On February 22, 2006, the infrared astronomical satellite ASTRO-F was launched by the M-V rocket No. 8 from the Uchinoura Space Center located in Kagoshima Prefecture, southern Kyushu, Japan. After successfully entering its initial orbit, ASTRO-F was renamed “AKARI” meaning “light” in Japanese. AKARI is Japan's first satellite dedicated to infrared astronomy. The spacecraft orbits the Earth once every 100 minutes at an altitude of about 700 km. The purpose of the AKARI mission is to survey the entire sky in infrared light. In addition, AKARI will also perform detailed observations of selected astronomical objects.
Nikon was responsible for AKARI's 68.5 cm-aperture reflecting telescope. An important feature of this telescope is that the mirrors are constructed from a new material, silicon carbide (SiC). On this occasion we interviewed Professor Hiroshi Murakami, project manager for the AKARI mission at the Japan Aerospace Exploration Agency's (JAXA) Institute of Space and Astronautical Science, about the significance of this new infrared All-Sky Survey, and the design and operation of AKARI.
Remaking the map of the infrared sky for the first time in 20 years
What is the objective of the AKARI (ASTRO-F) mission?
The AKARI infrared astronomical satellite, launched in February 2006 (artist's impression)
The purpose of AKARI (ASTRO-F) is to create a map of our Universe, for the first time in more than 20 years, using infrared light. In 1983, the world's first infrared astronomical satellite IRAS*1 was launched to carry out a pioneering infrared all-sky survey. The resulting database of infrared astronomical objects—essentially a map of the Universe—obtained from the IRAS observations motivated many subsequent observations from optical light, radio waves to X-rays, and contributed enormously to the innovation of astronomy. However, as one would expect, after 20 years, the IRAS database is no longer up to the task of serving as the basis for cutting-edge astronomy. For present-day astronomical observations, people may ask what has happened to the infrared—the map that is still used now is two decades old and shows neither faint astronomical objects nor the fine details and structure in brighter objects. Accordingly, the latest technology and research have been devoted to rectifying this situation by conducting high-sensitivity and high-precision infrared observations.
- *1IRAS (InfraRed Astronomical Satellite)
A joint project by NASA in the USA, NIVR in the Netherlands, and SERC in the UK. IRAS observed approximately 96% of the entire sky in 1983. It was equipped with a reflecting telescope of aperture 60 cm (effective aperture 57 cm) with a focal length of 5.5 m.
Why use infrared astronomy?
Infrared astronomy, began in 1960s, is a very important branch of astronomy. However, since most of the infrared radiation from space is absorbed by the Earth's atmosphere, it is difficult to make observations from the ground. Although large telescopes such as Subaru are effective, there are still many limitations to earthbound observations. Therefore, in order to observe the entire sky clearly, using long-wavelength infrared light, the only realistic option is to launch a telescope above the Earth's atmosphere into space. However, infrared observations from space have lagged behind those of X-ray and optical astronomy, because of the many technical challenges such as high performance infrared sensors required and the necessity for cooling equipment. Japanese astronomers have been considering space infrared observations since the late 1970s, and our first small mission was realized in 1995*2. Thereafter, many of the various technical problems associated with infrared observations from space were resolved, and the ASTRO-F project was born.
At that time, plans were already afoot in Europe and the United States for infrared astronomical satellites that would observe small areas of the sky to great degrees of precision. In Japan, we boldly decided to use the ASTRO-F mission to create a rejuvenated map of the entire sky, using high-sensitivity instruments to establish a legacy and basis for subsequent observations with future telescopes.
In the following 10 years, we solved the various technical issues, and now finally we can be proud to have launched our very own satellite dedicated for infrared observations.
- *2IRTS (InfraRed Telescope in Space)
Japan's first space infrared telescope with an aperture of 15 cm onboard the SFU (Space Flyer Unit), which was launched in March 1995 by an H-II rocket. The satellite was retrieved by the space shuttle in January 1996.
“Atmospheric windows” and AKARI's target observation wavelengths
Although most of the light from space is blocked by the Earth's atmosphere, some limited range of wavelength, such as visible light, short wavelength infrared light and radio waves, can penetrate through these “atmospheric windows”. The target of AKARI's observation is infrared radiation with wavelengths over the range of 2–180 μm. The longer part of this wavelength range, the so called “far-infrared” light, is almost completely absorbed by the Earth's atmosphere, and thus can only be observed from space.