radar astronomy

radar astronomy, application of radar to the determination of distances and planetary features within the solar system, such as rotation rates. A short burst of radio waves is transmitted in the direction of the object under study. The object reflects the radio waves back to earth, where they are detected by the same antenna that sent the signal. The time between sending the signal and receiving the "echo" can be precisely measured electronically. Since radio waves travel with the speed of light, the roundtrip distance from the earth to the object and back is then easily computed. This technique differs from radio astronomy in that the celestial object is here merely a passive reflector, rather than the actual source of the emission. The first yield of radar astronomy was a much improved value for the distance from the earth to the moon. Using more powerful transmitters, the distances to Venus and Mercury were also measured, as well as the planets' rotational periods and gross surface properties. Even greater precision is obtained by replacing the radio transmitter with a laser. During the Apollo project, special reflectors were installed on the moon; subsequently, by bouncing laser light off the moon the distance from the earth to the moon could be determined within centimeters. Radar observations are also useful for asteroids and comets whose orbits take them relatively near the earth. Much of the surface of Venus has been mapped by unmanned probes using radar altimeters to penetrate the cloud cover.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

radio and radar astronomy

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.

Learn more about radio and radar astronomy with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Radar astronomy

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.

Advantages

Control of attributes of the signal [i.e., the waveform's time/frequency modulation and polarization]
Resolve objects spatially;
Delay-Doppler measurement precision;
Optically opaque penetration;
Sensitive to high concentrations of metal or ice.

Disadvantages

Signal strength drops off very steeply with distance to the target.
Must have a relatively good ephemeris of the target before observing it.

Planetary

The following are a list of planetary bodies that have been observed by this means:

Mars - Mapping of surface roughness from Arecibo Observatory. The Mars Express mission carries a ground-penetrating radar.

Mercury - Improved value for the distance from the earth observed ( GR test). Rotational period, libration, surface mapping, esp. of polar regions.

Venus - first radar detection in 1960. Rotation period, gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.

Moon - first detection in 1945 - Surface roughness, mapping of shadowed regions near the poles.

Jupiter System - Galilean satellites

Saturn System - Rings and Titan from Arecibo Observatory, mapping of Titan's surface and observations of other moons from the Cassini spacecraft.

Earth - numerous airborne and spacecraft radars have mapped the entire planet, for various purposes. One example is the Shuttle Radar Topography Mission, which mapped the entire Earth at 30 m resolution.

Asteroids and comets

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.

Only a few comets have been studied by radar, including 73P/Schwassmann-Wachmann. There have been radar observations of more than 220 Near-Earth asteroids and over 100 Main belt asteroids.