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The Forefront of Space Science

Development and Future of Microgravity Experiment System Using a Balloon
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Dropping vehicle using a balloon

Our microgravity experiment method is based on the same principle as the others, namely “dropping an object from a height.” This is true even for the Space Shuttle, since it can be said that the Shuttle “continues to drop on its trajectory.” “Dropping” is the common principle to all the microgravity experiment methods. Among the methods, our unique feature is the use of a balloon. The balloon used is very different from regular ones, for example, those handed out in event sites. The balloon for the first trial system is planned to float at an altitude of 40km, almost the same altitude where the 1st and 2nd stages of the M-V Rocket are separated. The atmosphere becomes thin as going up higher. The density of the atmosphere at 40km altitude is about 1/300 that on the ground. Accordingly, the balloon must be very special to be able to ascend to that altitude. Fig. 1 is a photograph taken with a camera onboard an M-V Rocket at about 40km altitude. The clouds below the rocket must make you feel that the earth appears round at an altitude of 40km.

Figure 1
Figure 1. M-V-6 just before separation of 1st and 2nd stages at an altitude of about 40km,
nearly the same altitude reached by our balloon.

The reason for the persistent pursuit of altitude is as follows. If you drop an object in a denser atmosphere, it receives more atmospheric resistance. To produce an extended microgravity condition, the dropping speed increases in proportion to the lapsed time and, accordingly, the air resistance increases. If there is air resistance, the object in the container senses acceleration, even though very small. If the acceleration is great, it is difficult to obtain a good microgravity environment. For this reason, we have to conduct microgravity experiments in thinner air, that is, at higher altitude.

Even at higher altitudes there is a little air, so we cannot yet produce a pure microgravity environment. As a solution to the problem, we decided to float a ball-like container in the vehicle, and control the vehicle so that the container does not touch its body. With this method, we can obtain a good microgravity condition without imposing acceleration on the container. The vehicle’s body acts as a wind shelter, and perfect free-fall of the container is realized inside the vehicle. Because of these two factors (experimenting at higher altitude with lower air resistance and levitating the container inside the vehicle), a good quality microgravity environment is maintained for an extended duration of tens of seconds.

Fig. 2 shows the trial model of a vehicle built based on the above concepts. Although hard to tell from the photo, the vehicle has 16 gas thrusters that can steer the vehicle in all directions while it drops. The vehicle has a mechanism that detects the position of the levitation container inside the yellow cylinder-shape body and fires the gas thrusters automatically to prevent the container from touching the vehicle’s inner walls.

Figure 2
Figure 2. Photos in front of the vehicle under checkup before launch at the Sanriku Balloon Center
(the author can be seen at right in the back row)

The vehicle looks like a rocket at first glance because its top is streamlined to minimize air resistance. Unlike a rocket, however, it will not carry fuel to take off under its own power from the ground and requires the help of a balloon to ascend. The vehicle loads only approx. 2kg of compressed air, enough to allow it to adjust its position slightly during the drop but not enough to lift it by itself. Another difference from rockets is that the vehicle is reusable if recovered after an experiment. This allows us to conduct microgravity experiments safely and at a lower cost.

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