{"id":25194,"date":"2021-10-21T00:00:00","date_gmt":"2021-10-21T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=25194"},"modified":"2021-10-21T12:30:24","modified_gmt":"2021-10-21T12:30:24","slug":"ready-for-launch-the-promise-of-jwst","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2021\/10\/21\/ready-for-launch-the-promise-of-jwst\/","title":{"rendered":"Ready for launch: The promise of JWST"},"content":{"rendered":"\n

On the eve of its long-awaited launch, Govert Schilling reports on astronomers\u2019 expectations for the James Webb Space Telescope<\/p>\n\n

\"\"
JWST will enable scientists to see the stars that formed soon after the Big Bang and to search for traces of oxygen around exoplanets<\/figcaption><\/figure><\/div>\n\n

Finally, it\u2019s 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\u2019s 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. \u201cJWST has no competition,\u201d says long-time project scientist John Mather at NASA\u2019s Goddard Space Flight Center. \u201cThere is no other way to see what it can see.\u201d<\/p>\n\n

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.<\/p>\n\n

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.<\/span> \u201cThe science that emerged from the Hubble Space Telescope vastly outdid what we expected,\u201d says cosmologist Richard Ellis of University College London. \u201cI\u2019m confident that JWST will do the same.\u201d<\/p>\n\n

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In addition to its larger mirror and greater distance from Earth, JWST will be concentrating its observations on a different part of the spectrum to Hubble: the infrared<\/figcaption><\/figure><\/div>\n\n
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JWST\u2019s mirror provides over four times the resolution of Hubble, as these simulated infrared images of a quasar and its host galaxy \u2013 taken by JWST (top) and Hubble (bottom) \u2013 reveal<\/figcaption><\/figure><\/div>\n\n
Seeing infrared<\/strong><\/h5>\n\n

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\u2019t 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.<\/p>\n\n

There\u2019s 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\u2019s Spitzer Space Telescope, JWST is about a hundred times more sensitive and can also distinguish much finer detail in stellar spectra.<\/p>\n\n

Moreover, as astrochemist Ewine van Dishoeck (from Leiden Observatory in The Netherlands) explains, most complex molecules in the Universe leave their tell-tale \u2018fingerprints\u2019 in the infrared part of the spectrum. \u201cI\u2019m involved in a dozen JWST projects,\u201d she says, \u201cand I look forward to measuring the chemical make-up of the material \u2013 gases, ices, and silicates \u2013 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.\u201d<\/p>\n\n

According to van Dishoeck, the JWST will also study protoplanetary discs in exquisite detail. \u201cALMA [the Atacama Large Millimeter-submillimeter Array in Chile] can study the outer parts of these discs,\u201d she says, \u201cbut JWST will observe the inner parts, where most of the planets are born.\u201d Meanwhile, exoplanet researchers like Sara Seager, from the Massachusetts Institute of Technology, hope that the JWST\u2019s infrared instruments will be sensitive enough to detect oxygen and other so-called biomarkers in the atmospheres of planets orbiting neighbouring stars. \u201cThe most exciting discovery by JWST would be biosignatures in a rocky exoplanet atmosphere, but we have to get lucky,\u201d she says.<\/span><\/p>\n\n

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JWST will look at the inner parts of protoplanetary discs where planets are born<\/figcaption><\/figure>\n\n
Cosmic dawn<\/strong><\/h5>\n\n

Another key science goal of the new space telescope is to search for light from the first stars that formed after the Big Bang \u2013 \u2018witnessing cosmic dawn\u2019, as it has been described \u2013 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. \u201cThe redshift range [probed by JWST] very much represents the last missing piece in the jigsaw of cosmic evolution,\u201d says Ellis.<\/span><\/p>\n\n

According to the most recent estimates, the first galaxies emerged some 250 to 350 million years after the Big Bang, and Hubble can\u2019t see them because its instruments cannot probe beyond a wavelength of 1.6 micrometres. \u201cMy group did well in securing JWST time,\u201d says Ellis. \u201cWe 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.\u201d<\/span><\/p>\n\n

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\u2019s unsurpassed infrared capabilities. At infrared wavelengths, JWST can look below the clouds to see what is happening. \u201cThis 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,\u201d explains planetary scientist Naomi Rowe-Gurney from the University of Leicester. <\/span><\/p>\n\n

\"\"<\/figure><\/div>\n\n

Rowe-Gurney is involved in the first JWST observations of Uranus and Neptune that are scheduled for some time in 2022. \u201cBecause we haven\u2019t visited either planet since the Voyager 2 flybys in the late 1980s, we have a lot of unanswered questions,\u201d she says. \u201cAny 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!\u201d 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\u2019s orbit.<\/span><\/p>\n\n

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\u2019s 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)<\/span>\u2013a 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\u2019s Fine Guidance Sensor (FGS) \u2013 used for pointing and stabilising the space telescope \u2013 and the Near InfraRed Imager and Slitless Spectrograph (NIRISS).<\/p>\n\n

Into the cold<\/strong><\/h5>\n\n

To keep the instruments at a temperature of at most 50\u02da 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 \u2013 where the gravitational forces of the Sun and Earth exactly balance the centrifugal force \u2013 which is located 1.5 million kilometres behind Earth, as seen from the Sun.<\/span><\/p>\n\n

  • \"\"
    An image showing the sunshield after it had completed its critical testing. The sides of the primary mirror are folded into its launch configuration<\/figcaption><\/figure><\/li>
  • \"\"
    Technicians examine the JWST’s colossal 6.5m primary mirror<\/figcaption><\/figure><\/li>
  • \"\"
    Practising \u2018snow cleaning\u2019 on a test mirror using carbon dioxide \u2018snow\u2019<\/figcaption><\/figure><\/li>
  • \"\"
    Having completed tests, the JWST is packaged up for shipment to its launch site<\/figcaption><\/figure><\/li><\/ul>\n\n

    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.<\/span> Project scientist John Mather says this is the most critical part of the mission. \u201cWithout 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.\u201d<\/span><\/p>\n\n

    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 \u2018predecessor\u2019, the Hubble Space Telescope, outlives it \u2013 provided Hubble doesn\u2019t experience any major technological issues or breakdowns over the next decade.<\/span><\/p>\n\n

    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. \u201cIn my opinion all are worth building,\u201d says Mather. \u201cIt\u2019s only a question of how and when.\u201d<\/p>\n\n

    <\/div>
    \n

    Building the most expensive telescope ever<\/h4>\n\n\n\n

    What does it take to construct a telescope like JWST, and why have there been delays?<\/strong><\/p>\n\n\n\n

    \"\"
    The JWST\u2019s gigantic sunshield has been one of the most difficult elements to develop<\/figcaption><\/figure><\/div>\n\n\n\n

    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. \u201cWe were over-optimistic about schedule and budget,\u201d says project scientist John Mather of NASA\u2019s Goddard Space Flight Center, which manages the development of JWST.<\/p>\n\n\n\n

    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 \u2013 in particular with the development of the vulnerable sunshield \u2013 prime contractor Northrop Grumman eventually delivered the telescope in 2016, after which a long period of testing began.<\/p>\n\n\n\n

    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\u2019s total costs were just short of $10bn.<\/p>\n<\/div><\/section>\n\n

    \n
    \n
    \"\"<\/figure><\/div>\n<\/div>\n\n\n\n
    \n

    <\/p>\n\n\n\n

    Govert Schilling is an astronomy writer, science journalist and author of
    Ripples in Spacetime<\/em><\/p>\n<\/div>\n<\/div>\n\n

    Photos: NASA GSFC\/CIL\/Adriana Manrique Gutierrez, NASA\/J. Olmsted (STScI), M. Marshall (University of Melbourne), Stocktrek Images, Inc\/ Alamy Stock Photo, NASA\/Chris Gunn, NASA GSFC\/CIL\/Adriana Manrique Gutierrez, NASA\/Chris Gunn<\/p>\n","protected":false},"excerpt":{"rendered":"

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