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Explorer 9
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<p>Explorer 9, known as S-56A before launch, was a <a href="/facts/NASA/TIMCV0yV">NASA</a> satellite which was launched in February 1961 to study the density and composition of the upper <a href="/facts/Thermosphere/uhG50e2V">thermosphere</a> and lower <a href="/facts/Exosphere/7vTtk7qC">exosphere</a>. It was a reflight of the failed <a href="/facts/Explorer_S-56_(satellite)/bghx6Bxe">Explorer S-56</a> mission, and consisted of a 7 kg (15 lb), 3.66 m (12.0 ft) <a href="/facts/Weather_balloon/yYenKMYA">balloon</a> which was deployed into a <a href="/facts/Medium_Earth_orbit/7Qmz5XxG">medium Earth orbit</a>. The mission was conducted by <a href="/facts/NASA/TIMCV0yV">NASA</a>'s <a href="/facts/Langley_Research_Center/UrNAPV7g">Langley Research Center</a>. </p>