HII/L2 Mission


2.1 Orbits of Previous Infrared Missions

It is critical for infrared astronomical satellites to avoid radiation both from the sun and from the earth to minimize the heat load. This is a very difficult task for a satellite in a near-Earth orbit with low inclination (e.g., IRTS3).

For this reason, solar-synchronous orbits were chosen for IRAS1 and COBE2. The solar-synchronous orbit is the most suitable for these infrared survey missions among various near-Earth orbits, because (1) the avoidance angles for the sun and the earth do not change very much, and the heat input by radiation can be kept relatively small, and (2) one can survey the whole sky in half a year without violating the avoidance-angle condition. We also chose the solar-synchronous orbit for the Japanese infrared survey mission ASTRO-F/IRIS4.

Although the solar-synchronous orbit is suitable for survey missions, it is not suitable for observatory-type missions, since observations with long integration times are very difficult in such an orbit to avoid radiation from the earth.

ISO was launched into a high-earth elliptical orbit. The long orbital period (about 24 hours5) enables ISO to make long integration. But ISO has to go through the earth's radiation belt in every orbit, and a high dose of ionizing radiation makes stable operation of detectors very difficult.

Therefore, we have to go away from the earth to make observations with longer integration times at stable condition.


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Apri 18, 1998
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