View source for Radar astronomy

{{Short description|Observing nearby astronomical objects by analyzing reflected microwaves}}

'''Radar astronomy''' is a technique of observing nearby [[astronomical object]]s by reflecting [[radio wave]]s or [[microwave]]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 [[radar#Radar range equation|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 (planet)|Mercury]]<ref>{{cite conference |title=Radar and spacecraft ranging to Mercury between 1966 and 1988 |author=Anderson, John D. |author2=Slade, Martin A. |author3=Jurgens, Raymond F. |author4=Lau, Eunice L. |author5=Newhall, X. X. |author6=Myles, E. |journal=Proceedings of the Astronomical Society of Australia |conference=IAU, Asian-Pacific Regional Astronomy Meeting, 5th, Proceedings |location=Sydney, Australia |date=July 1990 |type=Held July 16–20, 1990 |publisher=Astronomical Society of Australia |issn=0066-9997 |volume=9 |issue=2 |pages=324 |bibcode=1991PASAu...9..324A }}</ref> and providing a refined value for the [[astronomical unit]].<ref name="SP4218"/> [[Radar imaging|Radar images]] provide information about the shapes and surface properties of solid bodies, which cannot be obtained by other ground-based techniques.

[[File:MillstoneHill.jpg|thumb|250px|Millstone Hill Radar in 1958]]
[[File:ADU-1000-4.jpg|thumb|250px|Early planetary radar [[Pluton (complex)|Pluton]], USSR, 1960]]

Relying upon high-powered terrestrial radars (of up to one [[megawatt]]),<ref>{{cite web |url=http://www.naic.edu/~nolan/radar/radarstatus.html |title=Arecibo Radar Status |author=<!--Staff writer(s); no by-line.-->  |access-date=22 December 2012}}</ref> radar astronomy is able to provide extremely accurate [[Astrometry|astrometric]] information on the structure, composition and movement of Solar System objects.<ref>{{cite web |url=http://echo.jpl.nasa.gov/introduction.html |title=Asteroid Radar Research Page |last1= Ostro  |first1= Steven |date=1997 |publisher= JPL |access-date=22 December 2012}}</ref> This aids in forming long-term predictions of [[asteroid impact|asteroid-Earth impacts]], as illustrated by the object [[99942 Apophis]].  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 Observatory|Arecibo Planetary Radar]]) suffered a structural cable failure, leading to the collapse of the main telescope in December of that year.<ref name="NSFrelease20-010">{{cite web |title=Giant Arecibo radio telescope collapses in Puerto Rico|url=https://www.theguardian.com/world/2020/dec/01/arecibo-radio-telescope-collapses-puerto-rico |website=www.theguardian.com |date=December 2020 |access-date=March 5, 2021 |language=en }}</ref>

There is one remaining radar astronomy facility in regular use, the [[Goldstone Solar System Radar]].

== 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 ==

The maximum range of astronomy by radar is very limited, and is confined to the [[Solar System]]. This is because the signal strength [[Inverse-square law|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.<ref name="Hey">{{Cite book  | last = Hey  | first = J. S.   | title = The Evolution of Radio Astronomy  | publisher = [[Paul Elek]] (Scientific Books)  | series = Histories of Science Series  | volume = 1  | date = 1973  }}</ref> 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&nbsp;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]] 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.<ref>{{cite journal |first=Jack |last=Mofensen |url=http://www.eagle.ca/~harry/ba/eme/index.htm |title=Radar echoes from the moon |archive-url=https://web.archive.org/web/20081029000712/http://www.eagle.ca/~harry/ba/eme/index.htm |archive-date=2008-10-29 |journal=Electronics |volume=19 |pages=92–98 |date=April 1946}}</ref><ref>{{cite journal |first=Zoltán |last=Bay |author-link=Zoltán Lajos Bay |title=Reflection of microwaves from the moon |journal=Hungarica Acta Physica |volume=1 |issue=1 |pages=1–22 |date=January 1947 |doi=10.1007/BF03161123 |doi-access=free |url=https://link.springer.com/content/pdf/10.1007/BF03161123.pdf}}</ref> 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 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, {{val|149467000|u=km}}.<ref name="SP4218">{{cite book |url=https://history.nasa.gov/SP-4218/sp4218.htm |title=NASA SP-4218: To See the Unseen - A History of Planetary Radar Astronomy |chapter=Chapter 2: Fickle Venus |chapter-url=http://history.nasa.gov:80/SP-4218/ch2.htm |first=Andrew J. |last=Butrica |publisher = NASA | date=1996 |access-date=2008-05-15 |archive-url=https://web.archive.org/web/20070823124845/https://history.nasa.gov/SP-4218/sp4218.htm |archive-date=2007-08-23 |url-status=live}}</ref>

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 {{val|149598500|500|u=km}} was determined for the [[astronomical unit]].<ref>
{{cite journal
 | last1 = Malling | first1 = L. R. |last2 = Golomb |first2 = S. W.
 | title = Radar Measurements of the Planet Venus
 | journal =  Journal of the British Institution of Radio Engineers| volume = 22 | issue = 4 | pages = 297–300
 | date = October 1961
 | url = https://ieeexplore.ieee.org/document/5259664
 | archive-url = https://web.archive.org/web/20180125020130/http://ieeexplore.ieee.org/document/5259664/
 | url-status = dead
 | archive-date = January 25, 2018
 | doi = 10.1049/jbire.1961.0121
 | format = PDF
}}</ref><ref>{{cite journal
 | title=The astronomical unit determined by radar reflections from Venus
 | last1=Muhleman |first1=Duane O. |last2=Holdridge |first2=D. B. |last3=Block |first3=N.
 | journal=Astronomical Journal |volume=67 |issue=4 |pages=191–203 |date=May 1962
 | doi=10.1086/108693 |bibcode=1962AJ.....67..191M
|doi-access=free }}  Using further analysis, this gives a refined figure of {{val|149598845|250|u=km}}.</ref>  Once the correct value was known, other groups found echos in their archived data that agreed with these results.<ref name="SP4218"/>

The Sun has been detected several times starting in 1959. Frequencies are usually between 25 and 38&nbsp;MHz, much lower than for interplanetary work. Reflections from both the photosphere and the corona were detected.<ref>{{Cite web |last=Ohlson |first=John E. |date=August 1967 |title=A RADAR INVESTIGATION OF THE SOLAR CORONA |url=https://ntrs.nasa.gov/api/citations/19680007049/downloads/19680007049.pdf |website=NASA Technical Reports Server}}</ref>

The following is a list of planetary bodies that have been observed by this means:
* [[Mercury (planet)|Mercury]] - Improved value for the distance from the earth observed ([[General relativity|GR]] test). Rotational period, [[libration]], surface mapping, esp. of polar regions.
* [[Venus]] - first radar detection in 1961. Rotation period, gross surface properties. The [[Magellan probe|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]], which mapped large parts of the surface of Earth at 30 m resolution.
* [[Mars]] - Mapping of surface roughness from [[Arecibo Observatory]]. The [[Mars Express]] mission carries a ground-penetrating radar.
* [[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-Huygens|Cassini]] spacecraft.

[[Image:Asteroid-Kleopatra-radar.png|thumb|250px|Computer model of asteroid [[(216) Kleopatra]], based on radar analysis.]]
[[Image:Radar images and computer model of asteroid 1999 JM8.jpg|thumb|250px|Radar images and computer model of asteroid {{mpl|1999 JM|8}}]]

== Asteroids and comets ==

Radar provides the ability to study the shape, size and spin state of asteroids and comets from the ground. [[Radar imaging]] 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,<ref name="detected"/> including [[73P/Schwassmann-Wachmann]].  There have been radar observations of 612 [[Near-Earth asteroid]]s and 138 [[Asteroid belt|Main belt asteroid]]s as of early 2016.<ref name="detected">{{cite web|url = http://echo.jpl.nasa.gov/asteroids/|title = Radar-Detected Asteroids and Comets|publisher = NASA/JPL Asteroid Radar Research|access-date = 2016-04-25}}</ref> By 2018, this had grown to  138 Main-Belt Asteroids, 789 Near-Earth Asteroids, also at that time 20 comets had been observed.<ref name="detected"/>

Many bodies are observed during their close [[flyby (spaceflight)|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 [[99942 Apophis|Apophis]] and other bodies.<ref name="NSFrelease20-010"/>  Being smaller the [[Goldstone Solar System Radar]] is less sensitive and unable to provide the same predictive capacity.

== Telescopes ==
* [[Arecibo Observatory]]
* [[Goldstone Deep Space Communications Complex]]
* [[RT-70]]
* [[Pluton (complex)|Pluton]]
* [[Deep Space Network]]<ref>{{Cite journal|last=Latifiyan|first=Pouya|date=April 2021|title=Space Telecommunications, How?|journal=Take off|location=[[Tehran]]|publisher=[[Civil Aviation Technology College]]|volume=1|pages=15, 16}}</ref>

== See also ==
{{Commons category}}
* [[Radar]]
* [[6489 Golevka]]
* [[4179 Toutatis]]

==References==
{{Reflist}}

== External links ==
* [http://www.planetary.org/blog/article/00003248/ How radio telescopes get images of asteroids] {{Webarchive|url=https://web.archive.org/web/20120125070636/http://www.planetary.org/blog/article/00003248/ |date=2012-01-25 }}
* {{cite web |url=http://www.naic.edu/%7Epradar/pradar.htm |title=Planetary Radar at Arecibo Observatory |publisher=NAIC |access-date=2008-05-15 |archive-date=2008-05-14 |archive-url=https://web.archive.org/web/20080514034816/http://www.naic.edu/%7Epradar/pradar.htm |url-status=dead }}
* {{cite web |url=http://deepspace.jpl.nasa.gov/technology/95_20/gold.htm |title=Goldstone Solar System Radar |publisher=JPL |access-date=2010-09-28 |url-status=dead |archive-url=https://web.archive.org/web/20101021074837/http://deepspace.jpl.nasa.gov/technology/95_20/gold.htm |archive-date=2010-10-21 }}
* {{cite web |url=http://echo.jpl.nasa.gov/ |title=JPL Asteroid Radar Research |author=Dr. Steven J. Ostro |author2=Dr. Lance A. M. Benner |name-list-style=amp |publisher=Caltech |date=2007 |access-date=2008-05-15}}
* {{cite web |url=http://www.cplire.ru/html/ra&sr/index.html  |title=Radar Astronomy and Space Radio Science |access-date=2008-05-15}}
* {{cite web |url=http://www2.ess.ucla.edu/~jlm/research/NEAs/intro.html |title=Introduction to Asteroid Radar Astronomy | author=Dr. [[Jean-Luc Margot]] |publisher=[[UCLA]] |access-date=2013-08-02}}
* [https://web.archive.org/web/20040608071121/http://echo.jpl.nasa.gov/~lance/binary.neas.html BINARY AND TERNARY NEAR-EARTH ASTEROIDS DETECTED BY RADAR]

{{Astronomy navbox}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{Authority control}}

[[Category:Observational astronomy]]
[[Category:Radar]]