Solar Impulse 2 headed to India Tuesday on the first major sea leg of its epic bid to become the first plane to fly around the world powered solely by the sun.
Pilot Bertrand Piccard kept the aircraft close to the Pakistani coast on the 1,465 kilometre (910 mile) journey across the Arabian Sea to Ahmedabad from the Omani capital Muscat.
The sea legs pose the greatest challenge for the Solar Impulse team as any loss over power over the water would leave the pilot no alternative but to bail out and await rescue by boat.
Much bigger crossings lie ahead as Piccard and fellow Swiss aviator Andre Borschberg, who alternate at the controls of the single-seat aircraft, traverse the great oceans.
The longest single leg will see one of them fly solo non-stop for five days and nights across the Pacific from Nanjing, China to Hawaii, a distance of 8,500 kilometres (5,270 miles).
Muscat was the first of 12 planned stops on the plane's journey around the world from Abu Dhabi, with a total flight time of around 25 days spread over five months.
"We hope that the people of Muscat will always remember that a solar airplane landed there," Piccard tweeted from the cockpit as he headed off for Ahmedabad.
The flight was expected to take around 16 hours, landing in India at around 1830 GMT.
Monday's maiden leg took Borschberg 13 hours and two minutes.
He spoke of an "emotional" trip, telling reporters he cruised at 6,000 metres (almost 20,000 feet) because the trip was "short".
- 'Flying into sustainable future' -
Piccard was flying even higher as he entered Pakistani airspace on Tuesday, at 8,230 metres (27,000 feet), his support team said.
The team hope to promote green energy with the circumnavigation attempt, which was ridiculed by the aviation industry when it was first unveiled.
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UN chief Ban Ki-moon hailed the venture and congratulated the pilots.
"With their daring and determination, we can all fly into a new sustainable future," his spokesman said.
The pilots' native Switzerland issued a special commemorative coin to mark the launch, which is the culmination of 13 years of research and testing.
The Si2 is the first solar-powered aircraft able to stay aloft for several days and nights.
It is powered by more than 17,000 solar cells built into wings that, at 72 metres, are longer than those of a Boeing 747 and approaching those of an Airbus A380 superjumbo.
Thanks to an innovative design, the lightweight carbon fibre aircraft weighs only 2.3 tonnes, about the same as a family 4X4 and less than one percent of the weight of the A380.
The propeller-driven craft has four 17.5-horsepower electric motors with rechargeable lithium batteries.
It will travel at 50-100 kilometres per hour (30-60 miles per hour), the slower speeds at night to prevent the batteries from draining too quickly.
The plane can fly on autopilot during rest breaks but both pilots have undergone intensive training to prepare them for the long solo flights.
They have learnt techniques, including yoga and self-hypnosis, to allow them to sleep for periods as short as 20 minutes but still wake up feeling refreshed.
While in the air, the pilots are linked to a control centre in Monaco where 65 weathermen, air traffic controllers and engineers are stationed. A team of 65 ground staff is travelling around the world with the two pilots.
From India the plane will head on to Myanmar, China, Hawaii and New York.
From there it will head on towards its starting point in Abu Dhabi via either southern Europe or North Africa, depending on weather conditions.