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Explorer 12
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<p>Explorer 12, also called EPE-A or Energetic Particles Explorer-A and as S3), was a <a href="/facts/NASA/TIMCV0yV">NASA</a> satellite built to measure the <a href="/facts/Solar_wind/BvuB3dT8">solar wind</a>, <a href="/facts/Cosmic_rays/uxCVdTKd">cosmic rays</a>, and the <a href="/facts/Earth%27s_magnetic_field/73G36wmx">Earth's magnetic field</a>. It was the first of the S-3 series of spacecraft, which also included Explorer 12, 14, 15, and 26. It was launched on 16 August 1961, aboard a <a href="/facts/Thor-Delta/XyulmLAY">Thor-Delta</a> launch vehicle. It ceased transmitting on 6 December 1961 due to power failure. </p>