Since Galileo first discovered the moons of Jupiter and the phases of Venus, telescopes have gotten larger, more accurate, and more powerful. They're now installed all around the world from mountaintop observatories to suburban backyards. And over those 350 years, all of them have battled the same enemy: our Earth’s atmosphere.
The thin layer of nitrogen, oxygen, and carbon dioxide that makes life possible on our planet makes observation of everything beyond the planet maddeningly difficult. The atmosphere absorbs a great deal of the light outside the visible part of the spectrum, blocking or severely attenuating information about the cosmos. Its turbulent motions distort what does get through. It scatters our own light back down into our eyes and instruments, making the night more like a slightly darker day, washing out all but the brightest celestial objects in a haze of light pollution.
We can reduce this atmospheric obfuscation slightly by situating our observatories at high altitudes, far from population centers. But it is not enough. If only we could open a hole in our gaseous shroud and peer through it...
...but obviously, we can't. As such, researchers can do the next best thing. By 1990, we had years of experience putting machines into orbit around the Earth, outside of the troublesome atmosphere. Some of these machines were even telescopes of a sort, but they pointed toward the Earth in the service of intelligence agencies rather than science. Putting an astronomical telescope in space was a natural solution, and it followed directly—and, according to some, very directly—from the spy satellite program.
The Hubble Space Telescope was launched in that year, and it continues to return images even today. After a repair mission to correct its notorious optical defect, it has surpassed expectations, gifting us with inspiring images of galaxies, gas clouds, colorful nebulae, and planets in formation.
The Hubble takes advantage of its position 360 miles above the surface to gather information that would be absorbed by the atmosphere. It sees mainly in the visible part of the spectrum, extended slightly into the near infrared and ultraviolet. But there is much information about the Universe that is invisible even to the Hubble Space Telescope—and that's where NASA's much hyped, two-decades-in-the-making, $8.8 billion-plus James Webb Telescope comes in.
The Universe in infrared
Given its limitations, the Hubble is part of a system of four telescopes in space collectively known as NASA's Great Observatories Program. The other devices are Compton Gamma-Ray Observatory (now retired), the Chandra X-ray Observatory, and the Spitzer Space Telescope. The Spitzer sees in infrared light, the part of the electromagnetic spectrum commonly known as heat. The others observe the most energetic wavelengths of light.
We get information about our terrestrial environment in several forms via personal sensors that correspond to most of them: sound through our ears; pressure and temperature through our skin; gravity through the semicircular canals in our middle ears; small molecules through smell and taste receptors. Light is perceived through our eyes.
In space, no one can hear you scream, and there's precious little to taste or feel. We get some data from the flux of various particles, but most of what we know about the cosmos comes in the form of light. Some of this light falls in the visible part of the spectrum and forms the images brought to us by optical telescopes. These range in quality from nearly all amateur hardware to the orbiting Hubble.
But there are crucial insights hiding in other parts of the electromagnetic spectrum, as well. The collection of microwave data led to the analysis of the cosmic microwave background, verifying key elements of the Big Bang model. The search for forming planets and advanced extraterrestrial civilizations is carried out using radio astronomy.
The Compton Gamma-Ray Observatory made many unique observations, including the discovery of an antimatter fountain in the Milky Way. X-ray observation is a central part of astronomy, and the Chandra Observatory continues to advance this field. Past achievements include the discoveries of black holes, galactic winds, and enormous X-ray-producing jets hundreds of thousands of light years long.
The infrared part of the spectrum, just as the others, carries unique types of information. We’ve known for a long time that our Universe is expanding and that the farther away something is, the faster it is receding from us. The velocity of light in a vacuum is always the same, so its color is shifted if there is a relative velocity between us and its source. If we are moving closer together, the spectrum is shifted toward smaller wavelengths, a so-called blue shift. And if the source of light is moving away from us, its spectrum is shifted toward longer wavelengths: a red shift.
The farthest (and therefore, oldest) objects in the Universe are receding the fastest, so they have the largest red shift. In fact, their spectrum is shifted so far that the visible light (and some of the UV) emitted by the oldest stars and galaxies reaches us as infrared light.
In this essence, the Webb Telescope being built by NASA and its partners is a more direct successor to the Spitzer Telescope rather than the Hubble. In short, the Webb will open up a whole new world of infrared astronomy when it launches in 2018. The telescope will be able to capture images of the very first stars and galaxies, formed only 200 million years after the Big Bang.
The beautiful shapes of modern galaxies take billions of years to evolve, as the stars of which they are composed arrange themselves under the influence of their mutual gravitational interaction. That’s a bit too long for us to wait if we want to study the evolution of a single galaxy. However, just as our view of the Sun shows us our star as it was eight minutes ago, our views of distant galaxies show us how they looked billions of years in the past. This allows us to study them in all stages of growth, from the early proto-galaxies to the mature spirals and ellipses in our galactic neighborhood.
The James Webb Space Telescope will be a major advance over all previous infrared observatories. Its primary mirror will be 50 times the area of the Spitzer Space Telescope, and its infrared images will have eight times the resolution (about the same resolution in the near-infrared as the Hubble has in the visible spectrum). This will allow the device to capture images of the structure of the first galaxies in the Universe with unprecedented detail. The wavelengths that the Webb will image can not be seen at all from the surface of the Earth because of our atmosphere.
But being able to see like a boss in the infrared will tell us much more than what the earliest galaxies looked like.
Much of the information about stars and their births is obscured by interstellar dust, which absorbs visible light. The Webb Telescope will be able to see through this dust, which is relatively transparent to infrared light. This is particularly useful in studying the creation of stars, as young stars tend to form in dusty environments.
Infrared imaging will also provide information about the formation of planetary systems around these young stars, as the material from which planets are formed is heated by the star and glows in the infrared.
All this is made possible by the extreme infrared sensitivity of the new observatory. This was made vivid by John Mather, the physics Nobelist who is guiding the Webb Telescope toward completion. He joined the project in 1995 after accepting an invitation from NASA. And at the most recentAAAS meeting in Washington, DC, he astounded the audience by remarking that the telescope would be able to detect the heat generated by a bumblebee a quarter-million miles away—the distance from the Earth to the Moon.
If some form of life is eventually discovered on an exoplanet, the first sign will likely be the presence of signal molecules (such as methane and oxygen) in its atmosphere. One of the main purposes of the James Webb Telescope is the study of the composition of exoplanet atmospheres.
The telescope will carry sophisticated spectrometers to measure the characteristic absorption lines produced by the atmospheres of other worlds as they pass in front of their stars. This means that they will measure the intensity of the starlight at different wavelengths as the light passes through the exoplanet atmosphere, searching for dips in this graph where the light is absorbed by a chemical in the atmosphere. Most molecules of interest have key absorption features in the infrared part of the spectrum, so the Webb is well-suited for this mission.
Where the Webb will live
Some of our space-based observatories are in Earth orbit, but not all. The Spitzer telescope is in an “Earth trailing” orbit, sharing our path around the Sun but far from the Earth itself. This is a good strategy for an infrared observatory, as it helps to keep the device cool and allows a wide view of the cosmos.
It is essential that a sensitive infrared telescope, like the Spitzer and Webb, be maintained as cold as possible to prevent heat radiation from its own structures from interfering with observations. So the Webb will be located far from the Earth as well, residing in two orbits at the same time. One of these orbits is around the Sun, but the other is around a location in space at which there is ... nothing, only a balance of forces.
The telescope will be situated near and make a slow orbit around the Lagrange point called L2, as the L2 point itself makes an orbit around the Sun. What is a Lagrange point? If you have two gravitating objects in space, in this case the Earth and Sun, these points are where you can put another object and have all three stay in place relative to each other. As the Earth orbits the Sun, the five Lagrange points all rotate with it as if they were all attached by rigid rods. In fact, the Lagrange points are such desirable neighborhoods that some small asteroids have moved in to L4 and L5. (L1, L2, and L3 are not as welcoming, as they are what is known as “metastable” points.)
The Lagrange points can also be suitable places to park an artificial satellite if you want it to stay in place (relative to the Earth) for a long time. We’ve already done this on several occasions, using L2 for the WMAP, Herschel, and Planck satellites.
The previous occupant of the L2 Lagrange point was the Herschel Space Observatory, named after William Herschel, who discovered infrared light around 1800. It ended its mission in 2013, after which it was sent into an elliptical orbit around the sun. The Herschel, operated by the European Space Agency with partners including NASA, was designed to see in the far infrared, where the most active star-forming galaxies emit the maximum energy. (The Webb is designed to be most sensitive in the near-infrared.)
As noted above, L1, L2, and L3 are metastable. Satellites parked here would drift away eventually unless occasional orbital corrections are made. You can think of the stable L4 and L5 points as the bottoms of hills—objects placed there will stay fixed unless bumped over the edge. The metastable points are like the centers of wide saddles. Objects not exactly centered will slowly fall away unless they are nudged back into position. These nudges are supplied to the Webb Telescope by small rocket engines, which are supplied by enough fuel to last 10 years. After this time, the Webb Telescope will drift away into retirement.
The idea of an orbit around an empty point in space may not be immediately intuitive. The discovery of the various types of orbits around L1, L2, and L3 is a reminder that celestial mechanics continued to progress after the eighteenth century. They were found in 1966 by Robert W. Farquhar, who was following up an idea that Arthur C. Clarke had in 1950.
Although there is no mass at the Lagrange point, there is a balance of forces from the Sun, Earth, and the centrifugal force that arises from being in a rotating reference frame with the Sun at the center. All these force vectors add up to create a kind of virtual gravitational attraction at the Lagrange point, as if there were a mass located there.
It will take the Webb Telescope a month-long journey of 1.5 million kilometers to reach its home at L2. It will settle into an orbit around L2 with a period of six months and a distance from the Lagrange point about the same as the distance from the Moon to the Earth.
One reason for this orbit, rather than a stationary existence at L2, is to keep the telescope out of the shadow of the Earth. Unlike the Hubble, which passes into and out of the shadow of the Earth every 90 minutes, the Webb Telescope will be able to make observations without interruption. In addition, the orbit around L2 will keep the Earth, Moon, and Sun in about the same direction, which will make it possible to shield the observatory from the heat emanating from all three bodies. At the same time, keeping the Sun in view of the hot side of the observatory is essential because of its need for solar power.
Webb Telescope technology
The Webb Space Telescope’s mission is to achieve unprecedented levels of precision in infrared astronomy. In order to accomplish this, a host of technological innovations are being applied. We'll briefly describe only a few of these.
Cooling
Again, it is essential for an infrared telescope to be kept as cold as possible. The visuals of the Webb Telescope are dominated by a tennis-court sized structure that will shield the observatory from the heat of the Sun and Earth. Although it looks solid, it's mostly empty—the heat shield exploits the unsurpassable insulating properties of the vacuum of space. The rest is a series of layers made from Kapton, a material similar to plastic food wrap, coated with highly reflective aluminum and doped silicon. Infrared radiation bounces back and forth between these layers and out the sides, while the vacuum prevents heat from being conducted through to the instruments.
The shield is highly effective: the telescope will operate at about 225 degrees Celsius below zero, colder than liquid nitrogen at Earth’s atmospheric pressure. The hot side of the satellite, by contrast, will be near the boiling point of water.
Four of the five layers are calculated to be sufficient for the cooling requirements of the telescope, but an extra layer is needed to compensate for the partial loss of shielding that may occur from the accumulated damage due to micrometeors.
Mirrors and Optics
The primary light-gathering mirror of the Webb Space Telescope is enormous, consisting of 18 hexagonal segments that fit together to make a mirror 6.5 meters across. A mirror this large was needed to make a telescope sensitive enough to see the details that the Webb is intended to see, but its sheer size presents some challenges for a device that is going to be put in a rocket.
The segmentation solves the problem of geometry: it allows the mirror to fold up to fit inside the spacecraft. The second problem is weight. If the Hubble’s mirror were to be enlarged to Webb proportions, it would simply be too heavy to launch. The solution was to fashion the Webb’s mirror segments from beryllium, a light, strong metal that holds its shape under extreme cold. From there, the segments are coated with gold for high infrared reflectivity.
At the recent AAAS meeting, Mather revealed that contracts for the construction of mirror segments were given to 12 companies that claimed to possess the necessary expertise. Only one of them was able to deliver.
Each hexagonal segment is equipped with six actuators that can position it to a precision of 1/10,000th the thickness of a human hair, plus an additional actuator attached to its center that can adjust its curvature. Adjusting all the mirrors individually can create a single large mirror with a perfect focus. NASA studied the problem of how to construct a large mirror to these precise tolerances over four years in collaboration with the Air Force and the National Reconnaissance Office, the agency in charge of spy satellites. The traditional alliance between space telescopy and espionage is alive and well.
Data and command transmission
The Webb Space Telescope is designed to never need service, and in fact it probably can't be serviced or repaired. The Hubble, specifically designed with astronaut-serviceable components, is currently the only satellite that is capable of being fixed or upgraded while in orbit.
In a conversation with Matt Greenhouse, who joined the project in 1997 and is project scientist for the Webb Telescope science instrument payload, he mentioned that it is not inconceivable that a refueling mission may be dispatched in a decade to extend the life of the Telescope. However, the cost-benefit decision for maneuvers like that would be made at a later date.
The Webb’s location at the L2 Lagrange point would make servicing challenging, but it means that it will always be approximately in the same location with respect to the Earth and always within sight of a ground station. We will be able to talk to the telescope and receive data from it continuously.
The planned mode of operation is to uplink command sequences and downlink data up to twice per day through the Deep Space Network, using ground stations in Australia, Spain, and California. The telescope can execute a sequence of pointing and observing commands autonomously. The Space Telescope Science Institute will upload a week’s worth of commands at one go and can make modifications daily as required.
The telescope will produce about 235 Gigabits of astronomical data every day that will need to be absorbed by the Deep Space Network. This requires a bandwidth higher than 10 megabits per second, a new threshold for communications to the L2 point. To get these high data rates meant changing the frequency used previously for communications through the space network and upgrading the entire communications infrastructure. A side effect of this will be a new communications standard to be used in future missions as well.
On the subject of data, the Webb Telescope will follow the policy in place for the Hubble, which is to embargo data for a one-year “proprietary period” for the benefit of investigators working on funded projects. After that, the data is to be released to the public.
A long struggle
The James Webb Space Telescope almost did not survive the combination of construction delays and the politics of science funding. Congress reacted to a ballooning budget and delays in schedule by nearly canceling it in 2011. Killing it probably would have received some support from scientists themselves.
Given the reality that any particular segment of the science budget is a zero-sum game, any large project will typically drain funds from a collection of smaller research projects. Typically, the victims are efforts in basic science that, while important, may not be easy to explain to the public. They're also unlikely to involve job-creating sub-projects scattered over several states that make them attractive to members of Congress. The Webb Telescope was not immune to this kind of friction, and 18 planetary scientists signed a letter opposing the possibility of its draining funding from other science projects.
But the project has survived these hurdles and is making excellent progress toward its 2018 launch date. Dr. Greenhouse pointed out in our conversation that, since the project’s restructuring in 2011, it has been under budget and on schedule. And as this was being written, the Webb Telescope has passed two milestones: the primary and secondary mirrors are now complete. NASA maintains a set of webcams where you can get a view into the cleanroom to watch the Telescope taking shape, live.
This progress, combined with the fact that it is the largest science project in US government history, has caused other areas of the government to take notice. The Webb Telescope may become a model for the organization of certain types of large government enterprises.
The mission of NASA is not purely science but one of public inspiration as well. The science community is well aware that the large sums of money spent on the International Space Station do not justify, in purely scientific terms, the relatively meager amount of actual research that results. It can be frustrating to contemplate the amount of serious space science that could be supported by the ISS budget.
But it is impossible to quantify the value of the inspiration gained from being able to watch, and talk to, astronauts floating in orbit and taking pictures of Earth. Is there any doubt that there are children in grade school today who are deciding to become scientists because of the men and women flying 249 miles above their heads?
Amber Straughn, the deputy project scientist for James Webb Space Telescope Science Communications, made it vividly clear how much importance NASA attaches to public education and outreach during her presentation at the recent AAAS meeting in Washington. She solved the mystery of why the photographs of telescope assembly on NASA’s websites are so beautiful: they are created by artists, and they are meant to transcend the function of mere laboratory record keeping.
Dr. Straughn also treated the audience to a showing of an amazing time-lapse record of the entire mirror assembly process and a photographic record of the time the DC Beltway was shut down to allow part of the telescope to be transported.
Mather, the physics Nobelist, considers the Webb Telescope “the most important project I could imagine working on." He believes it fits perfectly within NASA's missions, both practical and inspirational. “The public is extraordinarily interested in astronomy because it touches the question of our origins and the meaning of life,” he told Ars by e-mail.
In an age marked by pointless rage and petty obsessions, the James Webb Space Telescope should be a gleaming monument to our noblest aspirations. It will teach us about the origins of our Universe and find answers to questions that we have not yet thought to ask. It is the embodiment, in gold, Kapton, and beryllium, of the defiance of David Hilbert—the last century’s greatest mathematician and one of the creators of the theory of general relativity—a defiance against intellectual surrender that he expressed in words that remain inscribed on his tomb: