Hitomi (Japanese: ひとみ), also known as ASTRO-H and New X-ray Telescope (NeXT), was an X-ray astronomy satellite commissioned by the Japan Aerospace Exploration Agency (JAXA) for studying extremely energetic processes in the Universe. The space observatory was designed to extend the research conducted by the Advanced Satellite for Cosmology and Astrophysics (ASCA) by investigating the hard X-ray band above 10 keV. The satellite was originally called New X-ray Telescope; at the time of launch it was called ASTRO-H. After it was placed in orbit and its solar panels deployed, it was renamed Hitomi. The spacecraft was launched on 17 February 2016 and contact was lost on 26 March 2016, due to multiple incidents with the attitude control system leading to an uncontrolled spin rate and breakup of structurally weak elements.
Name The new name refers to the pupil of an eye, and to a legend of a painting of four dragons. The word Hitomi generally means "eye", and specifically the pupil, or entrance window of the eye – the aperture. There is also an ancient legend that inspires the name Hitomi. "One day, many years ago, a painter was drawing four white dragons on a street. He finished drawing the dragons, but without "Hitomi". People who looked at the painting said "why don't you paint Hitomi, it is not complete. The painter hesitated, but people pressured him. The painter then drew Hitomi on two of the four dragons. Immediately, these dragons came to life and flew up into the sky. The two dragons without Hitomi remained still". The inspiration of this story is that Hitomi is regarded as the "One last, but most important part", and so we wish ASTRO-H to be the essential mission to solve mysteries of the universe in X-rays. Hitomi refers to the aperture of the eye, the part where incoming light is absorbed. From this, Hitomi reminds us of a black hole. We will observe Hitomi in the Universe using the Hitomi satellite.
Objectives Hitomi's objectives were to explore the large-scale structure and evolution of the universe, as well as the distribution of dark matter within galaxy clusters and how the galaxy clusters evolve over time; how matter behaves in strong gravitational fields (such as matter inspiraling into black holes), to explore the physical conditions in regions where cosmic rays are accelerated, as well as observing supernovae. In order to achieve this, it was designed to be capable of:
Imaging and spectroscopic measurements with a hard X-ray telescope; Spectroscopic observations with an extremely high energy resolution using the micro-calorimeter; Sensitive wideband observations over the energy range 0.3–600 keV. It was the sixth of a series of JAXA X-ray satellites, which started in 1979, and it was designed to observe sources that are an order of magnitude fainter than its predecessor, Suzaku. Its planned mission length was three years. At the time of launch, two other large X-ray satellites were carrying out observations in orbit: the Chandra X-ray Observatory and XMM-Newton, both of which were launched in 1999.
Instruments The probe carried four instruments and six detectors to observe photons with energies ranging from soft X-rays to gamma rays, with a high energy resolution. Hitomi was built by an international collaboration led by JAXA with over 70 contributing institutions in Japan, the United States, Canada, and Europe, and over 160 scientists. With a mass of 2,700 kg (6,000 lb), At launch, Hitomi was the heaviest Japanese X-ray mission. The satellite is about 14 m (46 ft) in length. Two soft X-ray telescopes (SXT-S, SXT-I), with focal lengths of 5.6 m (18 ft), focus light onto a soft X-ray Spectrometer (SXS), provided by NASA, with an energy range of 0.4–12 keV for high-resolution X-ray spectroscopy, and a soft X-ray imager (SXI), with an energy range of 0.3–12 keV. Two hard X-ray telescopes (HXT), with a focus length of 12 m (39 ft), focus light onto two hard X-ray imagers (HXI), with energy range 5–80 keV, which are mounted on a plate placed at the end of the 6 m (20 ft) extendable optical bench (EOB) that is deployed once the satellite is in orbit. The Canadian Space Agency (CSA) provided the Canadian ASTRO-H Metrology System (CAMS), which is a laser alignment system that will be used to measure the distortions in the extendible optical bench. Two soft Gamma-ray detectors (SGD), each containing three units, were mounted on two sides of the satellite, using non-focusing detectors to observe soft gamma-ray emission with energies from 60 to 600 KeV. The Netherlands Institute for Space Research (SRON) in collaboration with the University of Geneva provided the filter-wheel and calibration source for the spectrometer.
Launch The launch of the satellite was planned for 2013 as of 2008, later revised to 2015 as of 2013. As of early February 2016, it was planned for 12 February, but was delayed due to poor weather forecasts. Hitomi launched on 17 February 2016 at 08:45 UTC into a low Earth orbit of approximately 575 km (357 mi). The circular orbit had an orbital period of around 96 minutes, and an orbital inclination of 31.01°. It was launched from the Tanegashima Space Center on board an H-IIA launch vehicle. 14 minutes after launch, the satellite separated from the launch vehicle. The solar arrays later deployed according to plan, and it began its on-orbit checkout.
Operations Measurements by Hitomi have allowed scientists to track the motion of X-ray-emitting gas at the heart of the Perseus cluster of galaxies for the first time. Using the Soft X-ray Spectrometer, astronomers have mapped the motion of X-ray-emitting gas in a cluster of galaxies and shown it moves at cosmically modest speeds. The total range of gas velocities directed toward or away from Earth within the area observed by Hitomi was found to be about 365,000 miles an hour (590,000 kilometers per hour). The observed velocity range indicates that turbulence is responsible for only about 4 percent of the total gas pressure.
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