A new cosmic dawn: peering across universe with NASA’s James Webb Space Telescope
07 Jan 2022
Taken from the January 2022 issue of Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.
With NASA’s James Webb Space Telescope finally launched, Keith Cooper explores the mission’s troubled past, its technological advances and the exciting future ahead for astronomy
The original deep-field image taken by the Hubble Space Telescope is one of the most iconic images in astronomy. Consisting of a mind-boggling number of distant galaxies set against a blanket of black, the picture is constructed via a series of observations that Hubble made in December 1995 of a small region in the constellation Ursa Major. Inspired by this timeless image, astronomers began planning a new mission to study the early universe – one that would see even further back in time, to 300 million years after the Big Bang when some of the first galaxies existed. But to do so required the biggest observatory ever to be conceived, one much larger than Hubble’s 2.4 m mirror. The answer: the Next Generation Space Telescope (NGST) – a huge spacecraft with a 6.5 m segmented primary mirror that promised a whole raft of new discoveries.
Excited by the NGST’s potential, US astronomers soon selected it as top priority for space-based missions in the 2000 Decadal Survey – a wishlist of future projects compiled by the US National Academies of Sciences, Engineering, and Medicine. Pegged for launch in 2007 at a cost of $1bn, in 2002 it was renamed the James Webb Space Telescope (JWST) after the former NASA administrator. Yet those dreams of a new telescope to study the evolution of galaxies and how stars and planets form quickly turned into a nightmare. The project’s budget spiralled so much that in 2011 the US House of Representatives moved to cancel it entirely, only for the troubled project to receive an eleventh-hour reprieve after scientists, the public and the media rallied to save it. As recently as 2018, when the cost was about to break the $8bn barrier, US Congress had to vote to provide it with more funds.
The JWST is now estimated to cost $9.7bn and part of the reason behind those ballooning costs was building a telescope of its enormous size. At 6.5 m, the JWST’s segmented primary mirror will be the largest ever sent into space. “When we started, we knew that we could more safely build a much smaller telescope,” says John Mather, the Nobel-prize-winning cosmologist who has been leading the project at NASA’s Goddard Space Flight Center since the mid-1990s. The problem, though, was that smaller wouldn’t do to carry out the transformative science that astronomers wanted.
The problem, though, was that smaller wouldn’t do to carry out the transformative science that astronomers wanted
And size was not the only tough requirement. The light from those early galaxies has been stretched by cosmic expansion. To see them, astronomers need a scope that can peer into near- and mid-infrared wavelengths. And to do that, the telescope needs to be stationed away from Earth’s thermal glow, around the L2 Lagrange point, with the Moon and Earth behind it. “Another thing is that to reach the infrared sensitivity that we need, the telescope has to be very cold,” Mather says. “Pretty soon you have a heck of a lot of hard technical challenges.”
Two decades of developmental hell have now passed and the JWST is finally ready to make its grand entrance. For most of the astronomy community, the spacecraft could not get off the ground soon enough. The scope was finally launched on 25 December 2021, at 7:20 a.m. EST from Europe’s Spaceport in French Guiana, South America – a much-awaited Christmas present for astronomers everywhere. Yet even once the payload was hurled skyward atop a column of fire aboard an Ariane 5 rocket, mission scientists did not immediately exhale in relief. While launch is usually the most dangerous part of a space mission, that is not the case with the JWST.
For NASA’s latest and most expensive eye in the sky, launch was the simple part. That is because before it can push the envelope of what science and space technology can achieve, it had to first overcome a dangerous deep-space unpacking: its 6.5 m primary mirror had to carefully unfold correctly and its tennis-court-sized sunshield needed unfurl – a key step of this task was perfectly achieved on 28 December. During this deployment period – which is still ongoing and will last a total of about 30 days to complete – if any of more than 300 things that could go wrong, do go wrong, the telescope’s capabilities will be limited at best. In the worst case scenario, the entire instrument could have been ruined, setting the field back decades. As it seemingly succeeds without a hitch, it should transform astronomy – all thanks to several technological marvels that make the JWST a unique and powerful instrument.
Layered protection: A test model of the giant sunshield that will keep the JWST cool. (Courtesy: NASA/Chris Gunn)
Layered protection: A test model of the giant sunshield that will keep the JWST cool. (Courtesy: NASA/Chris Gunn)Mirror symmetry
One of the biggest challenges facing engineers in building the JWST was the mission’s 6.5 m primary mirror. Constructing a mirror that size isn’t a problem per se, but it is an issue to fit it inside an Ariane 5 rocket that is just 4.57 m wide, without being too heavy to launch into space. The task of solving this particular issue fell to Lee Feinberg, the optical telescope element manager at NASA Goddard. “The primary mirror has a very elegant design,” says Feinberg. The essence of that design, he explains, is that the mirror is foldable: it comprises 18 hexagonal segments, with three of the segments on either side forming “wings” that fold out.
The JWST also has a secondary mirror 0.74 m across, plus a smaller tertiary mirror to remove the scope’s astigmatism and flatten the focal plane ready for its scientific instruments. Together, these three mirrors make up an arrangement known as an “off-axis three-mirror anastigmat” that corrects for spherical, coma and astigmatism errors, while providing the instrument with a larger field of view. But these capabilities contribute launch headaches of their own. “The real trick is that the booms holding the secondary mirror are 8 m long, so you also have to fit that inside the rocket,” Feinberg notes. “And then there’s the sunshield. So we had to fold up for all those reasons.”
The mirrors are made from a new type of gold-plated, optical-grade beryllium that remains stable at the telescope’s operating temperature of 36 K. This material, known as O-30, was created especially for the JWST by the materials firm Materion, and its advantages include a low mass and good technical performance at cryogenic temperatures. The material’s stiffness, for example, means that when the mirror is plunged into the freezing cold behind the telescope’s sunshield, it does not distort too much. Given all these factors, and the size of beryllium billets that were considered reasonable to work with, making a hexagonal segmented system that could fold up was “the best option”, Feinberg says.
A similar hexagonal system has operated on the twin 10 m Keck telescopes in Hawaii since the 1990s, and Feinberg acknowledges that his team learned a lot from Keck’s optical design. As at Keck, all 18 segments of the JWST’s primary mirror, as well as its secondary, have robotic actuators to nudge them into focus. However, while Keck has sophisticated wavefront sensors to align its segments, the JWST optical team decided that such a system would be too complex to operate autonomously in deep space. Instead, the telescope will use its science camera. “The first test image that we’ll get will actually be 18 separate stars, because of the 18 separate segments each acting like a telescope,” Feinberg explains. The telescope will then use its camera and a specially developed algorithm called “phase retrieval” to measure the shape of the wavefront and adjust the shape of the primary mirror until all 18 segments focus as one.
The design of the telescope’s mirrors is typical of the technology and techniques that had to be pioneered to make the JWST workable. “Across the board, we felt that for everything we did, there was no playbook,” Feinberg says. “We were really changing the way we were doing things.” A case in point: while the 2.4 m mirror on Hubble is contained within what is effectively a giant telescope tube assembly, the JWST’s mirrors are open to space. And protecting these mirrors from the heat and glare of the Sun is a tough challenge.
Transforming science – offering new views of other worlds
Delays to the James Webb Space Telescope (JWST) may have been a headache for NASA administrators, but they offered astronomers the chance to change the telescope’s mission beyond recognition. In the mid-1990s exoplanet science was in its infancy but during the JWST’s lengthy sojourn in development hell, NASA’s Kepler and TESS spacecraft, among others, got on with the business of discovering new planets outside our solar system. “People were just starting to make the first observations of [exoplanets], so we asked, how can we use [the JWST] to see them?” says John Mather from NASA’s Goddard Space Flight Center. “We now have our list of transiting exoplanets, and we wouldn’t have had that list even a few years ago – that’s a pretty clear benefit of being late.”
The JWST’s main exoplanet task will be to probe the atmospheres of these distant worlds as they pass between the telescope and their parent stars. During these transit events, the atoms and molecules in the exoplanet’s atmosphere will absorb some of the light from the parent star, creating telltale gaps in the star’s spectrum that the JWST can detect. And thanks to that painstakingly compiled target list, Mather says, they won’t have to be lucky to spot one. “We now know where to look for transiting exoplanets, and exactly when to look,” he adds.
Today we know of thousands of worlds beyond our solar system, and – all being well – the JWST will be in prime position to study their atmospheres. Indeed, nearly a third – 70 out of 286 – of the science proposals chosen for cycle 1 of JWST operations are related to exoplanets. One of the worlds that will fall under the JWST’s gaze during its first cycle of science observations is the hot, rocky planet LHS 3844b, which could harbour the first known volcanoes outside our solar system. Then there’s 55 Cancri e, a super-Earth with “weather” that may include lava raining from the sky, and HD 189733b – a hot, Jupiter-like planet that may have clouds and rain made from vaporized minerals. Astronomers are also keen to study a relatively young exoplanet, CT CHa b, which may be surrounded by a disc of gas and dust that is gradually accreting onto its surface.
A further target is the TRAPPIST-1 system, which consists of seven terrestrial exoplanets orbiting a red dwarf star 40 light-years away. Astronomers are planning to use the JWST to scrutinize this system in several ways, including a general reconnaissance of all seven worlds and two sets of observations on its third planet, TRAPPIST-1c, which may be in the system’s so-called habitable zone, where conditions may allow water to exist as a liquid on or near the surface. With many of these transiting, habitable-zone planets, the JWST will be looking for biomarkers – the absorption signatures of oxygen, water, carbon dioxide, ozone, methane and indeed anything else that could be produced by living creatures or indicate a potentially life-supporting environment. But the telescope’s exoplanet studies will go deeper, too. Using its infrared instruments, the JWST will peer into the dusty confines of star-forming nebulae and circumstellar discs around young stars, collecting data that should give scientists a better idea of how new planetary systems form.
from physicsworld.com 7/1/2022
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