Engineering the James Webb Space Telescope
AMY LO
Northrop Grumman Aerospace Systems
NASA’s James Webb Space Telescope (JWST) is the premier space telescope of its time. Set to launch in October 2018, it is designed to look at “first light” to image the formation of stars and galaxies. Building on the successes of the Hubble Space Telescope (HST), the JWST will look farther back in time—13.5 billion years, to the beginning of the universe—to dimmer, redder targets that were some of the very first objects to form in the universe.
The JWST is an engineering marvel unlike any space telescope before. The 6.5 meter aperture is accompanied by a 14 m × 22 m sunshield to passively cool the entire telescope portion of the observatory to cryogenic temperatures. Transported on an Ariane 5 launch vehicle, the JWST must fold up like an origami and travel 1.5 million km from Earth, to the Second Lagrange Point. At this distance, the JWST will not be serviceable like the Hubble and must deploy and operate flawlessly. The engineering required for this time machine is itself opening new pathways of technological capacity.
This paper describes the engineering needed to meet the JWST science goals, focusing on the precision design and testing required for the sunshield, which provides crucial protection to the telescope and scientific instruments. In addition, some of the challenges facing the alignment of the sunshield structure are discussed. The concluding section provides the latest status of the observatory and remaining steps to the launch in 2018.
CURRENT VS. NEWLY ENGINEERED CAPABILITIES
Wavelengths that are inaccessible from ground-based astronomical observations are accessed by launching telescope observatories into space. NASA has
launched a series of such observatories and significantly advanced understanding of the universe.
One of the greatest currently active space-based observatories is the Hubble Space Telescope. It has revolutionized just about every aspect of astronomy, making its greatest contributions in observations of the early universe. Figure 1 shows the HST view of the extreme deep field (XDF) of a small region in the Fornax constellation. This is one of the most sensitive images taken in the visible wavelength, and captures some of the oldest galaxies ever imaged—formed when the universe was just 450 million years old (it is now ~13.7 billion years old) (Illingworth et al. 2013).
Using data from the XDF, astronomers are able to probe the structure and organization of the beginning of the universe. But the galaxies that appear in the
FIGURE 1 Hubble extreme deep field (XDF), showing some of the oldest galaxies ever imaged. They appear as dim, red, fuzzy blobs. Figure in color at http://www.nap.edu/catalog/21825. Credit: NASA; European Space Agency (ESA); G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the Hubble Ultra Deep Field 2009 (HUDF09) team.
XDF are not the oldest galaxies ever formed: because of their distance and age, such galaxies are too dim to be captured by the HST and redder than the longest HST wavelength. The oldest, or the first, galaxies are thought to have formed around 200 million years after the Big Bang, or another 250 million years earlier than Hubble can see.
To enhance understanding beyond what can be learned from the HST, NASA has set out science goals for the JWST in four areas: first light and reionization (e.g., the formation of structures in the universe); assembly of galaxies (e.g., how galaxies are formed and what happens when small and large galaxies merge); the birth of stars and protoplanetary systems (e.g., the formation of stars and planetary systems); and planets and origins of life (e.g., planet formation, orbits, and habitable zones). Details of the JWST science goals are available from NASA (http://jwst.nasa.gov/science.html) and other sources (e.g., Gardner et al. 2006). The focus of this paper is on the engineering needed to support achievement of the first light goals.
As the successor to the HST, the James Webb Space Telescope is designed to have higher resolution, with optics seven times the surface area of the HST, making it 100 times more powerful. It will be able to probe the infrared portion of the electromagnetic spectrum to see these first light objects. Additionally, compared to the HST, the JWST has capabilities to 28 microns, versus the HST’s maximum wavelength of 2.5 microns. With these enhanced technical capacities, the JWST will be able to image objects as old as 13.5 billion years, or ~200 million years after the Big Bang.
ENGINEERING THE JAMES WEBB SPACE TELESCOPE
The four science goals may be distilled to the following JWST primary imaging requirements:
- targets anywhere in the sky
- faint targets (requiring high sensitivity with low background)
- small targets (requiring high resolution with low jitter)
- infrared objects (requiring a cool telescope to reduce background)
The mission architecture of JWST developed to meet these requirements necessitated a significant amount of technology development. NASA initiated Phase A technology development for the JWST in the late 1990s, and in 2007 ten JWST critical developments were brought to Technology Readiness Level (TRL) 6 (Figure 2), a criterion for program confirmation (Gardner et al. 2006).
Among the technologies developed for the JWST, this paper will focus on the sunshield (Figure 3).
FIGURE 2 James Webb Space Telescope (JWST) technology development items identified at the start of the flight program. All of the items completed Technology Readiness Level (TRL) 6 development by 2007. IR = infrared; MIRI = mid-infrared instrument; SIDECAR ASIC = System for Image Digitization, Enhancement, Control and Retrieval Application Specific Integrated Circuit. Credit: NASA/Goddard Space Flight Center.
THE JWST SUNSHIELD
Measuring 14 m × 22 m (the size of a tennis court) when deployed, the sunshield has to protect the sensitive telescope and instruments from visible and thermal radiation from the Sun, Earth, Moon, and the spacecraft itself.
Design, Function, and Performance
Unlike the HST barrel assembly forming a cylinder centered on the primary optics, the open JWST sunshield design leaves the telescope exposed to space to facilitate the passive cooling of the optics to cryogenic temperatures of 40 to 50 Kelvin. This design also provides shadow over the JWST’s pitch angles of +5° to −45° and roll angles of +5° to −5°.
The sunshield performs three major functions: (1) it shields the telescope from direct sunlight, earthlight, and moonlight, enabling the rejection of incident
FIGURE 3 Main elements of the James Webb Space Telescope, with the tennis court–sized sunshield shading the telescope and science instruments from intense solar radiation. Credit: Northrop Grumman Corporation.
solar radiation such that only ~1 part in 300,000 is transmitted; (2) the upper layers prevent stray light from entering the telescope; (3) the lower layers absorb and prevent the bouncing of light into the telescope. These features keep the telescope cold and limit background noise effects on the science images.
To meet these performance requirements, the sunshield needs to be carefully aligned to ensure that the deployed structure maintains its edges within a few centimeters of the nominal location. Detailed performance analyses were necessary, starting with the on-orbit environment, where perturbations to the deployed sunshield were assessed and controlled. Environmental factors that affect the sunshield on orbit are thermal distortion, composite dry-out, the elastic response of the sunshield structure under tension, and the inelastic response (or creep) of the sunshield structure under tension. These effects were carefully modeled and quantified.
Measurement and Testing
Rigorous processes are used to control the tolerances of relevant sunshield parts; manufacturing and as-built structure dimensions are examined and precisely
measured. Exposure of the observatory to the harsh launch vehicle environment can induce otherwise solidly attached parts to shift slightly at joints that are not bonded; these launch shifts are calculated based on design tolerances and allocated.
The need for deployment also affects the ultimate alignment of the sunshield, as the precision of the deployment is dependent on the details of the mechanical part performing the deployment. An extensive error budget has been constructed to account for these effects: environmental distortion, manufacturing and installation error, launch shift, and deployment repeatability.
A thorough set of alignment tests were baselined to measure and test the JWST sunshield in order to verify and/or demonstrate the values in the alignment error budget and quantify its on-orbit performance. Starting at the unit level and extending to the end of the observatory integration and test program, every major piece of the sunshield is measured, tested, and measured again to ensure that it has been properly characterized and its on-orbit performance is well understood.
FINAL STEPS TOWARD LAUNCH
As of the writing of this paper, most of the major components of the JWST sunshield are being manufactured; some subassemblies are undergoing testing and a high-fidelity pathfinder has recently completed deployment. The alignment program is well under way, measurements of the sunshield are being compiled, and pretest analysis is being done.
For the rest of the observatory, the telescope structure is complete and was shipped to Goddard Space Flight Center (GSFC) in August 2015. All mirrors have finished fabrication and are being installed at GSFC. The instruments in support of the four science goals have been integrated into their holding structure and are undergoing extensive testing. The spacecraft bus structure is complete and is in testing, as are many of its subsystems, preparing for integration into the spacecraft.
In 2017 the completed telescope portion, along with mirrors and instruments, will return to Northrop Grumman’s Space Park facility in Redondo Beach, California, to be integrated with the spacecraft and sunshield. The observatory will then undergo final testing and be shipped to French Guiana for launch, planned for October 2018.
REFERENCES
Gardner JP, Mather JC, Clampin M, Doyon R, Greenhouse MA, Hammel HB, Hutchings JB, Jakobsen P, Lilly SJ, Long KS, and 13 others. 2006. The James Webb Space Telescope. Space Science Reviews 123(4):485–606.
Illingworth GD, Magee D, Oesch PA, Bouwens RJ, Labbé I, Stiavelli M, van Dokkum PG, Franz M, Trenti M, Carollo CM, Gonzalez V. 2013. The HST extreme deep field (XDF): Combining all ACS and WFC3/IR data on the HUDF region into the deepest field ever. Astrophysical Journal Supplement 209(1):6–19.
