Hubble has been a boon to deep space exploration, gifting us iconic pictures of the skies and revealing new insights into the history of the early universe. For the next big step in space astronomy, NASA, ESA and the Canadian Space Agency (CSA) are raising the stakes even higher with one of their most ambitious projects in decades: building the largest space telescope ever ... the James Webb Space Telescope.
The James Webb Space Telescope, JWST for short, will have seven times the light-collecting capability of Hubble, span the size of a tennis court, and be so sensitive it could spot a single firefly a million kilometers away.
This "absolutely impressive piece of engineering," as NASA administrator Charles Bolden put it, includes technologies that make this spacecraft unlike any other and will allow us to learn about Earth-like exoplanets, help us understand how life began on Earth, and image the cosmos as it was only millions of years after the Big Bang, further back in time than ever before.
Why send a telescope into space?
The contemplation of celestial things will make a man both speak and think more sublimely and magnificently when he descends to human affairs. – Marcus Tullius Cicero, c. 30 BCE
Space telescopes can be extremely expensive. Hubble’s total operating costs (including a Shuttle visit to repair its main mirror) have long passed the 10 billion US dollar mark, and similarly the budget for JWST, originally set at $2 billion, is now closer to nine after a bump-up of its mirror size. Projects such as these can not only have a meaningful scientific output, but also produce iconic images that can inspire a generation. But why go through the hassle of operating a telescope in space, when we could build much larger ones on the ground at a fraction of the cost?
One reason why Hubble’s images became such a powerful part of the collective imagery is that, during its first years of operation, no telescope on the ground could remotely compete with Hubble’s capabilities at imaging faint and distant celestial objects, due to the way our atmosphere distorts incoming light before it reaches the ground. But in recent years, the advent of adaptive optics – a technique that can correct for atmospheric distortions in real time – has meant ground telescopes have caught up in many respects. With that being the case, is there still a point to space telescopes?
"[Adaptive optics] is pretty good, not perfect," Physics Nobel laureate and JWST senior project scientist John Mather told Gizmag. "Instead of a star looking like a big blur an arcsecond across, it now looks like a smaller blur with a sharp core. We are finally getting some real discoveries made with this technique. It can show fainter stars and galaxies, and it can show better maps of extended objects. But there’s still a bit of haze around things."
Both NASA and ESA already have definite plans for building a series of very large ground telescopes in the near future, including the $1.4 billion aptly-named Thirty Meter Telescope and the $1.3 billion, 42-m (138-ft) European Extremely Large Telescope (E-ELT). The latter would see first light as soon as 2018, the same year the Webb is scheduled to launch.
These and other upcoming ground-based behemoths will employ the latest in adaptive optics to try and image celestial objects as clearly as possible. However, these telescopes were never designed to replace a space telescope. Rather, it is more likely that these giant ground telescopes will be used to find targets for a space telescope to study in more detail. In fact, despite having to make do with a much smaller mirror, space telescopes can often see more clearly than their ground-based counterparts, no matter their size.
"Space telescopes can observe over the entire spectrum because they’re not blocked by anything," said Mather. "Ground based telescopes are hindered by weather, molecular and atomic absorption of the incoming light, turbulence in the atmosphere distorting the images, and thermal emission from both telescope and atmosphere. All of the space telescopes we have built and are building are designed to do things that can never be done from the ground. It’s too hard and takes too long to bother with anything less in space astronomy."
One specific area in which space telescopes have excelled over ground telescopes has been the detection of over a thousand planets outside the solar system. Most of that has been the work of the 1.4 m (55 in) Kepler space telescope.
"Space telescopes can provide very stable observations that are needed for certain observations such as finding planets by the transiting technique," interdisciplinary scientist at the Space Telescope Science Institute Massimo Stiavelli told us. "This is why Kepler – a small telescope – has found so many transiting planets beyond what would be feasible from the ground."
At 6.5 meters (21 ft) in diameter, the JWST mirror will dwarf Kepler’s and is expected to image exoplanets both in significantly greater detail and from much farther away. For that reason, the likely scenario once the new telescope is up and running is that Kepler will be used to scout the most promising candidates for the JWST to study in greater depth.
The Hubble legacy
As 25 years have passed since Hubble saw first light, it’s perhaps hard to remember what was the state of the art in telescopy at the time, and the sheer impact (scientific and otherwise) that this historic spacecraft has had.
Due to the Doppler effect, the stars and galaxies that are moving away from us the fastest (representative of the very early universe) appear shifted to a lower frequency of light, toward the infrareds. Unfortunately, the Earth’s atmosphere also emits and absorbs light in the infrared, creating a lot of noise that makes observations much more tricky. Because of this, before Hubble, astronomers knew little about the early stages of the universe.
After it was placed in low Earth orbit by the Space Shuttle Discovery in April 1990 Hubble, unhindered by the atmosphere, was able to produce an impressive array of colorful and insightful images from every corner of the universe, from some of the earliest known galaxies to the discovery of new moons in our solar system.
But perhaps its greatest merit was to inspire people around the world with wonderful, iconic pictures that have become part of the collective imaginary.
"Many fields in astronomy require high precision photometry, often in crowded regions," said JWST project scientist Jason Kalirai. "This can include observations of star clusters, of the Milky Way disk, of the center of our galaxy, or even in other nearby galaxies. Today, the premier tool to do that in Astrophysics is still the Hubble Space Telescope. Even though it’s a 2.4 meter [8 ft] telescope, and we have much bigger telescopes on the ground, Hubble is able to measure the brightnesses of faint stars and galaxies to unprecedented accuracy. The stability from space based observations for photometry is unmatched by ground based telescopes."
One of Hubble’s main strengths is its versatility: while other telescopes can only image the sky in visible, infrared or ultraviolet light, Hubble can do all three, partly because its instruments have been upgraded multiple times over the course of its lifetime. For all of its great merits, not even the next-generation James Webb Space Telescope will be as versatile as Hubble.
JWST: The next step for deep space exploration
In 1994, four years after the launch of Hubble, scientists began to ask what the next big space telescope would look like. Two years later, a committee of US astronomers recommended that NASA develop a second, more powerful space telescope that would focus on observations in the infrared spectrum. The new telescope would be able to see through dust and gas clouds, sport a primary mirror of approximately 4 m (13 ft) and operate a million miles away from Earth. Plans for what was then the Next Generation Space Telescope had begun.
Perhaps due to the mishaps of Hubble, getting funding for the new telescope was not straightforward. The mission was under review for cancellation by the United States Congress after about $3 billion had already been spent. But in November 2011, the United States Congress decided to provide additional funding, for a total of $8 billion, to complete the project.
The funding, however, was to be for a bigger telescope than initially planned – not a 4-meter, but a 6.5 meter telescope. This seemed incredibly ambitious, because it wasn’t clear how such a big telescope could even fit inside a rocket. Construction began in 2004, but by that time the instrument array had been heavily altered to fit the new specifications, and the telescope had changed its name to the James Webb Space Telescope.
James Edwin Webb was appointed by J.F. Kennedy as the second administrator of NASA in 1961, a chair he would keep until 1968. As a businessman and politician, he had no scientific or engineering background and was initially reluctant to take the job. Once he accepted the post, however, his expertise proved instrumental in producing continued political support and financial resources vital to the success of the Apollo program.
After retiring from NASA, Webb served as regent of the Smithsonian Institution. He passed away on March 27, 1992, two years after Hubble was launched, and two before planning for a new space telescope had begun.
Although it will aim to wow and inspire just like Hubble did, this next-generation telescope won’t just be a Hubble 2.0, as it was designed with different objectives in mind. While Hubble covered a wide range of the electromagnetic spectrum (from the near-infrared to ultraviolet), Webb will focus on the infrared, looking through the dust clouds rather than at them.
"The expanding universe stretches out the wavelength of the starlight into the infrared region," Mather told us. "The farther away one looks, the farther back in time one is seeing, and the longer the wavelengths we receive. To see farther back we have to use longer wavelengths, and they can’t be seen from the ground."
A telescope’s resolution depends on the ratio of the size of its mirror to the wavelength that is being observed. Since infrared light has a bigger wavelength than visible or UV light, a larger mirror than usual is required. According to NASA, the large mirror of the JWST means its resolution will be three times greater than Hubble’s and eight times greater than Spitzer’s. And because it can observe at longer wavelengths than any other telescope, the JWST will be the first to get peek at the universe as it was just 100 million years after the Big Bang.
Science objectives: From first light to origins of life
NASA, ESA and CSA have settled on four main science goals for the JWST. These are all strictly interrelated and, when taken together, lead us on a history tour of our universe.
The first objective on the list is the observation of "first light," the appearance of the very first generation of stars in the Universe.
The very early stars were up to 1,000 times more massive than our Sun, tens of millions of times brighter, and burned for only a few million years before either bursting into a supernova or collapsing into a black hole. Black holes would swallow stars and gas to become first mini-quasars, and then grow even further to become the supermassive black holes that today we see at the center of nearly every galaxy. The supernovae, on the other hand, would be massive enough that the immense pressures in their cores would fuse atoms, producing the heavy elements we find on Earth and elsewhere.
Both the supernovae and the mini-quasars will be detectable by the JWST, and it is expected that their observation will teach us more about how the very earliest galaxies evolved.
The second science goal of the space telescope is investigation on how early galaxies formed and evolved.
We have observed galaxies as far back as one billion years after the Big Bang and seen that some of those very early galaxies are inexplicably already very similar to ours. Others are irregular, and others form in peculiar shapes.
This part of the investigation will focus on understanding what controls the shape of galaxies and their evolution, how they form stars, how elements are formed and then redistributed through the galaxies, and the influence of the supermassive black holes on their evolution.
Next in the history of our universe and in the list of goals for JWST is learning about the formation of individual stars and planetary systems.
We still don’t know exactly how stars are being formed from clouds of gas and dust, why they tend to form in clusters, and what factors influence the formation of planets alongside them. Equally, we don’t know how dying stars disperse metals and other heavy elements for other stars to recycle in a process that is ultimately responsible for the variety of elements we find on Earth.
The JWST will try to answer these questions and track new stars up to the birth of early planetary systems. Studying them might also tell us why planetary systems seem so common around cooler and less massive stars than our Sun, and this could help us understand where we should be looking when searching for signs of rocky, Earth-like planets that might host life.
At the time the JWST was first conceived, scientists had no knowledge of the existence of planets orbiting foreign stars. Now that Kepler and other telescopes have discovered over a thousand exoplanets, a fourth goal has been tagged on: the detection and study of more of these distant worlds, along with the attempt to understand how life might evolve on them through observation of our own solar system.
As luck would have it, such observations are very well-suited to an infrared telescope like the JWST. The goal here will be to understand how dust and small objects combine into large ones that would become planets, and to learn more about the objects like asteroids and comets that delivered the necessary chemical precursors to life to the oceans.
For this objective, the JWST will first be used to carry out an in-depth investigation of objects within our solar system like asteroids, comets, outer planets and their moons to understand more about their composition. These objects have analogous in other planetary systems, so they help tell the story of how the planetary system formed.
Using that information, we could try to pinpoint what stars are the best candidates for hosting an Earth-like, possibly habitable planet.
Mather told us that imaging an exoplanet directly is extremely tricky because stars are approximately 10 billion times brighter the planets orbiting in their proximity. A biosignature on an Earth-sized planet would consist of a specific molecule like oxygen or chlorophyll, but this is outside the capabilities of JWST, so we’ll have to wait for the next generation of space telescopes (which could have mirrors of 15 to 20 meters across).
For planets that are roughly the size of Jupiter and are relatively close to us, however, scientists believe that about a day’s worth of observation could be enough to spot signs of carbon and water, as well as macro irregularities in the planet’s atmosphere similar to Jupiter’s "Great Red Spot".
Building a space-worthy telescope
A machine as sophisticated as a space telescope must be built with astounding precision, and needs to navigate plenty of constraints to make sure it can withstand the rigors of launch and of life in deep space.
Whereas Hubble had a single glass mirror, the main mirror of the JWST consists of 18 extremely lightweight hexagonal segments made of beryllium, a material that can handle the extreme temperatures of space and deforms less than glass. The mirror segments are coated with a fine layer of 24-karat gold, used in infrared telescopes because it reflects red light extremely well (according to NASA, this makes the mirror 98 percent reflective, compared to an ordinary mirror which is only about 85 percent reflective).
The mirror must be shaped to a very high degree of precision. To avoid distortions, the greatest imperfection must be a tiny fraction of the wavelength of infrared light. As a result, the mirror must be polished to such a degree that if a single mirror segment was the size of the entire continental United States, the largest imperfection on its surface could be no bigger than a couple of inches.
To complicate things, beryllium, just like any other material, deforms slightly when cooled to the mirror’s operational temperature of 40 K (-233° C, -388° F). Of course, what really matters is the smoothness at cold temperature, but that doesn’t necessarily correlate with smoothness at room temperature. So, the technicians need to go through the very time-consuming process of polishing the mirror at room temperature, test for imperfections at the operational temperature, try to guess where they may have gone wrong, and then repeat the process as long as necessary.
Housed behind the mirror, separated from the warm communications and control technology by a large sunshield, are the space telescope's four main scientific instruments. These are the Near-Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI) being developed as a partnership between Europe and the USA, and the Fine Guidance Sensor/Near-Infrared Imager and Slitless Spectrograph (FGS/NIRISS) contributed by the Canadian Space Agency.
The microshutter array is another of the telescope’s big innovations. One array contains 62,000 shutters that can selectively block out the light from objects the telescope is not targeting, so that the target can be imaged more effectively. Future space telescopes might take this very idea much further, going as far as building an extravagant, sunflower-shaped giant sunshade to image exoplanets directly while blocking out light from their host star.
Another big challenge specific to the JWST is climate control. Warm objects emit large amounts of infrared light, which could be picked up by the telescope’s instruments. For that reason, all of the instrumentation must work at very cold temperatures of 40 K (so they don’t pick up their own heat). Also, a five-layered, tennis court-sized sunshield protects the telescope from the heat and radiation of both the Sun and the Earth, with a sun protection factor of one million.
The shield is very effective at keeping out heat and radiation, but in the vacuum of space photons from the Sun strike the sunshield and create a tiny but noticeable wave of pressure. Although this phenomenon can be used to propel spacecraft, here the pressure is unwanted and needs to be constantly dealt with, because it could push the telescope away from its delicate parking spot or, even worse, flip the telescope and expose the instrumentation to the Sun’s heat. To prevent this, the telescope is equipped with reaction wheels and fuel to keep it on course and in the correct orientation at all times.
Firing a multi-billion telescope into space
Building such a sophisticated machine is difficult enough, but of course NASA engineers also had to find a way to put it into space. That meant making a tennis court-sized telescope fit inside the fairing of a rocket, all while keeping weight to a minimum and making this high-precision instrument rugged enough to survive the rigors of a space launch.
To solve the weight constraints, NASA harnessed new technologies that were not available when Hubble was being built. The best example of weight reduction is the primary mirror: while Hubble had a thick solid glass mirror with a mass of 1,000 kg (2,200 lb), the beryllium mirror segments of the JWST, together with their supporting frame, weigh a total 625 kg (1,375 lb).
To solve the size constraints, NASA engineers had to go for an even more radical solution – they had to find a way to pack a 6.5-meter mirror telescope inside the 5-meter fairing of an Ariane 5 rocket, which they did by adding a few joints here and there.
In October 2018, NASA will launch the rocket carrying the James Webb Space Telescope and, half an hour after launch, the telescope will separate from the last stage of the rocket and begin a month-long journey to its final destination one million miles away from Earth.
On its way there, solar panels will unfold, as will its five-layer sunshield. The main mirror will deploy over the following two weeks, after which the spacecraft will wait for two more months to be cooled down to the operational temperature of 40 K by the deployed sunshade.
From the ground, as the JWST is reaching its destination, operators will go through a painstaking process of aligning all of the mirror segments. Each segment can be moved freely on three axes, tilting, twisting and shifting to face the correct direction and position. Also, a pressure pad in the center of each segment can be moved like a piston, allowing operators to warp the shape of the segment to perfectly match the others so segments can operate as a single mirror.
Unlike Hubble, the JWST won’t be able to be serviced by humans. With the number of constraints already in place on the telescope, the choice was made not to design the telescope’s instruments for easy access by either humans or robots. The number of sharp edges would carry the risk of cutting a spacesuit, not to mention that the astronauts would need to travel four times farther from Earth than ever before to even reach the telescope.
The final destination of the James Webb is an orbit around the L2 Lagrangian point. If you were to trace a line from the Sun to the Earth and go on for about another million miles, the end point would be the L2 Lagrangian point. This is one of five spots where a body – be it an asteroid or a spacecraft – can sit and maintain its position relative to both the Sun and the Earth, because their gravitational forces are balanced out.
Of these five Lagrange points, two (L4 and L5) are stable, making it easier for small bodies like asteroids to accumulate there. L2, by contrast, is only semistable: like balancing a ping pong ball on top of a roof, a slight nudge in any direction will quickly lead it astray. So the JWST will need thrusters to keep correcting its orbit.
"The L2 point was chosen because it’s the first point where you can go where the Earth and the Sun are always in the same direction, so you can put up [the giant sunshield] and protect the telescope," says Mather.
Once every 22 days, a reaction control system will make sure that the telescope stays on course without drifting away into space. The correction needed is only of a few meters per second each year, requiring very small thrusters that had to be positioned on the warm side of the spacecraft so that firing them wouldn’t interfere with the heat-sensitive instrumentation.
Months ago, the four main instruments underwent cryogenic testing, and a full-scale engineering model demonstrated the precise folding and unfolding of the sunshield. Later this year, the 18 primary mirror segments will be mounted into the backplane along with the secondary mirror and support struts. Then, between 2016 and 2017, the primary and secondary mirrors will be integrated with the aft mirrors and the spacecraft will be linked to the sunshield. If everything goes according to plan, the telescope will be shipped to Kourou, French Guiana for launch in October 2018.
Engineers expect the limiting factor on the lifetime of the telescope to be the amount of fuel needed to keep the spacecraft on its intended orbit. The onboard instrumentation has been designed to operate for a minimum of five and a half years, but the craft is carrying fuel for a minimum of ten years, meaning it could last until the early 2030s.
By then, the next-generation space telescope will hopefully be ready.
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