The long duration balloon in flight shortly after launch. Click to enlarge.

The Balloon Illusion

January 2, 2002

On December 21, 2001 a long duration balloon was launched near McMurdo station in Antarctica. A balloon launch is different from a rocket launch. When the balloon is released it departs in complete silence. After release, the balloon rises in slow motion. Over four hours it rises to its final altitude of 120,000 feet or 24 miles (36 km) ascending an average of 500 feet per minute.( I have a hard time running uphill at 50 feet per minute.) Then it holds its altitude in the stratosphere while winds blow it on a circular path around Antarctica.

	A view of a balloon fully inflated above 100,000 ft. Click to enlarge.

The higher the balloon rises, the lower the air pressure at its elevation. The helium inside the balloon expands in the lower pressure environment. The helium bubble inside the balloon initially has a diameter of 150 feet (45m), when the balloon reaches cruising altitude it is a sphere 900 feet (270m) in diameter.

The volume of the balloon doubles after it rises to 18,000 feet (5500m) then doubles again by 35,000 feet(11,000m). Above 35,000 feet the volume doubles every 15,000 feet (4,500 m). By the time the balloon reaches 120,000 feet it has doubled its volume 8 times, so its volume is 2^8 or 256 times larger. As the balloon rises through the lowest layer of the atmosphere, the troposphere, the temperature decreases below -40 C. However, when the balloon enters the stratosphere the temperature remains approximately constant.

A Size-Distance Illusion

	To the naked eye, the balloon at altitude looks slightly smaller than the full moon. Click to enlarge.

I watched the balloon rise more and more slowly and then stop rising. This was an illusion. The balloon was still rising, and yet it appeared to stop. If the balloon were a constant size object it would get smaller and smaller as it moved away from me. However, the balloon was growing in diameter as it rose. If I see an object growing in size and there are no other clues to its distance, I assume it is moving toward me. At one altitude near 75,000 feet (23,000 m)as the balloon rises, it grows in volume at just the right rate to keep its apparent size constant. Even though it is moving away from me the balloon image stays the same size on my retina and I "see" it standing still. As the balloon rises at first it looks like it is getting smaller then above 75,000 feet it gets larger again.

At 120,000 feet the 900 foot diameter balloon has an angular diameter of 0.4 degrees. Its diameter is 80% the size of the sun or the full moon. Many pilots have seen these balloons and reported that they needed to change their flight path to avoid colliding with the balloon. Even though the balloon was over 20 miles higher than they were. There were no clues about the size of the balloon so they had no idea how far away it was.

Balloons as UFOs

The balloon is so far away that if you drive along the ground it stays in the same position relative to you. (Note that as you drive nearby trees rapidly move backwards, but distant mountains seem to stay in the same place.) To many people the balloon in the sky appears to follow them. When these high altitude balloons are launched over populated areas many UFO reports are made.

field notes