Yohkoh solar X-ray astronomy satellite. Artist's impression of the US/Japanese 'Yohkoh' ('Sunbeam') solar X-ray astronomy satellite in its orbital configuration. Yohkoh was launched into Earth orbit on 30 August 1991 by a Japanese M-3 rocket. There are two main instruments on the satellite: the Hard X-ray Telescope (HXT) for X-ray energies of 10-100 keV (thousand electron volts); and the Soft X-ray Telescope (SXT) for energies of 0.25- 3.00 keV. Both telescopes observe the entire solar disc. Yohkoh's main mission is to monitor and record high-energy events such as solar flares and sunspots in order to increase our understanding of the Sun's activity.