Editing Radar astronomy (section)

{{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>

As of 2023, there were two radar astronomy facilities in regular use, the [[Goldstone Solar System Radar]] and [[Yevpatoria RT-70 radio telescope|Evpatoria Planetary Radar]].<ref>{{Cite conference |last=Bezrukovs |first=Vladislavs |last2=Dugin |first2=Nikolai |last3=Skirmante |first3=Karina |last4=Jasmonts |first4=Gints |last5=Šteinbergs |first5=Jānis |date=2023 |title=The forward scatter radar method for detecting space objects using emission of extraterrestrial radio sources |url=https://conference.sdo.esoc.esa.int/proceedings/neosst2/paper/125 |conference=2nd NEO and Debris Detection Conference |language=en}}</ref>