Pulling back the curtain on the Universe
When it is launched, the James Webb Space Telescope will be able to look further back in time than we have ever seen. Dan Falk reports.
I’m standing in the second-floor viewing gallery in Building 29 at NASA’s Goddard Space Flight Centre, just outside Washington DC. On the other side of the enormous plate-glass window is the facility’s giant “clean room”, one of the largest in the world.
On the floor below, a couple dozen scientists and engineers are buzzing about, weaving around cranes, ladders, miles of cables and one very large robotic arm. With their all-white protective suits and face masks, the workers look like little snowmen. The suits aren’t for their protection, but for the protection of the delicate equipment they’re handling – because the machine they’re assembling is one of the most ambitious and expensive telescopes ever conceived. If all goes well, it will be launched into space on top of an Ariane 5 rocket a little more than four years from now. Eventually, from its desolate home 1.5 million kilometres from Earth, it will send back images and data that will revolutionise our picture of the cosmos.
It’s been nearly 25 years since the launch of the Hubble Space Telescope, and the hardy instrument is still going strong. But Hubble won’t last forever. Astronomers have been planning a larger, more ambitious telescope since the mid-1990s. That telescope is finally beginning to take shape in Building 29. Originally dubbed the Next Generation Space Telescope, it was later renamed in honour of James E. Webb, the man who served as NASA administrator back in the days of the Apollo Moon missions. Not that this is a solely American project: the Webb telescope is too big, too complex and too costly for any one country to go it alone, and the European Space Agency and the Canadian Space Agency are both playing a significant role. In all, more than 1,000 scientists and engineers, from at least 17 countries, are working on the project.
The biggest difference between the Webb and Hubble is sheer size: Hubble has a single mirror a bit less than 2.5 metres across, while Webb will use an array of 18 hexagonal mirrors. Arranged honeycomb-style, they’ll function as a single mirror 6.5 metres across (that’s a bit wider than the cabin of a jumbo jet). True, the largest ground-based telescopes in use today are bigger, with mirrors about 10 metres across – but 6.5 metres is still enough to make Webb by far the largest telescope ever planned for space.
The other crucial difference between Webb and Hubble is that, while Hubble works primarily in visible light, Webb is designed to work in the infrared. This long wavelength light passes right through the dust and gas clouds that can obscure Hubble’s view – one of the reasons infrared is the best way to study phenomena from ancient galaxies at the edge of the visible universe to stellar nurseries where new solar systems are taking shape. “Hubble is wonderful, but not quite wonderful enough,” John Mather, senior project scientist for Webb, put it recently. “There’s stuff just beyond what Hubble can see, that we really want to be able to pursue.”
Webb is often described as a successor to Hubble – but since it’s designed to probe the infrared, it might more accurately be thought of as a successor to the Spitzer Space Telescope, an infrared space observatory launched in 2003. But again, size is of the essence. Spitzer’s main mirror, at 85 centimetres across, will be dwarfed by Webb’s 6.5-metre reflector. On the day of my visit, engineers were using the clean room’s robotic arm to manipulate Webb’s secondary mirror – or rather, the “flight-spare” secondary, an exact duplicate of the telescope’s secondary mirror – designed to collect light from the massive primary and direct it back toward the telescope’s detectors. My guide for the day was Mark Clampin, observatory project scientist for Webb, and a veteran of several previous projects including Hubble. We watched as the robotic arm slowly lifted the flight-spare secondary mirror for a series of tests.
“Draw a line from the Sun to the Earth and keep going for a million miles – that’s basically where it’s located.”
“Just to give you some idea of the scale, the secondary mirror up there is about 10 centimetres smaller than the Spitzer Space Telescope’s primary mirror,” Clampin says. “So that gives you an idea of how big this telescope is.” Below the secondary mirror, and partially hidden by the clean room’s massive steel scaffolding, I can see the flight-spare “backplane” – the carbon-composite structure that will hold the mirrors in place. The robotic arm, Clampin explains, will be used to put each of the mirrors into place on the backplane, one at a time. The flight-spares – exact copies of the components that will travel into space – are essential as back-ups, in case anything happens to the actual flight hardware; plus, there’s always a risk of parts being damaged during testing. The actual primary mirror segments are kept under wraps. They were manufactured at Ball Aerospace in Colorado and were shipped to Goddard more than a year ago – they’re kept in sealed, nitrogen-filled steel chambers (which look rather like giant pots for cooking spaghetti). Still, one only needs to click on the Webb telescope’s website to see what the fully assembled mirror will look like. Cast from lightweight beryllium and coated with a microscopically thin layer of gold, the hexagonal mirror segments will look spectacular when they’re eventually deployed in space – if anyone were around to see them.
While Hubble circles the Earth some 500 kilometres up, the James Webb Space Telescope is heading for the “L2 Lagrange point”, located 1.5 million kilometres out in space. Back in the 18th century – long before anyone had imagined sending a telescope into space – the French mathematician Joseph-Louis Lagrange was working on what physicists call the “three-body problem”: If you have a pair of massive bodies like the Earth and the Sun, with each body’s motion dictated solely by the force of gravity, would there be any stable locations where you could place a third body and have it stay there, without drifting away? Lagrange found that, yes, there are five such points, and L2 is one of them.
“If you think of the Sun, and draw a line from the Sun to the Earth, and keep going for a million miles – that’s basically where it’s located,” Clampin explains. “We picked that because it’s a point that has a quasi-stable gravitational field. It’s a great place to be, for doing astronomy.” Great, but lonely: L2 is about four times more distant than the Moon. Hubble was serviced by astronauts four times once in orbit, but Webb will not feel human hands after launch. Everything has to work perfectly the first time.
But working so far from home has its advantages: L2 is so far from Earth that our planet never blocks the Sun’s light. That means the telescope will effectively be in “daytime” 24/7, with no troublesome day-night fluctuations in temperature. Even so, Webb’s infrared detectors need to be protected from the Sun’s heat – a dazzling stream of infrared radiation that would swamp the faint signals the telescope is designed to detect. Even the telescope’s own heat needs to be carefully managed. The telescope will have, in effect, a “hot side” that faces the Sun, and a “cold side” that faces deep space. The hot side will house the telescope’s communications equipment and electronics, while the mirrors and delicate infrared detectors will be on the cold side.
Separating the two halves of the telescope will be another unique feature – a giant, diamond-shaped sunshield. Dwarfing even the giant primary mirror, the sunshield is the telescope’s largest component, spanning an area about the size of a tennis court. It’s composed of five parallel layers of ultra-thin plastic film with a reflective metallic coating (which goes by the trade name of Kapton). Once deployed, it will block the Sun’s heat while also radiating the telescope’s own heat out into space. This way the cold side of the telescope will be kept down to 40 Kelvin – about 230 degrees below zero on the Celsius scale.
A cold telescope makes for great infrared observing – but also for staggering engineering hurdles. “This is one of the challenges … that the telescope has to work at 40K, but we polish the mirrors at room temperature,” Clampin says. The fine-polishing of the mirrors has to be carried out in stages: at Goddard, engineers will work on the mirror surfaces until a precision of 100 nanometres is reached – that’s about one-thousandth of the thickness of a human hair. Then the mirrors will be sent to the Marshall Space Flight Centre in Alabama, where they’ll be cooled in a cryogenic chamber that mimics the conditions the telescope will experience in space – with engineers noting exactly how the mirror’s shape changes as the temperature drops. Then the mirrors return to Goddard for a final tweak. “That way, the next time we cool it down to 40K, we’ll have the right prescription,” Clampin says. When the mirror is finally sent into space, the largest irregularities on its surface will be no more than 20 nanometres in size. If the mirror were scaled up to be the width of the continental United States, those defects would be less than two centimetres high.
And cryogenic testing is only a part of the challenge. At Goddard, I gawked at the machines that have been pushing and pulling on the telescope’s various components, to ensure that each piece of equipment can survive the launch. After all, being launched into space inside an Ariane 5 rocket is a bit like being strapped to a giant firecracker. There’s a lot of shaking and rattling. Goddard also has a massive centrifuge that can whirl objects around until they feel a pull equivalent to 15 times that of gravity – more than enough to simulate the g-forces experienced at launch. But the launch also produces a lot of sound – which is why there’s also an acoustics chamber, to blast the telescope’s parts with high-intensity sound waves. Ray Lundquist, one of the lead engineers for Webb, explained the chamber can produce sounds up to about 150 decibels, though 100 to 115 decibels are typical. What if I were unlucky enough to be in the chamber when it was cranked up to that level? “You’d pass out,” Lundquist assures me.
“We may get as close as a few million years after the Big Bang.”
The real excitement will begin when the James Webb Space Telescope unfolds, origami-like, from its launch vehicle, and makes its way to the L2 Lagrange point. And then when it starts recording data and sending it back to planet Earth. Some of the data will be coming from the most distant matter in the visible Universe – structures that formed perhaps a few million years after the Big Bang. In this quest, Webb’s use of infrared wavelengths is key: because the Universe is expanding, the light from these distant objects has been stretched – in astronomical jargon, the light has been “redshifted”. (Think of an ambulance driving past you – as it speeds away, its siren seems to emit a lower pitch sound.) Because of this redshift, light that would have been emitted at visible wavelengths is now shifted well into the infrared – and is ripe for detection by Webb.
For astronomers such as Marcia Rieke, that ancient light holds the promise of new insight into the Universe’s turbulent early years. Rieke, based at the University of Arizona, grew up reading science fiction and pondering the possibility of visiting distant stars and planets. “I was good at science, so I sort of gravitated toward physics and astronomy,” she says. She’s now the principal investigator for Webb’s Near Infrared Camera, known as NIRCam. It’s one of Webb’s four main detectors, and has been carefully designed to snare the light from those ancient structures. Exactly how far we can push back the clock, so to speak, is hard to say; it depends on how rapidly matter in the early Universe condensed into the first stars and galaxies. “We may get as close as a few million years after the Big Bang,” Rieke says.
Our models of the early Universe – specifically, those first few million years – are a bit sketchy. We know that gravity was the great choreographer; under its pull, and in spite of the Big Bang’s initial outward push, matter attracted matter; clouds of gas and dust spawned the first stars; those stars came together to form primordial galaxies. “I’m hoping that when it comes to things like looking at how galaxies assemble, that we really will be able to see the full sweep of cosmic history,” Rieke says. “We’d like to see the very first galaxies.”
Distant galaxies make appealing targets, but there are equally enticing objects to focus on closer to home. Webb will also be looking at the birth of planetary systems around stars in our own galactic neighbourhood. These days, of course “exoplanets” are a booming business; the Kepler observatory, a space telescope launched in 2009, has already found more than 1,000 planets orbiting stars beyond our solar system. Webb won’t compete with Kepler; rather, the two will function as a team. While Webb may well discover some new planets, “its bigger strength is as a planet characterisation machine,” says Ray Jayawardhana, an astronomer at York University in Toronto, and the author of a popular book on the search for exoplanets, Strange New Worlds. Webb will be able to tell us more about some of the exoplanets Kepler has discovered.
Thanks to its exquisite resolution, Webb will be able to discern some exoplanets as distinct objects, separated from their parent stars – what astronomers aptly call “direct imaging”. (Most exoplanets found to date were discovered using indirect methods. Kepler, for example, infers the existence of exoplanets by watching as the light of the parent star is periodically dimmed, as a planet passes in front.)
Giant planets, roughly the size of Jupiter or Saturn, will be easier for Webb to pick out, because of their girth. Such planets emit a fair bit of heat, meaning they radiate strongly in the infrared – which is what the telescope is designed to detect. A handful of exoplanets have already been directly imaged using ground-based instruments but Webb, with its greater resolution, will be much better at spotting an exoplanet in spite of the overwhelming glare of the parent star. But that’s not all: by monitoring a planet carefully for many hours, the telescope should reveal any regular changes in brightness – the sort of pattern one might expect if some irregularity in a planet’s atmosphere were periodically coming into view. (We know that the giant planets in our own Solar System have such features – think of the Great Red Spot on Jupiter.) “You might actually learn something about the storms in the atmospheres of these directly-imaged Jovian planets – that would be very cool,” says Jayawardhana.
“In the exoplanet business we’ve learnt to expect the unexpected.”
Any information about the atmospheres of these distant worlds would be a goldmine for astronomers – especially for those pondering the question of life beyond our own blue-white orb. Webb’s spectrograph will split an exoplanet’s light into its component colours, allowing scientists to look for the chemical signatures of water vapour or carbon compounds in its atmosphere, explains Jayawardhana. Again, these planets are most likely to be larger than Earth; the smaller the planet, the closer it has to be for Webb to detect it, and so the smaller the area of space in which to hunt for them. But slightly larger planets may well turn up in abundance, their atmospheres prime targets for study. “And that’s a very exciting prospect, because some of these ‘super Earths’ may well be rocky planets, with atmospheres that at least in principle allow for habitability,” says Jayawardhana. “That’s probably the most exciting thing that we’re planning for.” He emphasised the element of surprise. “In the exoplanet business, we’ve learnt time and again to expect the unexpected.”
Webb is, first and foremost, a scientific instrument – but like Hubble it holds the promise of producing images that resonate far beyond the scientific community. They won’t have the same flavour as Hubble’s images, though; in infrared light, everything looks different. In fact, by collecting these longer wavelengths of light, the telescope will be able to look through the clouds of dust and gas that, in visible light, would obscure whatever might lie behind them. The telescope will, in effect, be “pulling back the curtain” to reveal new celestial vistas. Consider Hubble’s best-known image – the Eagle Nebula, known as the “Pillars of Creation”. Deep in the interior of the nebula, new stars – and perhaps new planets – are being born. “Webb will allow you to peer into these objects in much more detail,” says Mark Clampin, my guide at Goddard. “So if you think of the Eagle Nebula, Webb will be able to ... look inside the nursery, if you like.”
The James Webb Space Telescope is big science, and it inevitably comes with a big price tag – which has gotten even bigger over the years. Initially estimated to cost between $US1-2 billion, the latest estimates put the figure at around $9 billion. There are, of course, some equally expensive science projects out there – the Large Hadron Collider comes to mind – but it’s still a lot of cash. And it’s been a bumpy ride: in the spring of 2011, Congress moved to pull funding from the project, but NASA fought back, and by autumn of that year, the funding was restored. The project has also taken longer than planners had originally thought. The launch had first been planned for 2011; the new date became 2018.
Will the Webb be the last of the big-budget space observatories? Perhaps. The project is so large and complex, that it’s “right at the limit of what people can do,” says Rieke. “And obviously from a cost perspective, it really is right at the limit.” And yet, as Jayawardhana points out, it’s all relative. Should we choose to one day send astronauts to Mars, the expense would almost certainly be tallied in hundreds of billions of dollars. One often hears how much good could come from that sort of money if it were spent here on Earth. There are various responses to such objections, but an internet video-blogger named Hank Green has as pithy a reply as any: “There are two ways to make the world a better place,” he says. “You can decrease the suck, and you can increase the awesome.” The Webb is a perfect example of increasing the awesome, he argues – and many astronomers (although not all) would agree.
Back at the Goddard Space Flight Centre, there is still plenty of nuts-and-bolts work to be done. The assembly and testing of the components will continue for another four years. Eventually, the mirror and the main instrument package will be shipped to Northrop Grumman’s “Space Park” complex in Los Angeles, where the giant sunshield will be integrated with the rest of the telescope. Eventually the whole shebang will be folded up and packed on to a barge bound for French Guiana and the European Spaceport on its coast. Then its million-mile odyssey will begin.