December 7, 2021

A new eye on the heavens

IT IS A majestic beast. Its primary mirror, a tessellation of golden hexagons, resembles a honeycomb sitting on a pile of silver-paper wrappers. But the mirror is six and a half metres across and the wrappers, each as big as a tennis court, are actually a sunshield. This shield divides the craft into a cold side and a hot side. On the cold side sit the primary, a tripod-mounted secondary that reflects light gathered by the primary back through a hole in its centre, and a pack of instruments behind that hole to parse and analyse the incoming light. The hot side carries a solar panel and the craft’s control systems. And all these things must fit into a rocket fairing a mere five metres across and then unfold in space into the shape above with nanometre precision.

The beast in question is the James Webb Space Telescope, named after NASA’s boss in the glory days of the 1960s, when the destination was the Moon and money was no object. The mirrors’ surfaces, made from gold-plated beryllium, are so smooth that, scaled up to the size of America, their irregularities would be mere centimetres high. Were the sunshield bottled and sold in Boots, it would have a protection factor of a million. One of the instruments behind the primary is so complicated that it has 250,000 tiny shutters, individually controlled to ensure that it is illuminated only by the correct narrow slice of the sky. No uncrewed science mission so sophisticated has previously been sent into orbit.

Tangled Webb
And none so late and so far over budget, either. The project that would become the JWST, as it is known for short, was first mooted in the 1990s. But it was not until 2001, when a survey conducted each decade by America’s National Academies of Sciences, Engineering and Medicine marked it as a priority, that work began in earnest. To provide many of the instruments, the launch vehicle and the launch site (the spaceport at Kourou, in French Guiana), the space agencies of Europe and Canada were quickly recruited, and in 2002 a launch date eight years away was set. After those years had passed, though, the launch was still six years away. And now, five years further on from that new date, the device remains on the ground. The price, meanwhile, has risen from $500m in 1996, when the idea was first proposed, to $9.7bn. And every day’s delay adds a million dollars to the bill.

Those delays, moreover, continue. The most recent scheduled launch date is December 22nd. That is a postponement, announced on November 22nd, from December 18th. But at least the thing is now at the launch site. Eventually, it must take off.

The Hubble Space Telescope, perhaps the JWST’s most famous predecessor, scans the universe mainly in the part of the spectrum visible to human eyes, making only a small foray into the infrared. But besides having a primary more than seven times as big as the Hubble’s, the JWST also has sensors optimised specifically for these longer wavelengths. The fact that the project was not simply cancelled at some point in its chequered career is testament to the unprecedented scientific potential which this infrared capability grants. It will enable the device to scry three crucial types of objects more effectively than the Hubble can: the old, the cold and the dusty.

The old objects are those from so long ago that their ultraviolet and optical-frequency emissions have been stretched by the expansion of the universe into the infrared’s longer wavelengths. A more mundane version of this phenomenon is the Doppler effect, noticeable when the pitch of a siren drops as the vehicle blaring it goes by. The cosmic equivalent is “redshift”, and becomes more pronounced the further back you go. The JWST will be sensitive to redshifted photons dating from 13.5bn years ago, a mere 300m years after the universe’s birth, and thus old enough to have come from the first generation of stars and galaxies.

The cold objects are those with temperatures as low as 100° above absolute zero (100K). All bodies in the universe radiate electromagnetic waves in a spectrum related to their temperature. For hot objects like stars this peaks in the wavelengths of visible light. In the case of cold bodies, though, it peaks in the infrared. Objects this cold include many of the ingredients of newly formed planetary systems, such as Earth-sized planets far from their parent stars.

The dusty objects are those hidden in so-called “dark” nebulae. These nebulae are composed of grains of matter that absorb visible light. But the effect of their dust on the infrared is less pronounced, meaning that the JWST will be able to scan their interiors in unprecedented detail, witnessing the nurseries of young stars which some of them contain, and the clouds of matter falling into the black holes found at the centres of others.

Pillars of creation
Accomplishing all this requires precision instruments. The many-shuttered device, called the Near-Infrared Spectrograph, is one. It will split incoming light into its constituent wavelengths to reveal information about its source. There is also the Near-Infrared Camera, which will generate most of the images that will, no doubt, adorn coffee cups and screensavers in years to come. The Fine Guidance Sensor/Near Infrared Imager and Slitless Spectrograph will provide reference points used to stabilise the telescope, and will study its brightest targets, including stars orbited by exoplanets. And the Mid-Infrared Instrument will look at wavelengths from five to 28 microns instead of the 0.6 to five microns to which the Near-Infrared devices are sensitive. This will let it see the oldest, coldest and dustiest objects of all.

The Mid-Infrared Instrument, a latecomer to the party, has proved particularly problematic. Capturing such wavelengths needs a detector even colder than the others. The sunshield ensures they are cooled below 40K, but the Mid-Infrared Instrument must be a far chillier 7K. It is therefore fitted with a helium-based “cryocooler” that takes it down to this temperature. And that has caused trouble.

Inventing the cryocooler proved expensive. In 2003, in an attempt to cut costs, NASA suggested that the Mid-Infrared Instrument be amputated to keep the rest of the telescope alive. After much petitioning by project scientists, it survived—and is indeed now regarded by astronomers as the most exciting of the JWST’s instruments. It has nevertheless continued to cause delays, and to eat up more than its fair share of the time and budget reserves held back for emergencies. By 2014, leaks in the cooler’s valves had swallowed up 18 of an allowance of 26 months of contingency time. And in 2015, 50% of the financial reserve went on staffing costs associated with cryocooler maintenance.

When the JWST does start sending back images, among the most eagerly awaited will be snapshots of the first few hundred million years after the universe was born. These may reveal how the first galaxies emerged from a flat and shapeless cosmos, as places with slightly higher densities of matter attracted other matter towards them, to form ever larger agglomerations. These galaxies will have hosted the first generation of stars, formed from hydrogen and helium created in the Big Bang. Those stars would then have become factories for heavier elements, formed by fusion in their cores and distributed by supernova explosions that ended the lives of many of them—though a few may survive, hiding in plain sight among their younger brethren in the galaxies of nowadays. Indeed, the JWST might find some.

A second stellar generation would have repeated this, producing yet-heavier elements as it did so and paving the way for a third generation, of which the sun (at 4.55bn years, almost exactly a third of the age of the universe) is part.

Stars of the first generation are thought likely to have had a huge impact on the rest of the cosmos by emitting radiation that blasted apart its primordial hydrogen atoms to create a plasma of electrons and protons. Unblasted, those atoms would have interfered with the passage of light. So the advent of stars made the universe transparent. This is the epoch of “reionisation”, sometimes called the cosmic dawn. The JWST should not only make this dawn visible, but also help understanding of the details of how it happened.

The new telescope will also peer into planetary origins. When stars form, they are surrounded by discs of leftover material that condense into planets and smaller bodies, such as asteroids and comets. The farther this material is from the star, the cooler it will be. And the frequencies the JWST will best detect tally nicely with those emitted by dust grains in the bits of the disc that are optimal for planet-forming.

Observations of these regions will put the Solar System in context. One problem faced by planetary scientists is information imbalance. Despite the discovery in recent decades of thousands of exoplanetary systems, these are known about in nothing like as much detail as the Solar System is. Yet generalising from this single, well-studied example is hazardous. For example, based on pre-existing assumptions about the Solar System no one expected to find gas giants akin to Jupiter and Saturn orbiting close to their parent stars. But that is actually quite common.
The JWST will look for exoplanets in two ways. It will take pictures of some of them directly—blocking out the parent star’s light with a special mask in order to do so. Others it will watch as they pass in transit in front of their parent stars, thus diminishing those stars’ apparent brightness.

The transit approach may be particularly fruitful. It will permit study of the composition of a planet’s atmosphere by analysing the light which travels through it during a transit. That will let people look for molecules of interest, such as methane, water and carbon dioxide. They will also be able to identify unusual weather patterns: sapphire clouds, for example, or iron rain.
Closer to home, the JWST will also be able to study objects in the Solar System itself, though only those from Mars outward, for looking at anything closer in would risk the device absorbing too much solar heat. Within this range it will examine the asteroids, comets and dwarf planets which survive from the Solar System’s earliest days, and aim to identify the composition and surface appearance of planetary moons as-yet-unvisited by spacecraft.

Cosmology’s dark side
This is already quite a long “to do” list, and will certainly keep the telescope busy for a while. But some researchers have plans to add yet more items to it. One of the biggest questions in modern cosmology, for example, is the nature of dark energy. This is the name given to an unknown 68% of the universe’s content, which is somehow responsible for its accelerating expansion. The JWST may be able to reveal more about this substance’s mysterious workings.

A big problem with understanding dark energy is that the expansion rate can be measured in two ways. This would be good if the answers agreed, but they don’t.

One approach, used since the 1920s and gradually refined since then, measures the distances to far-off stars and stellar explosions of known absolute brightness (so-called “standard candles”) by comparing that brightness with their perceived brightness from Earth. It then calculates the expansion rate from the redshift of this light. The other method looks at the cosmic microwave background, a sky-filling signal from the early universe discovered in the 1960s. Measuring irregularities in this allows the expansion rate at the time it formed (380,000 years after the Big Bang) to be determined. The current rate can then be calculated by feeding that number into an equation based on cosmologists’ understanding of fundamental physics.

The JWST should help with the first method by measuring the distances of stars up to five times farther away than is currently possible. That may either resolve the puzzle, by bringing it into line with the second, or—if the discrepancy persists—force a rethink of the basic science.

Another mysterious entity that the JWST may help explain is dark matter. This substance is reckoned by most researchers to make up a further 27% of the cosmos (the familiar matter of atoms and so on constitutes less than 5% of the total). It, too, is elusive, for it interacts with familiar matter only via the force of gravity.

Dark matter is believed crucial to galaxy formation. Measurements suggest that most, if not all, galaxies are really just accumulations of dark matter with a sprinkling of stars scattered through them. Dark matter’s true nature, however, remains disputed. Some think it consists of black holes left over from the universe’s early days. Others favour a set of as-yet-undiscovered subatomic particles. And a few holdouts believe it does not actually exist, and that the effects ascribed to it can be accounted for by clever tweaks to Einstein’s equations of gravity. By observing those effects in greater detail, the JWST may help distinguish between these alternatives.

Dark matter was first hypothesised in the 1930s, to explain odd behaviour by stars in the Milky Way and by galaxies in clusters. A more recent conundrum concerns Fast Radio Bursts (FRBs). These are unexplained flashes of radiation, first spotted in 2006, that last for milliseconds and are detectable from billions of light-years away. Over time, more and more FRBs have been discovered, with different sets of idiosyncrasies, suggesting that no one type of source is responsible for them all. The bulk, however, are likely to come from extremely young stars still embedded in the dust clouds whence they sprung. By peering into these dusty areas the JWST could pinpoint their locations more accurately.

All this does, however, depend on the telescope successfully deploying itself. Unlike the Hubble, which circles Earth, the JWST will orbit a point known as L2 (see diagram). This spot, one of five so-called Lagrange points where Earth’s gravitational field balances that of the sun, creating a well of stability, is a popular destination for space telescopes. In contrast to other Lagrange points, it is perpetually in Earth’s shadow, guaranteeing darkness and coolth for a telescope’s optimal operation.

Christmas unwrapping
Once it arrives at L2 the JWST will undergo a six-month process of unfolding and calibration, to prepare it for action. With over 300 mission-critical components, the failure of any one of which would doom the enterprise, ground-testing was a priority. Which was just as well, for the tests found many faults. Fasteners fell off the sunshield while it was shaken. The wrong solvent was used to clean the thrusters. Actuators did not fire as planned. And 501 bolts were deficient. All told, these problems cost nearly a billion dollars to fix, and delayed things by two and a half years.
Northrop Grumman, the project’s principal contractor, did not come out of all this covered in glory. Regular reports from the Government Accountability Office, set up by Congress to keep an eye on public spending, identified problems with workmanship and data collection.

In March 2018 NASA commissioned an independent review board to inspect the project. This recommended that Northrop Grumman review its internal processes. In July of that year, the firm’s chairman and chief executive agreed to take no profits from the project until the telescope was operational. For all the flaws their report identified, though, the board declared that the JWST “must now be completed within the principle that mission success is the top priority and is more important than cost and schedule”. The latest National Academies’ decadal survey, released on November 4th, thus recommends that in future targets should be set that would lead to a project’s automatic cancellation if not met. It emphasises, too, the importance of small- and medium-scale missions, which have been overshadowed by the likes of the delayed space telescope.

This survey is also the first to examine American astronomy’s culture. It emphasises the need to garner talent beyond the mainly white, male scientists who founded and still dominate the field (though many women have also made fundamental contributions, including Henrietta Leavitt, who established the idea of standard candles, and Vera Rubin, who provided evidence for dark matter). One small step for women will come in 2027, when the name of Nancy Grace Roman, NASA’s first chief of astronomy, will be emblazoned on the side of its next flagship telescope.

The worthiness of Webb himself for such an honour is also being reassessed. Eyebrows were raised in 2002, when his name was originally suggested, because the convention until then had been to call scientific spacecraft after scientists, not bureaucrats. More recently, aspects of his career, both before and during his time at NASA, have come under scrutiny. In the late 1940s he was an undersecretary in the Department of State, and as such is accused by some of having been involved in the Cold War “lavender scare”, an anti-gay witch-hunt that coincided with Joseph McCarthy’s anti-communist “red scare” activities, and which saw many State Department employees purged. Also, at least one NASA employee was dismissed on his watch for being gay. But Webb’s actual complicity in any of this is disputed, and, pending the conclusion of further investigations, NASA has rejected requests to rename the instrument.

The cloud of delay, meanwhile, has had silver linings. Scientific experiments inconceivable at the time of the telescope’s approval have subsequently been developed and integrated into the project. Exoplanet science, for example, was virtually non-existent in 1997. There is also a chance that the wait will permit the telescope’s lifetime to be extended. It will probably use up the thruster fuel needed to keep it on station within a decade. In the past, that would have been that. In light of advances in space technology, however, NASA has installed an arrangement which would let a service vessel dock and offer a top-up.

If all goes well on the 22nd—or whenever lift-off actually happens—and with the subsequent unwrapping and deployment, useful data should start returning to Earth some time in the middle of 2022. Regardless of the exact date, though, for the scientists involved it will be as if 20 years’ worth of Christmases have all come at once.