For Release: December 14, 1999
Lori J. Rachul
Glenn Researchers Contribute to Hubble Servicing Mission
The Hubble Space Telescope, one of the most prolific scientific instruments of the century, is about to get new, more durable insulating skin. Researchers at NASA Glenn Research Center have added their knowledge and experience with space-exposed materials to the task.
As part of the third Hubble servicing mission, whose main task will be to replace nonfunctioning gyroscopes, the astronaut crew of Space Shuttle Discovery, scheduled to launch December 16th at 12:13 a.m. EST, will also repair the telescope's multilayer insulation (MLI) blankets. The MLI blankets, which are made up of 17 thin layers of metallized plastic, cover nearly 80 percent of the telescope's surface and protect equipment from temperature extremes as the telescope moves into and out of Earth's shadow.
The outermost blanket layer, or skin, is made of supple, 127-micrometer thick, clear Teflon FEP (fluorinated ethylene propylene), which radiates heat. The Teflon's underside has an ultrathin coating of aluminum, which reflects light. Altogether, the MLI outer later is about the thickness of an ordinary resealable plastic freezer bag.
After the first servicing mission in 1993, Glenn Materials Engineer Kim de Groh traveled to NASA Goddard Space Flight Center, Greenbelt, MD, for a first look at the materials the astronauts had retrieved. She found, with her Goddard colleague, many tiny, barely visible cracks in the material.
During the second servicing mission, in 1997, the astronauts found over 100 cracks in the outer layer, including several long ones whose edges had curled back and exposed the blanket underlayer. The underlayer materials are shiny and reflect light well, but they do not radiate heat well, thus increasing the chance of overheating the telescope's equipment. The astronauts patched the longest cracks and brought back a tightly curled sample.
Shortly after that mission, Glenn materials engineers became members of the Goddard-lead Hubble MLI review board. In comments about the work of the review board, Dr. John Campbell, HST Program Manager at Goddard said, "The MLI review board is an excellent example of what NASA can achieve through intercenter cooperation. Their results, determining the damage mechanism and identifying a replacement material, were only possible with a strong, cohesive team with members from NASA, academia, and industry."
The returned samples were much more brittle than the original material. "The piece from the second servicing mission was so fragile that it tore into pieces, just from the handling necessary for retrieval," said Joyce Dever, Glenn materials engineer. Despite the embrittlement, the material's ability to reflect light and heat was nearly the same as the new material. Thus, cracked but essentially still-in-place material is able to thermally protect the telescope's equipment.
To duplicate the embrittlement here on Earth, the team had to expose the sample materials to the constituents of Hubble's low-Earth orbit environment sequentially, at several government and industry laboratories since no one laboratory is capable of duplicating it. The constituents include solar ultraviolet radiation, solar wind electrons and protons that are trapped along Earth’s magnetic field lines (Van Allen belts), x-rays from solar flares and atomic oxygen from Earth's atmosphere.
Both Glenn and Goddard researchers tested the exposed materials after thousands of heating and cooling cycles to simulate the Sun-shadow temperature swings the telescope experiences. "Although we were not able to duplicate the full extent of the damage, tests show that particle radiation from electrons and protons and nearly 40,000 thermal cycles act together to embrittle Teflon FEP," Dever said.
The review board selected 10 potential replacement materials for evaluation. The research team subjected the materials to space environmental simulation exposures roughly equal to 10 years in low Earth orbit, the expected remaining life of the telescope. Then the team tested them for their tendency to become brittle.
"Ironically, our team chose the same material as before but modified with a scrim, or fabric, bonded to the backside," said de Groh. Teflon FEP's optical properties, or its ability to reflect light and radiate heat, are significantly better than other materials considered. The scrim, with its fabric-like strands, will retard crack growth and help keep the outer layer in place even when it becomes brittle. The new outer layer should last the life of the telescope, until 2010.
Astronauts will bring back from the telescope fresh samples of both the original MLI and MLI used to repair the blankets in 1997. Researchers will then have samples with three, three and a half, seven and ten years of space exposure available for study. "We're really looking forward to studying the returned samples. It is very rare to be able to study materials that have been retrieved after being in the space environment for such a long time," said de Groh.
By examining these materials, researchers will learn more about the low Earth orbit environment and its effect on all kinds of materials. At Glenn, the information learned from these and other studies will be used to design more realistic space environment simulations for assessing the space-worthiness of materials.
Notes to Editors: Joyce Dever is one of the technical advisors who will support NASA mission control during the astronauts' extravehicular activities. She will be telecommunicating with mission control from her office at Glenn.
A NASA video file relating to this story will air on December 13 and 14 at noon EDT. NASA Television is available on GE-2, transponder 9C at 85 degrees West longitude, with vertical polarization. Frequency is on 3880.0 megahertz, with audio on 6.8 megahertz. Video File Advisories can be found at:
An image of a long crack taken during the second Hubble servicing mission is available at:
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