Astro-H (renamed Hitomi after launch) was the sixth in the series of successful Japanese X-ray observatories devoted to the study of energetic processes in celestial objects. Under development for a 2014 launch by the Japanese space agency JAXA jointly with NASA, the mission was designed to investigate the physics of the high-energy Universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV. Those instruments included a high-resolution, high-throughput spectrometer sensitive over 0.3 - 12 keV, enabled by a micro-calorimeter array located in a focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometer covering 5 - 80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; wide-field imaging spectrometer with moderate energy resolution, sensitive over 0.4 - 12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing soft gamma-ray Compton-camera type detector, sensitive in the 40 - 600 keV band, and capable of measuring polarization of hard x-ray sources.
The excellent energy resolution allows, among other investigations, detailed studies of hot (millions of degrees Kelvin) X-ray emitting gas in clusters of galaxies - shedding light on the formation of the largest structures in the Universe, and in supernova remnants - aimed to understand the formation and dispersion of chemical elements synthesized in massive stars. The broad bandpass is indispensable in studying the most energetic processes in the Universe, aimed at understanding nature's most extreme particle accelerators, responsible for generation of the most energetic cosmic rays.
Unfortunately, due to a failure of the attitude control system, the satellite ceased operations a few months after launch. Fortunately, prior to the failure, it provided excellent data on several celestial objects. One example was the observation of Perseus cluster of galaxies, giving us a taste for the tremendous power of high-resolution X-ray spectroscopy of extended objects.
The Japanese space agency is now planning to recover the scientific return from Hitomi by planning a re-flight of the mission. The re-flight will include only two of the four of Hitomi's instruments (the high-resolution X-ray spectrometer and the wide field imager), and is planned to take place in 2022.