GENEVA (AP) -- An experimental solar-powered plane took off from western Switzerland on Wednesday for a 24-hour test flight -- a key step in a historic effort to one day circle the globe using only energy collected from the sun.
The plane with its 262.5-foot (80-meter) wingspan left Payerne airfield shortly before 7 a.m. after overcoming an equipment problem that delayed a previous attempt, the Solar Impulse team said.
Clear blue skies on Wednesday allowed the prototype aircraft to soak up plenty of solar energy as it flew over the Jura mountains west of the Swiss Alps. The big question, however, was whether the plane's 12,000 solar cells could fill up its batteries with enough energy so the plane could fly through the night.
The flight is going "extremely well," said team co-founder Bertrand Piccard, a record-breaking balloonist whose father and grandfather also accomplished pioneering airborne and submarine feats.
"The goal of the project is to have a solar-powered plane flying day and night without fuel," Piccard said. "This flight is crucial for the credibility of the project."
By early afternoon, pilot Andre Borschberg had his oxygen mask on and was cruising at 19,680 feet (6,000 meters), after trying to dodge the low-level turbulence and thermal winds that are frequent in the mountains.
"Andre now reached 6000m in a cloudless sky ... this situation will last the entire afternoon," the control panel tweeted, adding later "batteries are nearly fully loaded. Sun rays now for climbing only."
He will take the plane to an altitude of 27,900 feet (8,500 meters) by Wednesday evening, when a decision will be made whether to continue through the night using solar power stored in its batteries.
Piccard told The Associated Press he was confident the plane will collect enough solar energy to theoretically fly through the night but was not exactly sure how energy efficient the plane was.
"We will most probably take the decision to go through the night," he said. "We'll have to be very careful, because then we have to see if the energy that we have in the batteries will be enough."
The plane is flying in loops in Swiss airspace and within gliding range of Payerne airport so it could land if it runs out of energy, he said. The solar plane needs a wide, concrete runway because of its wingspan.
Every aspect of the aircraft is monitored by engineers on the ground, with much of it fed onto the team's website and Twitter page.
If Borschberg, the plane's only pilot, decides to fly through the night, the plane will slowly descend to 4,920 feet (1,500 meters) before midnight and stay there until Borschberg attempts a dawn landing.
The 57-year old Swiss, a former fighter jet pilot, is wearing a parachute.
Piccard said Wednesday's test flight -- the third major step after its first 'flea hop' and an extended flight earlier this year -- will demonstrate whether the ultimate plan is feasible: to fly the plane around the world.
The team had hoped to make their 24-hour test flight last week when days in the northern hemisphere were even longer. But there was a problem with a key piece of communications equipment, forcing the team to ground the plane while modifications were made.
Piccard, who achieved the first nonstop circumnavigation of the globe in a balloon, the Breitling Orbiter III in 1999, said if successful, the next step will be a solar Atlantic crossing. That will be done in a second, lighter prototype, because it will involve new challenges and dangers, he said.
Although the goal is to show that emissions-free air travel is possible, the team says it doesn't see solar technology replacing conventional jet propulsion any time soon. Instead, the project is designed to test and promote new energy-efficient technologies.