On Aug. 30, 1991, Yohkoh, a mission of Japan’s Institute of Space and Astronautical Sciences in cooperation with the United States and the United Kingdom, was launched. Yoh-koh—the Japanese word for “sunbeam”—carried four instruments to study the Sun in soft and hard x-rays and at gamma ray energies. Engineers and scientists at the Solar & Astrophysics Laboratory at the ATC developed Yohkoh’s Soft X-ray Telescope (SXT).
In its 10 years and four months of solar observations, Yohkoh spawned numerous significant discoveries about the solar corona, solar flares and space weather, and gave rise to over 1,600 scientific publications and at least 53 Ph.D. degrees. It was the first spacecraft to have continuously observed the Sun in x-rays over an entire sunspot cycle, the roughly 11-year cycle in which the Sun goes from a period of relative calm to a time of numerous intense storms and sunspots and then back again, as illustrated in the composite photograph above.