The James Webb — NASA's next big telescope
Sen—Astronomers are looking forward to having a powerful new weapon in their armoury to help them gain answers to the Universe's greatest mysteries.
The James Webb Space Telescope ("JWST") will be an orbiting observatory to dwarf the remarkable Hubble and see almost back to the Big Bang and the beginning of time.
The $8.7 billion JWST, which underwent a bumpy ride from politicians over its funding, will have a honeycomb mirror 6.5 metres in diameter (21.3 ft) compared to Hubble's 2.5 metres. This will allow it to gather more than five and a half times as much light, though it will be looking at a different part of the spectrum to its optical forerunner—the infrared.
Hubble resembles the backyard reflecting telescopes that many amateur astronomers use, having a single primary or main mirror and a closed tube. By contrast, the mighty JWST's three-mirror optical system is an open design.
The primary mirror is made of 18 segments, made from beryllium metal and coated with gold, and three struts hold the secondary mirror that directs gathered light towards the telescope's scientific instruments. The whole assembly sits on a large tennis-court sized sunshield like a kind of cosmic raft, backed by solar panels and the spacecraft's control systems.
NASA is being partnered in building the JWST by the European and Canadian space agencies, plus five aerospace companies. The project, based at NASA's Goddard Space Flight Center in Maryland, has faced huge technical challenges. That is not least because the mirror and a five-layered sunshield that will protect it from the solar glare and keep it cold are so big that they will have to fly into space folded up when it launches in 2018, three years later than originally planned, aboard a heavy-lifting European Ariane 5 rocket from French Guiana.
Unlike Hubble, they will have to unfurl automatically to an accuracy of a millimetre to operate properly. What's more, the telescope will not sit in a low Earth orbit that is accessible to astronauts on servicing missions. Instead the JWST will be placed completely out of reach, 1.5 million km away (about 1million miles) in a gravitationally stable spot relative to the Earth called the Langrangian 2 point. There it will be unaffected by noise introduced by the heat from our own planet as it observes in cool infrared light.
From there it will view the first galaxies ever to form after the Big Bang more than 13 billion years ago and study how they were put together. It will also see deep inside cosmic dust clouds to observe newborn solar systems around other stars and also study exoplanets discovered by NASA's Kepler space telescope.
Natural cooling from the sunshade rather than a liquid coolant as used by previous infrared telescopes means the observatory should have a life of between five and 10 years.
Dr Heidi Hammel, one of the JWST project’s six interdisciplinary scientists and executive vice-president of the Association of Universities for Research in Astronomy, spoke to Sen about the importance of the JWST.
She said: "Hubble has done amazing things but it is just giving us a taste of what is going on. One of its iconic pictures was the dust fields that form the Eagle Nebula's Pillars of Creation. What infrared light does is it penetrates through that dust. Webb will push through the dusty veil and actually get inside those pillars and see those stars actually being formed. We know they are there. You'll have a whole other layer beyond what Hubble has been able to do."
When the JWST was first envisaged, it was known as the Next Generation Space Telescope but it was renamed to honour James Webb, an early NASA administrator who was pivotal in planning the historic Apollo missions and setting the agency's science agenda.
It was ranked NASA's top project in 2000 but ten years later, with much of the project nearing completion, the JWST was mired in controversy over its growing cost. An independent review in 2010, chaired by John Casani of NASA's Jet Propulsion Laboratory, criticised the telescope for being over budget and behind schedule. But it praised the progress made in developing difficult technologies and building cutting-edge hardware.
Dr Hammel believes the problems were due to insufficient early funding and the challenges of working on such an advanced observatory. She told Sen: "As Casani's report pointed out, even in 2008 people hadn't really wrapped their heads around how truly complex this mission was going to be. And they got the numbers wrong. The extra funding never materialised so it figures that the telescope would have to launch later and cost more. But the good news is that the new projected cost of $8.7 billion actually comes with a very high confidence level."
In 2011 a US Congress budget committee recommended axing the JWST as part of a $2billion trimming of NASA’s budget. However US politicians decided to continue supporting the observatory and work is far advanced on building it.
Major advances were made in the construction and testing of different elements of the JWST in 2014, including the framework that will hold the vast mirror segments.
The Integrated Science Instrument Module was stored in a cryogenic thermal vacuum chamber at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, for nearly four months at temperatures of -233°C (-387°F), similar to those it will experience in space.
And the protective sunshield, that must open out only once the telescope is in space, was successfully unfolded and separated in an important test.
In spring 2015, a model of the whole telescope, named Pathfinder, will begin cryogenic optical testing inside NASA's biggest thermal vacuum chamber, the giant Chamber A at Johnson Space Center in Houston.
The JWST's instruments
Four advanced instruments will be carried in the James Webb's main payload to analyse light collected by the space telescope from distant planets, stars and galaxies. They make up the Integrated Science Instrument Module, or ISIM.
The ISIM contains Europe's Mid-Infrared Instrument (MIRI), which is tuned to view the redshifted light of distant galaxies, newly forming stars, faint comets and objects in the Kuiper Belt beyond Pluto. This has been built and completed testing at the Rutherford Appleton Laboratory in the UK.
The Near-Infrared Camera (NIRCam) provided by the University of Arizona will make similar observations but be able to blot out brilliant objects, such as stars, in order to see fainter things around them.
The European Space Agency's Near-Infrared Spectrograph (NIRSpec) will split light from up to 100 galaxies at a time to analyse their spectra and tell what they are made of.
The Candian Space Agency is supplying the Fine Guidance Sensor Tunable Filter Imager (FGS- TFI) which, as the name suggests, will allow the JWST to point precisely in order to take detailed images.