1. Mission Planning

Planning the ASTRO-D mission started in early 1984. It was led by the ASTRO-D Working Group, which was set up under the Space Science Committee at ISAS, and involved virtually the entire X-ray astronomy community of Japan. By that time, the second X-ray astronomy satellite, TENMA, launched in 1983, was operational and the third one, ASTRO-C (post-launch name: GINGA), was under development for launch in 1987.

In planning mission, the following points were considered to be critical: X-ray astronomy missions should be carried out regularly with an optimum interval of about 5 years in order to ensure continuity of research and to allow steady growth of the community. Accordingly, the X-ray astronomy community desired to implement the ASTRO-D mission in early 1990s. Each mission should offer unique prospects for significantly deepening our understanding of current problems as well as for exploring new frontiers.

We believe that this strategy has been validated by the rapid growth of the Japanese X-ray astronomy community and steadily increasing scientific yield resulting from the past three X-ray astronomy satellites.

The mission concept of ASTRO-D was established on the basis of following considerations: The gas scintillation proportional counters having a superior energy resolution, a new technology developed at ISAS and used as the main instrument of TENMA, had opened a new field of "iron-line diagnostics" of X-ray sources. This achievement naturally directed us toward further development of X-ray spectroscopy. On the other hand, the scientific objectives of GINGA were to investigate rapid time variability of X-ray sources, as well as to go farther out of our galaxy; and observe extragalactic sources, in particular active galactic nuclei. For this purpose, a large-area proportional counter array, the largest of its kind so far, was chosen to be the main instrument of GINGA and was built in collaboration with UK scientists. A natural extension in this direction is to study the evolution of the universe by observing galaxies at cosmological distances. Based on these considerations, ASTRO-D was defined to be a mission for high sensitivity imaging and spectroscopy.

Focusing incident X-rays (imaging) was necessary in order to achieve the required sensitivity, orders of magnitude higher than GINGA, and to avoid source confusion caused by a myriad of faint sources. A lightweight X-ray telescope, an innovative technology developed by P. Serlemitsos of NASA Goddard Space Flight Center (GSFC) with whom the Nagoya group had closely collaborated previously, was chosen to be best suited for ASTRO-D. For the focal plane instruments, two types of detectors were adopted. One was a solid state detector with an excellent energy resolution (we later decided to use CCD for the solid state detector). The other was an imaging gas scintillation proportional counter whose basic technology we had acquired through the development of TENMA. These two types of detectors possessed complementary features to each other.