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{{Short description|European Space Agency scientific satellite}}
{{About|the satellite/catalogue|the Greek astronomer|Hipparchus|other uses|Hipparchus (disambiguation)}}
{{DISPLAYTITLE:''Hipparcos''}}
{{Infobox spaceflight
| name              = ''Hipparcos''
| names_list        = HIPPARCOS
| image             = Hipparcos-testing-estec.jpg
| image_caption     = Hipparcos satellite in the Large Solar Simulator, ESTEC, February 1988
| image_alt         = Hipparcos testing at ESTEC
| image_size        = 300px

| mission_type      = [[Astrometry|Astrometric]]<br/>[[Space observatory|observatory]]
| operator          = [[European Space Agency|ESA]]
| COSPAR_ID         = 1989-062B
| SATCAT            = 20169
| website           = http://sci.esa.int/hipparcos/
| mission_duration  = 2.5 years (planned)<br/>4 years (achieved)

| spacecraft        = HIPPARCOS
| spacecraft_type   = 
| spacecraft_bus    = 
| manufacturer      = [[Alenia Spazio]]<br/>[[Matra Marconi Space]]
| launch_mass       = {{cvt|1140|kg}} <ref name="rssdPdf">{{cite web|url=https://www.rssd.esa.int/SA/HIPPARCOS/docs/vol2_all.pdf|title=The Hipparcos and Tycho Catalogues|date=June 1997|publisher=ESA|access-date=2014-06-16|url-status=dead|archive-url=https://web.archive.org/web/20150616012158/http://www.rssd.esa.int/SA/HIPPARCOS/docs/vol2_all.pdf|archive-date=16 June 2015}}</ref>
| dry_mass          = {{cvt|635|kg}}
| payload_mass      = {{cvt|210|kg}}
| dimensions        = 
| power             = 295 [[watt]]s

| launch_date       = 8 August 1989, 23:25:53 [[Coordinated Universal Time|UTC]]
| launch_rocket     = [[Ariane 4|Ariane 44LP H10]] (V33)
| launch_site       = [[Guiana Space Centre|Centre Spatial Guyanais]], [[Kourou]], [[ELA-2]]
| launch_contractor = [[Arianespace]]

| entered_service   = August 1989
| disposal_type     = decommissioned
| deactivated       = August 1993
| last_contact      =

| orbit_reference   = [[Geocentric orbit]]<ref name="n2yo">{{cite web|url=http://www.n2yo.com/satellite/?s=20169|title=HIPPARCOS Satellite details 1989-062B NORAD 20169|publisher=N2YO|date=16 June 2015|access-date=16 June 2015}}</ref>
| orbit_regime      = [[Geostationary transfer orbit]]<br/><small>[[Geostationary orbit]] (planned)</small>
| orbit_periapsis   = {{cvt|500.3|km}}
| orbit_apoapsis    = {{cvt|35797.5|km}}
| orbit_inclination = 6.84°
| orbit_period      = 636.9 minutes
| orbit_rev_number  = 17830
| apsis             = gee

| telescope_type         = [[Schmidt telescope]]
| telescope_diameter     = {{cvt|29|cm}}
| telescope_focal_length = {{cvt|1.4|m}}
| telescope_wavelength   = [[visible light]]

| trans_band        = [[S band|S-Band]]
| trans_bandwidth   = 2-23&nbsp;[[kbit/s]]

| insignia          = Hipparcos insignia.png
| insignia_caption  = Legacy ESA insignia for the ''Hipparcos'' mission
| insignia_alt      = Hipparcos legacy mission insignia
| insignia_size     = 200px

}}

'''''Hipparcos''''' was a scientific [[satellite]] of the [[European Space Agency]] (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision [[astrometry]], the accurate measurement of the positions and distances of celestial objects on the sky.<ref name="GSM">{{cite book|last=Perryman|first=Michael|editor-first=Ramon|editor-last=Khanna|title=The Making of History's Greatest Star Map|date=2010|publisher=Springer-Verlag|location=Heidelberg|isbn=9783642116018|doi=10.1007/978-3-642-11602-5|series=Astronomers' Universe|bibcode=2010mhgs.book.....P|url=https://cds.cern.ch/record/1338896}}</ref> This was the first practical attempt at all-sky absolute [[stellar parallax|parallax]] measurement, something not possible with groundside observatories, and thus represented a fundamental breakthrough in astronomy.<ref name=brown2021>Brown 2021 "Microarcsecond Astrometry: Science Highlights from Gaia" https://www.annualreviews.org/content/journals/10.1146/annurev-astro-112320-035628</ref> The resulting high-precision measurements of the [[#Reference frame|absolute positions]], [[proper motion]]s, and parallaxes of stars enabled better calculations of their distance and [[tangential velocity]]; when combined with [[radial velocity]] measurements from [[spectroscopy]], astrophysicists were able to finally measure all six quantities needed to determine the motion of stars. The resulting '''''Hipparcos Catalogue''''',<ref>{{Cite web |title=Catalogues - Hipparcos - Cosmos |url=https://www.cosmos.esa.int/web/hipparcos/catalogues |access-date=2025-03-17 |website=Hipparcos |language=en-GB}}</ref> a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision ''Tycho Catalogue'' of more than a million stars was published at the same time, while the enhanced [[Tycho-2 Catalogue]] of 2.5 million stars was published in 2000. ''Hipparcos''{{'}}s follow-up mission,<ref>{{Cite journal |last=Gaia Collaboration |date=24 November 2016 |title=The Gaia mission |url=https://www.aanda.org/articles/aa/full_html/2016/11/aa29272-16/aa29272-16.html |journal=Astronomy & Astrophysics |volume=595 |issue=Gaia Data Release 1 |pages=36}}</ref> ''[[Gaia (spacecraft)|Gaia]]'', was launched in 2013.

The word "Hipparcos" is an [[acronym]] for '''High Precision Parallax Collecting Satellite''' and also a reference to the ancient Greek astronomer [[Hipparchus]] of Nicaea, who is noted for applications of [[trigonometry]] to astronomy and his discovery of the [[precession of the equinoxes]].

== Background ==
By the second half of the 20th century, the accurate measurement of [[star]] positions from the ground was running into essentially insurmountable barriers to improvements in accuracy, especially for large-angle measurements and systematic terms. Problems were dominated by the effects of the [[Earth]]'s [[atmosphere]], but were compounded by complex optical terms, thermal and [[gravity|gravitational]] instrument flexures, and the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967.<ref>{{cite journal|last=Lacroute|first=P.|journal=Transactions of the International Astronomical Union|title=Proceedings of the 13th General Assembly|volume=XIIIB|page=63|date=1967}}</ref>

Even other space telescopes including (but not limited to) [[Hubble]], are only capable of measuring ''relative'' astrometry, measuring positions and parallaxes by reference to nearby background stars/sources. Compiling a variety of such local, relative parallaxes results in regional errors, making distance comparisons between distant sky regions quite difficult. Hipparcos was the first project to engage in ''absolute'' astrometry: in addition to being an all-sky survey, Hipparcos introduced the concept of using two telescopes, separated by a wide "basic" angle, being directed onto a single focal plane. As a result, all measurements made by either telescope are connected to simultaneous measurements by the other. With a variety of transit angles on each individual source, it then becomes connected to many other parts of the sky. The final data reduction can then use these myriad distant sky connections to deduce a single global rigid [[#Reference frame|reference frame]] by which to anchor all position/parallax measurements.<ref name=brown2021/> (This is somewhat similar to how Earth [[geodetic datum|coordinate systems]] were anchored by a global [[triangulation network|network of triangles]].)

The mission was originally proposed to the French space agency [[CNES]], which considered it too complex and expensive for a single national programme and recommended that it be proposed in a multinational context. Its acceptance within the [[European Space Agency]]'s scientific programme, in 1980, was the result of a lengthy process of study and [[lobbying]]. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, and thus to place theoretical studies of stellar structure and evolution, and studies of [[galaxy|galactic]] structure and kinematics, on a more secure empirical basis. Observationally, the objective was to provide the positions, [[parallax]]es, and annual [[proper motion]]s for some 100,000 stars with an unprecedented accuracy of 0.002&nbsp;[[arcsecond]]s, a target in practice eventually surpassed by a factor of two. The name of the space telescope, "Hipparcos", was an [[acronym]] for ''High Precision Parallax Collecting Satellite'', and it also reflected the name of the ancient Greek astronomer [[Hipparchus]], who is considered the founder of [[trigonometry]] and the discoverer of the [[precession of the equinoxes]] (due to the Earth wobbling on its axis).

== Satellite and payload ==
[[File:Hipparcos-grid.gif|thumb|upright=1.0|right|Optical micrograph of part of the main modulating grid (top) and the star mapper grid (bottom). The period of the main grid is 8.2 [[micrometre]]s.]]

The spacecraft carried a single all-reflective, eccentric [[Schmidt telescope]], with an aperture of {{cvt|29|cm}}. A special beam-combining mirror superimposed two fields of view, 58° apart, into the common focal plane. This complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, and providing an [[Vignetting|unvignetted]] field of view of about 1° × 1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec (8.2 micrometre). Behind this grid system, an [[image dissector]] tube ([[photomultiplier]] type detector) with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of [[photon]] counts (with a sampling frequency of 1200 [[Hertz|Hz]]) from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Originally targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final ''Hipparcos Catalogue'' comprised nearly 120,000 stars<ref name="cat"/>{{rp|xiii}} with a median accuracy of slightly better than 0.001 arc-sec (1 milliarc-sec).<ref name="cat">{{cite web|title=The Hipparcos and Tycho Catalogues |url=https://www.cosmos.esa.int/documents/532822/552851/vol1_all.pdf|publisher=ESA|access-date=May 13, 2023}}</ref>{{rp|3}}

An additional [[photomultiplier]] system viewed a beam splitter in the optical path and was used as a star mapper. Its purpose was to monitor and determine the satellite attitude, and in the process, to gather photometric and [[astrometric]] data of all stars down to about 11th magnitude. These measurements were made in two broad bands approximately corresponding to B and V in the (Johnson) [[UBV photometric system]]. The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, which is a factor of 25 less than the main mission stars. Originally targeting the observation of around 400,000 stars, the resulting ''Tycho Catalogue'' comprised just over 1 million stars, with a subsequent analysis extending this to the [[Tycho-2 Catalogue]] of about 2.5 million stars.

The attitude of the spacecraft about its center of gravity was controlled to scan the [[celestial sphere]] in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun. The spacecraft spun around its [[Z-axis]] at the rate of 11.25 revolutions/day (168.75 arc-sec/s) at an angle of 43° to the [[Sun]]. The Z-axis rotated about the Sun-satellite line at 6.4 revolutions/year.<ref>{{cite journal|title=Hipparcos, The Input Catalogue
|author1=Gomez, A.E. |author2=Torra, J.|url=https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1987RMxAA..14..441G&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES|journal= Revista Mexicana de Astronomía y Astrofísica|volume=14|pages=441–447|date=1987|publisher=Universidad Nacional Autonoma de Mexico|bibcode=1987RMxAA..14..441G |access-date=14 May 2023}}</ref>

The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb. The solar array consisted of three deployable sections, generating around 300&nbsp;W in total. Two [[S band|S-band]] antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24&nbsp;[[kbit/s]]. An attitude and orbit-control subsystem (comprising 5-[[Newton (units)|newton]] [[hydrazine]] thrusters for course manoeuvres, 20-millinewton cold gas thrusters for attitude control, and [[gyroscope]]s for attitude determination) ensured correct dynamic attitude control and determination during the operational lifetime.

== Principles ==
Some key features of the observations were as follows:<ref name="perryman">{{cite book|title=Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data|author=Michael Perryman|chapter=The Hipparcos and Tycho Catalogues|url=https://assets.cambridge.org/97805215/14897/excerpt/9780521514897_excerpt.pdf|publisher=Cambridge University Press
|date=December 2008|access-date=May 14, 2023}}</ref><ref name=brown2021/>
* through observations from space, the effects of [[astronomical seeing]] due to the [[atmosphere]], instrumental gravitational flexure and thermal distortions could be obviated or minimised;
* all-sky visibility permitted a direct linking of the stars observed all over the celestial sphere;
* the two viewing directions of the satellite, separated by a large and suitable angle (58°), resulted in a rigid connection between quasi-instantaneous one-dimensional observations in different parts of the sky. In turn, this led to [[parallax]] determinations which are absolute (rather than relative, with respect to some unknown zero-point);
* the continuous [[ecliptic]]-based scanning of the satellite resulted in an optimum use of the available observing time, with a resulting catalogue providing reasonably homogeneous sky density and uniform astrometric accuracy over the entire celestial sphere;

[[File:Hipparcos-principles.gif|thumb|upright=1.0|Principles of the astrometric measurements. Filled circles and solid lines show three objects from one field of view (about 1° in size), and open circles and dashed lines show three objects from a distinct sky region superimposed by virtue of the large basic angle. Left: object positions at one reference epoch. Middle: their space motions over about four years, with arbitrary proper motion vectors and scale factors; triangles show their positions at a fixed epoch near the end of the interval. Right: the total positional changes including the additional apparent motions due to annual parallax, the four loops corresponding to four Earth orbits around the sun. The parallax-induced motions are in phase for all stars in the same region of sky, so that relative measurements within one field can provide only relative parallaxes. Although the relative separations between the stars change continuously over the measurement period, they are described by just five numerical parameters per star (two components of position, two of proper motion, and the parallax).]]

* the various geometrical scan configurations for each star, at multiple epochs throughout the 3-year observation programme, resulted in a dense network of one-dimensional positions from which the [[center of mass|barycentric]] coordinate direction, the [[parallax]], and the object's [[proper motion]], could be solved for in what was effectively a global [[least squares]] reduction of the totality of observations. The astrometric parameters as well as their [[standard error (statistics)|standard errors]] and [[correlation|correlation coefficients]] were derived in the process;
* since the number of independent geometrical observations per object was large (typically of order 30) compared with the number of unknowns for the standard model (five astrometric unknowns per star), astrometric solutions not complying with this simple five-parameter model could be expanded to take into account the effects of [[double star|double or multiple stars]], or non-linear photocentric motions ascribed to unresolved [[astrometric binaries]];
* a somewhat larger number of actual observations per object, of order 110, provided accurate and homogeneous photometric information for each star, from which mean magnitudes, variability amplitudes, and in many cases period and variability type classification could be undertaken.

[[File:Hipparcos-star-path.gif|thumb|upright=1.0|right|The path on the sky of one of the Hipparcos Catalogue objects, over a period of three years. Each straight line indicates the observed position of the star at a particular epoch: because the measurement is one-dimensional, the precise location along this position line is undetermined by the observation. The curve is the modelled stellar path fitted to all the measurements. The inferred position at each epoch is indicated by a dot, and the residual by a short line joining the dot to the corresponding position line. The amplitude of the oscillatory motion gives the star's parallax, with the linear component representing the star's proper motion.]]

== Development, launch and operations ==
{{unreferenced section|date=September 2012}}

The ''Hipparcos'' satellite was financed and managed under the overall authority of the [[European Space Agency]] (ESA). The main industrial contractors were [[Matra Marconi Space]] (now [[EADS Astrium]]) and Alenia Spazio (now [[Thales Alenia Space]]).

Other hardware components were supplied as follows:
* the beam-combining mirror by REOSC in [[Saint-Pierre-du-Perray]], France
* the spherical, folding, and relay mirrors by [[Carl Zeiss AG]] in [[Oberkochen]], Germany
* the external straylight baffles by [[EADS CASA|CASA]] in [[Madrid]], Spain
* the modulating grid by CSEM in [[Neuchâtel]], Switzerland
* the mechanism control system and the thermal control electronics by [[Dornier Satellite Systems]] in [[Friedrichshafen]], Germany
* the optical filters, the experiment structures and the attitude and orbit control system by [[Matra Marconi Space]] in [[Vélizy]], France
* the instrument switching mechanisms by [[Oerlikon-Contraves]] in [[Zürich]], Switzerland
* the image dissector tube and photomultiplier detectors by the Dutch Space Research Organisation ([[SRON]]) in the Netherlands
* the refocusing assembly mechanism by TNO-TPD in [[Delft]], Netherlands
* the electrical power subsystem by [[British Aerospace]] in [[Bristol]], United Kingdom
* the structure and reaction control system by Daimler-Benz Aerospace in [[Bremen]], Germany
* the solar arrays and thermal control system by Fokker Space System in [[Leiden]], Netherlands
* the data handling and telecommunications system by [[Saab Ericsson Space]] in [[Gothenburg]], Sweden
* the [[apogee kick motor]] by SEP in France

Groups from the Institut d'Astrophysique in [[Liège]], Belgium and the Laboratoire d'Astronomie Spatiale in [[Marseille]], France, contributed optical performance, calibration, and alignment test procedures; Captec in [[Dublin]], Ireland, and Logica in [[London]] contributed to the on-board software and calibration.

The ''Hipparcos'' satellite was launched (with the direct broadcast satellite [[TV-Sat 2]] as co-passenger) on an [[Ariane 4]] [[launch vehicle]], flight V33, from [[Guiana Space Centre|Centre Spatial Guyanais]], [[Kourou]], French Guiana, on 8 August 1989. It was launched into a [[geostationary transfer orbit]] (GTO), but the Mage-2 apogee kick motor failed to fire, and the intended [[geostationary orbit]] was never achieved. However, with the addition of further ground stations, in addition to ESA operations control centre at [[European Space Operations Centre]] (ESOC) in Germany, the satellite was successfully operated in GTO for almost 3.5 years. All of the original mission goals were, eventually, exceeded.

Including an estimate for the scientific activities related to the satellite observations and data processing, the ''Hipparcos'' mission cost about €600 million (in year 2000 economic conditions), and its execution involved some 200 European scientists and more than 2,000 individuals in European industry.

== Hipparcos Input Catalogue ==
The satellite observations relied on a pre-defined list of target stars. Stars were observed as the satellite rotated, by a sensitive region of the image dissector tube detector. This pre-defined star list formed the ''Hipparcos Input Catalogue'' (HIC): each star in the final ''Hipparcos Catalogue'' was contained in the Input Catalogue.<ref>{{cite journal|last=Turon|first=C.|display-authors=et al
|title=Properties of the Hipparcos Input Catalogue|journal=Astronomy & Astrophysics|volume=304| pages=82–93|date=1995|bibcode=1995A&A...304...82T}}</ref> The Input Catalogue was compiled by the INCA Consortium over the period 1982–1989, finalised pre-launch, and published both digitally and in printed form.<ref>{{cite book|last=Turon|first=Catherine|display-authors=et al|title=Hipparcos Input Catalogue, ESA SP-1136 (7 volumes)|publisher=European Space Agency|date=1992}}</ref>

Although fully superseded by the satellite results, it nevertheless includes supplemental information on multiple system components as well as compilations of radial velocities and spectral types which, not observed by the satellite, were not included in the published ''Hipparcos Catalogue''.

Constraints on total observing time, and on the uniformity of stars across the celestial sphere for satellite operations and data analysis, led to an Input Catalogue of some 118,000 stars. It merged
two components: first, a survey of around 58,000 objects as complete as possible to the following limiting magnitudes:
V&lt;7.9 + 1.1sin|b| for [[stellar classification|spectral types]] earlier than G5, and
V&lt;7.3 + 1.1sin|b| for [[stellar classification|spectral types]] later than G5 (b is the Galactic latitude). Stars constituting this survey are flagged in the ''Hipparcos Catalogue''.

The second component comprised additional stars selected according to their scientific interest, with none fainter than about magnitude V=13 mag. These were selected from around 200 scientific proposals submitted on the basis of an Invitation for Proposals issued by ESA in 1982, and prioritised by the Scientific Proposal Selection Committee in consultation with the Input Catalogue Consortium. This selection had to balance 'a priori' scientific interest, and the observing programme's limiting magnitude, total observing time, and sky uniformity constraints.

== Data reductions ==
{{unreferenced section|date=September 2012}}

For the main mission results, the data analysis was carried out by two independent scientific teams, NDAC and FAST, together comprising some 100 astronomers and scientists, mostly from European (ESA-member state) institutes. The analyses, proceeding from nearly 1000 Gbit of satellite data acquired over 3.5 years, incorporated a comprehensive system of cross-checking and validation, and is described in detail in the published catalogue.

A detailed optical calibration model was included to map the transformation from sky to instrumental coordinates. Its adequacy could be verified by the detailed measurement residuals. The Earth's orbit, and the satellite's orbit with respect to the [[Earth]], were essential for describing the location of the observer at each epoch of observation, and were supplied by an appropriate Earth ephemeris combined with accurate satellite ranging. Corrections due to [[special relativity]] ([[aberration of light|stellar aberration]]) made use of the corresponding satellite velocity. Modifications due to [[General relativity|general relativistic]] light bending were significant (4 milliarc-sec at 90° to the ecliptic) and corrected for deterministically assuming γ=1 in the [[PPN formalism]]. Residuals were examined to establish limits on any deviations from this general relativistic value, and no significant discrepancies were found.

== Reference frame ==
The satellite observations essentially yielded highly accurate relative positions of stars with respect to each other, throughout the measurement period (1989–1993). In the absence of direct observations of extragalactic sources (apart from marginal observations of quasar [[3C 273]]) the resulting rigid reference frame was transformed to an [[inertial frame of reference]] linked to extragalactic sources. This allows surveys at different wavelengths to be directly correlated with the ''Hipparcos'' stars, and ensures that the catalogue proper motions are, as far as possible, kinematically non-rotating. The determination of the relevant three solid-body rotation angles, and the three time-dependent rotation rates, was conducted and completed in advance of the catalogue publication. This resulted in an accurate but indirect link to an inertial, extragalactic, reference frame.<ref>{{cite journal|last=Kovalevsky|first=J.|display-authors=et al|title=The Hipparcos Catalogue as a Realisation of the Extragalactic Reference Frame|journal=Astronomy & Astrophysics|volume=323|pages=620–633|date=1997|bibcode=1997A&A...323..620K}}</ref>

[[File:Hipparcos-accuracies.gif|thumb|upright=1.0|right|Typical accuracies of the FK5, ''Hipparcos'', Tycho-1, and Tycho-2 Catalogues as a function of time. Tycho-1 dependencies are shown for two representative magnitudes. For Tycho-2, a typical proper motion error of 2.5 milliarc-sec applies to both bright stars (positional error at J1991.25 of 7 milliarc-sec) and faint stars (positional error at J1991.25 of 60 milliarc-sec).]] 
A variety of methods to establish this reference frame link before catalogue publication were included and appropriately weighted: interferometric observations of radio stars by [[VLBI]] networks, [[MERLIN]] and [[Very Large Array]] (VLA); observations of [[quasars]] relative to ''Hipparcos'' stars using [[charge-coupled device]] (CCD), photographic plates, and the [[Hubble Space Telescope]]; photographic programmes to determine stellar proper motions with respect to extragalactic objects (Bonn, Kiev, Lick, Potsdam, Yale/San Juan); and comparison of [[Earth rotation]] parameters obtained by [[Very-long-baseline interferometry]] (VLBI) and by ground-based optical observations of ''Hipparcos'' stars. Although very different in terms of instruments, observational methods and objects involved, the various techniques generally agreed to within 10 milliarc-sec in the orientation and 1 milliarc-sec/year in the rotation of the system. From appropriate weighting, the coordinate axes defined by the published catalogue are believed to be aligned with the extragalactic radio frame to within ±0.6 milliarc-sec at the epoch J1991.25, and non-rotating with respect to distant extragalactic objects to within ±0.25 milliarc-sec/yr.<ref name="perryman"/>{{rp|10}}

The ''Hipparcos'' and Tycho Catalogues were then constructed such that the resulting '''''Hipparcos'' celestial reference frame''' (HCRF) coincides, to within observational uncertainties, with the ''[[International Celestial Reference Frame]]'' (ICRF), and representing the best estimates at the time of the catalogue completion (in 1996). The HCRF is thus a materialisation of the [[International Celestial Reference System]] (ICRS) in the optical domain. It extends and improves the [[J2000]] ([[FK5]]) system, retaining approximately the global orientation of that system but without its regional errors.<ref name="perryman"/>{{rp|10}}

== Double and multiple stars ==
{{unreferenced section|date=September 2012}}

Whilst of enormous astronomical importance, [[double star]]s and [[multiple star]]s provided considerable complications to the observations (due to the finite size and profile of the detector's sensitive field of view) and to the data analysis. The data processing classified the astrometric solutions as follows:
* single-star solutions: 100,038 entries, of which 6,763 were flagged as suspected double
* component solutions (Annex C): 13,211 entries, comprising 24,588 components in 12,195 solutions
* acceleration solutions (Annex G): 2,622 solutions
* orbital solutions (Annex O): 235 entries
* variability-induced movers (Annex V): 288 entries
* stochastic solutions (Annex X): 1,561 entries
* no valid astrometric solution: 263 entries (of which 218 were flagged as suspected double)

If a binary star has a long orbital period such that non-linear motions of the photocentre were insignificant over the short (3-year) measurement duration, the binary nature of the star would pass unrecognised by ''Hipparcos'', but could show as a ''Hipparcos'' proper motion discrepant compared to those established from long temporal baseline proper motion programmes on ground. Higher-order photocentric motions could be represented by a 7-parameter, or even 9-parameter model fit (compared to the standard 5-parameter model), and typically such models could be enhanced in complexity until suitable fits were obtained. A complete orbit, requiring 7 elements, was determined for 45 systems. Orbital periods close to one year can become degenerate with the parallax, resulting in unreliable solutions for both. Triple or higher-order systems provided further challenges to the data processing.

=={{anchor|Hipparcos passband}} Photometric observations ==
{{unreferenced section|date=September 2012}}

The highest accuracy photometric data were provided as a by-product of the main mission astrometric observations. They were made in a broad-band visible light [[passband]], specific to ''Hipparcos'', and designated H<sub>p</sub>.<ref>{{cite journal
 | title=The Hipparcos and Tycho Photometric System Passbands
 | last=Bessell | first=Michael S.
 | journal=The Publications of the Astronomical Society of the Pacific
 | volume=112 | issue=773 | pages=961–965 | date=July 2000
 | doi=10.1086/316598 | bibcode=2000PASP..112..961B | s2cid=121605844 }}</ref> The median photometric precision, for H<sub>p</sub><9 magnitude, was 0.0015 [[Magnitude (astronomy)|magnitude]], with typically 110 distinct observations per star throughout the 3.5-year observation period. As part of the data reduction and catalogue production, new variables were identified and designated with appropriate [[variable star designation]]s. Variable stars were classified as periodic or unsolved variables; the former were published with estimates of their period, variability amplitude, and variability type. In total some 11,597 variable objects were detected, of which 8,237 were newly classified as variable. There are, for example, 273 [[Cepheid variable]]s, 186 [[RR Lyrae variable]]s, 108 [[Delta Scuti variable]]s, and 917 [[eclipsing binary star]]s. The star mapper observations, constituting the Tycho (and Tycho-2) Catalogue, provided two colours, roughly B and V in the Johnson [[UBV photometric system]], important for [[spectral classification]] and [[effective temperature]] determination.

== Radial velocities ==
{{unreferenced section|date=September 2012}}

Classical astrometry concerns only motions in the plane of the sky and ignores the star's [[radial velocity]], i.e. its space motion along the line-of-sight. Whilst critical for an understanding of stellar kinematics, and hence population dynamics, its effect is generally imperceptible to astrometric measurements (in the plane of the sky), and therefore it is generally ignored in large-scale astrometric surveys. In practice, it can be measured as a [[Doppler effect|Doppler shift]] of the spectral lines. More strictly, however, the radial velocity does enter a rigorous astrometric formulation. Specifically, a space velocity along the line-of-sight means that the transformation from tangential linear velocity to (angular) [[proper motion]] is a function of time. The resulting effect of secular or perspective acceleration is the interpretation of a transverse acceleration actually arising from a purely linear space velocity with a significant radial component, with the positional effect proportional to the product of the parallax, the proper motion, and the radial velocity. At the accuracy levels of ''Hipparcos'' it is of (marginal) importance only for the nearest stars with the largest radial velocities and proper motions, but was accounted for in the 21 cases for which the accumulated positional effect over two years exceeds 0.1 milliarc-sec. Radial velocities for ''Hipparcos Catalogue'' stars, to the extent that they are presently known from independent ground-based surveys, can be found from the astronomical database of the [[Centre de données astronomiques de Strasbourg]].

The absence of reliable distances for the majority of stars means that the angular measurements made, astrometrically, in the plane of the sky, cannot generally be converted into true space velocities in the plane of the sky. For this reason, astrometry characterises the transverse motions of stars in angular measure (e.g. arcsec per year) rather than in km/s or equivalent. Similarly, the typical absence of reliable radial velocities means that the transverse space motion (when known) is, in any case, only a component of the complete, three-dimensional, space velocity.

== Published catalogues ==
{| class="wikitable" border style="float:right; clear:right; margin-left:1cm; font-size:80%"
|+ Principal observational characteristics of the Hipparcos and Tycho
  Catalogues. ICRS is the International Celestial Reference System.
|-
! Property !! Value
|-
| colspan="2" style="text-align:center;"| Common:
|-
|&nbsp; &nbsp;Measurement period || 1989.8–1993.2
|-
|&nbsp; &nbsp;Catalogue epoch || J1991.25
|-
|&nbsp; &nbsp;Reference system || ICRS
|-
|&nbsp; &nbsp; &nbsp;• coincidence with ICRS (3 axes) || ±0.6 mas
|-
|&nbsp; &nbsp; &nbsp;• deviation from inertial (3 axes) || ±0.25 mas/yr
|-
| colspan="2" style="text-align:center;"| Hipparcos Catalogue:
|-
|&nbsp; &nbsp;Number of entries || 118,218
|-
|&nbsp; &nbsp; &nbsp;• with associated astrometry &nbsp; &nbsp;|| 117,955
|-
|&nbsp; &nbsp; &nbsp;• with associated photometry &nbsp; &nbsp;|| 118,204
|-
|&nbsp; &nbsp;Mean sky density || ≈3 per sq deg
|-
|&nbsp; &nbsp;Limiting magnitude || V≈12.4 mag
|-
|&nbsp; &nbsp;Completeness || V=7.3–9.0 mag
|-
| colspan="2" style="text-align:center;"| Tycho Catalogue:
|-
|&nbsp; &nbsp;Number of entries || 1,058,332
|-
|&nbsp; &nbsp; &nbsp;• based on Tycho data &nbsp; &nbsp;|| 1,052,031
|-
|&nbsp; &nbsp; &nbsp;• with only Hipparcos data &nbsp; &nbsp;|| 6301
|-
|&nbsp; &nbsp;Mean sky density || 25 per sq deg
|-
|&nbsp; &nbsp;Limiting magnitude || V≈11.5 mag
|-
|&nbsp; &nbsp;Completeness to 90 per cent || V≈10.5 mag
|-
|&nbsp; &nbsp;Completeness to 99.9 per cent || V≈10.0 mag
|-
| colspan="2" style="text-align:center;"| Tycho 2 Catalogue:
|-
|&nbsp; &nbsp;Number of entries || 2,539,913
|-
|&nbsp; &nbsp;Mean sky density: || 
|-
|&nbsp; &nbsp; &nbsp; • at b=0° || ≈150 per sq deg
|-
|&nbsp; &nbsp; &nbsp; • at b=±30° || ≈50 per sq deg
|-
|&nbsp; &nbsp; &nbsp; • at b=±90° || ≈25 per sq deg
|-
|&nbsp; &nbsp;Completeness to 90 per cent || V≈11.5 mag
|-
|&nbsp; &nbsp;Completeness to 99 per cent || V≈11.0 mag
|}

[[File:Hipparcos Catalogue equirectangular plot.svg|thumb|upright=1.0|right|Equirectangular plot of declination vs right ascension of stars brighter than apparent magnitude 5 on the Hipparcos Catalogue, coded by spectral type and apparent magnitude, relative to the modern constellations and the ecliptic]]

The final ''Hipparcos Catalogue'' was the result of the critical comparison and merging of the two (NDAC and FAST consortia) analyses, and contains 118,218 entries (stars or multiple stars), corresponding to an average of some three stars per square degree over the entire sky.<ref>{{cite journal|last=Perryman|first=M.A.C.|display-authors=et al|title=The Hipparcos Catalogue|journal=Astronomy & Astrophysics|volume=323
|pages=L49–L52|date=1997|bibcode=1997A&A...323L..49P}}</ref> Median precision of the five astrometric parameters (Hp<9 magnitude) exceeded the original mission goals, and are between 0.6 and 1.0 mas. Some 20,000 distances were determined to better than 10%, and 50,000 to better than 20%. The inferred ratio of external to standard errors is ≈1.0–1.2, and estimated systematic errors are below 0.1 mas. The number of solved or suspected double or multiple stars is 23,882.<ref>{{cite journal|last=Lindegren|first=L.|display-authors=et al|title=Double star data in the Hipparcos Catalogue|journal=Astronomy & Astrophysics
|volume=323|page=L53–L56|date=1997|bibcode=1997A&A...323L..53L}}</ref> Photometric observations yielded multi-epoch photometry with a mean number of 110 observations per star, and a median photometric precision (Hp<9 magnitude) of 0.0015 magnitude, with 11,597 entries were identified as variable or possibly-variable.<ref>{{cite journal|last=Van Leeuwen|first=F.|display-authors=et al|title=The Hipparcos Mission: Photometric Data|journal=Astronomy & Astrophysics|volume=323|pages=L61–L64|date=1997|bibcode=1997A&A...323L..61V}}</ref>

For the star mapper results, the data analysis was carried out by the Tycho Data Analysis Consortium (TDAC). The Tycho Catalogue comprises more than one million stars with 20–30&nbsp;milliarc-sec astrometry and two-colour (B and V band) photometry.<ref>{{cite journal|last=Høg|first=E.|display-authors=et al|title=The Tycho Catalogue|journal=Astronomy & Astrophysics|volume=323|pages=L57–L60|date=1997 |bibcode=1997A&A...323L..57H}}</ref>

The final ''Hipparcos'' and ''Tycho Catalogues'' were completed in August 1996. The catalogues were published by [[European Space Agency]] (ESA) on behalf of the scientific teams in June 1997.<ref>
{{cite book|last=European Space Agency|title=The Hipparcos and Tycho Catalogues|date=1997|isbn=978-92-9092-399-2|publisher=ESA Publications Division|location=Noordwijk, the Netherlands}}</ref>

A more extensive analysis of the star mapper (Tycho) data extracted additional faint stars from the data stream. Combined with old photographic plate observations made several decades earlier as part of the [[Astrographic Catalogue]] programme, the [[Tycho-2 Catalogue]] of more than 2.5 million stars (and fully superseding the original Tycho Catalogue) was published in 2000.<ref>{{cite journal|last=Høg|first=E. |display-authors=et al|title=The Tycho-2 Catalogue of the 2.5 million brightest stars|journal=Astronomy & Astrophysics|volume=355|pages=L27–L30|date=2000|bibcode=2000A&A...355L..27H}}</ref>

The ''Hipparcos'' and ''Tycho-1 Catalogues'' were used to create the [[Millennium Star Atlas]]: an all-sky atlas of one million stars to [[visual magnitude]] 11. Some 10,000 nonstellar objects are also included to complement the catalogue data.<ref>{{cite book|last=Sinnott|first=Roger|author2=Perryman, Michael|title=Millennium Star Atlas|publisher=Sky Publishing Corporation & European Space Agency|date=1997|isbn=978-0-933346-83-3}}</ref>

Between 1997 and 2007, investigations into subtle effects in the satellite attitude and instrument calibration continued. A number of effects in the data that had not been fully accounted for were studied, such as scan-phase discontinuities and micrometeoroid-induced attitude jumps. A re-reduction of the associated steps of the analysis was eventually undertaken.<ref name="FVL">{{cite book|last=Van Leeuwen|first=Floor
|title=Hipparcos, the New Reduction of the Raw Data|publisher=Springer, Dordrecht|date=2007|isbn=978-1-4020-6341-1}}</ref>

This has led to improved astrometric accuracies for stars brighter than Hp=9.0 magnitude, reaching a factor of about three for the brightest stars (Hp<4.5 magnitude), while also underlining the conclusion that the ''Hipparcos Catalogue'' as originally published is generally reliable within the quoted accuracies.

All catalogue data are available online from the [[Centre de données astronomiques de Strasbourg]].

== Scientific results ==
[[File:Milky Way 2005.jpg|thumb|upright=1.0|right|Artist's concept of the [[Milky Way]] galaxy, showing two prominent spiral arms attached to the ends of a thick central bar. ''Hipparcos'' mapped many stars in the solar neighbourhood with great accuracy, though this represents only a small fraction of stars in the galaxy.]]

The ''Hipparcos'' results have affected a very broad range of astronomical research, which can be classified into three major themes:

* the provision of an accurate reference frame: this has allowed the consistent and rigorous re-reduction of historical astrometric measurements, including those from Schmidt plates, meridian circles, the 100-year-old [[Astrographic Catalogue]], and 150&nbsp;years of Earth-orientation measurements. These, in turn, have yielded a dense reference framework with high-accuracy, long-term proper motions (the [[Tycho-2 Catalogue]]). Reduction of current state-of-the-art survey data has yielded the dense UCAC2 Catalogue of the [[United States Naval Observatory|U.S. Naval Observatory]] on the same reference system, and improved astrometric data from recent surveys such as the [[Sloan Digital Sky Survey]] and [[2MASS]]. Implicit in the high-accuracy reference frame is the measurement of [[gravitational lens]]ing and the detection and characterisation of double and multiple stars;
* constraints on [[stellar structure]] and [[stellar evolution]]: the accurate distances and luminosities of 100,000 stars has provided the most comprehensive and accurate data set of fundamental stellar parameters to date, placing constraints on internal rotation, element diffusion, convective motions, and [[asteroseismology]]. Combined with theoretical models and other data it yields evolutionary masses, radii, and ages for large numbers of stars covering a wide range of evolutionary states;
* Galactic kinematics and dynamics: the uniform and accurate distances and proper motions have provided a substantial advance in understanding of [[stellar kinematics]] and the dynamical structure of the solar neighbourhood, ranging from the presence and evolution of clusters, associations and moving groups, the presence of resonance motions due to the Galaxy's central bar and [[spiral arms]], determination of the parameters describing [[galaxy rotation curve|galactic rotation]], discrimination of the disk and halo populations, evidence for halo accretion, and the measurement of space motions of [[stellar kinematics|runaway stars]], [[globular clusters]], and many other types of star.

Associated with these major themes, ''Hipparcos'' has provided results in topics as diverse as Solar System science, including mass determinations of asteroids, [[Earth's rotation]] and [[Chandler wobble]]; the internal structure of [[white dwarf]]s; the masses of [[brown dwarfs]]; the characterisation of [[extra-solar planets]] and their host stars; the height of the Sun above the Galactic mid-plane; the [[age of the Universe]]; the stellar [[initial mass function]] and [[star formation]] rates; and strategies for the [[search for extraterrestrial intelligence]]. The high-precision multi-epoch photometry has been used to measure variability and stellar pulsations in many classes of objects. The ''Hipparcos'' and ''Tycho'' catalogues are now routinely used to point ground-based telescopes, navigate space missions, and drive public planetaria.

Since 1997, several thousand scientific papers have been published making use of the ''Hipparcos'' and ''Tycho'' catalogues. A detailed review of the ''Hipparcos'' scientific literature between 1997 and 2007 was published in 2009,<ref>{{cite book|last=Perryman|first=Michael|title=Astronomical Applications of Astrometry: Ten Years of Exploitation of the Hipparcos Satellite Data|publisher=Cambridge University Press
|date=2009|page=692|url=http://www.cambridge.org/9780521514897|isbn=978-0-521-51489-7}}</ref> and a popular account of the project in 2010.<ref name="GSM"/> Some examples of notable results include (listed chronologically):

* studies of Galactic rotation from [[Cepheid variable]]s <ref>{{cite journal|last=Feast|first=M. W.|author2=Whitelock, P. A.|title=Galactic kinematics of Cepheids from Hipparcos proper motions|journal=Monthly Notices of the Royal Astronomical Society|volume=291|issue=4|pages=683–693|date=1997|bibcode=1997MNRAS.291..683F|arxiv=astro-ph/9706293|doi=10.1093/mnras/291.4.683|doi-access=free |s2cid=17704884}}</ref>
* the nature of [[Delta Scuti variable]]s <ref>{{cite journal|last=Høg|first=E.|author2=Petersen, J. O.|title=Hipparcos parallaxes and the nature of delta Scuti stars|journal=Astronomy & Astrophysics|volume=323
|pages=827–830|date=1997|bibcode=1997A&A...323..827H}}</ref>

* studies of local [[stellar kinematics]]<ref>{{cite journal|last=Dehnen|first=W.|author2=Binney, J. J.|title=Local stellar kinematics from Hipparcos data|journal=Monthly Notices of the Royal Astronomical Society|volume=298|issue=2|pages=387–394|date=1998|doi=10.1046/j.1365-8711.1998.01600.x|doi-access=free |bibcode=1998MNRAS.298..387D|arxiv=astro-ph/9710077|s2cid=15936627}}</ref>
* testing the [[white dwarf]] mass–radius relation <ref>{{cite journal|last=Provencal|first=J. L.|display-authors=et al|title=Testing the white dwarf mass-radius relation with Hipparcos|journal=Astrophysical Journal|volume=494|issue=2|pages=759–767|date=1998|doi=10.1086/305238|bibcode=1998ApJ...494..759P|citeseerx=10.1.1.44.7051|s2cid=122724497 }}</ref>
* the structure and dynamics of the [[Hyades (star cluster)|Hyades cluster]]<ref>{{cite journal|last=Perryman|first=M. A. C.|display-authors=et al|title=The Hyades: distance, structure, dynamics, and age
|journal=Astronomy & Astrophysics|volume=331|pages=81–120|date=1998|bibcode=1998A&A...331...81P|arxiv=astro-ph/9707253}}</ref>

* kinematics of [[Wolf–Rayet star]]s and O-type [[runaway star]]s <ref>{{cite journal|last=Moffat|first=A. F. J.|display-authors=et al|title=Wolf-Rayet stars and O-star runaways with Hipparcos Kinematics
|journal=Astronomy & Astrophysics|volume=331|pages=949–958|date=1998|bibcode=1998A&A...331..949M}}</ref>

* [[subdwarf]] parallaxes: metal-rich clusters and the thick disk <ref>{{cite journal|last=Reid|first=I. N.|title=Hipparcos subdwarf parallaxes: metal-rich clusters and the thick disk|journal=Astronomical Journal|volume=115|issue=1|pages=204–228|date=1998|doi=10.1086/300167|bibcode=1998AJ....115..204R|doi-access=free}}</ref>
* fine structure of the [[red giant]] clump and associated distance determinations <ref>{{cite journal|last=Girardi|first=L.|display-authors=et al|title=Fine structure of the red giant clump from Hipparcos data, and distance determinations based on its mean magnitude|journal=Monthly Notices of the Royal Astronomical Society|volume=301|issue=1|pages=149–160|date=1998|doi=10.1046/j.1365-8711.1998.02011.x |doi-access=free |bibcode=1998MNRAS.301..149G|arxiv=astro-ph/9805127|s2cid=14940616}}</ref>
* unexpected stellar velocity distribution in the warped Galactic disk <ref>{{cite journal|last=Smart|first=R. L.|display-authors=et al|title=Unexpected stellar velocity distribution in the warped Galactic disk
|journal=Nature|volume=392|issue=6675|pages=471–473|date=1998|doi=10.1038/33096|bibcode=1998Natur.392..471S|s2cid=4338652}}</ref>

* confirming the [[Lutz–Kelker bias]] of parallax measurement <ref>{{cite journal|last=Oudmaijer|first=Rene D.|author2=Groenewegen, Martin A. T.|author3=Schrijver, Hans|title=The Lutz-Kelker bias in rigonometric parallaxes|journal=Monthly Notices of the Royal Astronomical Society|volume=294|issue=3|page=L41–L46|doi=10.1046/j.1365-8711.1998.01409.x|arxiv=astro-ph/9801093|bibcode=1998MNRAS.294L..41O|year=1998|doi-access=free }}</ref>
* refining the [[Oort constants|Oort]] and Galactic constants <ref>{{cite journal|last=Olling|first=R. P.|author2=Merrifield, M. R.|title=Refining the Oort and Galactic constants|journal=Monthly Notices of the Royal Astronomical Society|volume=297|issue=3|pages=943–952|date=1998|doi=10.1046/j.1365-8711.1998.01577.x|doi-access=free |bibcode=1998MNRAS.297..943O|arxiv=astro-ph/9802034|s2cid=196448}}</ref>
* Galactic disk dark matter, terrestrial impact cratering and the law of large numbers <ref>{{cite journal|last=Stothers|first=R. B.|title=Galactic disk dark matter, terrestrial impact cratering and the law of large numbers|journal=Monthly Notices of the Royal Astronomical Society|volume=300|issue=4|pages=1098–1104|date=1998|doi=10.1046/j.1365-8711.1998.02001.x|bibcode=1998MNRAS.300.1098S|doi-access=free}}</ref>
* vertical motion and expansion of the [[Gould Belt]]<ref>{{cite journal|last=Comerón|first=F.|title=Vertical motion and expansion of the Gould Belt|journal=Astronomy & Astrophysics|volume=351|pages=506–518
|date=1999|bibcode=1999A&A...351..506C}}</ref>

* the use of [[gamma-ray burst]]s as direction and time markers in [[SETI]] strategies <ref>{{cite journal|last=Corbet|first=R. H. D.|title=The use of gamma-ray bursts as direction and time markers in SETI strategies|journal=Publications of the Astronomical Society of the Pacific|volume=111|issue=761|pages=881–885|date=1999|doi=10.1086/316395|bibcode=1999PASP..111..881C|arxiv=astro-ph/9904268|s2cid=8349408}}</ref>
* evidence of a galaxy merger in the early formation history of the Milky Way <ref>{{cite journal|last=Helmi|first=A.|display-authors=et al|title=Debris streams in the solar neighbourhood as relics from the formation of the Milky Way|journal=Nature|volume=402|issue=6757|pages=53–55|date=1999|doi=10.1038/46980|bibcode=1999Natur.402...53H|arxiv=astro-ph/9911041|s2cid=2945433}}</ref>
* study of nearby [[stellar association|OB associations]]<ref>{{cite journal|last=de Zeeuw| first=P. T.|display-authors=et al|title=A Hipparcos census of the nearby OB associations|journal=Astronomical Journal |volume=117|issue=1|pages=354–399|date=1999|doi=10.1086/300682|bibcode=1999AJ....117..354D|arxiv=astro-ph/9809227|s2cid=16098861}}</ref>
* close approaches of stars to the [[Solar System]]<ref>{{cite journal|last=Garcia Sanchez|first=J.|display-authors=et al|title=Stellar encounters with the Oort Cloud based on Hipparcos data |journal=Astronomical Journal|volume=117|issue=2|pages=1042–1055|date=1999|doi=10.1086/300723|bibcode=1999AJ....117.1042G|doi-access=free}}</ref>
* studies of [[binary star]] orbits and masses <ref>{{cite journal|last=Söderhjelm|first=S.|title=Visual binary orbits and masses post Hipparcos|journal=Astronomy & Astrophysics|volume=341|pages=121–140
|date=1999|bibcode=1999A&A...341..121S}}</ref>

* the [[HD 209458]] planetary transits <ref>{{cite journal|last=Robichon|first=N.|author2=Arenou, F.|title=HD209458 planetary transits from Hipparcos photometry|journal=Astronomy & Astrophysics|volume=355 |pages=295–298|date=2000|bibcode=2000A&A...355..295R}}</ref>
* formation of the stellar [[Galactic halo]] and thick disk <ref>{{cite journal|last=Chiba|first=M.|author2=Beers, T. C.|title=Kinematics of metal-poor stars in the Galaxy. III. Formation of the stellar halo and thick disk as revealed from a large sample of non-kinematically selected stars|journal=Astronomical Journal|volume=119|issue=6|pages=2843–2865|date=2000|doi=10.1086/301409|bibcode=2000AJ....119.2843C
|arxiv=astro-ph/0003087|s2cid=16620828}}</ref>

* the local density of matter in the Galaxy and the [[Oort limit]]<ref>{{cite journal|last=Holmberg|first=J.|author2=Flynn, C.|title=The local density of matter mapped by Hipparcos|journal=Monthly Notices of the Royal Astronomical Society|volume=313|issue=2|pages=209–216|date=2000|doi=10.1046/j.1365-8711.2000.02905.x|doi-access=free |bibcode=2000MNRAS.313..209H|arxiv=astro-ph/9812404|s2cid=16868380}}</ref>
* [[ice age]] epochs and the Sun's path through the Galaxy <ref>{{cite journal|last=Gies|first=D.R.|author2=Helsel, J. W.|title=Ice age epochs and the Sun's path through the Galaxy|journal=Astrophysical Journal |volume=626|issue=2|pages=844–848|date=2005|doi=10.1086/430250|bibcode=2005ApJ...626..844G|arxiv=astro-ph/0503306|s2cid=14341982}}</ref>
* local kinematics of K and M giants and the concept of superclusters <ref>{{cite journal|last=Famaey|first=B.|display-authors=et al|title=Local kinematics of K and M giants from Coravel, Hipparcos, and Tycho-2 data. Revisiting the concept of superclusters|journal=Astronomy & Astrophysics|volume=430|issue=1|pages=165–186|date=2005|doi=10.1051/0004-6361:20041272|bibcode=2005A&A...430..165F|arxiv=astro-ph/0409579
|s2cid=17804304}}</ref>

* an improved reference frame for long-term Earth rotation studies <ref>{{cite journal|last=Vondrák|first=J.|author2=Stefka, V.|title=Combined astrometric catalogue EOC—3. An improved reference frame for long-term Earth rotation studies|journal=Astronomy & Astrophysics|volume=463|issue=2|pages=783–788|date=2007|bibcode=2007A&A...463..783V|doi=10.1051/0004-6361:20065846|doi-access=free}}</ref>
* the local stellar velocity field in the Galaxy <ref>{{cite journal|last=Makarov|first=V. V.|author2=Murphy, D. W.|title=The local stellar velocity field via vector spherical harmonics|journal=Astronomical Journal|volume=134|issue=1|pages=367–375|date=2007|doi=10.1086/518242|bibcode=2007AJ....134..367M|arxiv=0705.3267|s2cid=14934757}}</ref>
* Identification of two possible "siblings" of the Sun (HIP 87382 and HIP 47399), to be studied for evidence of exoplanets<ref>{{cite web |url=http://news.discovery.com/space/find-suns-sibling-find-lifes-cousin-120409.html |title=Search for Sun's Sibling Could Find Life's Cousin : Discovery News |publisher=News.discovery.com |date=2012-04-09 |access-date=2012-08-17 |archive-date=11 April 2012 |archive-url=https://web.archive.org/web/20120411195134/http://news.discovery.com/space/find-suns-sibling-find-lifes-cousin-120409.html |url-status=dead }}</ref>

=== The Pleiades distance controversy ===
One controversial result has been the derived proximity, at about 120 parsecs, of the [[Pleiades]] cluster, established both from the original catalogue<ref>{{cite journal|last=Van Leeuwen|first=F. |title=Hipparcos distance calibrations for 9 open clusters|journal=Astronomy & Astrophysics|volume=341|pages=L71–L74|date=1999|bibcode=1999A&A...341L..71V}}</ref> as well as from the revised analysis.<ref name="FVL"/> This has been contested by various other recent work, placing the mean cluster distance at around 130 parsecs.<ref>{{cite journal|last=Pinsonneault|first=M. H.|display-authors=et al|title=The problem of Hipparcos distances to open clusters. I. Constraints from multicolour main-sequence fitting|journal=Astrophysical Journal|volume=504|issue=1|pages=170–191|date=1998|doi=10.1086/306077 |bibcode=1998ApJ...504..170P|arxiv=astro-ph/9803233|s2cid=14224434}}</ref><ref>{{cite journal|last=Pan|first=X. P.|display-authors=et al|title=A distance of 133-137pc to the Pleiades star cluster|journal=Nature |volume=427|pages=326–328|date=2004|doi=10.1038/nature02296|pmid=14737161|bibcode=2004Natur.427..326P|issue=6972|s2cid=4383850}}</ref><ref>{{cite journal|last=Percival|first=S. M.|display-authors=et al |title=The distance to the Pleiades. Main sequence fitting in the near infrared|journal=Astronomy & Astrophysics|volume=429|issue=3|pages=887–894|date=2005|doi=10.1051/0004-6361:20041694 |bibcode=2005A&A...429..887P|arxiv=astro-ph/0409362|s2cid=14842664}}</ref><ref>{{cite journal|last=Soderblom|first=D. R.|display-authors=et al|title=Confirmation of errors in Hipparcos parallaxes from Hubble Space Telescope FGS astrometry of the Pleiades|journal=Astronomical Journal|volume=129|issue=3|pages=1616–1624|date=2005|doi=10.1086/427860|bibcode=2005AJ....129.1616S|arxiv=astro-ph/0412093|s2cid=15354711}}</ref>

According to a 2012 paper, the anomaly was due to the use of a weighted mean when there is a correlation between distances and distance errors for stars in clusters. It is resolved by using an unweighted mean. There is no systematic bias in the Hipparcos data when it comes to star clusters.<ref>{{cite journal|arxiv=1203.4945|title=XHIP-II: Clusters and associations|year=2012|author1=Charles Francis|author2=Erik Anderson|doi=10.1134/S1063773712110023|volume=38|issue=11|journal=Astronomy Letters|pages=681–693|bibcode = 2012AstL...38..681F |s2cid=119285733}}</ref>

In August 2014, the discrepancy between the cluster distance of {{val|120.2|1.5|u=parsecs (pc)}} as measured by ''Hipparcos'' and the distance of {{val|133.5|1.2|u=pc}} derived with other techniques was confirmed by parallax measurements made using [[Very-long-baseline interferometry|VLBI]],<ref>{{cite journal|last1=Melis|first1=C.|display-authors=et al|year=2014|title=A VLBI resolution of the Pleiades distance controversy|journal=Science|volume=345|issue=6200|pages=1029–1032|doi=10.1126/science.1256101|arxiv=1408.6544|bibcode=2014Sci...345.1029M|pmid=25170147|s2cid=34750246}}</ref> which gave {{val|136.2|1.2|u=pc}}, the most accurate and precise distance yet presented for the cluster.

=== Polaris ===
{{main|Polaris}}

Another distance debate set-off by Hipparcos is for the distance to the star Polaris.

=== Hipparcos-Gaia ===
Hipparcos data is recently being used together with [[Gaia (spacecraft)|Gaia]] data. Especially the comparison of the [[proper motion]] of stars from both spacecraft is being used to search for hidden binary companions.<ref>{{Cite journal |last1=Kervella |first1=Pierre |last2=Arenou |first2=Frédéric |last3=Mignard |first3=François |last4=Thévenin |first4=Frédéric |date=2019-03-01 |title=Stellar and substellar companions of nearby stars from Gaia DR2. Binarity from proper motion anomaly |url=https://ui.adsabs.harvard.edu/abs/2019A&A...623A..72K |journal=Astronomy and Astrophysics |volume=623 |pages=A72 |doi=10.1051/0004-6361/201834371 |arxiv=1811.08902 |bibcode=2019A&A...623A..72K |s2cid=119491061 |issn=0004-6361}}</ref><ref>{{Cite journal |last=Brandt |first=Timothy D. |date=2021-06-01 |title=The Hipparcos-Gaia Catalog of Accelerations: Gaia EDR3 Edition |journal=The Astrophysical Journal Supplement Series |volume=254 |issue=2 |pages=42 |doi=10.3847/1538-4365/abf93c |arxiv=2105.11662 |bibcode=2021ApJS..254...42B |s2cid=235187424 |issn=0067-0049 |doi-access=free }}</ref> Hipparcos-Gaia data is also used to measure the dynamical mass of known binaries, such as [[Substellar object|substellar]] companions.<ref>{{Cite journal |last1=Brandt |first1=Timothy D. |last2=Dupuy |first2=Trent J. |last3=Bowler |first3=Brendan P. |date=2019-10-01 |title=Precise Dynamical Masses of Directly Imaged Companions from Relative Astrometry, Radial Velocities, and Hipparcos-Gaia DR2 Accelerations |journal=The Astronomical Journal |volume=158 |issue=4 |pages=140 |doi=10.3847/1538-3881/ab04a8 |arxiv=1811.07285 |bibcode=2019AJ....158..140B |s2cid=119437089 |issn=0004-6256 |doi-access=free }}</ref> Hipparcos-Gaia data was used to measure the mass of the exoplanet [[Beta Pictoris b]] and is sometimes used to study other long-period [[Exoplanet|exoplanets]], such as [[HR 5183 b]].<ref>{{Cite journal |last1=Snellen |first1=I. A. G. |last2=Brown |first2=A. G. A. |date=2018-08-01 |title=The mass of the young planet Beta Pictoris b through the astrometric motion of its host star |url=https://ui.adsabs.harvard.edu/abs/2018NatAs...2..883S |journal=Nature Astronomy |volume=2 |issue=11 |pages=883–886 |doi=10.1038/s41550-018-0561-6 |arxiv=1808.06257 |bibcode=2018NatAs...2..883S |s2cid=256718410 |issn=2397-3366}}</ref><ref>{{Cite journal |last1=Venner |first1=Alexander |last2=Pearce |first2=Logan A. |last3=Vanderburg |first3=Andrew |date=2022-11-01 |title=An edge-on orbit for the eccentric long-period planet HR 5183 b |journal=Monthly Notices of the Royal Astronomical Society |volume=516 |issue=3 |pages=3431–3446 |doi=10.1093/mnras/stac2430 |doi-access=free |arxiv=2111.03676 |bibcode=2022MNRAS.516.3431V |issn=0035-8711}}</ref>

== People ==
{{unreferenced section|date=September 2012}}

* [[Pierre Lacroute]] ([[Observatory of Strasbourg]]): proposer of space astrometry in 1967
* [[Michael Perryman]]: ESA project scientist (1981–1997), and project manager during satellite operations (1989–1993)
* Catherine Turon (Observatoire de Paris-Meudon): leader of Input Catalogue Consortium
* Erik Høg: leader of the TDAC Consortium
* [[Lennart Lindegren]] ([[Lund Observatory]]): leader of the NDAC Consortium
* Jean Kovalevsky: leader of the FAST Consortium
* [[Adriaan Blaauw]]: chair of the observing programme selection committee
* Hipparcos Science Team: Uli Bastian, Pierluigi Bernacca, Michel Crézé, Francesco Donati, Michel Grenon, Michael Grewing, Erik Høg, Jean Kovalevsky, Floor van Leeuwen, Lennart Lindegren, Hans van der Marel, [[Francois Mignard]], Andrew Murray, Michael Perryman (chair), Rudolf Le Poole, Hans Schrijver, Catherine Turon
* Franco Emiliani: ESA project manager (1981–1985)
* Hamid Hassan: ESA project manager (1985–1989)
* Dietmar Heger: ESA/ESOC spacecraft operations manager
* Michel Bouffard: Matra Marconi Space project manager
* Bruno Strim: Alenia Spazio project manager

== See also ==
{{Portal|Spaceflight}}
* [[Gaia (spacecraft)|Gaia]], follow-up mission launched in 2013

== References ==
{{Reflist|30em}}

== External links ==
* [http://www.rssd.esa.int/Hipparcos/hipparcos.html The Hipparcos Space Astrometry Mission at ESA]
* [http://casu.ast.cam.ac.uk/casuadc Hipparcos and Tycho Catalogues at the CASU Astronomical Data Centre, Institute of Astronomy, University of Cambridge]
* [http://cdsarc.u-strasbg.fr/ftp/cats/I/239/ Hipparcos Main Catalogue and Tycho-1 Catalogue at the CDS in Strasbourg]
* [https://www.kaggle.com/datasets/konivat/hipparcos-star-catalog Hipparcos Astrometry Catalogue as CSV file]

{{Space observatories}}
{{European Space Agency}}
{{Orbital launches in 1989}}
{{Use dmy dates|date=July 2019}}
{{Use British English|date=January 2014}}

[[Category:Hipparcos| ]]
[[Category:Space astrometry missions]]
[[Category:Parallax]]
[[Category:European Space Agency satellites]]
[[Category:Space telescopes]]
[[Category:Satellites orbiting Earth]]
[[Category:Spacecraft launched in 1989]]