Editing Space Interferometry Mission (section)

==Mission==
[[Image:SIM Concept2006 lr.jpg|thumb|left|A 2006 artist's impression of the SIM Lite predecessor, SIM PlanetQuest, design]]
SIM Lite would have operated in an Earth-trailing [[heliocentric orbit]], drifting away from Earth at the rate of 0.1 [[astronomical unit|AU]] per year ultimately reaching a distance of 82&nbsp;million km from Earth. This would have taken approximately {{frac|5|1|2}}&nbsp;years. The Sun would have continuously shone on the spacecraft, allowing it to avoid the [[occultations]] of target stars and eclipses of the Sun that would occur in an [[Geocentric orbit|Earth orbit]].<ref name=northrop>"[http://www.st.northropgrumman.com/capabilities/space/science/sim.html SIM PlanetQuest] {{webarchive|url=https://web.archive.org/web/20070422144325/http://www.st.northropgrumman.com/capabilities/space/science/sim.html |date=22 April 2007 }}", Northrop Grumman, official site, 2004–2006. Retrieved 24 April 2007.</ref><ref name=mission>"[http://planetquest.jpl.nasa.gov/SIM/sim_mission.cfm The Mission] {{webarchive|url=https://web.archive.org/web/20060928220301/http://planetquest.jpl.nasa.gov/SIM/sim_mission.cfm |date=28 September 2006 }}", NASA, SIM Planetquest, ''Jet Propulsion Laboratory''. Retrieved 24 April 2007.</ref>

Had it been launched, SIM would have performed scientific research for five years.

===Planet hunting===
[[Image:NASASimLiteMissionCapabilities.jpg|thumb|alt=This chart depicts the potential number of habitable planets and other planets that SIM Lite might have detected. The number of one-Earth mass planets assumes 40% of mission time is assigned to the search.|This chart depicts the potential number of habitable planets and other planets that SIM Lite was expected to detect. The number of one-Earth mass planets assumes 40% of mission time is assigned to the search.]]
SIM Lite would have been the most powerful [[extrasolar planet]] hunting [[space telescope]] ever built.<ref name=davidson>Davidson, John et al., Ed., [http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/simBook2009/ SIM Lite Astrometric Observatory]  {{webarchive|url=https://web.archive.org/web/20100409140747/http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/simBook2009/ |date=9 April 2010 }}, [http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/SIMLiteBook/Front-Material-LR.pdf Executive Summary] {{webarchive|url=https://web.archive.org/web/20090827041834/http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/SIMLiteBook/Front-Material-LR.pdf |date=27 August 2009 }}, p. ix et seq., Jet Propulsion Laboratory 400–1360, 2009.  Retrieved 8 March 2010.</ref> Through the technique of [[interferometry]] the spacecraft would be able to detect Earth-sized planets.<ref name=davidson/> SIM Lite was to perform its search for nearby, Earth-like planets by looking for the "[[Methods of detecting extrasolar planets|wobble]]" in the parent star's apparent motion as the planet orbits. The spacecraft would have accomplished this task to an accuracy of one millionth of an [[arcsecond]], or the thickness of a [[Nickel (United States coin)|nickel]] viewed at the distance from Earth to the [[Moon]]. Titled the Deep Search, the planet hunting program was intended to search approximately 60 nearby stars for [[terrestrial planets]] (like [[Earth]] and [[Venus]]) in the habitable zone (where liquid water can exist throughout a full revolution (one "year") of the planet around its star). The Deep Search was to be the most demanding in terms of [[astrometry|astrometric]] accuracy, hence the name, Deep Search.<ref name=davidson/> This program would have used the full capability of the SIM Lite spacecraft to make its measurements.<ref name=davidson/>

A flexible search strategy<ref name=aas08>{{cite journal |date=2008 |title= Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest|journal=Publications of the Astronomical Society of the Pacific |volume=120 |issue=863 |pages=38–88 |doi=10.1086/525059 |last1=Unwin |first1=Stephen C. |last2=Shao |first2=Michael |last3=Tanner |first3=Angelle M. |last4=Allen |first4=Ronald J. |last5=Beichman |first5=Charles A. |last6=Boboltz |first6=David |last7=Catanzarite |first7=Joseph H. |last8=Chaboyer |first8=Brian C. |last9=Ciardi |first9=David R.|last10= Edberg|first10= Stephen J.|last11= Fey|first11= Alan L.|last12= Fischer|first12= Debra A.|last13= Gelino|first13= Christopher R.|last14= Gould|first14= Andrew P.|last15= Grillmair|first15= Carl|last16= Henry|first16= Todd J.|last17= Johnston|first17= Kathryn V.|last18= Johnston|first18= Kenneth J.|last19= Jones|first19= Dayton L.|last20= Kulkarni|first20= Shrinivas R.|last21= Law|first21= Nicholas M.|last22= Majewski|first22= Steven R.|last23= Makarov|first23= Valeri V.|last24= Marcy|first24= Geoffrey W.|last25= Meier|first25= David L.|last26= Olling|first26= Rob P.|last27= Pan|first27= Xiaopei|last28= Patterson|first28= Richard J.|last29= Pitesky|first29= Jo Eliza|last30= Quirrenbach|first30= Andreas|bibcode = 2008PASP..120...38U |arxiv = 0708.3953 |s2cid= 10797266|display-authors= 8}}</ref> tunes SIM Lite's mass sensitivity at each star to a desired level in the habitable planet search.  The value of η<sub>Earth</sub> (Eta_Earth), the fraction of stars carrying Earth-analog planets, will be estimated by the [[Kepler Mission]] some time before SIM Lite launches. One strategy for a habitable planet search is to do a 'deeper' search (i.e. to lower mass sensitivity in the habitable zone) of a smaller number of targets if Earth analogs are common. A 'shallower' search of a larger number of targets could have been done if Earth analogs are rarer. For example, assuming that 40% of mission time is allocated for the planet search, SIM Lite could have surveyed:

* 65 stars for planets down to one Earth mass, in scaled 1 AU orbits, OR
* 149 stars for planets down to two Earth masses, in scaled 1 AU orbits, OR
* 239 stars for planets down to three Earth masses, in scaled 1 AU orbits.

Aside from searching for Earth-sized planets SIM Lite was scheduled to perform what has been dubbed the "Broad Survey". The Broad Survey would have looked at approximately 1,500&nbsp;stars to help determine the abundance of [[Neptune]]-mass and larger planets around all star-types in Earth's sector of the [[Milky Way]].<ref name=davidson/>

[[Image:Earthlike planet-browse.jpg|thumb|left|SIM Lite would have been able to detect Earth-sized planets, such as in this artist's rendering.]]
A third part of the planet finding mission was the search for [[Jupiter]]-mass planets around young stars. The survey would have helped scientists understand more about solar system formation, including the occurrence of [[hot Jupiters]].<ref name=davidson2>Davidson, John et al., Ed., ., ''[http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/simBook2009/ SIM Lite Astrometric Observatory]  {{webarchive|url=https://web.archive.org/web/20100409140747/http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/simBook2009/ |date=9 April 2010 }}'', [http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/SIMLiteBook/Chapter-2-LR.pdf Chapter 2] {{webarchive|url=https://web.archive.org/web/20090827042502/http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/SIMLiteBook/Chapter-2-LR.pdf |date=27 August 2009 }}, Jet Propulsion Laboratory 400–1360, 2009.  Retrieved 8 March 2010.</ref> This portion of the planet hunt was designed to study systems with one or more Jupiter mass planets before the system has reached long term equilibrium.<ref name=davidson2/> Planet hunting techniques using a star's radial velocity cannot measure the regular, tiny to-and-fro wobble motions induced by planets against the strong atmospheric activity of a youthful star. It is through the techniques pioneered by [[Albert A. Michelson]] that the SIM would have been able to execute its three primary planet-finding missions.

The mission's planet finding component was set up to serve as an important complement to the future missions designed to image and measure terrestrial and other exoplanets. SIM Lite was to perform an important task that these missions will not be capable of: determining planet masses.<ref name=davidson3>Davidson, John et al., Ed., ''[http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/simBook2009/ SIM Lite Astrometric Observatory] {{webarchive|url=https://web.archive.org/web/20100409140747/http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/simBook2009/ |date=9 April 2010 }}'', [http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/SIMLiteBook/Chapter-3-LR.pdf Chapter 3] {{webarchive|url=https://web.archive.org/web/20090827042120/http://planetquest.jpl.nasa.gov/SIM/keyPubPapers/SIMLiteBook/Chapter-3-LR.pdf |date=27 August 2009 }}, Jet Propulsion Laboratory 400–1360, 2009.  Retrieved 8 March 2010.</ref> Another task that the SIM was envisioned to perform for the future missions will include providing the orbital characteristics of the planets.<ref name=davidson3 /><ref name=tanner>{{cite journal |bibcode=2005AAS...206.1404C |title=Space Interferometry Mission (SIM) PlanetQuest's Discovery Space and Potential Synergy with Terrestrial Planet Finder (TPF): I. Detection of Terrestrial Planets in the Habitable Zone |last1=Catanzarite |first1=J. |last2=Tanner |first2=A. |last3=Shao |first3=M. |volume=206 |date=2005 |pages=453 |journal=American Astronomical Society Meeting 206}}</ref> With this knowledge other missions can estimate the optimal times and projected star–planet separation angles for them to observe the terrestrial (and other) planets SIM has detected.

===Stellar mass===
[[Image:White dwarf-590.jpg|thumb|right|White dwarfs, imaged by NASA's Hubble Space Telescope]]
Another key aspect of SIM Lite's mission was determining the upper and lower limits of star's masses. Today, scientists understand that there are limits to how small or large a star can be. Objects that are too small lack the internal [[pressure]] to initiate [[thermonuclear fusion]], which is what causes a star to shine. These objects are known as [[brown dwarf]]s and represent the lower end of the stellar mass scale. Stars that are too large become unstable and explode in a [[supernova]].<ref name=masses>[http://planetquest.jpl.nasa.gov/SIM/simLiteScience/blackHoles/ Stars, Neutron Stars, & Black Holes] {{webarchive|url=https://web.archive.org/web/20100325134503/http://planetquest.jpl.nasa.gov/SIM/simLiteScience/blackHoles/ |date=25 March 2010 }}, NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.</ref><ref name=masses2>[http://planetquest.jpl.nasa.gov/SIM/scienceMotivations/neutronStars/ Stars, Neutron Stars, & SIM Lite] {{webarchive|url=https://web.archive.org/web/20100614201119/http://planetquest.jpl.nasa.gov/SIM/scienceMotivations/neutronStars/ |date=14 June 2010 }}, NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.</ref>

Part of the SIM's mission was to provide pinpoint measurements for the two extremes in stellar mass and evolution. The telescope will not be able to measure the mass of every star in the Galaxy, since there are over 200&nbsp;billion, but instead, it will take a "population census."<ref name=masses/> Through this technique, SIM will be able to output accurate masses for representative examples for nearly every star type, including brown dwarfs, hot [[white dwarf]]s, [[Red giant|red giant star]]s, and elusive [[black hole]]s.<ref name=masses/> Current space telescopes, including NASA's [[Hubble Space Telescope]], can accurately measure mass for some types of stars, but not all. Estimates put the range for stellar mass somewhere between 8% the mass of the [[Sun]] and in excess of 60&nbsp;times the mass of the Sun.<ref name=masses/><ref name=masses2/> The entire study was to focus on [[binary star]] systems, stars coupled through a mutual gravitational attraction.<ref name=masses/><ref name=masses2/>

===Galactic mapping===
[[Image:Milky Way 2005.jpg|thumb|left|How scientists think the Milky Way is shaped]]
Interferometric measurements of stellar positions over the course of the mission would have permitted SIM to precisely measure the distances between stars throughout the [[Milky Way]]. This would have allowed astronomers to create a "roadmap" of the Galaxy, answering many questions about its shape and size.<ref name=dark1>[http://planetquest.jpl.nasa.gov/SIM/simLiteScience/darkMatterGalaxies/ The Milky Way and Dark Matter] {{webarchive|url=https://web.archive.org/web/20100530064440/http://planetquest.jpl.nasa.gov/SIM/simLiteScience/darkMatterGalaxies/ |date=30 May 2010 }}, NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.</ref><ref name=dark2>[http://planetquest.jpl.nasa.gov/SIM/scienceMotivations/darkMatter/ SIM Lite and Dark Matter] {{webarchive|url=https://web.archive.org/web/20100614193918/http://planetquest.jpl.nasa.gov/SIM/scienceMotivations/darkMatter/ |date=14 June 2010 }}, NASA, SIM Lite, Jet Propulsion Laboratory. Retrieved 9 March 2010.</ref>

Currently, astronomers know little about the shape and size of our galaxy relative to what they know about other galaxies; it is difficult to observe the entire Milky Way from the inside. A good analogy is trying to observe a marching band as a member of the band.<ref name=guide/> Observing other galaxies is much easier because humans are outside those galaxies. Steven Majewski and his team planned to use SIM Lite to help determine not only the shape and size of the Galaxy but also the distribution of its mass and the motion of its stars.<ref name=guide>Silberg, Bob. "[http://planetquest.jpl.nasa.gov/news/sim_guideToGalaxy.cfm Building a better guide to the Galaxy] {{webarchive|url=https://web.archive.org/web/20070509153845/http://planetquest.jpl.nasa.gov/news/sim_guideToGalaxy.cfm |date=9 May 2007 }}", NASA, SIM PlanetQuest, 14 February 2006, ''Jet Propulsion Laboratory''. Retrieved 24 April 2007.</ref>

SIM Lite measurements of Milky Way stars were to yield data to understand four topics: fundamental galactic parameters, the [[Oort Limit]], disk mass potential, and mass of the Galaxy to large [[radii]].<ref name=mainUVA>"[http://astsun.astro.virginia.edu/~rjp0i/takingmeasure/index.shtml Main Page] {{webarchive|url=https://web.archive.org/web/20070406221823/http://astsun.astro.virginia.edu/~rjp0i/takingmeasure/index.shtml |date=6 April 2007 }}", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.</ref> The first, fundamental galactic parameters, was aimed at answering key questions about the size, shape and the rotation rate of the Milky Way.<ref name=parameters>"[http://astsun.astro.virginia.edu/~rjp0i/takingmeasure/keyproject1.shtml Fundamental Galactic Parameters] {{webarchive|url=https://web.archive.org/web/20070206060234/http://astsun.astro.virginia.edu/~rjp0i/takingmeasure/keyproject1.shtml |date=6 February 2007 }}", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.</ref> The team hoped to more accurately determine the distance from the Sun to the [[Galactic Center]]. The second topic, the Oort Limit, would have attempted to determine the mass of the galactic disk.<ref name=oort>"[http://astsun.astro.virginia.edu/~rjp0i/takingmeasure/keyproject2.shtml Oort Limit] {{webarchive|url=https://web.archive.org/web/20070818102238/http://astsun.astro.virginia.edu/~rjp0i/takingmeasure/keyproject2.shtml |date=18 August 2007 }}", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.</ref>

The third project topic was disk mass potential. This topic was designed to make measurements of the distances to disk stars as well as their proper motions. The results of the third topic of study were to be combined with the results of the fundamental galactic parameters portion of the study to determine the Solar System's position and velocity in the galaxy.<ref name=diskmass>"[http://www.astro.virginia.edu/~rjp0i/takingmeasure/keyproject3.shtml Disk Mass Potential] {{Webarchive|url=https://web.archive.org/web/20131103040752/http://www.astro.virginia.edu/~rjp0i/takingmeasure/keyproject3.shtml |date=3 November 2013 }}", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.</ref> The final topic dealt with dark matter distribution in the Milky Way. SIM data was to be used to create a [[three-dimensional space|three-dimensional]] model of mass distribution in the Galaxy, out to a radius of 270 [[kiloparsec]]s (kps). Astronomers were to then use two different tests to determine the galactic potential at large radii.<ref name=radii>"[http://www.astro.virginia.edu/~rjp0i/takingmeasure/keyproject4.shtml Mass of the Galaxy to Large Radii] {{Webarchive|url=https://web.archive.org/web/20131103040529/http://www.astro.virginia.edu/~rjp0i/takingmeasure/keyproject4.shtml |date=3 November 2013 }}", Taking Measure of the Milky Way: A SIM PlanetQuest Key Project, University of Virginia, site updated 1 June 2005. Retrieved 25 April 2007.</ref>

===Dark matter===
[[File:UniverseComposition.svg|thumb|right|The gray portion of this pie graph shows the estimated distribution of dark matter in the universe.]]
[[Dark matter]] is the matter in the universe that cannot be seen. Because of the gravitational effect it exerts on stars and galaxies, scientists know that approximately 80% of the matter in the universe is dark matter.<ref name=dark1/><ref name=dark2/> The spatial distribution of [[dark matter]] in the universe is largely unknown; SIM Lite would have helped scientists answer to this question.

The strongest evidence for dark matter comes from galactic motion.<ref name=dark1/><ref name=dark2/> Galaxies rotate much faster than the amount of visible [[matter]] suggests they should; the gravity from the ordinary matter is not enough to hold the galaxy together. Scientists theorize that the galaxy is held together by huge quantities of dark matter.<ref name=dark1/><ref name=dark2/> Similarly, clusters of galaxies do not appear to have enough visible matter to gravitationally balance the high speed motions of their component galaxies.

Besides measuring stellar motions within the Milky Way, SIM Lite was to measure the internal and average galactic motion of some of the neighboring galaxies near the Milky Way.<ref name=dark1/><ref name=dark2/> The telescope's measurements were to be used in conjunction with other, currently available, data to provide astronomers with the first total mass measurements of individual galaxies. These numbers would enable scientists to estimate the spatial distribution of dark matter in the local group of galaxies, and by extension, throughout the universe.<ref name=dark1/><ref name=dark2/>