RADIO AND RADAR ASTRONOMY

study of celestial bodies by examination of the radio-frequency energy they emit or reflect. In 1932 the U.S. radio engineer Karl Jansky, investigating radio disturbances interfering with transoceanic telephone service, found a source of cosmic static he attributed to interaction between electrons and ions (charged atoms) in interstellar space. He located this source in the direction of the centre of the Milky Way Galaxy. The amateur astronomer Grote Reber, also of the U.S., found in the 1930s and 1940s that radio-frequency radiation came from all along the plane of the Galaxy, with a strong maximum, secondary to that of the galactic centre, in the constellation Cygnus. During the 1940s, astronomers of several nations began to make radio observations. After 1945, large antennas (huge dish-shaped reflectors), improved receivers and data-processing methods, and the use of radio interferometers (devices in which the mutual interference or reinforcement of signals is made to yield information) enabled astronomers to study fainter sources and to obtain greater detail. Radio waves penetrate much of the gas and dust in space as well as the clouds of planetary atmospheres. Radio astronomers can therefore obtain a much clearer picture of the central region of the Galaxy, behind clouds of small particles, than is possible by means of optical observation. A better understanding of the Milky Way has resulted from measurements of radio signals produced at the 21-centimetre wavelength by cold clouds of interstellar neutral hydrogen atoms distributed in its spiral arms. Radio astronomers have also discovered various hitherto unknown cosmic objects such as quasars (distant galaxies that release enormous amounts of energy primarily in the form of radio waves) and pulsars (neutron stars that emit highly rhythmic radio pulses). In recent years these and other related objects have been studied at extremely high resolution by means of very-long-baseline interferometry, a technique involving the synchronized observation of a cosmic radio source with multiple-dish interferometers located many kilometres apart. It is possible to send radio signals to astronomical phenomena relatively close to the Earthe.g., meteor trails, the Moon, the nearby planetsand measure the reflections; this use and study of deliberately reflected radio signals constitutes the basis of radar astronomy. It provides one of the astronomer's few means of actively experimenting; unlike most other scientists, he must often simply watch and wait for natural events to observe. In 1946 astronomers in Hungary and the U.S. detected for the first time radar signals bounced off the Moon. Since the 1940s radar has been used to study meteors; a meteor leaves behind it in the upper air an ionized column of gas from which a signal can be reflected. Radar observations have revealed several daytime meteor streams that could never have been observed optically. In 1958 astronomers in the U.S. obtained the first radar echoes from another planet, Venus. Radar reflections from planets provide an accurate measurement of their distances, from which other distances in the solar system can be calculated. Radar techniques made it possible to penetrate the dense cloud cover surrounding Venus and reveal valleys and enormous mountains on the planet's surface. Decisive evidence for the correct rotation period of Venus and of Mercury was also provided by radar. Radio and radar studies of the Moon revealed the sandlike nature of its surface before landings were made. In addition, radio observations have contributed greatly to knowledge about the Sun. As the receiver is tuned to higher frequencies, the radiation received comes from increasing depths within the Sun's outer layers, which can thus be studied separately. Flares and spots on the Sun are strong radio sources.