On January 16, 2003, the Space Shuttle Columbia launched on mission STS-107. At T plus 82 seconds, with the orbiter rocketing upwards at 1,870 miles per hour, a briefcase-sized chunk of insulating foam broke off from the external fuel tank and struck Columbia’s left wing. During reentry on February 1, hot gasses entered the wing through the damaged area of the orbiter’s thermal protection system, causing devastating structural failure that led to the destruction of Columbia and the deaths of the seven crew members onboard.

The space shuttle’s external tank is coated in insulating foam that must be checked for defects. To accomplish this, NASA turned to the developing field of terahertz imaging.
After the Columbia disaster, NASA grounded the space shuttles for more than a year as it worked on new safety protocols to ensure that such a tragedy would not happen again. As part of the preparations for the Return to Flight mission, the Agency required a method for detecting potentially hazardous defects in the external tank’s sprayed-on insulating foam prior to launch.


NASA Langley Research Center scientists suspected that a new imaging technology called terahertz imaging had the potential to accurately find flaws in the foam on the external tank. Terahertz radiation—lying between microwaves and far infrared on the electromagnetic spectrum—offers imaging capabilities similar to X-rays, but unlike X-rays, terahertz radiation is non-ionizing and thus safe for frequent human use. Terahertz wavelengths can be used to see through many materials and reveal defects like cracks, voids, and density variations. They can be used to image or as an anomaly detector, or both at the same time.

Terahertz can
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Picometrix, of Ann Arbor, Michigan, was at the forefront of the emerging field of terahertz imaging. In 2000, Picometrix introduced the world’s first commercial terahertz system, the T-Ray 2000. The T-Ray 2000 was based upon the company’s patented fiber coupling system, but was a non-integrated, workbench-mounted system, which rendered it fine for the research market but impractical for NASA’s manufacturing quality control needs. Langley researchers asked the company via Small Business Innovation Research (SBIR) agreements to quickly redesign the terahertz systems to be more integrated and deployable into a manufacturing environment.

Based on the success of that new prototype system, the company was next asked to deliver a more compact, self-contained terahertz system, the T-Ray QA-1000, and NASA purchased five of the systems for inspecting the external fuel tanks as they were being manufactured by Lockheed Martin. The QA-1000’s long, optical fiber umbilicals enabled the system’s terahertz sensors to scan the tank from top to bottom. The systems were deployed at NASA’s Michoud Assembly Facility and at Marshall Space Flight Center. Langley’s original unit was later retrofitted with a similar higher speed delay stage that was also capable of imaging thicker foam.

“This was significant. In addition to the company’s patented fiber coupling system that makes Picometrix systems unique, they can also inspect thicker material at substantially higher speed with our T-Ray systems versus others terahertz systems,” says Irl Duling III, company director of terahertz business development.

Picometrix became a wholly owned subsidiary of Advanced Photonix Inc. (API), also of Ann Arbor, Michigan, in 2005. The company’s terahertz systems—including its latest, highly compact and rugged T-Ray 4000 systems—were later adopted by Kennedy Space Center as a diagnostic tool for scanning the orbiter’s thermal tiles for the remaining shuttle flights. The systems offered an effective way of not only inspecting the tiles for hidden damage, but also of precisely locating components underneath the tiles that were in need of attention—without the costly removal and replacement of extra tiles which often happened before the use of the T-Ray 4000.

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