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The Forefront of Space Science

Aiming for a Much Higher Sky
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Then, ISAS appeared on the scene. In the 1990s, ISAS initiated development of a thin-film, high-altitude balloon. Generally, 20 µm thickness film is used for balloons for scientific experiments. Nonetheless, ISAS started to produce balloons using 6 µm thickness film, the thinnest film then available. For reference, a plastic shopping bag is about 10 to 20 µm in thickness and plastic wrap about 10 µm. In the U.S., small balloons made of 6 µm film were used to sound wind conditions in the upper atmosphere before launching large balloons carrying heavy observational instruments. Because a small balloon’s empty weight is light, it can go up to the same height as a large one. The ISAS researchers tried to increase the size of balloons to send them even higher. There are light instruments of around 1 kg carried by balloon such as an ozone sounder. The weight per 1 m2 of 20 µm film is about 20 g, so the empty weight of a 100,000 m3 balloon becomes 300 kg. Thus, when carrying light instruments, the arrival altitude is determined by the balloon’s weight.

While satellites fly at much higher altitudes, however, there are no flying vehicles that can fly in the range between balloon’s and satellite’s altitudes. It is extremely difficult to lower the altitude of satellites because air resistance increases. The only possible method to stay in the mesosphere above 50 km altitude is to make a high altitude balloon.

To perform ozone observation at higher altitudes, we started to develop a light balloon from every detail. Other simultaneous developments included new equipment to weld thin film continually, weight reduction of onboard instruments by using surface mounted devices, a new method to release the balloon without damaging the thin film, etc. In 1997, we succeeded in sending a 120,000 m3 balloon to an altitude of 50.2 km.

The last and most difficult hurdle was to make the balloon film thinner. In collaboration with a resin maker, we found that polyethylene using metallocene catalyst is appropriate to make thinner film. Eventually, the film maker’s efforts produced a polyethylene film of 3.4 µm thickness, while a balloon maker improved its skill to fabricate a large balloon by welding pieces of film together. In 2002, a 60,000 m3 balloon reached an altitude of 53.0 km, successfully breaking the balloon altitude record for the first time in 30 years. It was 1/25 the volume compared to one in 1972, proving that thin film was very effective. Metallocene-based polyethylene was indispensable for thinner film. It had already been developed in the 1970s. At the time, however, there was too much focus on balloon enlargement. We should have pursued lightweight material and balloon enlargement simultaneously.

Since then, ISAS has led the way by breaking its own records again. Our first task was to make the film even thinner. To this end, we needed to develop a film fabrication machine itself. In collaboration with a film manufacturer, we first modified an existing machine, but in the end we decided to build a dedicated machine. In 2003, we were able to produce a polyethylene film of 2.8 µm thickness and the following year we successfully launched a 5,000 m3 balloon. After this initial success, we had a hard time. Some balloon launches failed because of holes in the film surface that appeared during the ascent. We speculated two causes: that the film material itself was weaker than forecast; or that the force imposed on the film was greater than estimated. We looked into the problems from various angles, such as the possibility that the film had weaker areas, or that it tore due to a weak load over an extended time period. However, we were unable to identify the cause. It was very difficult to estimate by calculation the force imposed on the film because of various factors, such as the balloon’s elastic deformation caused by wind before full expansion, or the movement of wrinkles on the film. Finally, we estimated the required intensity based on our experimental rule derived from a comparison of successful and unsuccessful flights and reinforced the film with protective tape.

One reinforcement was to add another layer of film in the balloon’s top area. The main purpose of this measure was to strengthen the area touched by the spooler during launch, but another purpose was to protect the balloon’s top in low altitude before inflation. Before inflation, the gas only fills part of the balloon head. Since the entire balloon is lifted by the inflated part alone, large force is applied. So, we doubled the film in the area. This technique has been employed for typical scientific balloons when their empty weight or payload weight is large. Our attempt was the first case for a thin-film scientific balloon.

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