In 1991, Per Lindstrand and entrepreneur Richard Branson attempted the first crossing of the Pacific Ocean in a hot-air balloon.
To do this, they had to break the world distance record and fly nearly 11,000 kilometers from Japan to Canada.
The balloon had to be the largest ever constructed.
Hot-air balloons vary greatly, but shapes like this are inefficient.
They use a lot of material, but don't hold a lot of air.
The ratio between their surface area and volume is poor.
The volume of a hot air balloon is crucial to its flight.
As the air inside a balloon is heated, it becomes less dense.
This makes it less dense than the air outside, creating lift so the balloon can fly.
And because it's hot air that maintains lift, volume is important if a balloon is intended for sustained flight.
Hot-air balloons tend to be approximately spherical, because this represents the most economical surface-area-to-volume ratio.
Volume of the Pacific Flyer
But to cross the Pacific, the Virgin Pacific Flyer would have to have the biggest volume ever.
Per Lindstrand, Aeronautical Engineer - "In our case, we needed to fly for three days, which meant about six tons of propane had to be carried, and with that you can calculate the volume, and that came out as 2.6 million cubic foot. That is about 75,000 cubic meters."
Using the formula for the volume of a sphere - four-thirds pi r cubed - engineers could calculate the dimensions of the balloon.
It would have a radius of 26 meters, and require around 8500 square meters of material to build.
The Virgin Pacific Flyer became the largest hot air balloon ever flown - big enough to swallow a jumbo jet.
And the adventure was a success, with the balloon setting a new world record by its near-11,000-kilometer journey over the Pacific Ocean.