In the not-too-distant future, an innocuous U.S. military satellite might aim its antenna at a spacecraft recently launched by another nation or private company, silently sweeping the orbital newcomer with a covert radio-frequency (RF) beam. Seconds later, technicians at a ground station would receive a highly detailed image of the new space platform, plus complete identification of materials that make up its structure and payload. Such possibilities stem from gov- ernment-laboratory and private-company research into the electromagnetic spectrum's terahertz (10~(12) Hz.) region—a frequency band in the "gray zone" between RF and light. These signals—also called "T-waves" by some researchers—range from about 100 GHz. (0.1 terahertz or THz.) to roughly 10 THz., a region of the spectrum loosely referred to as "submillimeter," because wavelengths are on the order of a millimeter long or less. Some definitions set the low-frequency end of "submillimeter" at 300 GHz. Frequencies above 10 THz. are in the far infrared band, still well below the visible-light spectrum.
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