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STS-114: Space Shuttle Return to Flight
NASA’s Linda Clark Looks Through Walls to Ensure Shuttle Safety
June Malone/Martin Jensen
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256.544.0034)
News Release: 05-102

Linda Clark NASA engineer Linda Clark can see through walls.

She laughs at the superhero comparison. After all, the rest of her team can do it too. Clark works in the Safety and Mission Assurance Directorate at NASA’s Marshall Space Flight Center in Huntsville, Ala. Her team has spent two years developing non-invasive technologies to evaluate the Shuttle External Tank’s thermal protection system. Their goal? To help make STS-114: Space Shuttle Return to Flight the safest mission possible.

The technologies, backscatter radiography and Terahertz imaging, are used to evaluate the integrity of the spray-on foam insulation in a non-destructive manner. Instead of cutting apart materials for study, Clark's team uses the techniques to simply look inside, through the wall of foam -- probing for internal anomalies without raising blade or blowtorch.

"If you break your arm, you get an X-ray. If you’re pregnant, you get an ultrasound,” she says. "Such common medical procedures are closely related to our work. We’re using state-of-the-art technologies to help ensure the safety and integrity of the External Tank.”

The tank -- the Shuttle’s 154-foot-long “backbone” -- is covered with an insulating spray-on foam, which helps maintain the tank’s interior temperature and prevents buildup of ice on the exterior. Because foam debris poses a potential risk during launch and the climb to orbit, NASA is developing safer techniques for applying and inspecting its exterior coating.

Enter the non-destructive evaluation team. They first study the foam-covered tank using backscatter radiography, which involves exposing a component to X-rays and collecting the electromagnetic waves that "scatter" or bounce back from the part. These secondary X-rays are generated by interaction between the initial X-ray beam and the material it is passing through. This phenomenon, known as "Compton scattering," is named for Albert Compton, the space scientist who first observed it in 1923. Conventional radiography systems involve transmitting an X-ray through the subject -- a human body or hardware component -- to be captured digitally or on film. Such systems require access to both sides of the subject -- not possible with non-destructive evaluation of foam applied to a Shuttle External Tank.

Backscatter radiography, developed by the University of Florida in Gainesville, provides imagery comparable to regular X-rays, but only requires access to one side of the part. The technology, originally intended to detect landmines, today is being developed by Lockheed Martin and NASA to inspect the Space Shuttle's entire thermal protection system.

NASA is also exploring Terahertz imaging for this purpose. Developed by Picometrix of Ann Arbor, Mich., the technique uses lightning-quick laser pulses to generate high-frequency radio waves that occupy the part of the electromagnetic spectrum between microwaves and infrared light. Until recently, electronic systems supporting the Terahertz range were not commonly available and commercial applications were largely unexplored. Today, a number of companies produce Terahertz imaging equipment, serving industries including security, communications, biomedicine and non-destructive evaluation.

Clark’s team has matured and industrialized both technologies, delivering "field ready" systems to support inspections in the factory at NASA's Michoud Assembly Facility in New Orleans. After conducting successful inspections on several tanks, the team's goal now is to evolve both technologies into certified inspection techniques. In coming months, the team will perform rigorous testing to meet Shuttle certification requirements -- helping to assure the safety of the Shuttle and its crew as they return to flight to complete the International Space Station and pave the way for new exploration of the Solar System.

A native of Birmingham, Ala., Clark grew up a mathematics whiz in an athletics-driven household that included five older brothers. Mentored by her brother Johnnie, also an engineer at the Marshall Center, Clark gravitated toward a career in engineering. She joined Marshall as a co-op student in 1986. Three years later, she obtained a bachelor's degree in materials engineering from the University of Alabama in Birmingham, and went to work full-time in the Non-Destructive Evaluation Department of Marshall’s Engineering Directorate. For more than 12 years, Clark has supported non-destructive research and engineering efforts at the Michoud facility and at NASA’s Kennedy Space Center, Fla.

Clark, who also holds a 1998 master's degree in management from the Florida Institute of Technology in Melbourne, strives to pass on what she’s learned. She has served as a mentor in the Marshall Center's Summer Teacher Enrichment Program, which provides high school science and math teachers with hands-on NASA lab experience. She also has volunteered as a mentor for NASA’s Summer High School Apprenticeship Research Program, or SHARP, which brings exceptional students to work at NASA centers -- discovering firsthand the potential career opportunities in science and engineering.

"Just giving some time to a young person -- sharing a part of who you are, what you've learned, what you've accomplished -- can make a world of difference in their lives," she says. "It demonstrates to them how they too can realize their dreams."

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