On the eve of its long-awaited launch, Govert Schilling reports on astronomers’ expectations for the James Webb Space Telescope
Finally, it’s about to happen. All being well, on 18 December, the 6.5-tonne James Webb Space Telescope (JWST) will leave Earth, ready to study the Universe in unprecedented detail. After being launched on a European Ariane 5 rocket, astronomy’s new eye on the sky promises to yield new insights into the origin and evolution of planets, stars, galaxies and the Universe as a whole. “JWST has no competition,” says long-time project scientist John Mather at NASA’s Goddard Space Flight Center. “There is no other way to see what it can see.”
Sporting a segmented primary mirror 6.5m in diameter and equipped with sensitive cameras and spectrographs, the James Webb Space Telescope has often been called the successor of the famous Hubble Space Telescope (HST). Indeed, with a light-collecting area almost seven times that of Hubble, JWST is much more sensitive, enabling astronomers to peer further into space and time, and to discern much finer detail in star clusters, stellar nurseries, and galaxies alike.
One important thing that JWST has in common with Hubble is the international character of the project. NASA led the construction of the telescope; the European Space Agency (ESA) provided part of the instrumentation and takes care of the launch, and Canada is a third partner. Also, just like Hubble, the JWST is a truly multi-purpose instrument that will leave its mark on almost every field of astronomy. “The science that emerged from the Hubble Space Telescope vastly outdid what we expected,” says cosmologist Richard Ellis of University College London. “I’m confident that JWST will do the same.”
Seeing infrared
The most important difference between Hubble and JWST (apart from the primary mirror diameter) is that the new space telescope is designed to observe a different set of wavelengths in the electromagnetic spectrum. While Hubble gained fame for its revolutionary discoveries at every colour of the spectroscopic rainbow (plus ultraviolet (UV) and near-infrared), JWST can’t see UV, violet, indigo, blue, green or yellow. Instead, it is sensitive to everything between orange light and the midinfrared, corresponding to wavelengths between 600 nanometres and 28.3 micrometres.
There’s good reason for that. Stars are born in the dark and relatively cold interiors of dust-laden clouds of molecular gas that are completely opaque at optical wavelengths. In the (mid-)infrared, however, astronomers can peer through the absorbing dust to witness the birth of new suns and planetary systems. And compared to earlier infrared observatories like NASA’s Spitzer Space Telescope, JWST is about a hundred times more sensitive and can also distinguish much finer detail in stellar spectra.
Moreover, as astrochemist Ewine van Dishoeck (from Leiden Observatory in The Netherlands) explains, most complex molecules in the Universe leave their tell-tale ‘fingerprints’ in the infrared part of the spectrum. “I’m involved in a dozen JWST projects,” she says, “and I look forward to measuring the chemical make-up of the material – gases, ices, and silicates – from which planets are built, and to compare that to the composition of the atmospheres of extrasolar planets. JWST will provide an enormous step forward.”
According to van Dishoeck, the JWST will also study protoplanetary discs in exquisite detail. “ALMA [the Atacama Large Millimeter-submillimeter Array in Chile] can study the outer parts of these discs,” she says, “but JWST will observe the inner parts, where most of the planets are born.” Meanwhile, exoplanet researchers like Sara Seager, from the Massachusetts Institute of Technology, hope that the JWST’s infrared instruments will be sensitive enough to detect oxygen and other so-called biomarkers in the atmospheres of planets orbiting neighbouring stars. “The most exciting discovery by JWST would be biosignatures in a rocky exoplanet atmosphere, but we have to get lucky,” she says.
Cosmic dawn
Another key science goal of the new space telescope is to search for light from the first stars that formed after the Big Bang – ‘witnessing cosmic dawn’, as it has been described – and to study the formation and evolution of galaxies. Here, too, infrared astronomy is key. The reason is that light from the earliest phases in cosmic history has been strongly redshifted by the expansion of the Universe by the time it arrives at Earth. In other words: what is emitted in the ultraviolet (the fierce radiation of the very first generation of massive stars) can only be observed in the infrared. “The redshift range [probed by JWST] very much represents the last missing piece in the jigsaw of cosmic evolution,” says Ellis.
According to the most recent estimates, the first galaxies emerged some 250 to 350 million years after the Big Bang, and Hubble can’t see them because its instruments cannot probe beyond a wavelength of 1.6 micrometres. “My group did well in securing JWST time,” says Ellis. “We hope to get spectra of promising examples. Also, tracking the composition of the gas as a function of look-back time is one of our goals.”
Closer to home, JWST has much to offer the investigation of our own Solar System. In particular, the study of the giant planets will greatly benefit from JWST’s unsurpassed infrared capabilities. At infrared wavelengths, JWST can look below the clouds to see what is happening. “This tells us about the weather and winds, as well as the temperature and chemistry of the atmosphere, which are all really important for learning how these planets work,” explains planetary scientist Naomi Rowe-Gurney from the University of Leicester.
Rowe-Gurney is involved in the first JWST observations of Uranus and Neptune that are scheduled for some time in 2022. “Because we haven’t visited either planet since the Voyager 2 flybys in the late 1980s, we have a lot of unanswered questions,” she says. “Any questions we can answer with JWST will ultimately help motivate a future dedicated mission to one or both of these ice giants. So look out for some amazing images and groundbreaking science from my two favourite planets!” In addition, JWST will be a great instrument to study the composition and characteristics of minor Solar System bodies like asteroids, comets and ice dwarfs in the Kuiper Belt, beyond Neptune’s orbit.
Fulfilling all these goals is the task of a suite of four big, complicated scientific instruments. Astronomers expect jaw-dropping photographs from the US-built Near-InfraRed Camera (NIRCam), JWST’s most important camera, which operates at wavelengths between 0.6 micrometres and 5 micrometres. Detailed spectra in the same wavelength range will be obtained by the European Near-InfraRed Spectrograph (NIRSpec). Europe also built part of the Mid-InfraRed Instrument (MIRI)–a combination of a camera and a spectrograph covering wavelengths longer than 5 micrometres. Finally, the Canadian Space Agency (CSA) contributed an instrument that combines Webb’s Fine Guidance Sensor (FGS) – used for pointing and stabilising the space telescope – and the Near InfraRed Imager and Slitless Spectrograph (NIRISS).
Into the cold
To keep the instruments at a temperature of at most 50˚ above absolute zero (lest their own heat radiation would wreck the observations), JWST is fitted with a huge sunshield, measuring 20m by 14m. It is composed of five extremely thin layers of Kapton (a polyimide film developed by DuPont), coated with silicon-doped aluminium. And to keep it away from the infrared emission of our home planet, the JWST will not orbit Earth like Hubble does. Instead, it will essentially orbit the Sun, describing a so-called halo orbit around L2, the second Lagrange point in the Sun-Earth system – where the gravitational forces of the Sun and Earth exactly balance the centrifugal force – which is located 1.5 million kilometres behind Earth, as seen from the Sun.
An image showing the sunshield after it had completed its critical testing. The sides of the primary mirror are folded into its launch configuration 
Technicians examine the JWST’s colossal 6.5m primary mirror 
Practising ‘snow cleaning’ on a test mirror using carbon dioxide ‘snow’ 
Having completed tests, the JWST is packaged up for shipment to its launch site
Unlike the crewed spacecraft that have to dock with the International Space Station (ISS), JWST can basically be launched at any convenient time. At the time of writing, the launch date is 18 December. Within weeks of the launch, while cruising to its remote vantage point, JWST will deploy both its fragile sunshield and its folded primary mirror, which consists of 18 gold-plated hexagonal segments. Project scientist John Mather says this is the most critical part of the mission. “Without them, nothing else works. But our deployment has been tested many times, and we have redundant ways to set off every actuator and turn every motor.”
Keeping JWST in orbit around L2 will require the incidental firing of its 16 thrusters. Given the amount of on-board fuel, the operational lifetime of the telescope is likely be limited to around 10 years. So unless engineers find a way to robotically refuel JWST, in principle it is possible that its ‘predecessor’, the Hubble Space Telescope, outlives it – provided Hubble doesn’t experience any major technological issues or breakdowns over the next decade.
Meanwhile, astronomers are already discussing plans for the successor to the James Webb Telescope. Four amazingly powerful observatories are on the drawing board, competing for priority and funding. “In my opinion all are worth building,” says Mather. “It’s only a question of how and when.”
Building the most expensive telescope ever
What does it take to construct a telescope like JWST, and why have there been delays?
When work on an infrared successor to the Hubble Space Telescope began in 1996, a quarter of a century ago, the plan was to build an 8m instrument that would launch in 2007 and would have a price tag of just $500m. “We were over-optimistic about schedule and budget,” says project scientist John Mather of NASA’s Goddard Space Flight Center, which manages the development of JWST.
By 2005, cost estimates had risen to a few billion dollars, and NASA called for a redesign with a smaller primary mirror. Still, thanks to many technical setbacks and associated delays – in particular with the development of the vulnerable sunshield – prime contractor Northrop Grumman eventually delivered the telescope in 2016, after which a long period of testing began.
In early 2020, the COVID-19 pandemic caused more delays: NASA had to give priority to completing its Perseverance Mars rover in time for its planned July launch. Eventually, in late August 2021, the JWST was ready for shipment to the European launch base in French Guiana. By then, the project’s total costs were just short of $10bn.
Govert Schilling is an astronomy writer, science journalist and author of
Ripples in Spacetime