Study of celestial bodies by measuring the energy they emit or reflect at radio wavelengths. It began in 1931 with Karl Jansky's discovery of radio waves from an extraterrestrial source. After 1945, huge dish antennas, improved receivers and data-processing methods, and radio interferometers let astronomers study fainter sources and obtain greater detail. Radio waves penetrate much of the gas and dust in space, giving a much clearer picture of the centre and structure of the Milky Way Galaxy than optical observation can. This has allowed detailed studies of the interstellar medium in the Galaxy and the discovery of previously unknown cosmic objects (e.g., pulsars, quasars). In radar astronomy, radio signals are sent to near-Earth bodies or phenomena (e.g., meteor trails, the Moon, asteroids, nearby planets) and the reflections detected, providing precise measurement of the objects' distances and surface structure. Because radar waves can penetrate even dense clouds, they have provided astronomers' only maps of the surface of Venus. Radio and radar studies of the Moon revealed its sandlike surface before landings were made. Radio observations have also contributed greatly to knowledge about the Sun. Seealso radio telescope.
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Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the echoes. This research has been conducted for four decades. Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an active one. Radar systems have been used for a wide range of solar system studies. The radar transmission may either be pulsed and continuous.
The strength of the radar return signal is proportional to the inverse fourth-power of the distance. Upgraded facilities, increased transceiver power, and improved apparatus have increased observational opportunities.
Radar techniques provide information unavailable by other means, such as testing general relativity by observing Mercury, and providing a refined value for the astronomical unit. Radar images provide information about the shapes and surface properties of solid bodies, which cannot be obtained by other ground-based techniques.
The extremely accurate astrometry provided by radar is critical in long-term predictions of asteroid-Earth impacts, as illustrated by the object 99942 Apophis. In particular, optical observations measure very accurately where an object appears on the sky, but cannot measure the distance accurately at all. Radar, on the other hand, directly measures the distance to the object (and how fast it is changing). The combination of optical and radar observations normally allows the prediction of orbits at least decades, and sometimes centuries, into the future.
The following are a list of planetary bodies that have been observed by this means:
Radar provides the ability to study shape, size and spin state of asteroids and comets from the ground. Radar imaging has produced images with up to 7.5-m resolution. With sufficient data, the size, shape, spin and radar albedo of the target asteroids can be extracted.