TIGER in Antarctica - Q&A

Why is a balloon a better option to use than launching a satellite into space?
Cost. The least expensive satellites now cost about $120 million dollars (which includes instruments, spacecraft, and the rocket). TIGER is much, much less, even including the expense of coming to Antarctica. There are real advantages to satellites that balloons will never beat, but until we get approved for a spacecraft, we can refine our methods and still do good science for a fraction of the cost. Balloons allow one to test new instruments without the full expense of putting them in space.
Congratulations on your successful launch! Are the LDB and ULDB the same?
No, the LDB (Long Duration Ballooning) program (which is what TIGER is currently flying) uses what is called a "zero-pressure" balloon, where the helium gas is at the same pressure as the atmosphere, and some helium leaks out as the balloon cools. This type of balloon is used for flight of several days all over the world, but can only be pushed to flights of several weeks (like TIGER) at latitudes where the Sun does not set.
ULDB (Ultra Long Duration Ballooning) will use a new design, where the balloon is sealed, at a higher pressure than the local atmosphere, and somewhat pumpkin-shaped. It should be capable of flight of more than 100 days from any latitude. There have been test flights, but there hasn't been a successful ULDB science flight yet. TIGER had been selected for the first ULDB flight, but delays in the program caused us to shift our sights to LDB.
How does the new TIGER balloon compare with the balloons used in the past?
This is the first year that 40 million cubic foot zero-pressure balloons have been launched from Antarctica (the new launch vehicle allows the bigger balloons to be launched). Before this, we (and others) flew on 28 million cubic foot (mcf) balloons. But both the 40 mcf and 28 mcf balloons are identical to balloons that have flown for years from other parts of the world.
When you launch the balloon, does it go straight up for a while and then start circling around the south pole or does it start circling the pole right away, even as it is going up?
The winds at different heights can point in just about any direction, and can have very different speeds. The balloon has no engine, it just travels where the wind blows it. In the case of TIGER, at ground level we started heading southeast, then drifted northeast as we moved up through the atmosphere. At about 80,000 feet we were heading south and it wasn't until we were nearly at our maximum altitude of 128,000 feet that we started heading due west, which will take us around the pole. It took about 3 and a half hours to get all the way up there.
Are conditions good to launch the weather balloon? Has it been launched yet?
The winds at 127,000 feet are not ready yet. Tuesday, December 11 [2001], looks like the first chance to launch the balloon.
Does the failure of two Ultra-Long Duration Balloons (ULDB) (which caused the two-year delay in TIGER) put any doubts in your head about these two ULBD balloons being able to work successfully?
We are not actually going to fly TIGER on the new ULDB balloon design, which is designed for more than 100-day flights. We are going to fly on a standard balloon design, which has been flown successfully more than 100 times, although there are still failures. The thirty day flight is about the limit of the standard balloon, however. And we can only get a flight that long because we are going to stay in daylight the entire time. Every sunset requires hundreds of pounds of ballast, and every night requires hundreds of pounds of batteries (we get our power solely from solar arrays).
Why did you decide to do the research from the South Pole rather than the North Pole?
Because we can circle the pole and stay over land the entire time (we also stay in daylight the entire time, which is why we want to be near the pole). We also don't have to worry about overflying Russia, for example, although that is easier now than it used to be.
How does the weather help or hurt your experiment?
In order to launch the payload, we need fairly special weather conditions. The ground winds need to be light (3-8 knots), but not zero, and from a steady direction. And there needs to be no "shear layer" in the lower atmosphere, where the wind changes direction and/or velocity in a small region of altitude. These constraints come from the way the extremely large balloon has to be launched.
Do the wind patterns and or temperature patterns affect your experiment?
Other than the ground winds needed for launch, we also need the winds at 120,000 feet (the altitude we'll fly at) to be stable and "circumpolar". This is another way of saying that they just circle around the pole, so our instrument will circle the continent and come back to almost directly overhead. This circular wind pattern will start about December 10, and this is what sets the first possible day we can launch. And, while we have to watch the temperature of our instrument so that it doesn't get too hot or too cold, the ground temperatures don't affect it at all.
Does the ozone hole help and/or hinder your experiment?
The major problem with the ozone hole on our experiment is that the extra ultraviolet light degrades the plastic of the balloon and parachute, and they are worried that it will get too weak over a 30 day flight. It's one of the reasons why we might not be able to go twice around the pole.
What was the distance covered by the balloon, and the distance of its landing point from its launch point?
I've seen two numbers calculated for the distance traveled. If you just take the average instantaneous speed times the number of hours, you get about 10,400 miles, but this includes some small loops in which we were moving but not really getting anywhere. If you just measure the length of the trajectory on a map, the answer is about 8800 miles. It landed about 290 miles from the launch site.
Could you say how much ground control was exercised over the balloon during the course of the flight? For example, were you able to direct it to fly two circuits, or was that in the lap of the gods?
The balloon was cut down by control. We knew from the beginning that we wanted to try two circuits (which had never been tried before). We could have cut it down after one circuit, but after it finished the first, everything still looked OK, so we let it go. Because of gradual altitude loss and the way the winds were going, I don't think we could have made a third circuit. Even if we had, we would have had to leave the instrument on the ice over the winter, and we didn't want that.
What do you think these balloons are capable of, in terms of flight duration? Could they fly three or even more circuits?
Three circuits would be extremely tough for this balloon design (called a zero-pressure balloon because it is essentially an open bag with the same pressure inside and out). But NASA is developing a new design that has a sealed, pressurized balloon that should be good for more than 100 days. For more information on this program, you can check the ULDB website (ULDB stands for Ultra Long Duration Balloon).
Could you please explain what is meant by the term "zero pressure balloon"? Does it mean that the helium is exercising the same pressure against the balloon's skin as the outside air?
Yes, that is it exactly.
If it were open to the atmosphere, there would be zero pressure but the helium would diffuse into the atmosphere. Can you shed some more light on the physics of the balloon?
A zero pressure balloon IS open to the atmosphere at the bottom. The correct name should be "zero-differential pressure balloon", because the helium inside does have pressure. It is at exactly the same pressure as the atmosphere outside the balloon. This puts a minimum amount of stress on the balloon plastic.
The balloon traps the helium gas. The gas, being lighter than air, rises and gets trapped underneath the film that makes up the balloon. The helium does diffuse somewhat into the atmosphere, but this is actually a pretty slow process (at the opening, there is local diffusion of helium, but it tends to migrate back upwards into the balloon because it is lighter than air).
The major helium loss occurs because of day/night temperature differences (the helium shrinks and expands, and when it expands, some of it "flows" out the bottom). We don't have that problem in Antarctica; actually, because there are some daily temperature variations due to the changing angle of the Sun, the problem does exist in 24 hour sunlight, it just is a lot less. But every day the balloon sags lower and lower in the atmosphere. At some point, it goes too low to do useful science. It becomes a danger to air traffic and must be cut down. This is why NASA is developing sealed, superpressure balloons.
How does this record of flight duration compare with all other unmanned balloon flights?
There have been very small pressurized balloons that have flown for 100 days, I think, but this is the longest flight with an instrument as big as TIGER (which weighed two tons). A more typical "Long Duration Balloon" (LDB) flight is 10-15 days, and the longest previous flight was 26 days. The 26-day flight was one very slow circuit around Antarctica.
Can you tell me about the previous record holder?
The previous record was an instrument called TopHat, which flew from McMurdo last year for 26 days (one slow rotation around the pole).

National Aeronautics and Space Administration

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