STEREO: Difference between revisions

| header = Animation of STEREO{{'s}} trajectory

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 will take 347 days to complete one revolution of the Sun and Spacecraft B will take 387 days. The A  spacecraft/Sun/Earth angle will increase at 21.650° per degree/year. The B  spacecraft/Sun/Earth angle will change −21.999 degrees° 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 sunSun 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 will continue to separate from each other at a combined rate of approximately 44 degrees° per year. There are no ''final'' positions for the spacecraft. They achieved 90 degrees° separation on January 24, 2009, a condition known as [[quadrature (astronomy)|quadrature]]. This is 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 degrees° 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}}</ref>

Even as the angle increases, the addition of an Earth-based view, ''e.g.'', from the [[Solar Dynamics Observatory]], will still provide full-Sun observations for several years. In 2015, contact was lost for several months when the STEREO spacecraft passed behind the Sun. They will then start to approach Earth again, with closest approach sometime in 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>

On October 1, 2014, contact was lost with STEREO-B during a planned reset to test the craft's automation, in anticipation of the aforementioned solar "conjunction" period. The team originally thought that the spacecraft had begun to spin, decreasing the amount of power that could be generated by the solar panels. Later analysis of the received telemetry concluded that the spacecraft was in an uncontrolled spin of about three degrees3° per second; this was too rapid to be immediately corrected using its [[reaction wheel]]s, which would become oversaturated.<ref name="stereonews20161023" /><ref name="nasa20151211" />

After a silence of 22 months, contact was regained at 22:27&nbsp;UTC on August 21, 2016, when the Deep Space Network established a lock on STEREO-B for 2.4 &nbsp;hours.<ref name="nasa20160822">{{cite web |url=http://www.nasa.gov/feature/goddard/2016/nasa-establishes-contact-with-stereo-mission |title=NASA Reestablishes Contact with STEREO Mission |publisher=NASA |last=Fox |first=Karen C. |date=August 22, 2016 |accessdate=August 22, 2016}}</ref><ref name="stereonews20161023">{{cite web |url=http://stereo-ssc.nascom.nasa.gov/new.shtml |title=What's New |work=STEREO Science Center |publisher=NASA |date=October 11, 2016 |archiveurl=https://web.archive.org/web/20161023133257/https://stereodata.nascom.nasa.gov/new.shtml |archivedate=October 23, 2016}}</ref><ref>{{cite conference |url=https://amostech.com/TechnicalPapers/2017/Poster/Geldzahler.pdf |title=A Phased Array of Widely Separated Antennas for Space Communication and Planetary Radar |conference=Advanced Maui Optical and Space Surveillance Technologies Conference. September 19–22, 2017. Wailea, Maui, Hawaii. |first1=Barry |last1=Geldzahler |first2=Chris |last2=Bershad |first3=Robert |last3=Brown |first4=Rachel |last4=Cox |first5=Richard |last5=Hoblitzell |first6=John |last6=Kiriazes |first7=Bruce |last7=Ledford |first8=Michael |last8=Miller |first9=Gary |last9=Woods |first10=Timothy |last10=Cornish |first11=Larry |last11=D'Addario |first12=Faramaz |last12=Davarian |first13=Dennis |last13=Lee |first14=David |last14=Morabito |first15=Philip |last15=Tsao |first16=Jason |last16=Soloff |first17=Ken |last17=Church |first18=Paul |last18=Deffenbaugh |first19=Keith |last19=Abernethy |first20=William |last20=Anderson |first21=John |last21=Collier |first22=Greg |last22=Wellen |display-authors=1 |pages=13–14 |date=2017 |bibcode=2017amos.confE..82G}}</ref>

Engineers planned to work and develop software to fix the spacecraft, but once its computer was powered up, there would only have been about two 2&nbsp;minutes to upload the fix before STEREO-B entered failure mode again.<ref name="businside20160823">{{cite news |url=http://www.businessinsider.com/stereo-b-spacecraft-rescue-mission-2016-8 |title=NASA may have less than 2 minutes to rescue its long-lost spacecraft |work=Business Insider |first=Dave |last=Mosher |date=August 23, 2016 |accessdate=August 24, 2016}}</ref> Further, while the spacecraft was power -positive at the time of contact, its orientation would drift, and power levels fall. Two-way communication was achieved, and commands to begin recovering the spacecraft were sent through the rest of August and September.<ref name="stereonews20161023" />