By Paul Rincon
Science reporter, BBC News, Seattle
The Hubble Space Telescope is a hard act to follow.
JWST will look at the infrared spectrum in space
Since it was launched in 1990, the telescope has become one of the most important instruments in the history of astronomy, making critical discoveries that have vastly enriched our understanding of the cosmos.
John Mather is only too aware of this legacy. He is senior project scientist on Hubble's designated successor, the James Webb Space Telescope (JWST).
Dr Mather, 60, has been involved with JWST from the start and is busy directing construction of it at Nasa's Goddard Space Flight Center in Maryland.
He is also a Nobel Laureate - Nasa's first. Mather shared the 2006 Nobel Prize in Physics with George Smoot for their work on the Cosmic Background Explorer (Cobe) satellite.
Cobe detected subtle temperature variations in the cosmic microwave background radiation - the "afterglow of the Big Bang".
These variations pointed to the density differences that ultimately gave rise to the first galaxies and stars, something Stephen Hawking called "the most important discovery of the century, if not of all time".
Speaking to BBC News at the American Astronomical Society's winter meeting in Seattle, Dr Mather said there were great expectations for JWST from the astronomical community and the public.
"Everybody needs the successor to Hubble. If it were not here, people would be moaning and crying into their beer every day. So we know that we have to have something," he explained.
"It was identified as the top priority for the nation - so we're doing it."
But the James Webb Space Telescope, named after a former administrator of Nasa, is not going to repeat the science carried out by its predecessor.
While Hubble gazed at the Universe in optical and ultra-violet wavelengths, JWST will look primarily in the infrared.
"This telescope extends the science that Hubble has pioneered, but it covers different wavelength regions," Dr Mather said.
"The infrared is where the new science seems to be, and where this mission has a special and unique advantage."
Hubble will be a tough act to follow
Infrared astronomy is particularly important for understanding about the processes that went on in the early Universe.
Distant objects in the Universe are moving away from us at very fast speeds - and this has an interesting effect on the light they emit. It gets shifted to longer wavelengths: the "red" part of the spectrum.
The infrared is therefore essential to seeing the farthest - and therefore the earliest - objects to form in the Universe, a consideration that was one of the most important driving factors behind the design of the telescope.
"We know what the theories are predicting about these early objects, we know how bright and how far away they are supposed to be and what kind of telescope it would take to see them," said John Mather.
"So we said, 'it can't be a small telescope, it has to be a big one, and it has to work at certain infrared wavelengths'."
Eagle Nebula: In the infrared (L) and at Hubble's visible wavelengths
Infrared wavelengths are also good for seeing through the cocoons of dust that often obscure stars and planets in the process of formation.
But infrared light does not penetrate the Earth's atmosphere very well. This was one reason why an infrared telescope was needed in space, says Dr Mather.
In addition, the telescope needs to be cold, otherwise it will emit its own infrared radiation, swamping faint astronomical signals.
"It's designed to operate at cryogenic temperatures, on the order of 40 Kelvin (-233C, -388F)," said Dr Mark Clampin, observatory project scientist for JWST.
"By doing that, we get the background of the telescope down so that we're looking at red light coming from the early Universe."
The large shield that visually dominates the spacecraft is designed to block light from the Sun, Earth and Moon that would otherwise heat up the telescope.
For this to work, it cannot be put in low-Earth orbit like Hubble. It must be in an orbit where all three of these objects are in about the same direction.
The most convenient place is "L2", one of five gravitational balance points in space, where it can stay fixed in the same spot relative to the Earth and the Sun.
JWST's primary mirror will be 6.6m (22ft) in diameter, compared with Hubble's 2.4m (7.9ft) mirror.
This is important because it determines the amount of light the telescope can collect, and therefore its ability to detect dim objects.
"Hubble is a conventional telescope. It has a 2m primary mirror with a secondary mirror. JWST has a segmented primary mirror comprised of 18 individual elements," said Mark Clampin.
JWST's mirror will be much bigger than Hubble's
"It gives us a lot more flexibility during its lifetime because we can actually tweak up the performance of the telescope.
"A lot of the technology builds on what we've done with Hubble, but takes it to the next level."
Although its primary mission is due to last five years, it is hoped this will be extended as it has been again and again in the case of Hubble.
It is even possible that when Nasa's new manned vehicle, the Crew Exploration Vehicle (CEV) enters use, JWST could be serviced to upgrade it and extend its life.
The mission aims to examine every stage in cosmic history and the science priorities are organised into four main themes:
"We think we know the initial conditions of the Universe now, at least in a statistical way," said John Mather.
- the end of the cosmic "Dark Ages"
- the assembly of galaxies
- the birth of the first stars and protoplanetary systems
- planetary systems and the origin of life
"We have international agreement that our theories work up until the first stars start to form. The minute that interesting things start to happen, that stars light up and galaxies are forming, then all of those theories are no longer sufficient.
"We get into the complexity of immense chaotic processes. It's like going from climate prediction to tornado prediction."
Many aspects about the formation of the first stars are unknown, as are the processes by which the earliest clusters of stars started to organise themselves into galaxies like our own Milky Way.
"We'd like to do cosmic archaeology," said John Mather.
"Astronomers are lucky in the sense that we can see things as they were by looking very far away. Light travels so slowly that it takes a long time to get here from those distant objects.
"People think that light travels fast. But for cosmologists, it's just the right speed. It lets us look back into the earliest Universe if we can build the right kind of telescope."
JWST should also be able to detect extrasolar planets through the transit technique and examine the formation of planetary systems, a puzzle that has occupied astronomers for more than 50 years. And it even aims to shed light on the origins of life.
"We may be able to see something about the atmospheric chemistry of planets. If the planet's small enough, we may be able to learn about organic chemistry on an Earth-like object," said John Mather.
"We need luck on this one. We need people to go and survey all the nearby stars for planets and select the best targets. Maybe we'll get lucky."
The project has survived through some turbulent times at Nasa.
As the agency has been re-structured to pay for President Bush's Vision for Space Exploration - which involves returning humans to the Moon by 2020 - science budgets have been slashed, resulting in many robotic missions being dropped.
JWST has been protected, partly through the patronage of influential executives within Nasa.
The telescope is due to launch on a European Ariane 5 from Kourou in 2013. With its tennis court-sized sunshield and huge mirror, JWST is so big it has to be folded up to get it into the rocket fairing.
The process of unfurling it in space is extremely complicated, and engineers are modelling this through computer simulations.
"Once we get to orbit, we first put out the transmitter and solar panels," said Mark Clampin.
"Then we deploy the sunshield so we can protect the primary mirror, then we unfold two wings with additional segments for the primary mirror, followed by the secondary mirror."
"It's very different from anything that's been done in the past."
Dr Mather said: "It's an elaborate process, but a butterfly comes out of the cocoon and unfolds its wings."
He said he hoped the telescope would eventually prove as important to astronomy as Hubble has been.
"Hubble has been so brilliantly successful," he said, "and we just know we can do more with something better."
THE JAMES WEBB SPACE TELESCOPE
JWST is named after James E Webb, Nasa Administrator during the Apollo lunar exploration era; he served from 1961 to 1968
It will be placed 1.5m km from Earth, at Lagrange Point 2, an area of gravitational balance that keeps it in a Sun-Earth line
The telescope will be shaded from sunlight by a shield, enabling it to stay cold, increasing its sensitivity to infrared radiation
Three principal instruments will gather images of the Universe in the infrared region of the spectrum
These will yield new information about how stars and galaxies first formed a few hundred million years after the Big Bang