still

The speed of light might be quick but it is finite. So when they picked up that record-breaking gamma-ray burst, they detected something that happened just a few hundred million years after the Big Bang: possibly some kind of giant star exploding with so much energy that it far outshone the invisible primordial galaxy which was its home.

According to Marcia Rieke, principal investigator for the James Webb's Near Infra Red Camera (NIRCam), it is difficult for the current generation of telescopes to say anything other than that the colossal explosion happened.

GRB 090423 (Gemini Observatory/NSF/AURA/D.Fox/A.Cucchiara)
Light from the furthest single object yet detected by telescopes

She told BBC News: "JWST would be able to see the galaxy which the gamma-ray burst resided in" and that "it could likely also get a spectrum of its stars".

This information would help astronomers narrow down what kind of star was responsible for the blast.

Early in the Universe's history, most galaxies were much smaller than they are now and many of the stars were much larger.

The very first stars in the Universe are thought to have been monsters.

They began to form between one and two hundred million years after the Big Bang. At this time, the universe was a dark void of hydrogen and helium gas which was the cooling debris of the Big Bang itself.

But here and there, primordial stars started to condense and ignite in denser patches of the blackness.

Brief but brilliant

Theorists calculate that because these stellar giants were formed from just hydrogen and helium, they could have grown to be as massive as 300 suns. These simple ingredients would have also made them much hotter, brighter and much shorter lived than our Sun.

"All of these things have been calculated and predicted in tremendous detail and we have the most amazing computer movies of what might have been," says James Webb chief project scientist and Nasa's first Nobel Laureate, John Mather.

Webb telescope's Mid-Infrared Instrument (Nasa)
Instruments for JWST are being prepared in the US and in Europe

"What would be extremely exciting is to find out if any of them are true."

Even the James Webb is unlikely to have the power to penetrate to the very earliest individual stars. It will not resolve them as single objects at such a distant epoch.

However, John Mather is optimistic that if first generation stars formed in groups of a few hundred, the telescope will find these "micro-galaxies".

He adds that there is another way the James Webb will detect them.

"A star lasts three million years when it's one of these. It blows up in what we call a supernova explosion which means that it's millions of times brighter than it was. So we should be able to see an individual supernova coming from almost the visible edge of the Universe."

Listen again to the scientists and engineers working on JWST in The New Galileosprogramme. The next episode will be broadcast on BBC Radio 4 on Thursday 7 May, 2009. Andrew will be visiting the Large Binocular Telescope



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