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Radio is electromagnetic radiation (EMR) with a wavelength of 1 mm or above (100000 km is sometimes cited as a maximum), i.e., in the frequency range of 3 kHz to 300 GHz. Infrared is adjacent to it on the high-frequency end. The low frequency end, e.g., past 100000 km wavelength, is sometimes called subradio. Radio astronomy, which began in the 1930s/1940s, has become a significant portion of astrophysical research. Other uses of radio include communication with research spacecraft, and in planetary science, the use of radar and "radio science", a term for miscellaneous radio-based measurements, often borrowing the spacecraft's communications radio, such as for gravity sounding.
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