How strong must a space telescope be to survive both in the environment of the Earth and in the cold and airless environment of space? Paul Geithner, the assistant project manager – technician for the James Webb Space Telescope at NASA's Goddard Space Flight Center in Greenbelt, Maryland, answered some questions about the design challenges of the telescope's construction and the challenge of testing that has endured in the years prior to launch. James Webb Space Telescope, or Webb, is NASA's next infrared space observatory, to be launched in 2019.
Q: What kind of conditions must Webb and its instruments endure?
Paul: The entire observatory must survive by mechanically pressing the conditions of the violent vibration of the launch. In addition to this, the "cold" half of the observatory ̵
Webb has to survive years in space, exposed to radiation from the sun and the galaxy.
Q: Why are the vibration tests so important and how does it show that Webb is ready for the rigors of the launch?
Paul: Vibration tests are performed for two reasons. One reason is to validate that Webb can handle the rigorous shake it will experience while driving a rocket into space, and the other reason is to verify the fabrication of Webb's construction and prove that the design was designed and assembled correctly.
We use two complementary methods of vibration. For lower frequencies of vibration, that is, from about 5 hertz (cycles per second) to 100 hertz, we place the hardware on a surface, basically a large metal plate, which is mounted on the bearings so that it can move forward and backwards, and this surface is connected to essentially a large electromagnet that generates the shaking motion.
For higher frequencies, more than 100 hertz, it is very difficult or impossible to achieve the necessary vibration with a large vibration table system, so instead we put the hardware in an acoustic chamber. This is a thick-walled room with large speakers that produce literally deafening sound levels.
Taken together, the vibration table and the acoustic chamber produce the vibration environments we need to properly test the Webb. In general, for a one-of-a-kind article, such as Webb, the vibration levels we submit to field tests are roughly double the duration of the mission. These tests give us confidence that Webb has been assembled correctly, will survive the actual flight and will work as designed once in space.
Q: Why are cryogenic tests so important? How do you see that Webb is ready for the tensions of space?
Paul: Super cold or "cryogenic" tests are part of the demonstration and verification that Webb's instruments and components work as they should and will work correctly once at the second Lagrange point (L2) on Earth. Point L2 is 1 million miles from Earth.
We put the Webb telescope hardware in a large vacuum chamber, close the door, remove all the air and then apply extremely cold liquid nitrogen and helium through the pipe that intersects the surface of the thin "shells" that they are nested in the style of Russian dolls inside the vacuum chamber.
The shells are also called shrouds, and they are very cold. The outside is close to 77 Kelvin (approximately minus 321 degrees Fahrenheit / minus 196 degrees Celsius, the temperature of liquid nitrogen). The inner shell runs between 10 and 20 Kelvin (between minus 442 degrees Fahrenheit / minus 263 degrees Celsius and minus 424 degrees Fahrenheit / minus 253 degrees Celsius, the temperature of the cold helium gas). Anything huddled within the shrouds will radiate its latent heat to them and it will get really cold too.
The effect is similar to what happens on a clear night, when the heat of the previous day radiates freely in the night sky. In the morning, the temperature can be quite cold. Think of the desert, where the skies are typically dry and clear. Although it is very hot during the day, it becomes cold at night because the heat radiates from the surface.
Q: Why does Webb need a "sunscreen" and what kind of protection does it provide?
Paul: The instruments are shaded from the Sun by a deployable parasol the size of a five-layered tennis court. The parasol consists of deployable barriers and gauze polyimide membranes, essentially special plastic sheets (DuPont Kapton), each approximately one thousandth of an inch thick and coated with reflective aluminum and protective silicone. Basically, it looks like a giant five-layered silver comet in space.
We need a lens hood to keep the telescope and instruments cool because Webb is an infrared telescope, which means it sees infrared light. Infrared light is light that is of slightly longer or redder wavelengths than visible light. We can not see it with our eyes, but we can feel it as radiant heat.
For an infrared telescope to be as sensitive as possible, its optical and scientific instruments must be very cold, so its own heat does not blind them to the faint infrared signals they are trying to observe from astronomical objects. In the space and in the shade of the sun next to the parasol, the telescope and the scientific instruments will face the extreme cold of deep space and they will cool a lot.
Q: What materials were used to build Webb and how do these Webb materials increase? resilience?
Paul: We use beryllium for many of Webb's mirrors and some of the structures because it is light, rigid, resilient and dimensionally stable (stops shrinking and expanding) at the operating temperature of the telescope. Beryllium changes a lot of dimensions with temperature, but it practically stops when it falls below a temperature of 100 degrees Kelvin (minus 280 degrees Fahrenheit or minus 173 degrees Celsius).
We use many other materials at Webb, including aluminum for some things, stainless steel for fasteners, titanium for structures and fasteners, invar (an alloy) for structural nodes and many other metals. We also have non-metallic compounds such as graphite-epoxy for most structures and silicon carbide ceramics for one of the scientific instruments (the near-infrared spectrograph – NIRSpec).
Q: Webb's orbit at the second Lagrange point (L2) of Earth is beyond the protective covering of Earth's magnetic field, which means that the telescope is more susceptible to solar radiation and solar flares . How is Webb isolated from these threats?
Paul: The Earth's magnetic field acts as a deflector shield of protons and electrons that spew all the time from the Sun. The protection of satellites within the Earth's magnetic field includes placing aluminum panels similar to metals between the electronics and the space environment, implementing a good connection to electrical ground and making the electronic components resistant to radiation. Because Webb is outside the Earth's magnetic field, it will be bombarded by charged particles that flow from the Sun, so it needs additional protection. These charged particles are hard on electronic components and can accumulate on surfaces to accumulate static charge that can cause harmful discharges.
Webb will also be vulnerable to the occasional massive "belches" of the Sun that occur with solar and coronal eruptions. massive ejections, which are phenomena in which the Sun releases slugs of perhaps a few years of protons and electrons in just a few hours. To allow Webb to weather the stormy solar weather and normal "pleasant days," almost all of its electronic components are protected inside metal boxes and behind multiple layers of metal or metal-coated film.
The electronics on the cold side of Webb's parasol have some benefit of being behind the five layers of the shield, which are coated with aluminum. The electronic components inside the spacecraft bus, which is oriented towards the Sun, are hardened, shielded and grounded. Webb used proven and true design practices and satellite building codes to ensure that it will survive and function in the hardness of the L2 environment.
Q: Webb was not designed to receive service, but it could be repaired or resupplied during a robotic operation. Service mission?
Paul: Possibly, some Webb robotic service could be possible. A robot could deal with Webb in the same place where it was connected to the Ariane launch vehicle, which is the launcher's interface ring on the spacecraft bus that faces the Sun, and then add fuel to its propulsion tank. Since Webb is an exquisitely sensitive infrared observatory, and much of it is at cryogenic temperatures, the opportunities and benefits of the service are limited.
The James Webb Space Telescope is the first infrared space observatory in the world for the next decade. A ground breaking mission for engineers and astronomers, Webb will solve the mysteries of our solar system, look beyond distant worlds around other stars and explore the mysterious structures and origins of our universe and our place in it. Webb is an international program run by NASA with its partners, the European Space Agency (ESA) and the Canadian Space Agency (CSA).
Webb Telescope emerges from Camera A at JSC
Houston TX (SPX) December 4, 2017
The combined scientific instruments and optical element of the telescope recently completed about 100 days of testing cryogenic inside Johnson's Chamber A, a massive thermal vacuum test chamber in the center. Johnson's scientists and engineers subjected Webb to a series of tests designed to ensure that the telescope worked as expected in an extremely cold and airless environment similar to space. … read more
Webb Telescope at NASA
Stellar Chemistry, The Universe and All Within It
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