HII/L2 Mission

2.2 Sun-Earth Lagrangian liberation point L2

First, we propose one of the five Sun-Earth Lagrangian liberation points (L2) as an orbit for an observatory-type infrared mission.

  
Figure 1: Sun-Earth Lagrangian liberation points.

Figure 1 shows the five Lagrangian liberation points of the Sun-Earth system. The nearest points to the earth among them are L1 and L2 points, which are about km from the earth. They are on a straight line connecting the sun and the earth, and are thus metastable. But it requires a very small (several m s^-1 year^-1) to keep a satellite in orbit around them¹⁰.

Among them, L2 (hereafter S-E L2; the point at the opposite side of the sun from the earth) is the best place for infrared astronomy for the following reasons:

1. Effective cooling by radiation
   The apparent size of the earth, which is one of the dominant heat source for satellites near the earth, becomes small ( 30'), and all of the heat sources (Sun, Earth, Moon) are almost in the same direction. Hence, it is very easy to shield the telescope from the heat sources, and the radiative cooling becomes very efficient. Our simple estimate (Figure 2) shows that, if the telescope is properly shielded from the solar radiation, the temperature of the telescope can be cooled as low as 20 K only by the radiative cooling.
2. Long integration time
   Since the sun and the earth are in almost the same direction, large areas of the sky can be observed. With the help of a stable gravitational condition at the S-E L2, very long integration is possible.

  
Figure 2: Temperature profile of a model payload with an effective shielding system. It is cooled only by radiation.

Therefore, we conclude that S-E L2 is the best orbit for observatory-type infrared missions.