MONTGOLFIER Infrarouge (MIR)
The Infrared Montgolfi&eagrave;re is a long-duration stratospheric balloon designed to sustain flight above 25 km altitude by capturing solar energy during the day and infrared radiation from the ground and clouds at night, eliminating the need for burners or gas propulsion.
Concept and Development
Conceived in 1976 by researchers at the Service d'Aéronomie, the MIR addressed the need for long-duration balloons capable of carrying payloads of several tens of kilograms to study planetary-scale variations in stratospheric composition when no suitable satellite existed. The concept evolved from earlier attempts with superpressure balloons and stratospheric pressurized designs, which proved either impractical or unsuccessful.
The innovation involved using a montgolfi&eagrave;rea hot-air balloonheated by natural energy rather than a burner. Alain Hauchecorne developed a thermal model to test feasibility. Initial experiments with tetrahedral black plastic bags and 320 m³ fabric balloons demonstrated that daytime stratospheric flight was achievable, with internal air temperatures reaching 90100°C above ambient.
To sustain flight through the night, the balloon was constructed as a two-part envelope: the lower half with maximum emissivity and absorptivity to capture upwelling infrared radiation, and the upper half with minimal emissivity to prevent heat loss toward space. Thermal modeling demonstrated that maintaining a temperature difference of 12°C between the balloon interior and ambient air could sustain a 5,0006,000 m³ balloon above 17 km even at night, with buoyancy proportional to this temperature differential relative to the surrounding atmosphere.
Prototype Design and Construction
In 1977, CNES and the Service d'Aéronomie began constructing an MIR prototype. Initial designs using 68 micron polyester films proved overly complex; the final design employed commercially available materials: 12-micron polyester film (Mylar) with aluminized outer surface for the upper half, and 15-micron polyethylene film for the lower half. The balloon adopted an inverted teardrop shape with a rigidified opening at the base for air intake. Assembly involved heat bonding for polyester and welding for polyethylene, with a specially developed cold adhesive tape joining the two half-envelopes.
The first 5,800 m³ prototype was tested on October 14, 1977, but capsized after separation from the carrier balloon due to insufficient gondola mass. A second test on December 19, 1977, with added ballast, succeeded spectacularly. The balloon climbed to 23 km, descended to 17 km overnight, and rose again at sunrise, demonstrating that neither gas nor burner could sustain stratospheric flight. It subsequently traveled over France into Turkey in 66 hours, proving the concept's validity.
Flight Operations and Performance
Over subsequent decades, multiple campaigns from locations spanning the Arctic to the tropics revealed significant latitude-dependent performance patterns. At mid-latitudes such as Pretoria (26°S), winter flights encounter the westerlies with temperatures of -40° to -50°C, yielding average lifespans of fourteen days. Arctic operations proved decidedly favorable, with the low polar vortex temperatures (-75° to -85°C) far below the highest cloud tops, though flights were limited by safety requirements to 1228 days.
Tropical stations including Bauru (22°S) and Latacunga (1°S, Ecuador) yielded the best results, with some flights exceeding two months. The Seychelles station (5°S) demonstrated similar potential. The most hazardous region for MIRs was Micronesia in boreal winter, where the cold tropopause (-85°C) and dense cirrus clouds created what researchers called the "Stratospheric Fountain." Flight duration from equatorial sites depended critically on wind patterns governed by the 26-month Quasi-Biennial Oscillation: balloons launched under westerly winds achieved average flights of 31 days, while those under easterly winds lasted only 1014 days.
Failure Modes and Technical Issues
MIR descents, apart from mandatory automatic separations, typically occurred when flying over high, cold clouds with low infrared emission. The most significant technical problem emerged with 45,000 m³ MIRs introduced after 1994: unpredictable rapid descents of 45 m/s at sunset, sometimes followed by stabilization or re-ascent. Investigation revealed that a reduction in the lower intake sleeve diameterrequired for valve installationprevented adequate air intake to compensate for volume loss during cooling. Though the sleeve diameter was increased for subsequent flights, verification of improvement was complicated by later setbacks.
Secondary hazards included electrical discharges called blue jets between thunderstorms and the ionosphere. Long electrical cables between gondolas were removed after documented electrical breakdowns. In 2008, a mysterious MIR failure over Angola was attributed possibly to aluminum coating vaporization from intense electrical current between the service gondola and balloon.
Current Status and Future Prospects
Since its inception in 1976, the MIR has undergone numerous modifications in volume, architecture, materials, and payloads, achieving operational status with significant scientific results in lower stratospheric chemistry and climate research. Ongoing challenges with unpredictable descents over high, cold storm systems prompted temporary suspension of MIR flights. Mitigation strategies include corrected intake sleeve dimensions, payload lightening through satellite transmission and instrument miniaturization, and electrical decoupling between service gondolas and balloons. The future of the program depends on CNES's safety philosophy, though the MIR remains a unique tool for studying the stratosphere beyond the reach of satellites.
List of all MIR balloons launched (incomplete)
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