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The Yohkoh Space Observatory

The Yohkoh satellite , an observatory for studying X-rays and gamma-rays from the Sun, was launched from Kagoshima, Japan (picture at right) on August 30, 1991. Yohkoh is a project of the Institute for Space and Astronautical Sciences, Japan; Yohkoh (the spacecraft) was built in Japan, but the observing instruments have contributions from the United States and from the United Kingdom.

Why should the Sun make X-rays and gamma-rays? The visible surface of the Sun is hot - some 6,000 degrees, but that is not hot enough for X-radiation. The photosphere actually appears dark in the Yohkoh soft X-ray images, which show instead the extremely hot corona that envelops the Sun and extends far out into space. Yohkoh's main scientific objectives deal with finding explanations for the solar X-ray and gamma-ray emission and the interesting phenomena seen only in this kind of radiation.

The Mu-3SII rocket sending Yohkoh into orbit. The launch site is at the southern tip of Japan's southernmost main island, Kyushu, at the village of Uchinoura on the Ohsumi peninsula. The Pacific Ocean can be seen in the background.

An exploded view of the Yohkoh launch vehicle. The first rocket stage, on the right, has two solid-fuel boosters; the third stage is spherical. It and Yohkoh itself (the square box at the extreme left, roughly 2 m long and 1 m wide), are encased in a two-piece "shroud" to protect them during the high-speed ascent through the Earth's atmosphere.

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