The launch of the James Webb Space Telescope on 25 December 2021

The launch of the James Webb Space Telescope on 25 December 2021

Credit: ESA/CNES/Arianespace

Commentary: Professor Bunker on the James Webb Space Telescope

Astronomy and astrophysics

Professor Andy Bunker has been involved with the James Webb Space Telescope since 2004 when he was named as one of seven scientists on the newly-formed Instrument Science Team for NIRSpec, overseen by the European Space Agency (ESA). Here he gives a potted history of the JWST and shares his excitement for a revolution in observational astronomy.  

On Christmas day 2021, a new era in astronomy started: the James Webb Space Telescope (JWST) was launched from French Guiana in South America. After a faultless launch on an Ariane 5 rocket, Webb left Earth with exactly the right velocity for its month-long journey, meaning that less of the precious propellant on board would be used than expected in reaching the final orbit around the Sun – which in turn translates to a longer science mission.

Webb orbits at the second Lagrange point (L2) in the Earth-Sun system, about a million miles from Earth. This stable orbit keeps the Sun, Earth and Moon on the same side of the telescope. With the deployment of a large sun shield (the size of a tennis court, but consisting of 5 layers the thickness of a human hair) the telescope optics are kept constantly in shadow. This reduces the background light, and increases the sensitivity, without the need for a big and heavy telescope tube, as was used on Webb's predecessor, the Hubble Space Telescope. Hubble has been doing excellent science for more than 30 years, and continues to do so, but Webb has a significantly bigger mirror than Hubble's 2.5m-diameter (which was set by the size of the Space Shuttle cargo bay used to deploy Hubble).

Optimised instruments

Webb's mirror is 6.5m, and segmented into 18 hexagons so that it could fold up to fit within the nose fairing of the Ariane 5 rocket. As well as significantly larger light-gathering power, Webb is in a much more thermally-stable and low-background environment at L2 compared with Hubble (which is in low Earth orbit, passing into sunlight every 90 minutes). With the low-background light conditions and cooler temperatures at L2, Webb has the sensitivity observe at much longer wavelengths than Hubble, and its instruments are optimised for this. Three of the four main instruments work in the near-infrared (1-5microns), and include a camera (NIRCam), a low-spectral-resolution slit-less spectrograph with a wide field of view (NIRISS), and a multi-object spectrograph (NIRSpec). A fourth instrument, MIRI, works at even longer wavelengths (out to 24 microns in the mid-infrared).

Having this large telescope in the low-background conditions at L2 means that Webb is 10-100 times more sensitive than Hubble, and opens up a new parameter space in observational astronomy. However, a mission out to L2 carries more risks than one in low-Earth orbit – if anything goes wrong, a repair mission cannot be launched, unlike Hubble which has repeatedly been upgraded and fixed by Space Shuttle visits. Everything had to work right first time for Webb, and there were some terrifying days in the weeks after when the telescope unfolded and the sun shield deployed. It was a great relief when these complex operations went smoothly, with the many actuators working as planned, and the telescope reached its final L2 orbit.

Ready for science

Since then, Webb has cooled down to its final operating temperature of 50K, and undergone 5 months of commissioning of the many different observing modes. The image quality looks excellent, with diffraction-limited performance at most wavelengths, and the sensitivity is in line with (and in some cases better than) that predicted before launch. Having had its mirror segments aligned and focussed, and all four of its instruments tested, Webb was declared ready for science operation at the start of July 2022. Early release observations have already generated great excitement, with beautiful images including a massive cluster gravitationally lensing the light from galaxies behind it into arcs. Spectroscopy of some of these lensed galaxies with Webb have enabled measurement of the temperature of the inter-stellar gas in very distant galaxies, showing this is much hotter than in more nearby galaxies, with many papers posted within days by different groups analysing this public data and trying to explain this early discovery.

A long history

The James Webb Space Telescope has had a long history, with early proposals for a ‘next generation space telescope’ to succeed Hubble dating back to 1996. My involvement with Webb started in 2004 when I became one of seven scientists on the newly-formed Instrument Science Team for NIRSpec, overseen by the European Space Agency (ESA). While the Webb mission is NASA-led, ESA is a major contributor, providing NIRSpec and half of the MIRI instrument along with the Ariane 5 launch vehicle. The Canadian Space Agency also contributes NIRISS and the Fine Guidance Sensor to the Webb mission.

The NIRSpec instrument is complex and novel. Spectrographs in astronomy often employ a slit or an optic fibre to increase sensitivity by cutting down the background light while allowing photons through from the target astronomical object. To build up statistics (for example, redshift surveys of galaxies) astronomers often want to observe many objects in the same field simultaneously, and such multi-object spectroscopy has been commonly used in ground-based observatories since the 1990s, where we can easily laser-cut slit masks for spectrographs to target fields of objects. However, until now from space we have been limited to single-object slit spectroscopy with Hubble, or much less sensitive ‘slit-less spectroscopy’. NIRSpec has four arrays of micro-shutters, and we can command different configurations of these 250,000 windows to open, enabling up to a few hundred spectra to be taken in the same field at once – a huge multiplex gain.

A new view

Webb is likely to reshape our understanding of many topics, including the study of exo-planets around other stars, and star formation in our own galaxy. My own field of research is finding very distant galaxies, seen when the universe was young, to understand their formation and evolution. By measuring the wavelengths of emission and absorption lines of various elements, we can determine the redshifts and distances – the ability of Webb to work at infrared wavelengths beyond Hubble's capability means we can push the high redshift frontier to earlier epochs.

Indeed, from just the first few images that Webb took, there are candidate distant galaxies (based on their colours from images in different filters) well beyond the current record of redshift 11; that means a look-back time of 13.3 billion years, seeing the Universe when it was less than 3% of its current age. These candidates will need spectroscopic confirmation of their redshifts (something NIRSpec can achieve), but it looks certain that Webb will extend our knowledge of the most distant galaxies to epochs within a few hundred million years of the Big Bang. An exciting prospect is to look for the first generation of stars that form from just the hydrogen and helium made in the minutes after the Big Bang – without the spectral signatures of heavier elements such as oxygen, nitrogen and carbon which are commonly seen in spectra of galaxies and which were made by nucleosynthesis within stars later on.

A revolution in observational astronomy

In October, the NIRSpec Instrument Science Team will be obtaining spectra of galaxies in the Hubble Ultra Deep Field, the most sensitive picture of the sky ever taken. My team was the first to analyse the Hubble images of this small patch of sky back in 2004, and now with Webb we have the ability to study the spectra of the faintest objects in these images, to measure their distances, the rate at which they are forming stars, and the amount of heavy elements which have been produced. We hope to answer questions on the birth and early history of galaxies which went on to evolve into galaxies such as our own Milky Way.

The coming months and years are likely to see a revolution in observational astronomy thanks to the unprecedented capabilities of the James Webb Space Telescope. Let's hope that the new parameter space opened up leads to many unexpected discoveries!