EXOSAT

<h2 id="history-of-exosat">History of Exosat</h2>
During the period from 1967 to 1969, the <a href="/facts/European_Space_Research_Organisation/bbNlK2fW">European Space Research Organisation</a> (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 <a href="/facts/Cos-B/FzjRbfyd">Cos-B</a> 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<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a> 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.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>
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.
It was originally planned to launch on an <a href="/facts/Ariane_1/eNnQsLD8">Ariane 1</a> but concerns over delays to the rocket resulted in it being transferred to a <a href="/facts/Thor-Delta/XyulmLAY">Thor-Delta</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a><a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>

<h2 id="satellite-operations">Satellite operations</h2>
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 <a href="/facts/Stray_light/7TAs3tR4">stray-light</a> shields for the telescopes and star trackers, respectively.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>
The orbit of Exosat was different from any previous <a href="/facts/X-ray_astronomy/KAQ6WxgQ">X-ray astronomy</a> satellite. To maximize the number of sources occulted by the <a href="/facts/Moon/zKHqEMPb">Moon</a>, a highly eccentric orbit (e ~ 0.93) with a 90.6 hr period and an inclination of 73° was chosen.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> The initial apogee was 191,000 km and perigee 350 km. To be outside the <a href="/facts/Earth/HLLmDfj0">Earth</a>'s radiation belts, the scientific instruments were operated above ~50,000 km, giving up to ~76 hr per 90 hr orbit.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> 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.

<h2 id="external-links">External links</h2>
<ul>
<li>Spaceflight portal</li></ul>
<ul><li><a href="http://sci.esa.int/science-e/www/area/index.cfm?fareaid=32">ESA's X-ray Observatory (EXOSAT at ESTEC, ESA)</a> on the internet</li>
<li><a href="https://heasarc.gsfc.nasa.gov/docs/exosat/exosat.html">Data archive</a> at NASA <a href="https://heasarc.gsfc.nasa.gov">High Energy Astrophysics Science Archive Center (HEASARC)</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Hoff HA (Aug 1983). "EXOSAT - The new extrasolar X-ray observatory". J Brit Interplan Soc (Space Chronicle). 36 (8): 363–7. Archived from the original on 2012-08-29. <a href="https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes" target="_blank">https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Taylor BG, Andresen RD, Peacock A, Zobl R (Mar 1981). "The Exosat mission". Space Sci. Rev. 30 (1–4): 479–94. Bibcode:1981SSRv...30..479T. doi:10.1007/BF01246069. S2CID 120475350. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Hoff HA (Aug 1983). "EXOSAT - The new extrasolar X-ray observatory". J Brit Interplan Soc (Space Chronicle). 36 (8): 363–7. Archived from the original on 2012-08-29. <a href="https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes" target="_blank">https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Dickson, David (11 March 1983). "Ariane Loses One to NASA". Science. 219 (4589): 1202. doi:10.1126/science.219.4589.1202. ISSN 0036-8075. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Calder, Nigel (1992). Giotto to the Comets. London: Presswork. p. 65. ISBN 0-9520115-0-6. <a href="0-9520115-0-6" target="_blank">0-9520115-0-6</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Hoff HA (Aug 1983). "EXOSAT - The new extrasolar X-ray observatory". J Brit Interplan Soc (Space Chronicle). 36 (8): 363–7. Archived from the original on 2012-08-29. <a href="https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes" target="_blank">https://web.archive.org/web/20120829162441/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8339971AH&q=&uid=788028604&setcookie=yes</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">White NE, Peacock A (1988). "The Exosat observatory". Societa Astronomica Italiana, Memorie. 59 (1–2): 7–31. Bibcode:1988MmSAI..59....7W. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">White NE, Peacock A (1988). "The Exosat observatory". Societa Astronomica Italiana, Memorie. 59 (1–2): 7–31. Bibcode:1988MmSAI..59....7W. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
</ol>

EXOSAT open-in-new

EXOSAT