BBC Sky at Night Magazine
Explainer: The JWST
NASA’s successor to Hubble will reveal the early Universe in unprecedented detail
Building on Hubble’s legacy
Anew era of space observation is expected to begin in March 2021. Provided there are no further delays, Hubble’s heir, the long-awaited James Webb Space Telescope (JWST), should launch aboard an Ariane 5 rocket from French Guiana.
With its tennis court-sized sunshield and 6.5m primary mirror flat-packed inside the launcher like a ship in a bottle, the scope will separate from the rocket half an hour after take-off and deploy within a day.
Over the following 30 days it will travel over 1.5 million km to a gravitationally stable outpost called Lagrange 2 (L2). This lies on a straight line from the Sun to Earth and beyond, so that JWST will be locked into Earth’s yearly orbit around the Sun. This means that JWST – unlike Hubble, which orbits Earth, going in and out of our shadow every 90 minutes – will have unobstructed views of the Universe. However, the new telescope’s far-flung location also means that, unlike Hubble, it will be unserviceable.
JWST should start its science observations six months after launch. It will hover at L2 for the next 5.5 to 10 years, and scientists hope it will give us glimpses of the early Universe that we have never seen before. It will be one of the largest, most powerful scopes and should give us new insights into every phase of the
Universe’s history, from the first dust clouds to our Solar System’s formation. NASA, one of JWST’s collaborators along with the European Space Agency (ESA) and the Canadian Space Agency, is keen to point out that JWST is Hubble’s successor rather than a replacement. When Hubble was launched 30 years ago, it was the first space-based optical scope and has given us unprecedented views of the Universe. However, it looks at the optical, ultraviolet and near-infrared wavelength ranges. James Webb will look between visible red and mid-infrared light, peering much further back into the early Universe.
Light from the earliest luminous objects travels so far in an expanding Universe that by the time it reaches us,
its wavelengths have been stretched or ‘redshifted’. This means that the earliest Universe is observable only in the infrared part of the spectrum.
Peering into the past
JWST’s larger mirror will enable it to collect over six
times the light that Hubble can, with a field of view 15
times the area of Hubble’s near-infrared camera and spectrometer (NICMOS). Its primary aim is to probe the so-called ‘end of the dark ages’ after the Big Bang,
when the Universe began to fill with ‘first light’ from
newly ignited stars. It should be able to look back to 100–250 million years after the Big Bang.
But, like Hubble, it is also a general purpose
observatory and will specifically look at the birth and
assembly of galaxies, the effects of black holes and the origins of life. Scientists hope JWST will help us better understand the Universe’s size and geometry, throwing light on dark matter and dark energy, and helping us understand the ultimate fate of the Universe.
Its high resolution means that JWST could give better insights into the Milky Way and our neighbouring galaxies, “extending the work started
by Hubble outwards significantly”, according to ESA.
Similarly, its resolution will enable scientists to see how planetary systems form.
To do all this, JWST will have just four scientific
instruments: a near-infrared spectrograph (NIRSpec) which can observe 100 objects simultaneously; a near-infrared camera (NIRCam); a combined midinfrared camera and spectrograph (MIRI) which has a
cryocooler to keep its temperature at –266˚C; and a fine guidance system and wide-field imager (FGS/NIRISS)
that includes a mode for exoplanet spectroscopy.
There are already dome projects planned for JWST. One task will be to observe the atmospheres of potentially habitable, rocky exoplanets in the seven-planet system of TRAPPIST-1, 39 lightyears from Earth. This system was discovered by the Spitzer Space Telescope, which was retired in January. Like James Webb, it specialised in the infrared range, but JWST will be 1,000 times more powerful. Compared to Hubble, JWST is 100 times more powerful and will work alongside it in its final years.
Named after the NASA administrator who oversaw the start of the Apollo Program, JWST has been almost a quarter of a century in the making. Scientists hope it will remain on schedule for its current launch.