In this article, I would like to introduce, citing research examples, how Japan's first infrared astronomical satellite AKARI accomplished the all-sky survey by mid-infrared (Fig. 1) and how the research will advance in the future.
What is infrared observation of the universe?
Infrared is an electromagnetic wave of wavelength 2 to 200µm, which is longer than visible light (about 0.5µm wavelength). Infrared observation's objectives are ultimately the origins of the universe and of life. The universe is expanding and so the further away galaxies are, the more quickly they move away from us. For this reason, light coming from more distant (i.e., old) galaxies has greater red shift with their wavelength extended. Thus, we can observe them in infrared. In addition, infrared can identify thermal emissions and bright lines from solid particles (dust) and organic molecules - the materials of planets and life in a broad sense -, which drift in outer space. Thermal emissions and bright lines cannot be caught in visible light.
Twenty-seven years ago, the U.S., the U.K. and the Netherlands jointly launched the infrared astronomical satellite IRAS. For the first time in the world, the satellite observed the whole sky in infrared and provided us with a variety of new discoveries including infrared galaxies and clues to the formation of the planet system. These results changed our view of the universe. As AKARI has better sensitivity and spatial resolution than the IRAS, it is rewriting the all-sky infrared data, performing more comprehensive research using far more detection samples than those discovered in the IRAS era.
Infrared astronomical satellite AKARI
AKARI carries a 68.5cm-aperture telescope with two observation instruments mounted on the focal plane: the InfraRed Camera (IRC) covering the wavelength 2 to 26µm; and the Far-Infrared Surveyor (FIS) covering the wavelength 50 to 180µm. The telescope and focal plane instruments are cooled up to 6K (about minus 267 deg. C) by a refrigerator system for their operation and use.
AKARI's orbit is a sun-synchronous polar orbit at an altitude of 700km. The satellite is flying on the border of the day and night above the earth. In this position, its solar array panel always orients to the day (i.e., the Sun) while its telescope always looks at the night sky in opposite direction to the earth. Since the orbital plane shifts little by little to circle the earth axis over a period of one year, the telescope can watch the whole sky in a half-year. AKARI's original mission plan was to first complete a far-infrared all-sky survey with FIS from this orbit and then perform an observatory-style observation mainly using IRC. By utilizing IRC during the far-infrared survey, however, we were able to obtain useful astronomical data of the whole sky in mid-infrared as well. After completing the satellite and instrument design, I started researching ways to achieve a mid-infrared all-sky survey.