NASA engineers unsealed the vault-like, 40-foot diameter, 40-ton door of Chamber A at Johnson Space Center in Houston on November 18, signaling the end of nearly 100 days of cryogenic testing for the agency’s James Webb Space Telescope. The cryogenic vacuum test began when the chamber was sealed shut on July 10, 2017. Scientists and engineers at Johnson put Webb’s optical telescope and integrated science instrument module (OTIS) through a series of tests designed to ensure the telescope functioned as expected in an extremely cold, airless environment akin to that of space.
“After 15 years of planning, chamber refurbishment, hundreds of hours of risk-reduction testing, the dedication of more than 100 individuals through more than 90 days of testing, and surviving Hurricane Harvey, the OTIS cryogenic test has been an outstanding success,” said Bill Ochs, project manager for the James Webb Space Telescope at NASA’s Goddard Space Flight Center in Greenbelt MD. “The completion of the test is one of the most significant steps in the march to launching Webb.”
The tests included an important alignment check of Webb’s 18 primary mirror segments, to make sure all of the gold-plated, hexagonal segments acted like a single, monolithic mirror. This was the first time the telescope’s optics and its instruments were tested together, though the instruments had previously undergone cryogenic testing in a smaller chamber at Goddard. Engineers from Harris Space and Intelligence Systems, headquartered in Melbourne FL, worked alongside NASA personnel for the test at Johnson. “The Harris team integrated Webb’s 18 mirror segments at Goddard and designed, built and helped operate the advanced ground support and optical test equipment at Johnson,” said Rob Mitrevski, vice president and general manager of intelligence, surveillance and reconnaissance at Harris. “They were a key, enabling part of the successful Webb telescope testing team.”
Engineers spent about a week removing air from the chamber before beginning to bring the chamber, the telescope and the telescope’s science instruments down to cryogenic temperatures—a process that took about 30 days. During cooldown, Webb and its instruments transferred heat to surrounding liquid nitrogen and cold gaseous helium shrouds in Chamber A. Webb remained at “cryo-stable” temperatures for around 30 more days before the engineering team began to warm the chamber back to ambient conditions on Sept. 27.
“With an integrated team from all corners of the country, we were able to create deep space in our chamber and confirm that Webb can perform flawlessly as it observes the coldest corners of the universe,” said Jonathan Homan, project manager for Webb’s cryogenic testing at Johnson. “I expect [Webb] to be successful, as it journeys to Lagrange point 2 [after launch] and explores the origins of solar systems, galaxies, and has the chance to change our understanding of our universe.”
While Webb was inside the chamber, insulated from both outside visible and infrared light, engineers monitored it using thermal sensors and specialized camera systems. The thermal sensors kept tabs on the temperature of the telescope, while the camera systems tracked the physical position of Webb to see how its components very minutely moved during the cooldown process. Monitoring the telescope throughout the testing required the coordinated effort of every Webb team member at Johnson. “This test team spanned nearly every engineering discipline we have on Webb,” said Lee Feinberg, optical telescope element manager for the Webb telescope at Goddard. “In every area there was incredible attention to detail and great teamwork, to make sure we understand everything that happened during the test and to make sure we can confidently say Webb will work as planned in space.”
In space, the telescope must be kept extremely cold, in order to be able to detect the infrared light from very faint, distant objects. Webb and its instruments have an operating temperature of about 40 K. Because the Webb telescope’s mid-infrared instrument (MIRI) must be kept colder than the other research instruments, it relies on a cryocooler to lower its temperature to less than 7 K.
To protect the telescope from external sources of light and heat (like the Sun, Earth and Moon), as well as from heat emitted by the observatory, a five-layer, tennis court-sized sunshield acts like a parasol that provides shade. The sunshield separates the observatory into a warm, sun-facing side (reaching temperatures close to 185 degrees Fahrenheit / 85 degrees Celsius) and a cold side (minus 400 degrees Fahrenheit / minus 240 degrees Celsius). The sunshield blocks sunlight from interfering with the sensitive telescope instruments.
Webb’s combined science instruments and optics will next journey to Northrop Grumman Aerospace Systems in Redondo Beach CA, where they will be integrated with the spacecraft element, which is the combined sunshield and spacecraft bus. Together, the pieces form the complete James Webb Space Telescope observatory. Once fully integrated, the entire observatory will undergo more tests during what is called “observatory-level testing.” This testing is the last exposure to a simulated launch environment before flight and deployment testing on the whole observatory.
Webb is expected to launch from Kourou, French Guiana, in the spring of 2019.