The James Webb Space Telescope, a joint project of NASA, the European Space Agency (ESA), and the Canadian Space Agency, is planned to be launched in 2018 on an ESA Ariane 5 rocket. The Northrop Grumman Corporation and its partner ATK have announced that the final component of the Primary Mirror Backplane Assembly (PMBA), called the backplane support frame, is now complete. The PMBA is the backbone that will support the telescope’s 6.5-meter segmented primary mirror and ensure the exact alignment of each of the gold-coated beryllium mirror segments.
Northrop Grumman is responsible for the design and development of Webb’s sunshield, telescope, and spacecraft. ATK is now adding the backplane support frame to the backplane center section, which it completed in April 2012, and the two backplane wing assemblies, which it completed in March. ATK has manufactured the thousands of composite parts of the PMBA using lightweight graphite components, connected with precision metallic fittings made of invar and titanium.
The PMBA center section will support 12 of the 18 primary mirror segments, while the two wings to be attached to each side of the center section will support three mirror segments each. This arrangement will allow the primary mirror to be folded up within the nose cone of the Ariane 5 launch vehicle and be fully deployed later, when the spacecraft is well beyond the Earth’s atmosphere. The newly built component, the backplane support frame, will connect the PMBA center section and the wings, as well as eventually support the aft optics subsystem of the telescope.
The full PMBA structure will be approximately 8 meters tall, 7 meters wide, and 4 meters deep. It will have a mass of 970 kilograms, but will support a mass over three times its own, in the form of the mirror segments and key optical subsystems of the telescope.
Being the backbone of a multi-billion dollar space telescope that will not only surpass the optical resolving power of the Hubble Space Telescope, but also the infrared resolving power of the Spitzer Space Telescope, the PMBA is expected to provide unprecedented thermal stability. This is vital, as the assembly must ensure the Webb Telescope’s optics are precisely aligned for its deep space operational environment of less than 40 Kelvin and also maintain that alignment through the physical stresses of launch and the subsequent cooling of the telescope from Earth ambient temperatures (of around 300 Kelvin).
Should the mirror fail to be precisely aligned when it reaches its mission destination near the Sun-Earth Lagrange point 2—a distance of 1.5 million kilometers from Earth—it would not be feasible to undertake a manned repair mission, as was the case with the Hubble Space Telescope.
Next steps in construction are to deliver the PMBA components to Marshall Space Flight Center later this year for extreme cryogenic testing and then return them to Northrop Grumman’s facilities for static structural testing before all components (the center, the wings, and the support frame) are fully integrated and ready to support the remaining components of the Webb Telescope.
The James Webb Space Telescope site contains more details about the construction of the telescope, including short Behind the Webb video clips about the construction of the PMBA center section and the two PMBA wings.
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