The Webb Space Telescope Finishes Cryogenic Testing

Vacuum chambers come in many shapes and sizes, from small, hermitically sealed microelectronics to huge space systems test chambers to the 4km long LIGO chambers. On December 1st, the James Webb Space Telescope emerged from the large Chamber A test facility at Johnson Space Center, which is a great piece of vacuum technology.

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NASA’s James Webb Space Telescope sits inside Chamber A at NASA’s Johnson Space Center in Houston after having completed its cryogenic testing on Nov. 18, 2017. This marked the telescope’s final cryogenic testing, and it ensured the observatory is ready for the frigid, airless environment of space. Credits: NASA/Chris Gunn

The James Webb Space Telescope will be a large infrared telescope with a 6.5m primary mirror. The scheduled launch date is in the spring of 2019. It will be lifted on an Arian 5 rocket from French Guiana.

Before a space instrument is put into service, it must be rigorously tested in a space simulation chamber. NASA’s Johnson Space Center’s Chamber A was prepared. The vault-like, 40-foot diameter, 40-ton door was sealed shut on July 10, 2017, beginning the scheduled 100 days of cryogenic testing for NASA’s James Webb Space Telescope in Houston.

There are two important reasons that Webb needs to be tested in a space simulation chamber. First is that the telescope will be operating in a far orbit at cryogenic temperatures. Webb was designed so that the instrument will come to thermal and mechanical stability to known dimensions in the cold of deep space. Secondly, the infrared equipment must be tested in a cryogenic chamber because objects at room temperature emit infrared energy that would swamp the instruments, so putting the space telescope in a cryogenic chamber all but eliminates background infrared energy.

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Engineers watch as Chamber A’s colossal door closes at NASA’s Johnson Space Center in Houston.
Credits: NASA/Chris Gunn

The chamber evacuation sequence started on July 20, 2017. Engineers began to bring the chamber, the telescope and its science instruments down to cryogenic temperatures a process that took about 30 days. The cold gaseous helium shroud inside Chamber A is the innermost of two shrouds used to cool the Webb telescope down to the temperatures at which it will operate while in orbit. This shroud sits inside an outer liquid nitrogen shroud.  During cool down, Webb and its instruments transferred their heat to surrounding cold gaseous helium and liquid nitrogen shrouds. Liquid nitrogen reaches 77 Kelvin (minus 321 degrees Fahrenheit/minus 196 degrees Celsius), while the cold gaseous helium in the shroud gets as low as 11 Kelvin (minus 440 Fahrenheit/minus 262 Celsius).

Webb remained at “cryo-stable” temperatures for about another 30 days. The thermal sensors kept track of the temperature of the telescope, while the specialized camera systems monitored the physical position of Webb’s components as they moved during the cool down process. These 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.

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NASA’s James Webb Space Telescope cools to cryogenic temperatures by radiating heat through the surrounding vacuum, to the liquid nitrogen and cold gaseous helium shrouds. The outside dimensions of Chamber A are 19.8m (65ft) in diameter and 33.6m (120ft) in height. The chamber is equipped with staged roughing pumps, valved turbo molecular and cryo absorption pumps, and 20 K (-424 oF) helium refrigeration units. The high vacuum pumps speeds are rated at 2 x 107 liters/sec condensibles and 3 x 105 liters/sec noncondensables at 1 x 10-6 torr pressure.

 

Systems testing continued and on Sept. 27, the engineers began to warm the chamber back to near room temperature, before pumping the air back into it and unsealing the door. This was done slowly and on the 18th of November, the chamber door was unsealed.

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