This is the fourth installment in a four-part series of conversations with Paul Geithner Deputy Project Manager - Technical for the James Webb Space telescope at NASA's Goddard Space Flight Center in Greenbelt, Maryland about different aspects of the telescope. This installment focuses on the importance of keeping Webb's telescope and scientific instruments at extremely cold temperatures.
The James Webb Space Telescope will gaze into the cosmos in infrared light, which will help unlock many mysteries. Using infrared light, the Webb telescope will be able to look farther back in time than previous telescopes, and will allow scientists to look through dust to see stars forming inside. Paul provides answers to questions about the kind of freezing temperatures the Webb telescope will endure in space.
Q: What kind of conditions does the Webb telescope and instruments need to withstand?
Paul: The Webb telescope has to survive the mechanically stressing conditions from the violent vibration of launch, and the 'cold' half of the observatory—the telescope and scientific instruments—have to survive the thermal shrinkage that occurs cooling down from room temperature to the cryogenic temperatures at which they operate. This is a significant engineering challenge in that the Webb's telescope and scientific instruments operate at extremely cold temperatures but they are built at room temperature. Things typically shrink as they get cold, and materials shrink differently at various rates with temperature. We have to build the Webb telescope precisely wrong at room temperature so that it shrinks to precisely the right shape and dimensions when it's cold. Moreover, it has to survive the stress of things shrinking and expanding when we test it cold and warm it back up again on the ground, and when it goes cold for good when it goes into space.
The Webb telescope also has to survive years in space exposed to radiation from the sun and from the galaxy.
Q: Where is the Webb telescope going to be in space?
Paul: The Webb telescope will be in an L2 orbit. L2 is short-hand for the second Lagrange Point, a wonderful accident of gravity and orbital mechanics, and the perfect place to park the Webb telescope in space. There are five so-called "Lagrange Points" - areas where gravity from the sun and Earth balance the orbital motion of a satellite. Putting a spacecraft at any of these points allows it to stay in a fixed position relative to the Earth and sun with a minimal amount of energy needed for course correction. The L2 point is about a million miles from Earth: if you draw a line from the Sun through the Earth, and keep going a million miles, that’s where L2 is. For more information, visit: http://www.nasa.gov/topics/universe/features/webb-l2.html
Q: Why is cryo-testing so important?
Paul: Super cold or "cryogenic" tests are part of demonstrating and verifying that instruments and components operate like they should and will operate properly once at the L2 point.
Q: How does a cryo-test test work?
Paul: We put the Webb telescope's hardware into a big vacuum chamber, close the door, pump all the air out, then run liquid nitrogen and extremely cold helium gas through plumbing that criss-crosses the surface of thin 'shells' nested Russian-doll style inside the vacuum chamber. Because the shells (a.k.a. shrouds) inside the chamber are very cold—the outer one approaching 77 kelvin (the temperature of liquid nitrogen) and the inner one running between 10 and 20 kelvin (the temperature of the cold helium gas we circulate)—anything nestled inside them will radiate their latent heat to them and get really cold too. The effect is like that on a clear night when the heat from the previous day radiates freely into the night sky and by morning the temperature can be quite cold. Think the desert, where skies are typically dry and clear and even though it's scorching hot during the day it gets frigid at night.
In our cryogenic tests and when talking about the Webb telescope, we usually use the Kelvin scale when referring to temperature because it makes more scientific and engineering sense than the more familiar Fahrenheit and Celsius scales. Photography enthusiasts and people who notice “color temperatures” on light bulb packages will recognize degrees Kelvin, or K. Named after the famous 19th century physicist and engineer Lord Kelvin (William Thomson), the Kelvin scale uses the same increment as the Celsius scale but zero is pegged at absolute zero, which is the theoretical coldest temperature in physics. So, absolute zero is 0 K or -273.15 degrees Celsius (C) or -459.67 degrees Fahrenheit (F). The freezing point of water, which is 0 C or 32 F, is 273.15 K. One handy thing about using an absolute temperature scale like the Kelvin scale is that temperatures and heat content of things can be directly related, meaning for example that something that is 300 kelvin is literally “twice as hot,” i.e., has twice the heat energy, as something that is 150 Kelvin. The total absence of heat energy is absolute zero, which is 0 K, helium turns liquid at 4 K, oxygen is a solid at about 54 K, Nitrogen is a liquid at about 77 K, dry ice (solid carbon dioxide) is about 217 K or -109 F, and of course water ice melts at 273.15 K, or 0 C or 32 F. Room temperature—72 F—is about 295 K or about 22 C, and boiling water is 212 F or 100 C or 373.15 K.
Q: How long do cryo-tests last?
Paul: The test just completed in the fall of 2014 on the integrated science instrument module lasted about 4 months, and was run around-the-clock. Cryo-vacuum testing takes time, not only because of all the parts that need to “exercised” to see that they all move, focus, turn, and otherwise work and that the software used to operate them works, but also because it takes weeks just to create a vacuum and cool-down a big test article to a steady and frigid temperature. Also, once you start, the operation is 24/7 until it’s over, so you have to coordinate shifts and make sure people don’t get burned-out, which could lead to costly or even dangerous mistakes, and you have to be ready for “what-if” scenarios in advance.
Q: Can you describe how the Primary Mirror was cryo-tested?
Paul: The individual primary mirror segments were tested at the X-ray & Cryogenic Facility at NASA's Marshall Space Flight Center in Huntsville, Alabama over the course of ten weeks in 2013. During the test, the primary mirror segments were chilled to around -379 degrees Fahrenheit, or about 45 kelvin.
By cryotesting the mirrors, NASA can verify that they will respond as expected at their frigid operating temperature once in space and therefore will focus light correctly. While the mirrors were being tested, engineers took detailed measurements of how the shape of each one changed as they cooled and that they indeed achieved the correct curvature or “figure” when cold.
The Webb telescope is the world's next-generation space observatory and successor to NASA's Hubble Space Telescope. The most powerful space telescope ever built, the Webb telescope is designed to observe the most distant objects in the universe, provide images of the first galaxies formed and study unexplored planets around distant stars. The Webb telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.