View source for EXOSAT

{{Short description|Space observatory}}
{{Expand language|topic=|langcode=fr|date=June 2021}}
{{Infobox spaceflight
| name                  = EXOSAT
| image                 = exosat.jpg
| image_caption         = EXOSAT
| mission_type          = Astronomy
| operator              = [[European Space Agency|ESA]]
| website               = {{url|http://www.esa.int/export/esaSC/120394_index_0_m.html}}
| COSPAR_ID             = 1983-051A
| SATCAT                = 14095
| mission_duration      = 3 years
| spacecraft_bus        =
| manufacturer          = [[Messerschmitt-Bölkow-Blohm|MBB]]
| dry_mass              =
| launch_mass           = {{convert|510.0|kg|lb|abbr=on}}
| power                 = 165.0&nbsp;watts
| launch_date           = {{start-date|26 May 1983, 15:18:00|timezone=yes}}&nbsp;UTC
| launch_rocket         = [[Delta 3000|Delta 3914]] D169
| launch_site           = [[Vandenberg Air Force Base|Vandenberg]] [[Vandenberg AFB Space Launch Complex 2|SLC-2W]]
| launch_contractor     =
| disposal_type         =
| deactivated           =
| last_contact          =
| decay_date            = {{end-date|5 May 1986}}
| orbit_epoch           = 26 May 1983, 11:18:00 UTC<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1983-051A |title=EXOSAT |work=NASA Space Science Data Coordinated Archive |publisher=[[NASA]] |access-date=October 23, 2017}}</ref>
| orbit_reference       = [[geocentric orbit|Geocentric]]
| orbit_regime          = [[Low Earth orbit|Low Earth]]
| orbit_periapsis       = {{convert|347|km|mi|abbr=on}}
| orbit_apoapsis        = {{convert|191,709|km|mi|abbr=on}}
| orbit_inclination     = 72.5&nbsp;degrees
| orbit_semimajor       =
| orbit_eccentricity    = 0.93428
| orbit_period          = 5,435.4&nbsp;minutes
| orbit_RAAN            =
| orbit_arg_periapsis   =
| orbit_mean_anomaly    =
| orbit_mean_motion     =
| apsis                 = gee
| insignia              = EXOSAT insignia.png
| insignia_caption      = Legacy ESA insignia for the ''EXOSAT'' mission
| insignia_alt          = ''EXOSAT'' mission logo
| insignia_size         = 180x180px
}}
The '''European X-ray Observatory Satellite''' ('''EXOSAT'''), originally named '''HELOS''', was an X-ray telescope operational from May 1983 until April 1986 and in that time made 1780 observations in the [[X-ray]] band of most classes of astronomical object including [[active galactic nuclei]], [[stellar coronae]], [[cataclysmic variable]]s, [[white dwarfs]], [[X-ray binaries]], [[cluster of galaxies|clusters of galaxies]], and [[supernova remnants]].
This [[European Space Agency]] (ESA) satellite for direct-pointing and lunar-occultation observation of X-ray sources beyond the [[Solar System]] was launched into a highly eccentric orbit (apogee 200,000&nbsp;km, perigee 500&nbsp;km) almost perpendicular to that of the Moon on 26 May 1983. The instrumentation includes two low-energy imaging telescopes (LEIT) with Wolter I X-ray optics (for the 0.04–2 keV energy range), a medium-energy experiment using Ar/CO<sub>2</sub> and Xe/CO<sub>2</sub> detectors (for 1.5–50 keV), a Xe/He gas scintillation spectrometer (GSPC) (covering 2–80 keV), and a reprogrammable onboard data-processing computer. Exosat was capable of observing an object (in the direct-pointing mode) for up to 80 hours and of locating sources to within at least 10 arcsec with the LEIT and about 2 arcsec with GSPC.<ref name=Hoff>{{cite journal |author=Hoff HA |title=EXOSAT - The new extrasolar X-ray observatory |journal=J Brit Interplan Soc. (Space Chronicle) |date=Aug 1983 |volume=36 |issue=8 |pages=363–7 |url=http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes |url-status=dead |archive-url=https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes |archive-date=2012-08-29 }}</ref>
==History of Exosat==
During the period from 1967 to 1969, the [[European Space Research Organisation]] (ESRO) studied two separate missions: a European X-ray observatory satellite, as a combined X- and gamma-ray observatory (Cos-A), and a gamma-ray observatory (Cos-B). Cos-A was dropped after the initial study, and [[Cos-B]] was proceeded with.
Later in 1969 a separate satellite (the Highly Eccentric Lunar Occultation Satellite - Helos) was proposed. The Helos mission was to determine accurately the location of bright X-ray sources using the lunar occultation technique. In 1973 the observatory part of the mission was added, and mission approval from the European Space Agency Council was given<ref name=Taylor>{{ cite journal |vauthors=Taylor BG, Andresen RD, Peacock A, Zobl R |title=The Exosat mission |journal= Space Sci. Rev. |volume=30 |issue=1–4 |date=Mar 1981 |pages=479–94 |doi=10.1007/BF01246069 |bibcode = 1981SSRv...30..479T |s2cid=120475350 }}</ref> for Helos, now renamed Exosat.
It was decided that the observatory should be made available to a wide community, rather than be restricted to instrument developers, as had been the case for all previous ESA (ESRO) scientific programmes. For the first time in an ESA project, this led to the approach of payload funding and management by the Agency. Instrument design and development became a shared responsibility between ESA and hardware groups.
In July 1981 ESA released the first Announcement of Opportunity (AO) for participation in the Exosat observation programme to the scientific community of its Member States. By 1 November 1981, the closing of the AO window, some 500 observing proposals had been received. Of these, 200 were selected for the first nine months of operation.<ref name=Hoff/>
Exosat was the first ESA spacecraft to carry on board a digital computer (OBC), with its main purpose being scientific data processing. Spacecraft monitoring and control were secondary. To provide the data handling subsystem with an exceptional flexibility of operation, the OBC and Central Terminal Unit were in-flight reprogrammable. This flexibility far exceeded any other ESA spacecraft built up to then.
==Satellite operations==
Each of the three axes were stabilized and the optical axes of the three scientific instruments were coaligned. The entrance apertures of the scientific instruments were all located on one face of the central body. Once in orbit the flaps which cover the entrances to the ME and LEIT were swung open to act as thermal and [[stray light|stray-light]] shields for the telescopes and star trackers, respectively.<ref name=Hoff/>
The orbit of Exosat was different from any previous [[X-ray astronomy]] satellite. To maximize the number of sources occulted by the [[Moon]], a highly eccentric orbit (e ~ 0.93) with a 90.6 hr period and an inclination of 73° was chosen.<ref name=White>{{ cite journal |vauthors=White NE, Peacock A |title=The Exosat observatory |journal=Societa Astronomica Italiana, Memorie |volume=59 |issue=1–2 |year=1988 |pages=7–31 |bibcode=1988MmSAI..59....7W }}</ref> The initial apogee was 191,000&nbsp;km and perigee 350&nbsp;km. To be outside the [[Earth]]'s radiation belts, the scientific instruments were operated above ~50,000&nbsp;km, giving up to ~76 hr per 90 hr orbit.<ref name=White/> There was no need for any onboard data storage as Exosat was visible from the ground station at Villafranca, Spain for practically the entire time the scientific instruments were operated.
==References==
{{reflist}}
==External links==
{{Portal|Spaceflight}}
*[http://sci.esa.int/science-e/www/area/index.cfm?fareaid=32 ESA's X-ray Observatory (EXOSAT at ESTEC, ESA)] on the internet
*[http://heasarc.gsfc.nasa.gov/docs/exosat/exosat.html Data archive] at NASA [http://heasarc.gsfc.nasa.gov High Energy Astrophysics Science Archive Center (HEASARC)]
{{Space observatories}}
{{European Space Agency}}
{{Orbital launches in 1983}}
{{Use British English|date=January 2014}}
{{DEFAULTSORT:Exosat}}
[[Category:European Space Agency satellites]]
[[Category:X-ray telescopes]]
[[Category:Space telescopes]]
[[Category:1983 in spaceflight]]
[[Category:Spacecraft launched in 1983]]