Infrared Dawn: The Next Space Telescope Will Be Hubble x 100

The James Webb Space Telescope will see out to the universe’s edge.

A thin layer of gold on each of the James Webb Space Telescope’s 18 mirror segments reflects mostly infrared light. (Drew Noel/NASA)
Air & Space Magazine

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Assuming all goes according to plan, Webb will see first light in early 2019. It will be sent to orbit about the L2 Lagrange point, a gravitationally stable location 930,000 miles from Earth. The orbit was selected because at that distance, the telescope’s sunshield can block heat from both the sun and Earth, preventing it from interfering with Webb’s observations. Besides, an orbit around the sweet spot where sun and Earth gravity are in balance requires little fuel to maintain. Though designed for a minimum mission life of five years, not including travel and cool-down time, Webb will carry enough propellant to hold that orbit for at least a decade.

Mike Menzel, Webb’s mission systems engineer, says the telescope’s engineering challenges have been what he always knew they’d be: its cryogenic temperature (Webb will operate at 50 Kelvin, or -370 Fahrenheit) and the need to make both the sunshield and the primary mirror deploy. “Things tend to ring like a bell when they get cold,” Menzel says. “Vibrations carry through these structures. You’ve got a six-meter telescope and you’ve got all these actuators on the spacecraft” for adjusting the mirrors. “Spinning wheels. Cryo-pumps. You’ve got to make sure their vibrations don’t jitter the telescope.” Menzel cues up a video clip on a monitor, showing a “sine vibration test” submitting a Webb component to forces similar to those it will be exposed to during launch. “Optical engineers get nauseated when they watch this,” he says, chuckling.

Soon after launch from Kourou, French Guiana, the giant observatory will begin to carefully unfold itself from its ESA-supplied Ariane 5 launcher. On its straight shot to L2, Webb will unfurl its solar arrays just 33 minutes after leaving Earth. Two days later, the five layers of the tennis-court-size sunshield will deploy, a slow, careful unspooling that will take about 72 hours. The flower-like primary mirror deploys on day 11, less than halfway before the observatory reaches its destination on day 29. Meanwhile, “almost three and a half metric tons of material have to be cooled to around 50 degrees Kelvin,” Menzel says. “We’ve never launched anything of this size to that temperature.”

Another stringent requirement for a telescope that will see back to the dawn of time: The surface of each primary mirror segment must stay within 25 nanometers of its smooth, polished shape. (A sheet of paper has a thickness of about 100,000 nanometers.) To this end, manufacturer Ball Aerospace made the segments from beryllium, a lightweight, toxic element, chosen because at very low temperatures it behaves so predictably. When lowered to cryogenic temperatures, beryllium, like most materials, warps; unlike other materials, beryllium warps in a consistent, easy-to-predict way. During cryo-testing, the mirrors were subject to drastic fluctuations while being repeatedly cooled from room temperature (at which the electrically heated Hubble operates) to its operational temperature in space. Engineers made a topographic map of the surface of each mirror segment. “Where it went high, we polished low, and where it went low, we polished high, at room temperature,” says Ball’s Mark Bergeland. “So every subsequent time they go cold, that nulls itself out and you get that nice, smooth optical surface.”

Besides its ability to pick up faint, faraway objects, Webb’s infrared capability offers another benefit: Infrared light penetrates the molecular clouds of gas and dust better than visible light does. Those clouds are where stars form. “We initially thought of having an infrared-optimized telescope to look for highly redshifted, very distant galaxies,” says Jonathan Gardner, the Webb’s deputy senior project scientist. “It also turns out to be very good at studying the formation of stars and planets in our own galaxy.”

The birth of stars and their proto-planetary systems is one of the four main science areas that Webb’s suite of near- and mid-infrared imagers and spectrographs will study. The other three are: the formation of galaxies; exoplanets and their potential for harboring life; and the universe’s re-ionization, when the first light shone, a mere 500 million years after the Big Bang.

In 2012, California Institute of Technology astronomer Richard Ellis headed up the Hubble Ultra Deep Field project. That survey looked farther back than any other toward the cosmic dawn, whetting astronomers’ appetites for what Webb will show them. “We found there are galaxies seen when the universe was, say, seven or eight hundred million years old that were not recent arrivals; they’d already been there for one or two hundred million years,” says Ellis. “That’s very exciting, because that means that there are even earlier objects that are currently beyond reach…. James Webb will be able to find them.”

For a younger generation of astronomers, the Webb telescope has been part of the conversation for as long as they’ve been in the field. Zachory Berta-Thompson finished his Ph.D. at Harvard only last year. Now a post-doctoral fellow at the Massachusetts Institute of Technology, he’s excited about the Webb’s capabilities for researching the conditions of planets orbiting distant stars. Using Webb’s Near-Infrared Imager and Slitless Spectrograph, NIRISS, astronomers will be able to collect the light emissions as an exoplanet moves in front of its host star, and from that data, determine the composition of the planet’s atmosphere.

Like Hubble, the Webb telescope will be primarily an open observatory: Anyone can submit a proposal to use it. Hubble’s time is allocated in 90-minute increments because that’s how long it takes the telescope to orbit Earth. The Webb, traveling around the sun in sync with Earth, will take a year to complete one orbit, so the allocation committee will have to dole out its use in some not-yet-determined slice of time, according to Gardner.

He says Hubble’s committee receives six to eight times as many proposals as it can accommodate. Dunlop, who with Ellis was on the team that obtained the Ultra Deep Field images from Hubble, believes the competition for the Webb will be even tougher than that. He already has Ph.D. candidates doing prep work for the proposals they’ll submit in a few years, using computer modeling to project what present-day galaxies must have looked like billions of years earlier, based on calculations of the expansion of the universe, the lifespan of stars, and other factors. They’ll hope to use Webb observations to confirm or refute their theories.

A global explosion of interest in astronomy has made the process of applying for time on one-of-a-kind observatories far more competitive than it was when Dunlop started his career, in the early 1980s. “The research community is at least 10 times bigger than it was two decades ago,” he says, and the increase has driven a “sociological evolution” in which investigators must band together into ever-larger teams to increase their likelihood of winning observing time. “When I started in astronomy I wrote a lot of one- and two-author papers,” says Dunlop. “Now they rarely have less than 10. A lot of the big astronomy papers now typically have like a hundred authors.”

For the Ultra Deep Field, Ellis and Dunlop won the observing time on Hubble in part by promising to make their data public after only three months. But their applications for followup research with Spitzer have been declined twice. Ellis suspects that one reason is the allocation committee’s awareness that the Webb telescope will be a far more efficient tool for the kind of infrared survey they want to make.

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