Pioneer 5 (also known as Pioneer P-2, and Thor Able 4) was a spin-stabilized space probe in the NASA Pioneer program used to investigate interplanetary space between the orbits of Earth and Venus.
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Before the first rocket missions, the corona could be observed only in white light during the eclipses, while in the last fifty years the solar corona has been photographed in the EUV and X-rays by many satellites (Pioneer 5, 6, 7, 8, 9, Helios, Skylab, SMM, NIXT, Yohkoh, SOHO, TRACE, Hinode).