To study some of the hottest regions in the universe, the Japanese Aerospace Exploration Agency has launched the coldest instrument ever flown. Chilled to six-hundredths of a degree above absolute zero, the X-ray Spectrometer-2 is one of six devices carried by a Japanese-NASA satellite now gearing up to study high-energy emissions from such sources as the hot gas expelled by supernovas, the energetic material spiraling into black holes, and the warm gas among stars and between galaxies.