DTM Visits the James Webb Space Telescope, the Next Successor of Hubble
A number of DTM scientists, including staff scientists Alan Boss, Alycia Weinberger, postdocs Tri Astraatmadja, Sergio Dieterich, Miki Nakajima, Erika Nesvold, research trainee Maggie Thompson, former postdoctoral fellow Jackie Faherty, and two young science explorers, visited NASA Goddard on November 7 to witness the partially assembled James Webb Space Telescope (JWST).
JWST is a red and infrared space telescope (0.6-28.5 µm) whose launch is scheduled for October 2018 from French Guiana. It is a successor of the Hubble Space Telescope (0.1-2.5 µm), the first major optical telescope to be placed in space in 1990. This new space telescope’s scientific goals include observing first light from the stars and galaxies after the Big Bang, understanding star and planet formation, detecting Earth-like exoplanets, and investigating planetary atmospheres that would provide essential information of the planetary surfaces and their habitability.The primary mirror of the JWST. Photo by Miki Nakajima, DTM.
When we arrived at NASA Goddard's visitor center, we were kindly greeted by Briana Horton. After taking a number of pictures with tiny versions of NASA spacecrafts and space telescopes, we headed for the building where JWST is currently being assembled where we met Dr. Mark Clampin, director of the astrophysics division, and Lee Feinberg, JWST optical telescope element manager. The building has the largest clean room in the world at 1.3 million cubic feet, which is indeed necessary to assemble and test this huge telescope. Through a large window, we saw a fully assembled golden primary mirror, as well as the secondary mirror. The primary mirror is much larger at 6.5 m in diameter than that of Hubble at 2.4 m so that it can collect more photons at a time. However, this larger mirror makes it more challenging to send the telescope to space. To solve this issue, the primary mirror, which consists of 18 hexagonal segments, are partly foldable, making the volume of JWST much smaller during the launch than it is in its full shape. Once it enters into its orbit after launch, this origami-like telescope will unfold to its full size.A 3/4 view of the top of the JWST. Photo by NASA.
As you may notice in the photo above, the primary mirror appears shiny and golden, which Clampin explained to us is because it is gold covering mirrors made of ultra-lightweight beryllium. A proposed alternative was a special type of glass that turned out to be much heavier and more expensive to send to space than beryllium. Furthermore, this special type of glass has a larger coefficient of thermal expansion, indicating that this glass is prone to expand as the temperature increases and could have influenced JWST observations. Since the primary mirror is expected to have tiny temperature heterogeneity, this special type of glass could have distorted and interfered with observations if the coefficient was large enough. Thus, beryllium turned out to be more ideal material for this mirror.The JWST will actually orbit the Sun 1.5 million kilometers away from the Earth at what is called the second Langrange point, or L2. Photo by NASA.
Given that its temperature during operation will be below 50K, Clampin mentioned cryogenic testing was also extremely important for JWST development. This low temperature will minimize the thermal emission from the telescope itself, which could have interfered with JWST’s infrared observations.
Clampin also showed us a huge cryogenic facility that is cooled by helium, and pointed out the importance of the telescope’s five sun-shielding layers that will keep JWST cold. JWST will be placed in the L2 point, which is an equilibrium point located at 1.5Mkm from the Earth on the opposite side of the sun. At this specific point, the combined gravitational pull due to the Sun and Earth equals the centrifugal force. The important point here is that the Sun, Earth, and JWST will stay in line, so that the sun-shields will efficiently block light and heat from these bodies.
At the end of the tour, Clampin showed us a gigantic “shaker,” which literally shakes JWST in order to test whether it can endure a series of traumatic shaking experiences, including an airplane landing and a spacecraft launch. The progress of JWST is constantly updated on NASA’s website. If you are interested in seeing JWST in person, you should visit Goddard fairly soon, because JWST will be transported to Johnson Space Center in early 2017.
This time-lapse shows the assembly of the primary mirror of NASA's James Webb Space Telescope. Assembly was completed on February 3, 2016. Video by NASA.
I enjoyed this tour very much. Last time I saw JWST in February 2016, it consisted of only a few hexagonal segments. Now, the primary mirror is fully assembled. Clampin told us that NASA engineers have been working without a break, literally 24/7, by having three shifts a day. I am very much looking forward to obtaining new data from JWST to understand strange new worlds.
Written by Miki Nakajima, November 23, 2016
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