For decades NASA has been sending big balloons to high-altitudes as a low-cost way of gathering atmospheric data, but in their current form they do have their limitations. One of which is sustaining long duration flights at certain latitudes. NASA is trying to overcome this with its Super Pressure Balloon (SPB) designed to eventually remain airborne for more than 100 days at a time. The floating observatory is edging closer to this goal, coming down to land on Saturday after 46 days in the air to set a new mid-latitude flight duration record for a balloon of its type.
At the moment, NASA and other researchers use what are known as zero pressure balloons to probe the mysteries of the atmosphere, and by extension our world and the universe as a whole. The data they collect with onboard instruments can help inform our understanding of atmospheric chemistry, but the problem is they can only fly for a maximum of two weeks at a time, with their endurance particularly limited at mid-latitudes.
This is because during the day time, the sun heats up the helium inside the balloon, causing it to expand and start to escape. Then after the sun goes down, the balloon and gas inside starts to cool and the balloon descends, sometimes by as much as 50,000 ft (15.2 km). The balloon therefore has to release ballast, around eight percent a day, to maintain altitude, which limits how long it can remain in the air for.
It's not so much of a problem over the polar regions where more constant weather conditions allow them to fly for up to two weeks at a time, but mid-latitude conditions aren't so accomodating. This is where NASA's Super Pressure Balloon (SPB) comes into play. The 18.8-million-cubic-foot (532,000-cubic-meter) balloon uses a closed system where no gases or ballast is released and the helium is pressurized, which allows it to float along at a more consistent altitude and, hopefully, for far longer periods at mid-latitudes.
The recently concluded voyage was the Super Pressure Balloon's second test flight, following a 32-day jaunt last year. The mission began back on May 17, launching from Wanaka, New Zealand, and came to an end on Saturday, July 2. This 46 day, 20 hour and 19 minute duration makes it the longest mid-latitude flight of a large scientific research balloon and marks an important milestone in NASA's efforts to bring these research tools to previously unexplored regions of our atmosphere.
"We're extremely pleased with the flight time we achieved with this mission, far and away the longest mid-latitude flight of a NASA heavy-lift balloon to date," says Debbie Fairbrother, NASA's Balloon Program Office chief. "We'll continue to strive for even longer duration flight, 100 days or more, and what we learn from this year's mission will help take us there."
The mission was brought to an end in Peru after operators picked up on considerable altitude variations in its last few weeks, the kind of behaviour you'd expect with a zero pressure balloon. This took place at night and was particularly notable as it flew over cold storms in temperatures as low as -80° C (-112° F), causing the ballon to dip as low as 80,000 ft (24.4 km) when it is engineered to fly at 110,000 ft (33.5 km).
The team says a possible explanation for these fluctuations is that some helium may have escaped during one of the more severe storms, before the hole was somehow plugged. NASA is now in the process of retrieving the balloon to dig into its secrets.
"Balloons are thermal vehicles, and some altitude variance isn't uncommon during periods of extreme cooling and heating," says Fairbrother. "Given the occasional periods of altitude variation we noted, and at times the magnitude we observed, we're eager to retrieve the balloon and payload so we can analyze the flight data and balloon."
While this was primarily a test flight, it did see NASA tick off a number of firsts. It was the first balloon to complete a mid-latitude circumnavigation and the first time the SPB has carried a science payload during mid-latitude flight. The Compton Spectrometer and Imager (COSI) allowed a science team at the University of California, Berkeley, to detect their first gamma ray burst, which lasted nearly 10 seconds. These types of flashes are indicative of high-energy explosions in far-away galaxies, perhaps coming from supernovas or the formation of black holes.
"At its core, this was always a test flight," says Fairbrother. "We're looking forward to the this next phase of analysis. We'll apply any lessons learned to future missions as we continue to eye our 100-day duration goal."