Since 1977, the CNES (National Center for Space Studies) has been developing the MIR (Montgolfière Infrarouge) hot-air balloon for long-duration scientific flights in the stratosphere. By day, the MIR balloon flies at an altitude of 28 km to 32 km, and by night, between 18 km and 22 km depending on the quantity of infrared rays rising in the flight zone and the temperature of the air at the flight level. The balloons can then carry a payload of about 50 kg for several weeks. The trajectory follows the circulation of stratospheric winds, enough to circle the Earth more than once…
MIR is an “open” hot-air balloon with a helium complement at take-off. Thanks to their aluminum covering, these 35,000 to 45,000-cubic-meter balloons are heated exclusively by the sun during the day or by terrestrial infrared rays at night. This phenomenon follows the Earth, which is heated in its mass during the day by solar radiation, and gives back this heat at night in the form of invisible infrared radiation (this infrared radiation from the ground or clouds provides only a very weak thrust, estimated at 4 gr/m3). This “passive” heating system allows the air inside the balloon to remain hotter than the surrounding air, which gives a certain lift to this flying object.
Assembled by Zodiac International, the MIR balloon is composed of two distinct hemispheres made of materials that offer an adequate compromise between their thermo-optical properties and the balance of their mass:
The upper part is made of aluminized Mylar of 12 to 16 µm forming a cavity to absorb rising infrared radiation and preventing any re-emission towards the sky.
The lower part is made of 15 µm linear polyethylene, a material that is transparent for infrared rays and resistant when the balloon is exposed to a cold environment (temperature below -80°C) during its flight.
The average lifespan of a hot-air balloon is 15 to 20 days.
On December 8, 1988, an infrared hot-air balloon took off from Pretoria in South Africa. It successfully circled the Earth twice in 50 days. More recently in 2001, a hot-air balloon circled the Earth three times in 70 days.
Infrared hot-air balloons are powerful instruments for scientific measurements, and their low-carbon flights could inspire many other applications in aeronautics or outer space.
