GENEVA - An experimental solar-powered plane whose Swiss makers hope someday to fly around the globe soared into uncharted territory Wednesday - the cold, dark night.

The team of adventurers and engineers behind the Solar Impulse project are already celebrating an aviation milestone for the longest solar flight after keeping the single-seat prototype aloft for almost 15 hours.

But with the goal of 24 hours of nonstop flight, the team set its sights on keeping the sleek aircraft with a 207-foot wingspan in the air until Thursday morning.

Pilot Andre Borschberg "will stay up there as long as possible," said Bertrand Piccard, the project's co-founder.

"Hopefully he will still be in the air at sunrise tomorrow. That is the challenge."

Borschberg took off from Payerne airfield into the clear blue sky shortly before 7 a.m. Wednesday, allowing the plane to soak up plenty of sunshine and fly in gentle loops over the Jura mountains west of the Swiss Alps.

As the sun set, technicians hoped the Solar Impulse's batteries - charged from the 12,000 solar cells fixed to the wings and body - would keep the four-engine plane engines airborne through the night. The batteries would begin charging again at dawn.

Earlier in the afternoon, Piccard told The Associated Press the flight was going "extremely well."

A record-breaking balloonist whose father and grandfather also accomplished pioneering airborne and submarine feats, Piccard has become the figurehead for the project and will be one of two pilots when it eventually takes off for its round-the-world attempt in 2013, with a scheduled five stops along the way.

Piccard said the night flight was a key step toward achieving that ultimate feat.

"The goal of the project is to have a solar-powered plane flying day and night without fuel," he said. "This flight is crucial for the credibility of the project."

At 9:30 p.m., Piccard told reporters that strong winds had pushed the plane off course, cutting the safety margin by one hour.

"If there is one hour missing tomorrow morning, it will be that hour," he said.

Should Borschberg or the team decide that it looks like the Solar Impulse won't make it through the night, the pilot will have the difficult task of landing the fragile aircraft before the batteries run out.

Borschberg was circling in Swiss airspace, first at 28,000 feet and then gently easing down through the night - always within gliding range of Payerne airport, so he can land if the plane runs out of energy, Piccard said. Still, the 57-year-old former Swiss fighter pilot is wearing a parachute - just in case.