With the launch of the James Webb Space Telescope (JWST), mankind’s understanding of the universe — and its origins — will increase exponentially.
Originally called the Next Generation Space Telescope (NGST) and renamed in September 2002 to honor former NASA administrator James Webb, the JWST represents an international collaboration of partners including NASA, the Canadian Space Agency (CSA), the European Space Agency (ESA), aerospace manufacturer Northrop Grumman, and the Space Telescope Space Institute, which will operate the telescope after it is launched.
What makes the JWST different from the space-based telescopes that came before it, like the history-making Hubble Space Telescope? For one thing, Hubble, which was launched in 1990, is an optical telescope; the JWST is an infrared telescope. As lightwaves travel through the ever-expanding universe, they get “stretched,” meaning they shift to longer, redder energy wavelengths. At some point, once-visible light from the most distant stars in the universe shifts to infrared wavelengths that can no longer be detected by optical telescopes such as Hubble. The JWST is specifically designed to capture images of those infrared lightwaves and analyze them using state-of-the-art spectroscopy.
The JWST will not be the first time scientists have used infrared technology to explore the universe. In 1983 NASA launched its groundbreaking Infrared Astronomical Satellite (IRAS) into orbit, making it the world’s first space-based infrared telescope. A joint project engineered by the USA, Netherlands, and United Kingdom, it orbited 559 miles above the Earth in a mission that lasted 10 months and observed over 250,000 infrared sources in the 12, 25, 60 and 100 micrometer wavelengths. The success of that mission led to the installation of a helium-cooled infrared telescope onboard the Space Shuttle Challenger in 1985 (STS-51) and eventually resulted in development of the Spitzer Space Telescope, which was launched in 2003.
In between those milestones, the European Space Agency, working with NASA and Japan’s Institute of Space and Astronautical Science (ISAS), launched the Infrared Space Observatory (ISO) in November 1995 on a three-year mission designed to observe approximately 30,000 infrared sources, perform imaging in the 2.5 to 240 micrometer range, and spectroscopy in the 2.5 to 196.8 micrometer range, and transmit the data back to Earth in real time. And in 1997, NASA gave the Hubble optical telescope infrared capability by equipping it with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) during Servicing Mission 2 (STS-82).
NICMOS, a combined imaging device and spectrometer designed and built by Ball Aerospace & Technologies Corp., featured three mercury cadmium telluride near-infrared detectors that were bonded to sapphire substrates and designed to operate in the 0.8 to 2.5 micrometer wavelength. The NICMOS, which operated from 1997 to 1999 before running out of coolant, and then again from 2002 to 2008 following the installation of a new cryogenic cooling system during Servicing Mission 3B (STS-109), was eventually replaced in 2009 with the Wide Field Camera 3 (WFC3) during Servicing Mission 4 (STS-125). Although not strictly an infrared instrument — it also had a UV and optical channel capable of recording images in the 200 to 1000 nm wavelength range — the WFC3 did have a near-infrared detector designed to capture images in the 800 - 1700 nm wavelength range. Although limited in its infrared capability compared to NICMOS (1700 nm vs. 2500 nm), the WFC3 could be thermoelectrically cooled, eliminating the need for cryogenic cooling.
Arguably the most technologically advanced scientific instrument ever launched into space, the JWST will not only combine the best aspects of the Hubble and Spitzer space telescopes, it will far exceed them, beginning with the size of its primary mirror. Webb’s mirror will be 6.5 meters in diameter compared to Hubble’s 2.4-meter mirror and Spitzer’s compact 0.8-meter mirror.
Because a mirror that size is too large to fit inside any current launch vehicle, it will consist of 18 individual hexagonal-shaped segments made of lightweight beryllium that will unfold and automatically adjust to shape once in orbit. Each segment will be vacuum vapor deposition coated with a thin layer of gold just 1000 angstroms (100 nanometers) thick. To put that into perspective, given the density of gold at room temperature (19.3 gm/cm3), that works out to 48.25g of gold — roughly the same mass as a golf ball — to coat a surface area of 25m2. Why gold? Superior reflectivity. Gold will reflect 98-percent of the infrared light collected, whereas a material such as aluminum typically only reflects about 85-percent of visible light.
The JWST is designed to orbit the L2 point, 1.5 million kilometers above the Earth. The farther away from Earth’s atmosphere a telescope is, the less elements there are to negatively impact the quality of the data being collected. It will also be far enough away from Earth’s protective magnetic field where high-energy cosmic rays could interfere with its signals or create electrical charges that could potentially damage the telescope’s sensitive instruments. As additional insurance, the JWST has been designed with special shielding and conductive materials to prevent voltage from accumulating and damaging the craft’s sunshields and subsystems. The telescope will make one complete orbit around L2 every 198 days...in case you want to watch for it.
The telescope’s sunshield, which is approximately the size of a tennis court (21.197m × 14.162m), is by far the largest element of the JWST. Consisting of five layers of silicon-coated Du-Pont™ Kapton®, each layer less than 1mm thick, the sunshield’s primary purpose is to separate the cold side of the telescope, where the instrumentation is housed, from the side facing the sun. The maximum temperature layer 1 can withstand is 383K (~231°F), while layer 5 can withstand a maximum temperature of 221K (~ -80°F) and a minimum temperature of 36K (~ -394°F). Since infrared detectors prefer cool temperatures and any heat generated by the JWST’s onboard systems could pollute the infrared signals being collected, the telescope’s preferred operating temperature is under 50K (~ -370°F).
Given the size of the sunshield and how thin its materials are, one of the engineering challenges faced by its designers was making it strong enough to withstand the rigors of space travel. They achieved that by creating an ingenious system of supporting ribs that will provide the necessary structural stability without becoming brittle. The system will also tolerate small rips and tears caused by space debris without failing.
In terms of technology, the JWST can be broken down into three sections: the Integrated Science Instrument Module (ISIM), the Optical Telescope Element, and the Spacecraft Element.
The Science Instruments
The ISIM contains the JWST’s four main scientific instruments: the Near-Infrared Camera (NIRCam), the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor/Near-Infrared Imager and Slitless Spectrograph (FGS/NIRISS).
The Near-Infrared Camera, built by the University of Arizona and Lockheed Martin, will perform two important functions. The first is to capture images in the 600nm to 5000nm wavelength range using a 10,000-second exposure (approximately 2.8 hours). Designed to operate at 37K (~ -393°F), it will observe and record light produced by some of the first stars and galaxies formed in the universe following the Big Bang. Its other important function is to constantly monitor the performance of the Primary Mirror’s 18 segments, ensuring that the telescope remains in focus.
The Near-Infrared Spectrograph, contributed by the European Space Agency (ESA), is unique in that it can simultaneously analyze as many as 100 objects in a 3 arcminute × 3 arcminute field of view in the 600nm to 5000nm wavelength range. It can do this thanks to an innovative system of four arrays of programmable slit masks containing roughly 250,000 micro shutters, each measuring just 100 × 200 microns. The NIRSpec has four operational modes: Multi-Object Spectroscopy (MOS), Integral Field Unit (IFU) Mode, High-Contrast Slit Spectroscopy (SLIT), and Imaging Mode (IMA). Like the Near Infrared Camera, it will be used to analyze light collected from the origins of the universe.
The Mid-Infrared Instrument is designed to work as both a camera and a spectrograph and picks up where the near-infrared instruments leave off, capturing and analyzing light in the 5000nm to 28000nm wavelength range. The key to its performance in this area is its arsenic-doped silicon detectors, also known as Focal Plane Modules (FPMs), which have a resolution of 1024 × 1024 pixels. The MIRI, which is cryogenically cooled to 7K (~ -447°F), also contains a Low-Resolution Spectrometer equipped with germanium metal and zinc sulfide prisms that can analyze light in the 5000nm to 12000nm wavelength range. It is also equipped with coronagraphs, giving it the ability to study exoplanets.
Finally, the Fine Guidance Sensor/ Near Infrared Imager and Slitless Spectrograph, built by the Canadian Space Agency, is designed to observe light in the 800nm to 5000nm wavelength range and performs two functions. The Fine Guidance Sensor provides the JWST’s sense of direction, aiming it at designated targets. The Near Infrared Imager and Slitless Spectrograph, which is equipped with a 2048 × 2048 pixel mercury cadmium array and has a 2.2 ft × 2.2 ft field of view, is designed to detect and analyze exoplanets.
The Optical Telescope Element (OTE) is, as its name implies, the eyes of the JWST. According to NASA, it consists of the 18 hexagonal segments that make up the 6.5-meter primary mirror; the 0.74-meter circular secondary mirror; the tertiary and fine steering mirrors; the primary mirror’s backplane assembly and main backplane support fixture, which also houses the instrument module; the thermal management subsystem; aft deployable ISIM radiator (ADIR); and the spacecraft’s wavefront sensing and control system.
The final piece of the puzzle is the Spacecraft Element, which consists of the sunshield and the Spacecraft Bus. In adition to supporting the entire 6500 kg mass of the telescope, the Spacecraft Bus, which is made of graphite composite material, houses the JWST’s six major subsystems, namely the electrical power subsystem, the attitude control subsystem, the communication subsystem, the command and data handling subsystem, the propulsion subsystem, and the thermal control subsystem.
The primary function of the electrical power subsystem is to convert the energy collected by the solar panels into the electrical power required by the other subsystems. The attitude control subsystem manages the telescope’s orientation and stability in orbit. The communication subsystem will handle the transmission of data and command signals through NASA’s Deep Space Communication Network. The command and data handling subsystem contains the JWST’s main computer and Command Telemetry Processor (CTP), as well as its Solid State Recorder (SSR) data storage device. The propulsion subsystem consists of the rockets and fuel tanks needed to aim the telescope and keep it in its proper orbit. And the thermal control subsystem is designed to control the four Deployable Radiator Shade Assemblies and maintain critical operating temperatures aboard the spacecraft.
Based on the extraordinary amount and quality of data collected by the Hubble and Spitzer Space Telescopes, the sense of anticipation and excitement regarding what we might learn from the JWST is understandably high. Its projected mission duration is 5 - 10 years and within that time, scientists hope to not only learn more about the origins and formation of our universe, but to gather valuable information on other mysteries such as black holes, supernovae, baby galaxies, and distant planets that might hold the potential for supporting life.
Regardless of what it discovers, like the fictional Starship Enterprise in the popular Star Trek television series, the JWST will give us the very real capability to scientifically go “where no man has gone before.”
This article was written by Bruce A. Bennett, Editor, Photonics & Imaging Technology, SAE Media Group (New York, NY).
- Ivanova, Nelly. 2012. The James Webb Space
- Telescope Science Guide. Baltimore: Space
- Telescope Science Institute (STScl).