When borders aren't the problem, the duration of a balloon flight is limited by physics.
As a balloon rises, its helium expands. By the time it reaches its targeted altitude—in excess of 22 miles—the gas has expanded to fill the volume of the balloon, typically 40 million cubic feet. (The National Scientific Balloon Facility Web site notes that you could fit two Boeing 747s back to back inside the envelope.) The balloon has enough lift to rise still higher; however, it can expand no more and would burst if it continued to ascend. To prevent that, polyethylene tubes serve as vents, permitting surplus helium to escape. The excess lift vanishes, and the balloon flies near the desired altitude.
The night falls. The balloon cools, contracts in volume, and sinks. To prevent it from descending, ground controllers send a command to drop ballast. When the sunlight of the following morning warms it anew, it rises again—and because it has dropped ballast and therefore lost weight, it vents still more helium. Such cycles can continue only as long as the balloon has ballast. Once the ballast is gone, it descends for good.
If there were a way to interrupt the warming and cooling cycle, a balloon flight could last longer. One alternative is to travel to a place where the sun never sets. For the past 10 years, NASA's balloonists have launched from a base in Antarctica, where during the south polar summer the sun shines continuously. The balloons stay aloft for as long as two weeks, and several years ago, one of these longer flights produced headlines.
Just after Christmas in 1998, a mission known as Boomerang (for "Balloon Observations of Millimetric Extragalactic Radiation and Geophysics"!) was launched from Williams Field, six miles from McMurdo Station in Antarctica. Boomerang was intended to further the work begun by the Cosmic Background Explorer satellite, which in 1992 made historic observations (with instruments first tested on balloons) of microwaves that fill all space with a thin electronic fog, the faint remnant of the Big Bang. COBE measured very slight differences in temperature—variations as small as 0.0001 degree—among various parts of the sky. Boomerang provided a finer measure, with high enough resolution to show the size of the "hot" and "cold" patches on the sky. According to Andrew Lange of the California Institute of Technology, one of the chief investigators on the project, "The Boomerang map shows structures that are the right size to have evolved into galactic superclusters, so for the first time there's a visible link between the embryonic universe and the present universe." The measurements also support the leading theory of origin of the universe (see "The Big Push," below).
"This was a real high point in balloon science," recalls Grindlay. "It made the cover of Nature." (That, for scientists, is what making the cover of Rolling Stone is for rock musicians.)
On reason that Boomerang could produce such phenomenal results is that it stayed up for 10 days. Moreover, Boomerang, appropriately enough, returned almost to its starting point because of circumpolar winds that stay nearly constant in latitude. Boomerang remained at or near 79 degrees south and traveled 5,000 miles. A radio signal then brought it down within 30 miles of its launch site.
If momentous discoveries can be had from a 10-day balloon mission, imagine what scientists could do with a balloon that stays aloft 10 times that long. NASA's balloon program office is at work on a new science balloon that could fly as long as 100 days. The Ultra Long Duration Balloon would be the first real alternative to satellites.
The ULDB is sealed and does not vent helium. The envelopes of conventional balloons are too delicate to with stand the pressure of expanding helium, but the ULDB is designed to be stronger. Mike Smith of Raven Industries, which is assembling ULDBs, explains that instead of expanding in sunlight and contracting at night, the ULDB is able to maintain a constant volume while the pressure increases and decreases.
The ULDB takes its strength from "tendons," long cords that run from top to bottom like lines of longitude on a globe, dividing the balloon into narrow zones. Within each zone, the plastic film relieves stress by bulging outward. Artists' renderings show only a few such tendons and give a ULDB the appearance of a pumpkin. Actually there are some 300 of them. The internal pressure is low, only around 0.03 pound per square inch, but a ULDB is as large as a football field and has a lot of square inches. "Even with the low pressure it's very tight," says NASA's ULDB project manager, Steve Smith. "The tendons are like guitar strings."