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Radar astronomy is a technique of observing nearby
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...
s by reflecting
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (shor ...
s or
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency rang ...
s off target objects and analyzing their reflections. Radar astronomy differs from '' radio astronomy'' in that the latter is a passive observation (i.e., receiving only) and the former an active one (transmitting and receiving). Radar systems have been conducted for six decades applied to 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 In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. T ...
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 The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
. Radar images provide information about the shapes and surface properties of solid bodies, which cannot be obtained by other ground-based techniques. Relying upon high-powered terrestrial radars (of up to one megawatt), radar astronomy is able to provide extremely accurate
astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. Hist ...
information on the structure, composition and movement of Solar System objects. This aids in forming long-term predictions of asteroid-Earth impacts, as illustrated by the object
99942 Apophis 99942 Apophis is a near-Earth asteroid and potentially hazardous asteroid with a diameter of that caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit Earth on April&nbs ...
. In particular, optical observations measure where an object appears in the sky, but cannot measure the distance with great accuracy (relying on parallax becomes more difficult when objects are small or poorly illuminated). 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. In August 2020 the Arecibo Observatory ( Arecibo Planetary Radar) suffered a structural cable failure, leading to the collapse of the main telescope in December of that year. There is one remaining radar astronomy facility in regular use, the Goldstone Solar System Radar.


Advantages

* Control of attributes of the signal .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

The maximum range of astronomy by radar is very limited, and is confined to the Solar System. This is because the signal strength drops off very steeply with distance to the target, the small fraction of incident flux that is reflected by the target, and the limited strength of transmitters. The distance to which the radar can detect an object is proportional to the square root of the object's size, due to the one-over-distance-to-the-fourth dependence of echo strength. Radar could detect something ~1 km across a large fraction of an AU away, but at 8-10 AU, the distance to Saturn, we need targets at least hundreds of kilometers wide. It is also necessary to have a relatively good
ephemeris In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ve ...
of the target before observing it.


History

The Moon is comparatively close and was detected by radar soon after the invention of the technique in 1946. 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 The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
, 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 led 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, . The first unambiguous detection of Venus was made by the Jet Propulsion Laboratory on 10 March 1961. JPL established contact with the planet Venus using a planetary radar system from 10 March to 10 May 1961. Using both velocity and range data, a new value of was determined for the
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
. Once the correct value was known, other groups found echos in their archived data that agreed with these results. The following is a list of planetary bodies that have been observed by this means: * 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. * Earth - numerous airborne and spacecraft radars have mapped the entire planet, for various purposes. One example is the
Shuttle Radar Topography Mission The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a near-global scale from 56°S to 60°N, to generate the most complete high-resolution digital topographic database of Ea ...
, which mapped large parts of the surface of Earth at 30 m resolution. * Mars - Mapping of surface roughness from
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science Fo ...
. The Mars Express mission carries a ground-penetrating radar. * Jupiter System -
Galilean satellites The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupite ...
* Saturn System - Rings and Titan from
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science Fo ...
, mapping of Titan's surface and observations of other moons from the Cassini spacecraft.


Asteroids and comets

Radar provides the ability to study the shape, size and spin state of asteroids and comets from the ground.
Radar imaging Imaging radar is an application of radar which is used to create two-dimensional images, typically of landscapes. Imaging radar provides its light to illuminate an area on the ground and take a picture at radio wavelengths. It uses an antenna and ...
has produced images with up to 7.5-meter resolution. With sufficient data, the size, shape, spin and radar albedo of the target asteroids can be extracted. Only 19 comets have been studied by radar, including 73P/Schwassmann-Wachmann. There have been radar observations of 612 Near-Earth asteroids and 138
Main belt asteroid The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
s as of early 2016. By 2018, this had grown to 138 Main-Belt Asteroids, 789 Near-Earth Asteroids, also at that time 20 comets had been observed. Many bodies are observed during their close flyby of Earth. While operational the Arecibo Observatory provided information about Earth threatening comet and asteroid impacts, allowing impact and near miss predictions decades into the future such as those for Apophis and other bodies. Being smaller the Goldstone Solar System Radar is less sensitive and unable to provide the same predictive capacity.


Telescopes

*
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science Fo ...
*
Goldstone Deep Space Communications Complex The Goldstone Deep Space Communications Complex (GDSCC), commonly called the Goldstone Observatory, is a satellite ground station located in Fort Irwin in the U.S. state of California. Operated by NASA's Jet Propulsion Laboratory (JPL), its ...
* RT-70 *
Pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
* Deep Space Network


See also

* Radar *
6489 Golevka 6489 Golevka is an Apollo, Mars-crosser, and Alinda asteroid discovered in 1991 by Eleanor F. Helin. Its name has a complicated origin. In 1995, Golevka was studied simultaneously by three radar observatories across the world: Goldstone in C ...
*
4179 Toutatis 4179 Toutatis, provisional designation , is an elongated, stony asteroid and slow rotator, classified as a near-Earth object and potentially hazardous asteroid of the Apollo asteroid and Alinda asteroid groups, approximately 2.5 kilometers in d ...


References


External links


How radio telescopes get images of asteroids
* * * * *

{{Authority control Observational astronomy Radar