The Infrared Astronomical Satellite (IRAS1, 1983) opened a new era of infrared astronomy. IRAS made the first all-sky survey in the mid- and far-infrared, and detected some 250,000 sources. Next, the Cosmic Background Explorer (COBE2) was launched in 1989, and made the first absolute photometric measurements of the whole infrared sky.
The infrared astronomical community in Japan launched a small infrared survey mission, the Infrared Telescope in Space (IRTS3) in 1995, and now promotes the ASTRO-F/IRIS4 (Infrared Imaging Surveyor) project. ASTRO-F/IRIS is also a survey type mission, and has much better sensitivity and spatial resolution than those of IRAS. ASTRO-F/IRIS is scheduled to be launched in 2003, and is expected to detect millions of sources, and to discover many interesting objects.
To study in detail the interesting objects found in these survey missions, an observatory-type mission is needed. The first observatory-type infrared mission was the Infrared Space Observatory (ISO5), which was launched in 1995. The success of ISO demonstrated the effectiveness of an infrared observatory-type satellite. The Space Infrared Astronomical Facility (SIRTF6), which is now scheduled for launch in 2001, will also make many important observations.
But the mirror size of these missions (60 cm for ISO, and 85 cm for SIRTF) is much smaller than those of optical and radio telescopes, and hence the spatial resolution of infrared observations from space has been very poor. Thus, infrared astronomers have longed for observatory-type missions with much larger telescopes to make high resolution observations.
High spatial resolution is also important to achieve good sensitivity in the far-infrared, since the sensitivity of a well-designed space-borne mission is limited by confusion of sources7. In this case, longer integration does not improve the detectivity, and better spatial resolution with a large aperture telescope is the only way to improve the sensitivity.
Two large missions for infrared and sub-mm regions are now proposed for the launch by 2010. They are the Next Generation Space Telescope (NGST8) and the Far-Infrared and Sub-mm Space Telescope (FIRST9). They are powerful observatories, but their telescopes are only moderately cooled, and thermal radiation from the telescopes degrade the sensitivity especially in mid- and far-infrared.
Therefore, there has been a great demand for an observatory-type mission with a large aperture cooled telescope for mid- and far-infrared astronomy.
