Future dust research and observation from space
As discussed above, infrared observations with cooled telescopes onboard satellites are very effective in investigating the nature of dust and discovering new types of dust. The Spitzer Space Telescope now in service is good at obtaining infrared spectra from faint celestial objects and we expect progress in detailed studies for each type of dust. Meanwhile, our ASTRO-F is good at observing broader regions in the universe with its broad wavelength coverage. The satellite should be able to investigate changes in abundance of various dust types as shown in Fig. 1 under different environments in interstellar space, and drive forward research on the formation, growth and destruction of dust.
Finally, I would like to comment on the significance of dust observations in the X-ray region. The dust band observed in the infrared provides an important clue to understand the structure of dust as shown in the PAH bands (Fig. 1). As suggested in Fig. 2 and 3, however, we cannot explore the individual composition of dust. This is partly because the band of solid substances depends on various factors including shape, size and degree of crystallization. Until now, dust composition has been estimated based on the amount of gases observed in interstellar space and their differences from the composition of the Sun. This approach, however, is based on a unilateral assumption that the intrinsic composition of the interstellar matter was the same as the composition of the Sun. In fact, if we obtain X-ray spectra, we can directly estimate the amount of elements contained in dust based on observation that detects the scattered light by dust or separates the absorption by gases from that by dust. X-ray observations have so far been performed with GINGA and other satellites. The importance of dust observation in the X-ray wavelength is rising, and the approach is expected to advance greatly dust research in the future.