radar astronomy
| Dictionary: radar astronomy |
n.
The branch of astronomy that studies bodies in the solar system by analyzing the reflections of radio waves sent from Earth.
| 5min Related Video: radar astronomy |
| Sci-Tech Encyclopedia: Radar astronomy |
A powerful astronomical technique that uses radar echoes to furnish otherwise-unavailable information about bodies in the solar system. By comparing a radar echo to the transmitted signal, information can be obtained about the target's size, shape, topography, surface bulk density, spin vector, and orbital elements. While other astronomical techniques rely on passive measurement of reflected sunlight or naturally emitted radiation, the illumination used in radar astronomy is a coherent signal whose polarization and time modulation or frequency modulation are tailored to meet specific scientific objectives. Through measurements of the distribution of echo power in time delay or Doppler frequency, radar achieves spatial resolution of a planetary target despite the fact that the radar beam is typically much larger than the angular extent of the target. This capability is particularly valuable for asteroids and planetary satellites, which appear as unresolved point sources through optical telescopes. Moreover, the centimeter-to-meter wavelengths used in radar astronomy readily penetrate cometary comas and the optically opaque clouds that conceal Venus and Titan, and also permit determination of near-surface roughness (abundance of wavelength-scale rocks), bulk density, and metal concentration in planetary regoliths. See also Asteroid; Satellite (astronomy); Saturn; Venus.
A radar telescope is esssentially a radio telescope equipped with a high-power transmitter (a klystron vacuum-tube amplifier) and specialized instrumentation that links the transmitter, low-noise maser receiver, high-speed data-acquisition computer, and antenna together in an integrated radar system. Planetary radars, which must detect echoes from targets at distances from about 106 km for closely approaching asteroids and comets to more than 109 km for Saturn's rings and satellites, are the largest and most sensitive radars on Earth. See also Klystron; Maser; Radar; Radio telescope.
| Columbia Encyclopedia: 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.
| Wikipedia: 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 or 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.
Contents |
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.
History
The moon is comparatively close, and was detected by radar soon after its invention, in 1946.[1][2] Measurements included surface roughness and later mapping of shadowed regions near the poles.
The next easiest target is Venus. This was a target of great scientific value, since it could provide an unambiguous way to measure the size of the astronomical unit, which was needed for the nascent field of interplanetary spacecraft. In addition such technical prowess had great public relations value, and was an excellent demonstration to funding agencies. So there was considerable pressure to squeeze a scientific result from weak and noisy data, which was accomplished by heavy post-processing of the results, utilizing the expected value to tell where to look. This lead to early claims (from Lincoln Laboratory, Jodrell Bank, and Vladimir A. Kotelnikov of the USSR) which are now known to be incorrect. All of these agreed with each other and the conventional value of AU at the time.[3]
The first un-ambiguous detection of Venus was made by JPL on 10 March 1961. A correct measurement of the AU soon followed. Once the correct value was known, other groups found echos in their archived data that agreed with these results.[3]
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 1961. Rotation period, gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.
- 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.
See also
References
- ^ J. Mofensen, Radar echoes from the moon, Electronics, vol. 19, pp. 92-98; April, 1946
- ^ Z. Bay, "Reflection of microwaves from the moon," Hung. Acta Phys., vol. 1, pp. 1-22; April, 1946.
- ^ a b Andrew J. Butrica (1996). "NASA SP-4218: To See the Unseen - A History of Planetary Radar Astronomy". NASA. http://history.nasa.gov/SP-4218/sp4218.htm. Retrieved 2008-05-15.
External links
- "Planetary Radar at Arecibo Observatory". NAIC. http://www.naic.edu/%7Epradar/pradar.htm. Retrieved 2008-05-15.
- "Goldstone Solar System Radar". JPL. http://www332.jpl.nasa.gov/group8/. Retrieved 2008-05-15.
- Dr. Steven J. Ostro and Dr. Lance A. M. Benner (2007). "JPL Asteroid Radar Research". Caltech. http://echo.jpl.nasa.gov/. Retrieved 2008-05-15.
- "Radar Astronomy and Space Radio Science". http://www.cplire.ru/html/ra&sr/index.html. Retrieved 2008-05-15.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Learn More
 | radar telescope |
| altitude (measure) |
| radar (mechanical device – in optics, electronics) |
 | What does a radar do? Read answer... |
| What is Amature radar and phalcon radar? Read answer... |
| What mysteries are in astronomy? Read answer... |
Help us answer these
 | When astronomy born? |
| Branches of astronomy? |
| Who made astronomy? |
Post a question - any question - to the WikiAnswers community:
Copyrights:
 |
 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
 |
| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
 |
| Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/. Read more |
 |
| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Radar astronomy". Read more |
Related answers
- What is astronomy?
- RADAR or radar?
- What does a astronomy do?
- Astronomy about you?
- What do you do with astronomy?
- What is radar?
- What is a radar?
ADVERTISEMENT
Answer these
- Who is astronomies?
- What is the radar for?
- What radar do?
- What are the the branches of astronomy?
- What are the angles in astronomy?
Mentioned in
- radar telescope
- altitude (measure)
- radar (mechanical device – in optics, electronics)
- antenna (in electronics)
- Edward Mills Purcell (American physicist)
- RADAR
- Meteor
- Alexander Zaytsev
- Project Diana
- Year 1959 (in Science & Technology)
- Martin Ryle
- Scott Hudson
- Aleksandr Leonidovich Zaitsev
- Steven J. Ostro
