The aircraft, which flies without a drop of fuel, took off at 7:05 am (0605 GMT), piloted by Andre Borschberg, and flew away into a clear sky headed for Ouarzazate in southern Morocco.
“The sky is magnificently beautiful and I am pretty confident of arriving at the destination,” Borschberg told AFP by satellite telephone from his cockpit. “Mother Nature seems to be more favourable than the last time.”
When Borschberg on June 13 made his first attempt to cross the desert on the final stage of a trip that has taken him from his native Switzerland to Spain and then to Morocco, he had to turn round because of strong winds and turbulence near the Atlas mountains.
The plane, which was flying over the Atlantic towards the port city of Casablanca at about 62 kilometres per hour (38.6 mph), was awaited at around 11:30 pm (2230 GMT) in Ouarzazate, where the Moroccan authorities plan to build the largest solar power station in the world.
“I can see far away the Moroccan coast in a superb blue… Today everything seems possible. In Ouarzazate, the weather forecast is good,” Borschberg said on the phone.
Speaking of his foiled bid the previous week, he said that people “should not talk of failure, but of experience. It’s training, you learn a lot of things.”
The flight was expected to be the most challenging Solar Impulse has yet faced because of the arid, baking hot nature of the terrain and the proximity of the mountains, which are more than 3,000 metres (9,800 feet) high.
The giant high-tech aircraft, which has the wingspan of a jumbo jet but weighs no more than a medium-sized car, is fitted with 12,000 solar cells feeding four electric motors driving propellors.
Ouarzazate is 550 kilometres (340 miles) from the Moroccan capital. The flight is expected to take about 16 hours because of the slow speed of the prototype aircraft and the unpredictable climate.
The flight has been jointly organised by the Swiss Solar Impulse company and the Moroccan agency for solar energy (Masen). When he reaches Ouarzazate, Borschberg will fly over the future site of the solar power complex.
Masen is responsible for building a power station with an initial capacity of 160 megawatts and plans to raise this capacity to about 500 MW to 2015.
Last month, the solar-powered plane made the 2,500-kilometre (1,550-mile) journey from Madrid to Rabat, its longest to date and its first between continents, after an inaugural flight to Paris and Brussels last year.
The flights are intended as a rehearsal for the goal of a round-the-world trip in 2014 by an updated version of the plane.