HIWIND (High-altitude Interferometer WIND Experiment)

The NASA Super Pressure Balloon (SPB) is a large, sealed, high-altitude balloon engineered to maintain a constant positive internal pressure relative to the surrounding atmosphere. This positive pressure allows it to preserve a nearly constant volume, which in turn provides remarkable altitude stability, especially valuable during the day/night cycles encountered at mid-latitudes. This contrasts significantly with conventional zero-pressure (ZP) balloons, which experience considerable altitude variations due to thermal changes from solar exposure and night cooling.

The SPB is specifically designed to address this limitation, enabling extended-duration flights in regions where traditional ballooning has been constrained. The overall shape of the balloon is an oblate spheroid, meaning it resembles a slightly flattened sphere-squashed at the top and bottom-with the height measuring about 60% of its diameter. The structure of the balloon is composed of many individual vertical panels called gores. Each gore stretches from the top to the bottom of the balloon and has edges that are heat-sealed together. These sealed joints incorporate a strong, lightweight tendon or rope that also runs from top to bottom, contributing to the balloon's structural integrity. Under internal pressure, the gores form a slightly curved and lobed shape, resulting in the balloon taking on a characteristic "pumpkin" appearance when fully inflated. The balloon is filled with a precise amount of helium lifting gas, carefully calculated in relation to the payload mass to be carried and the target pressure altitude, which is approximately 33.5 kilometers (or roughly 110,000 feet). As the balloon ascends through the atmosphere, the decreasing ambient pressure allows the helium to expand within the sealed structure without compromising its form, thereby helping it to float at a predetermined altitude. The SPB serves as a platform for scientific missions that require long-duration, high-altitude observations that are impractical from polar regions or space, such as certain Earth science investigations and instrument development for future satellites. Its ability to maintain altitude with minimal fluctuation and its compatibility with mid-latitude flight paths greatly expand the potential for balloon-borne research missions.

The design and performance of the SPB are being validated through a series of incremental test flights. Currently, the SPB remains in its qualification phase, during which all payloads flown are considered "missions of opportunity". This designation implies that while these payloads can achieve valuable scientific objectives, they are secondary to the primary goal of testing the balloon's capabilities. Consequently, there is an inherent risk that payloads could be lost due to potential failures in the balloon system. Such missions are essential for advancing the SPB's development, as they provide real-world data on performance and reliability under various conditions. The insights gained from these flights are crucial for refining the technology and ensuring the success of future long-duration scientific missions.

THE PAYLOAD

HIWIND stands for High-altitude Interferometer WIND Experiment and is the first balloon-borne Fabry-Perot interferometer specialled conceived for measure the daytime thermospheric winds by monitoring the neutral wind induced Doppler shift in the airglow emission O 630 nm.

The instrument measures four meters and weighs about 720 kilograms. Inside the central body -in an environmentally sealed pressure vessel- are located the different elements of the interferometer which form the optical bench: the lenses, an etalon chamber, the filter wheel and a CCD. The cylindrical body is covered by reflective space blankets during flight to provide protection from the extreme thermal environment that exists in the stratosphere. Near the top of the instrument, four light port tubes pointed upwards allow light measurements to be taken in four directions simultaneously to determine wind velocity and direction. The tubes are oriented at an elevation angle of 50º from the horizon to look at wind velocities well above HiWind's flight altitude.

The solar panels are continuously positioned to face the Sun during flight to provide power and shade critical areas of the instrument from intense solar radiation. A large radiator at the rear allowed precise thermal control of sensitive components inside the payload.

Pointing information is provided via differential GPS and a sun detector developed by NASA to drive a motorized rotator located between the payload and the balloon.

Balloon launched on: 4/16/2025 at 22:50 UTC
Launch site: Wanaka Airport, Otago, New Zealand  
Balloon launched by: Columbia Scientific Balloon Facility (CSBF)
Balloon manufacturer/size/composition: Super Pressure Balloon Aerostar - 18.793.000 cuft
Flight identification number: 750NT
End of flight (L for landing time, W for last contact, otherwise termination time): 5/4/2025
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): + 17 d
Landing site: Sank in the Pacific Ocean in a point located 285 miles SE of Chatham Islands

The balloon was launched by dynamic method from Wanaka, New Zealand, at 22:50 UTC on April 16th, 2025. Initially after the launch, the balloon floated across New Zealand before shifting to lower latitude and heading with the prevailing winds to the east. The balloon crossed southamerica without trouble but after starting its overflight of the Southern Atlantic Ocean the craft began to experience losses of altitude at night greater than expected.

On its 16th day aloft it completed a full circumnavigation to the southern hemisphere but shortly after that milestone was reached the mission ended. The balloon and the payload sank in the Pacific Ocean in a point located 285 miles SE of Chatham Islands, 1000 miles east of New Zealand's coast. Total flight time was more than 17 days.

• HAO's HiWind Balloon has Launched NCAR website
• NASA Balloon at Float, Begins Southern Hemisphere Journey NASA website
• NASA Scientific Balloon on Track to Cross South America NASA website