Editing Radar astronomy (section)

== 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,<ref>{{cite journal 
  |title=Radar Echoes from Venus: Advances in several arts made possible this experiment in radio astronomy performed during the IGY
  |last1=Price |first1=R |last2=Green Jr |first2=PE |last3=Goblick Jr |first3=TJ |last4=Kingston |first4=RH
  |last5=Kraft Jr |first5=LG |last6=Pettengill |first6=GH |last7=Silver |first7=R |last8=Smith |first8=WB
  |journal=Science
  |volume=129
  |issue=3351
  |pages=751--753
  |year=1959
  |publisher=American Association for the Advancement of Science
  |url=https://www.science.org/doi/pdf/10.1126/science.129.3351.751}}</ref> Jodrell Bank,<ref>{{cite journal
  |title=Radio echo observations of Venus
  |last1=Evans |first1=JV |last2=Taylor |first2=GN
  |journal=Nature
  |volume=184
  |issue=4696
  |pages=1358--1359
  |year=1959
  |publisher=Nature Publishing Group UK London}}</ref> and Vladimir A. Kotelnikov of the USSR<ref>{{cite journal
  |title=Radar contact with Venus
  |last1=Kotelnokov |first1=VA
  |journal=Journal of the British Institution of Radio Engineers
  |volume=22
  |issue=4
  |pages=293--295
  |year=1961
  |publisher=IET
  |url=http://vak.rutgers.edu/Chapters_T2/020_060%20Radar%20Contact%20with%20Venus%20(1961).pdf}}</ref>) 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
| url-access = subscription
 }}</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}}]]