"I remember that people never thought they'd be able to fly. After the planes set flight records, those critics were silenced," says Del Frate. But critics still take issue with solar airplanes.
"Critics tend to point out that these airplanes are fragile," says Del Frate. NASA's Pathfinder plane was damaged inside a NASA hangar by wind blowing through the door, he says. "We build in the necessary strength and no more. They're light and very minimal on material -- for a reason."
It's hard to imagine paying $20 million for a plane as thin as a wafer, but that's about what solar airplanes cost. According to Del Frate, the solar panels alone account for about half the cost. But to put it in perspective, a Boeing 747 starts at $234 million [source: Boeing].
The planes are not heavy lifters; the strongest built to date can carry one pilot. "If they hardly carry any payload, what's the point?," says Del Frate, summarizing what critics say. He points out that solar planes can carry sensors and cameras, which are light and are getting lighter. "Look at all your cell phone can do. It hardly weighs anything."
So far, solar planes need special flight conditions. While the batteries can carry them through night and the shade, the planes can't take off or fly in storms. They can't take off in strong wind. They can't stay in cumulus clouds or turbulent layers of the sky.
"Critics will point out they're only useful nine months out of the year, and they're right," says Del Frate. During winter, the planes struggle to stay up, with days being short and nights being long. Because the sun is close to the horizon, and the solar panels usually point straight up, the plane struggles to collect enough sunlight to stay aloft. Designers are angling and placing solar panels to catch the sun no matter where it is -- and some are planning folding planes.