Nearly 500 years ago, an expedition started by explorer Ferdinand Magellan sailed entirely around the world for the first time, using only the power of wind.
Now, a pair of Swiss adventurers and pilots are about to complete humanity's first round-the-world voyage in a plane powered only by the sun.
Yet, for the pilots of the Solar Impulse 2, about to embark on its final leg from Cairo to Abu Dhabi, the trip has always been more about showcasing the potential of solar energy than setting aviation records.
Pilots Andre Borschberg and Bertrand Piccard, who've alternated manning the flights around the world, want to raise awareness about climate change, showing what can be done using nothing but renewable energy.
"The most important thing isn't to make world records," Piccard said last year. "It's to show what we can do with clean technologies," he said, ones that could simultaneously reduce carbon dioxide emissions and stimulate economic growth.
The Solar Impulse 2 plane and flight is a "technology demonstrator," said Bob Van der Linden, the curator of aeronautics at the Smithsonian Institution's Air and Space Museum. "It was never intended to be a pioneering plane." It's intent instead, he said, was to advance the science and technology of solar power.
Van der Linden said many of the other first round-the-world flights were more about aviation records. These included the first round-the-world flight by Army Air Service pilots in 1924, the first non-stop flight by Dick Rutan and Jeana Yeager in 1986, the first non-stop balloon flight in 1999 by Piccard and Brian Jones and the first solo non-stop flight in 2005 by Steve Fossett.
He said the Solar Impulse 2 round-the-world solar-powered flight is not a record aeronauts long sought, unlike these others.
And there really aren't any more aviation records left, he added. The prime time for records was the 1920s and 30s, when aviation was in its infancy and aviation technology exploded between the world wars.
Still, Van der Linden said the Solar Impulse flight reminds people of the age of discovery and the great explorers such as Magellan centuries ago.
"By focusing the attention on the flight, they succeeded in that," he said.