HII/L2 Mission

2.3 Technical Feasibility

One more advantage of the S-E L2 orbit is that we can launch relatively large mass into the orbit in spite of its large distance from the earth.

   

Class 2 t class 3 t class 4 t class

W₀ 2,800 kg 5,700 kg 7,600 kg

300 kg 500 kg 700 kg

2,500 kg 5,200 kg 6,900 kg

Table 1: Launching capability of the H-IIA series. W₀ is the mass of a satellite which can be launched into a transfer orbit to S-E L2, and is the mass of fuel required to put the satellite into a halo orbit around S-E L2 from the transfer orbit.

Table 1 shows the estimated mass which the Japanese latest launching vehicle, H-IIA, can carry into the S-E L2 orbit. The H-IIA series consists of three types of vehicles with different launching capabilities. Even the smallest of them, 2 t class, which can launch a 2 t satellite into the geosynchronous orbit, can put a 2.5 t satellite into the S-E L2 orbit. In other words, we can launch heavier satellites into the S-E L2 orbit than those into the geosynchronous orbit.

In spite of the large distance to the S-E L2 point, down link from a satellite to the earth is not a serious problem, since one ground station can contact the satellite for 8 hours a day. With a 40 cm aperture high-gain antenna on-board the satellite and the 64 m dish of the Japanese deep space ground station at Usuda, about 3 Gbytes of data per day can be sent from the satellite to the ground.

One point we should mention here is that the S-E L2 point itself is not a good orbit; since the sun and the earth are in the same direction, the command receivers, which have to receive signals from the earth, suffer large noise from the sun. Hence halo or Lissajous orbits around the S-E L2 are more appropriate for infrared missions.¹⁰