On Wednesday, NASA revealed the golden honeycomb structure under careful construction at the Goddard Space Center, marking one of the most significant milestones in the life of the James Webb Telescope.
The array of mirrors will be used to capture images from across our solar system and the universe around it. At 21 feet (6.4 meters) wide, the telescope will be the largest ever sent into space, and will observe the earliest galaxies in the universe. It is about 2.7 times larger in diameter than the Hubble telescope, and weighs about one-half of Hubble’s mass at 14,300 pounds (6,500 kg).
NASA live-streamed the unveiling of the mirror, which is made of 18 beryllium segments. They were integrated at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, after being manufactured by Ball Aerospace. The brightly colored coating is a layer of vaporized gold, added to the mirrors in order to improve their reflection of infrared light. The shape of the mirrors is controlled to 0.6 micrometers, in order to be less than a fraction of the wavelengths which it will observe.
The bulk of the weight of the telescope is in the mirror. Beneath it will float layers of Kapton, a commercially available film, coated with aluminum and silicon: these are the sunshields, which will protect the mirrors from the light and heat coming off both the Sun and the Earth.
The spacecraft bus is slung underneath the shields, as is the Earth-facing antenna. The solar panels that power the telescope are a fraction of the size of the sun shield and, naturally, face the opposite direction. A wavefront sensing and control system developed by Ball Aerospace ensures that the segmented mirror performs like a single mirrored surface, by detecting and correcting for an optical errors.
The Webb telescope is undergoing more rigorous testing than the Hubble telescope was, and more stringent testing at NASA than is required during the manufacturing of the parts. This stems from the development of the Hubble Telescope, on which a figure used for setup ended up being wrong since the same information was used for primary testing and final checks.
The James Webb telescope will hover at the second Sun-Earth Lagrange point, a stable orbit in which it will be able to remain at a position consistent with the Earth and Sun. Six gyroscopes are used to sense the orientation of the telescope. Like those used in the Chandra X-Ray Observatory, these are “wine glass gyroscopes,” which use crystal bowls to measure minute amounts of flexing vibration or “jitter.” Reaction wheels – essentially flywheels – are used to turn the telescope to point at particular areas of space.
The observatory is equipped with four scientific instruments in total, the Near Infrared Camera, Near-Infrared Spectrograph, Mid-Infrared Instrument, and Near Infrared Imager and Slitless Spectrogaph, which is integrated with the Fine Guidance Sensor (for guidance!) and will be used to examine exoplanets.
The telescope is scheduled to launch on board an Ariane 5 rocket in 2018, marking the culmination of a project started in July of 2011. Because the surface area of the mirror is larger than any rocket built on Earth, it will fold up for launch. Launch will be the most difficult part of the mission for the relatively fragile beryllium mirrors, so they will be subjected to vibration testing after the entire telescope has been constructed and integrated on Earth.
Once there, it will need to be able to survive orbiting 1 million miles away from the Earth without service, since it will be further away than the reach of any planned crewed vehicle. The James Webb telescope carries enough fuel for 10 years in space, with mission assurance testing designed to guarantee five years of useful scientific operation.