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The Forefront of Space Science

Aiming at a Highly Accurate Satellite Structure
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In case of a satellite using an optical telescope

The mirror plane error requirement on a radiowave antenna is 0.1mm to a few millimeters. In case of telescopes using shorter wave infrared (wavelength 800nm to 1mm) or visible light (wavelength 380 to 780nm), however, the accuracy requirement jumps to the order of micro to nano meters. The material used for the mirror is glass and it is polished to bring the mirror error below 1/20th of the wavelength used. I mentioned above that aperture enlargement is a common desire of observation researchers. If we increase the mirror size, however, it would deform under its own weight on the ground. For large terrestrial telescopes, we operate them by correcting the deformations that vary according to direction. With telescopes in space, however, no gravitational deformation occurs. Thus, we must consider use condition of a no-gravity environment when manufacturing and polishing telescopes on the ground. A 3.5m-diameter aperture telescope will be mounted on the infrared observation satellite SPICA (planned to be launched around 2017). This is the largest that can be stowed in the rocket fairing. In fact, it looks as though the satellite is mounted on the telescope, rather than vice versa.

Since SPICA’s infrared telescope will observe infrared from far-distant stars, infrared emission from the telescope itself cannot be allowed. To that end, the telescope must be cooled to an extremely low temperature (45K). The telescope will be manufactured and polished at room temperature. After launching at room temperature, the telescope will be cooled to almost absolute zero temperature by many mechanical refrigerators over time in outer space. This imposes severe manufacturing and operating conditions on us. Specifically, we have to produce a mirror that will not deform under changes in temperature of even 300 deg. C, nor suffer structural damage. We are considering the use of silicon carbide (SiC) with its small thermal expansion coefficient for the mirror. Another technical challenge is the method of retaining the mirror.

Figure 3
Figure 3. Infrared observation satellite SPICA

After launch

After surmounting the technical problems above and enduring the rocket launch environment, the satellite is successfully input into outer space. Once in orbit, however, the satellite faces minute vibration environments called “disturbances.EThe source of these disturbances are gyros, control wheels, refrigerators, motors driving the solar-array paddle to track the Sun while orbiting the earth, motors operating observation instruments, etc. The disturbance forces and torques pass to the satellite structure and affect the pointing performance of mission instruments. In particular, when vibration goes through the satellite’s structural part, the vibrational magnitude is amplified due to structural resonance. Unfortunately, it is difficult to accurately forecast and control the magnitude.

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