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'''STEREO''' ('''Solar TErrestrial RElations Observatory''') is a [[Sun|solar]] observation mission.<ref>{{cite web |url=http://www.nasa.gov/missions/highlights/schedule.html |title=NASA Launch Schedule |publisher=NASA |date=September 20, 2006 |access-date=September 20, 2006}}</ref> Two nearly identical spacecraft ('''STEREO-A''', '''STEREO-B''') were launched in 2006 into orbits around the Sun that cause them to respectively pull farther ahead of and fall gradually behind the Earth. This enabled [[stereoscopic]] imaging of the [[Sun]] and solar phenomena, such as [[coronal mass ejection]]s.
Contact with STEREO-B was lost in 2014 after it entered an uncontrolled spin preventing its solar panels from generating enough power, but STEREO-A is still operational.
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The two STEREO spacecraft were launched at 00:52 UTC on October 26, 2006, from Launch Pad 17B at the [[Cape Canaveral Air Force Station]] in Florida on a [[Delta II]] 7925-10L launcher into highly [[ellipse|elliptical]] [[geocentric orbit]]s. The [[Apsis|apogee]] reached the Moon's orbit. On December 15, 2006, on the fifth orbit, the pair swung by the Moon for a [[gravity assist]]. Because the two spacecraft were in slightly different orbits, the "ahead" (A) spacecraft was ejected to a [[heliocentric orbit]] inside Earth's orbit, while the "behind" (B) spacecraft remained temporarily in a high Earth orbit. The B spacecraft encountered the Moon again on the same orbital revolution on January 21, 2007, being ejected from Earth orbit in the opposite direction from spacecraft A. Spacecraft B entered a heliocentric orbit outside the Earth's orbit. Spacecraft A took 347 days to complete one revolution of the Sun and Spacecraft B took 387 days. The A spacecraft/Sun/Earth angle will increase at 21.650° per year. The B spacecraft/Sun/Earth angle will change −21.999° per year. Given that the length of Earth's orbit is around 940 million kilometres, both craft have an average speed, in a rotating geocentric frame of reference in which the Sun is always in the same direction, of about 1.8 km/s, but the speed varies considerably depending on how close they are to their respective aphelion or perihelion (as well as on the position of Earth). Their current locations are shown [http://stereo.gsfc.nasa.gov/where.shtml here].
Over time, the STEREO spacecraft continued to separate from each other at a combined rate of approximately 44° per year. There were no ''final'' positions for the spacecraft. They achieved 90° separation on January 24, 2009, a condition known as [[quadrature (astronomy)|quadrature]]. This was of interest because the mass ejections seen from the side on the limb by one spacecraft can potentially be observed by the ''in situ'' particle experiments of the other spacecraft. As they passed through Earth's [[Lagrangian point]]s {{L4}} and {{L5}}, in late 2009, they searched for [[Trojan (celestial body)|Lagrangian (trojan) asteroids]]. On February 6, 2011, the two spacecraft were exactly 180° apart from each other, allowing the entire Sun to be seen at once for the first time.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/stereo/news/entire-sun.html |title=First Ever STEREO Images of the Entire Sun |publisher=NASA |editor-first=Holly |editor-last=Zell |date=February 6, 2011 |access-date=February 8, 2011 |archive-date=January 20, 2019 |archive-url=https://web.archive.org/web/20190120215904/https://www.nasa.gov/mission_pages/stereo/news/entire-sun.html |url-status=dead }}</ref>
Even as the angle increases, the addition of an Earth-based view, e.g., from the [[Solar Dynamics Observatory]], still provided full-Sun observations for several years. In 2015, contact was lost for several months when the STEREO spacecraft passed behind the Sun. They then started to approach Earth again, with closest approach in August 2023. They will not be recaptured into Earth orbit.<ref name="nasa20151211">{{cite web |url=http://www.nasa.gov/feature/goddard/saving-nasas-stereo-b-the-189-million-mile-road-to-recovery |title=Saving STEREO-B: The 189-million-mile Road to Recovery |publisher=NASA |last=Sarah |first=Frazier |date=December 11, 2015}}</ref>
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==Mission benefits==
[[File:Delta II 10L fairing installation around STEREO spacecraft.jpg|thumb|STEREO spacecraft in Delta II fairing]]
The principal benefit of the mission was [[stereoscopic]] images of the Sun. Because the satellites are at different points along the Earth's orbit, but distant from the Earth, they can photograph parts of the Sun that are not visible from the Earth. This permits NASA scientists to directly monitor the far side of the Sun, instead of inferring the activity on the far side from data that can be gleaned from Earth's view of the Sun. The STEREO satellites principally monitor the far side for [[coronal mass ejection]]s — massive bursts of [[solar wind]], solar [[Plasma (physics)|plasma]], and magnetic fields that are sometimes ejected into space.<ref name=CBC>{{cite news |url=
Since the radiation from coronal mass ejections, or CMEs, can disrupt Earth's communications, airlines, power grids, and satellites, more accurate forecasting of CMEs has the potential to provide greater warning to operators of these services.<ref name=CBC /> Before STEREO, the detection of the [[sunspot]]s that are associated with CMEs on the far side of the Sun was only possible using [[helioseismology]], which only provides low-resolution maps of the activity on the far side of the Sun. Since the Sun rotates every 25 days, detail on the far side was invisible to Earth for days at a time before STEREO. The period that the Sun's far side was previously invisible was a principal reason for the STEREO mission.<ref>{{cite magazine |url=http://www.time.com/time/health/article/0,8599,2046569,00.html |archive-url=https://web.archive.org/web/20110209073113/http://www.time.com/time/health/article/0,8599,2046569,00.html |url-status=dead |archive-date=February 9, 2011 |title=NASA Images the Entire Sun, Far Side and All |magazine=[[Time (magazine)|Time]] |last=Lemonick |first=Michael |date=February 6, 2011 |access-date=February 8, 2011}}</ref>
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STEREO program scientist [[Madhulika Guhathakurta]] expected "great advances" in theoretical [[solar physics]] and space weather forecasting with the advent of constant 360° views of the Sun.<ref>{{cite news |url=http://content.usatoday.com/communities/ondeadline/post/2011/02/sun-shines-in-twin-probes-first-360-degree-images/1 |title=Sun shines in twin probes' first 360-degree images |work=USA Today |last=Winter |first=Michael |date=February 7, 2011 |access-date=February 8, 2011}}</ref> STEREO's observations are incorporated into forecasts of solar activity for airlines, power companies, satellite operators, and others.<ref>{{cite news |url=https://www.bbc.co.uk/news/science-environment-12365083 |title=Stereo satellites move either side of Sun |work=BBC News |date=February 6, 2011 |access-date=February 8, 2011}}</ref>
STEREO has also been used to discover 122 [[eclipsing binaries]] and study hundreds more [[variable star]]s.<ref name="variable">{{cite news |url=http://www.astronomy.com/news-observing/news/2011/04/stereo%20turns%20its%20steady%20gaze%20on%20variable%20stars |archive-url=https://web.archive.org/web/20140531213218/http://www.astronomy.com/news-observing/news/2011/04/stereo%20turns%20its%20steady%20gaze%20on%20variable%20stars |url-status=dead |archive-date=May 31, 2014 |title=STEREO turns its steady gaze on variable stars |work=[[Astronomy (magazine)|Astronomy]] |agency=Royal Astronomical Society |date=April 19, 2011 |access-date=April 19, 2011 }}</ref> STEREO can look at the same star for up to 20 days.<ref name="variable"/>
On July 23, 2012, STEREO-A was in the path of the CME of the [[solar storm of 2012]]. This CME, if it were to collide with Earth's magnetosphere, is estimated to have caused a geomagnetic storm of similar strength to the [[Carrington Event]], the most intense geomagnetic storm in recorded history.<ref name="July2012CME">{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2014/23jul_superstorm/ |title=Near Miss: The Solar Superstorm of July 2012 |publisher=NASA |date=July 23, 2014 |access-date=July 24, 2014}}</ref> STEREO-A's instrumentation was able to collect and relay a significant amount of data about the event without being harmed.
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[[File:Stereo mission.jpg|thumb|upright=1.2|Instrument locations on STEREO]]
Each of the spacecraft carries cameras, particle experiments and radio detectors in four instrument packages:
* '''Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)''' has five cameras: an extreme ultraviolet imager (EUVI) and two white-light [[coronagraph]]s (COR1 and COR2). These three telescopes are collectively known as the Sun Centered Instrument Package or SCIP. They image the solar disk and the inner and outer [[solar corona|corona]]. Two additional telescopes, [[heliospheric imager]]s (called the HI1 and HI2), image the space between Sun and Earth. The purpose of SECCHI is to study the 3-D evolution of [[coronal mass ejection]]s through their full journey from the Sun's surface through the corona and interplanetary medium to their impact at Earth.<ref name=inst>{{cite web |url=http://www.nasa.gov/mission_pages/stereo/spacecraft/index.html |title=STEREO Spacecraft & Instruments |publisher=NASA |date=March 8, 2006 |access-date=May 30, 2006 |archive-date=May 23, 2013 |archive-url=https://web.archive.org/web/20130523040216/http://www.nasa.gov/mission_pages/stereo/spacecraft/index.html |url-status=dead }}</ref><ref>{{cite journal |title=Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) |journal=Advances in Space Research |first1=R. A. |last1=Howard |first2=J. D. |last2=Moses |first3=D. G. |last3=Socker |first4=K. P. |last4=Dere |first5=J. W. |last5=Cook |volume=29 |issue=12 |pages=2017–2026 |date=June 2002 |doi=10.1016/S0273-1177(02)00147-3 |bibcode=2002AdSpR..29.2017H |hdl=2268/21196 |url=http://orbi.ulg.ac.be/handle/2268/21196 |hdl-access=free }}</ref> The principal investigator for SECCHI was Russell Howard.
* '''In-situ Measurements of Particles and CME Transients (IMPACT)''', to study [[solar energetic particles|energetic particles]], the three-dimensional distribution of solar-wind electrons and interplanetary magnetic field.<ref name=inst /><ref>{{cite journal |title=IMPACT: Science goals and firsts with STEREO |journal=Advances in Space Research |first1=J. G. |last1=Luhmann |first2=D. W. |last2=Curtis |first3=R. P. |last3=Lin |first4=D. |last4=Larson |first5=P. |last5=Schroeder |display-authors=etal |volume=36 |issue=8 |pages=1534–1543 |date=2005 |doi=10.1016/j.asr.2005.03.033 |bibcode=2005AdSpR..36.1534L}}</ref> [[Janet Luhmann]] was the principal investigator for IMPACT.
* '''PLAsma and SupraThermal Ion Composition (PLASTIC)''', led by [[Antoinette Galvin]], to study the plasma characteristics of [[proton]]s, [[alpha particle]]s and [[heavy ion]]s.<ref name=inst />
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==See also==
{{Portal|Spaceflight}}
* [[Advanced Composition Explorer]] (ACE), launched 1997, still operational
* [[Heliophysics]]
* [[Living With a Star]] (NASA program), still ongoing {{as of|
** [[Solar Dynamics Observatory]] (SDO), launched 2010.
** [[Parker Solar Probe]], launched August 2018.
* [[Solar and Heliospheric Observatory]] (SOHO), launched 1995, still observational {{as of|
* [[Solar Orbiter]] (SolO), launched February 2020.
* [[TRACE]], launched 1998.
* ''[[Ulysses (spacecraft)|Ulysses]]'', spacecraft launched in 1990.
* ''[[Wind (spacecraft)|Wind]]'', spacecraft launched 1994, still operational {{as of|
* [[Zooniverse#Solar Stormwatch|Zooniverse – Solar Stormwatch]]
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* Comets
** [https://science.nasa.gov/science-news/science-at-nasa/2007/01oct_encke/ Tail of Comet Encke removed by CME] by Science@NASA
** [http://sungrazer.nrl.navy.mil/index.php?p=news/secchi_5yrs STEREO/SECCHI Comets: The First 5 Years] {{Webarchive|url=https://web.archive.org/web/20120401090124/http://sungrazer.nrl.navy.mil/index.php?p=news/secchi_5yrs |date=April 1, 2012 }} by the Sungrazer Project
{{Sun spacecraft}}
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