Irene Klotz To see back to the universe’s first galaxies, engineers had to develop 10 new technologies for the James Webb Space Telescope.
Irene Klotz
When it came to inventing the successor to the Hubble Space Telescope, an instrument intended to image the universe’s first stars and galaxies, engineers faced the tyranny of the rocket fairing: how to fit a 269 ft.2 primary mirror—more than seven times the surface area of Hubble’s—into a 56-ft.-tall X 18-ft.-dia. payload shroud.
Resolving that conundrum with a segmented, foldable mirror—a first for an astronomical observatory—was among the most visible technology development initiatives needed for NASA’s most ambitious and most expensive astrophysics project.
Over the decades, teams working on the James Webb Space Telescope (JWST) ended up creating 10 new technologies, some of which already have been incorporated into various programs. Others, such as a five-layer gossamer sunshield that unfolds to the size a tennis court, are flying for the first time on the JWST, which is due to launch on Dec. 18. Here’s a look at the inner workings of Webb.
Webb is designed to image and analyze longer wavelengths of light than Hubble, which will allow it to peer inside dust-shrouded stellar nurseries and to scan the atmospheres of planets beyond the Solar System. “The ultimate holy grail is to find a temperate exoplanet where we could look for life,” says Gunther Hasinger, science director with the European Space Agency (ESA), a partner in the program.
Sensitive to long-wavelength visible light, near-infrared and mid-infrared radiation, the JWST is capable of detecting objects 400 times fainter than what can be observed by current ground and space-based telescopes. Its four main science goals are to search for the first light after the Big Bang; study how galaxies and black holes form and evolve; observe the life cycles of stars and protoplanetary systems; and probe the atmospheres of exoplanets.
“James Webb is about 100 times more sensitive than Hubble, so the images will look much fuller, much denser,” Hasinger says. “Because you have a 6.5-m-[dia.] (21.3-ft.) mirror, compared to [Hubble’s] 2.4-m mirror, the images also [will be] three times sharper. I think the James Webb images will blow our mind.”
Scaling up Hubble’s glass mirror to meet the JWST’s 21.3-ft.-dia. requirement was not an option, as it would have been too heavy to launch. Designers settled on 18 hexagonal-shaped mirror segments made of beryllium, a lightweight, strong metal that holds its shape at cryogenic temperatures. Each 4.3-ft.-dia. mirror segment weighs about 46 lb.
To fit in the rocket fairing, the mirror segments were installed on a structure that folds up like a drop-leaf table for launch. Once deployed, the segments will be aligned in orbit to form a single optical surface measuring 21.3 ft. in diameter. “Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair,” says Lee Feinberg, Webb Optical Telescope Element Manager at NASA Goddard Space Flight Center. “Engineers and scientists working on the Webb telescope literally had to invent how to do this.”
To catch the light emitted by the oldest stars—radiation that has shifted into the infrared due to the expansion of space—Webb’s mirror not only had to be big but also cold. “If Webb’s mirror was the same temperature as Hubble’s, the light from distant galaxies would be lost in the infrared glow of the mirror,” notes NASA.
The JWST, which is designed to operate at -364F, will be positioned in deep space nearly 1 million mi. from Earth at L2, the second of five Lagrange Points where gravity from the Sun and Earth balance the orbital motion of a satellite.
Ball Aerospace, under contract to JWST prime contractor Northrop Grumman, began manufacturing the JWST’s mirror segments in 2003. The mirror is coated in gold, which better reflects infrared light. The gold is covered by a thin layer of protective glass.
Wavefront Sensing and Control
To integrate the JWST mirror segments into a single unit, engineers developed a technology known as wavefront sensing and control. “We need to be able to get our optical performance to within 50 nanometers,” says Sandra Irish, mechanical systems lead structures engineer at Goddard.
The process involves using one of the telescope’s cameras to take 18 images of a star, one from each mirror segment. Computer algorithms then analyze the images to determine the overall shape of the primary mirror and calculate the adjustments needed to bring the image into focus.
The technology was spun off into a technique to diagnose eye conditions, map eye movements and improve LASIK procedures.
Sunshield
Stationing the JWST in deep space is only part of the solution for keeping the observatory cold enough to detect faint, infrared radiation from objects some 13.5 billion light years away.
To shield the telescope from heat and light sources, including the Sun, Earth and Moon, as well as to dissipate heat generated by the observatory, the JWST includes a novel five-layer, kite-shaped sunshield. The shield, which unfolds to become about the length of a tennis court, separates the observatory into a warm, Sun-facing side, with temperatures close to 400F, and a cold side operating at -185F.
The layers, each less than half the thickness of a piece of paper, are made of Kapton, a commercially available polyimide film that has been coated with aluminum. The Sun-facing side of the two hottest layers also have what is known as a “doped-silicon” (or treated silicon) coating to reflect the Sun’s heat back into space. Unlike other infrared space telescopes that are actively cooled, the JWST’s sunshield is a passive system, with no consumables for cooling.
Each successive layer of the sunshield is cooler than the one below. Heat radiates out from between the layers, with the vacuum between the layers providing insulation. Once on orbit, the observatory will be positioned so that the Sun, Earth and Moon are always on one side, with the sunshield acting as an umbrella to shade the telescope mirrors and instruments from the warmer spacecraft electronics and the Sun.
The membranes are folded 12 times to fit within the payload shroud of the Arianespace Ariane 5 rocket that will be the JWST’s ride to orbit. ESA is providing launch services as part of an approximately $800 million contribution to the JWST project. The Canadian Space Agency also is a partner, having invested about $160 million. NASA has spent nearly $10 billion on the project, which has been in development for 25 years.
The sunshield support structure contains more than 7,000 parts, including springs, bearings, pulleys and magnets, as well as hundreds of custom-fabricated pieces. Unfurling the sunshield is among some 40 major deployments needed for the JWST to operate.
The Backplane
Building a lightweight, composite structure to hold the observatory’s primary mirror presented some unique challenges to the JWST team. The support structure, known as the backplane, not only has to operate at -370F but also needs to remain essentially motionless so the mirrors and science instruments can operate as intended.
The 2,180-lb. structure was designed to be steady down to 32 nanometers—about 1/10,000 the diameter of a human hair—as temperatures swing between -406F and -343F. In addition to holding the primary mirror and other telescope optics, the backplane also carries the module containing all of the JWST’s scientific instruments, for a total hardware load of 2.5 tons.
Near-Infrared and Mid-Infrared Detectors
Detecting faint emissions from distant galaxies, stars and planets requires large arrays for efficient sky surveys. The technology development effort for the JWST yielded arrays that are larger format and more sensitive.
The observatory is outfitted with two types of detectors: four-megapixel near-infrared (IR) mercury-cadmium-telluride detectors for wavelengths of 0.6-5 microns, and one-megapixel mid-IR silicon-arsenic detectors for 5-29 microns.
The near-IR detectors, each of which has about 4 million pixels, were made by Teledyne Imaging Sensors in California, and the mid-IR detectors, with about 1 million pixels, were manufactured by Raytheon Vision Systems, also in California.
All the JWST’s detectors have the same sandwich-like architecture consisting of a thin semiconductor absorber layer, a layer of indium that interconnects to join each pixel in the absorber layer to the readout and a silicon readout integrated circuit to read out millions of pixels using a manageable number of outputs.
Cryogenic Data-Acquisition Integrated Circuit
Each of the JWST’s science instruments has detectors to convert light from astronomical objects into electrical signals, a technique that is not new. But converting analog signals into digital data at cryogenic operating temperatures is. The JWST team developed a low-noise, cryogenic application-specific integrated circuit that features a micro-processor with extremely low power dissipation and a 16-bit analog-to-digital converter that generates noise comparable to conventional warm electronics.
Mid-Infrared Instrument Cryocooler
The JWST is outfitted with four science instruments: the Near-Infrared Spectrograph (NIRSpec), which can determine temperature, mass and chemical composition of 200 objects simultaneously; the Near-Infrared Camera (NIRCam), which is designed to image the most distant objects in near IR; the Near-Infrared Imager and Slitless Spectrograph (Niriss), which provides the temperature, mass and chemical composition of objects; and the Mid-Infrared Instrument (MIRI), which observes cold, distant objects and provides spectroscopy mapping.
MIRI has its own cooler to reduce its operating temperature to -448F, just above absolute zero. Led by JWST prime contractor Northrop Grumman and NASA’s Jet Propulsion Laboratory, the MIRI cryocooler is a complex system that NASA says has never been flown in space. The development effort was hampered by manufacturing and development delays, including a redesign to resolve a problem with a leaky valve in the cryocooler cold-head assembly.
The cryocooler is basically a sophisticated refrigerator with its pieces distributed throughout the observatory, says NASA. The primary piece—the Cryocooler Compressor Assembly (CCA)—is a heat pump consisting of a precooler that generates about 0.25 watts of cooling power at about 14K (-434F) (using helium gas as a working fluid) and a high-efficiency pump that circulates refrigerant (also helium gas) cooled by conduction with the precooler to MIRI.
Webb’s cryocooler includes a precooler that uses three stages of pulse-tube cooling, as compared with heritage systems that have only two stages. In addition, the separation between the precooler and the cooling hardware is several meters, rather than centimeters, NASA says.
The only moving parts in the cryocooler are the pair of two-cylinder piston pumps in the CCA that are horizontally opposed, which cancels out most vibration. As a closed system, the cryocooler does not use liquid helium or any other coolant. Its operational life is limited only by wear to its moving parts (the pumps) and the longevity of its electronics.
Microshutters
Another major technology effort for the JWST focused on microshutters, which are tiny windows the width of a few hairs that allow scientists to block out unwanted light so only the most distant stars and galaxies can be detected by the Airbus-led NIRSpec instrument.
Beginning at Technology Readiness Level 0—the JWST is the first to use the technology—work on the microshutters stalled for a decade due to an acoustic damage issue. The Microshutter Assembly consists of tiny cells measuring 100 by 200 microns (about the width of 3-6 human hairs) that are arranged in four waffle-like grids. Each grid, which is about the size of a postage stamp, contains more than 62,000 shutters designed to be individually opened or closed to view or block a portion of the sky.
With the microshutters, NIRSpec will become the first spectroscopic space instrument that can make high-resolution observations of up to 100 objects simultaneously.
In addition to operating in extreme cold, the microshutters, which are made of silicon nitride, had to demonstrate that they could operate reliably without fatigue. “To build a telescope that can peer farther than Hubble, we needed brand-new technology,” says Murzy Jhabvala, chief engineer of Goddard’s Instrument Systems and Technology Division. “We’ve worked on this design for over six years, opening and closing the tiny shutters tens of thousands of times in order to perfect the technology.”
In testing, the microshutters were opened and closed more than 200,000 times, more than double their design life.
Heat Switches
Tailored to work at -388F, the JWST includes heat switches to protect its instruments from contamination during cool-down as well as to decontaminate them in the event of a problem. The switches are designed to temporarily break the thermal path from the instruments to their radiators, allowing power-efficient warming of the instruments.
The JWST is designed to last 5-10 years, with propellant for station-keeping the limiting factor. “We’re really good, once the telescopes are up there, about saving fuel . . . and extending their lifetimes,” says Thomas Zurbuchen, NASA’s associate administrator for science.
In addition to Northrop Grumman, which provided the spacecraft, designed and built the deployable sunshield, and integrated the system, and Ball, which handled the optical design, mirrors, wavefront sensing and control design, and algorithms, U.S. partners in the project include L3 Harris, the University of Arizona and NASA’s Jet Propulsion Laboratory.
The telescope’s 181-lb. Mid-Infrared Instrument (MIRI), on balance beam, was installed in the science instrument module at NASA’s Goddard Space Flight Center. Credit: Chris Gunn/NASA
The compressor assembly for the MIRI cryocooler was installed in a vacuum chamber for testing. MIRI’s arsenic-doped silicon detectors, sensitive to mid-infrared wavelengths of 5-28 microns, operates just a few degrees above absolute zero. NASA/JPL-Caltech
The Near-Infrared Camera will collect light in detectors that feature pixilated purple mercury-cadmium-telluride film. The detector is pictured here without optical baffles. Credit: University of Arizona/NASA
Ball Aerospace engineered a scaled telescope testbed to develop and demonstrate the technology to operate 18 mirror segments as one giant mirror. Chris Gunn/NASA
The JWST’s NirSpec instrument includes more than 248,000 microshutters, each 100 X 200 microns, that can be individually operated to view or block a portion of the sky. Credit: NASA
Prime JWST contractor Northrop Grumman stacked and unfurled a full-size test unit of the JWST’s sunshield in 2016. Credit: Chris Gunn/NASA
The telescope’s carbon-fiber support structure features two foldable wings so the observatory can fit inside an Ariane 5 rocket fairing for launch. Credit: Chris Gunn/NASA
The JWST’s gold-coated, 18-segment primary mirror was removed from a clean room at Goddard Space Flight Center after a 2016 test of its curvature. Credit: Chris Gunn/NASA
