After an unscheduled stopover of more than three weeks in Japan, the Solar Impulse 2 aircraft is on its longest and riskiest leg of the journey around the globe.

It is now entering its first night over the Pacific, as opposed to the controlled airspace above ground flown so far.

The solar generators have been switched off and the craft will run on the power stored in the batteries which is nearly full at present.

The eighth leg of the solar-powered flight will take 120 hours before the next stop at Hawaii.

After 15 hours of flight covering 1,038 kms, Swiss pilot Andre Borschberg, 62, who is cruising at around 17,000 feet, is well beyond the eight hours of flight permitting the craft to return to Nagoya from where it took off.

From here on, he will have to bail out on a dinghy under any emergency.

Borschberg tweeted: "Feeling a bit lonely up here, but very excited about the rest of the flight."

Sunday's (28 June) window of flight opportunity was the last to avoid being stuck in Japan for a year. Bad weather has already seen two aborted take-offs.

If the craft reaches Hawaii, it will have the distinction of being the longest-duration solo flight in aviation history, as well as that of a craft powered only by the Sun.

Borschberg took off on the 7,900-km journey from Nagoya, in central Japan, at around 3 am local time on Sunday (18:03 GMT).

"We are now waiting for it to reach the point of no return before sending out an official press release at about 10 am local time," Solar Impulse spokeswoman Elke Nuemann said when the craft took off.

The weather looked good for now but could still change, she said.

Once over the ocean, if it fails to soak up enough rays to fully charge its batteries and make it through the night, the pilot could be forced to bail out.

Bertrand Piccard, chairman and co-founder of Solar Impulse, will fly the next leg from Hawaii to the US mainland after which the craft will continue across North America, before attempting to fly over the Atlantic.

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Challenges of solar flight

Solar Impulse 2 is powered by more than 17,000 solar cells built into the craft's wings which at 72m are longer than that of a Boeing 747.

During day the plane will rise to an altitude of 9,000m when the sun will charge its batteries but at night the plane will glide down to 1,000m and run on the energy stored in its four lithium ion batteries. By the end of the night it will have only around 5-10% battery charge.

High cirrus clouds can limit the amount of energy harvested by the solar cells.

The flight at night can also encounter the trade winds blowing to the west which can slow it down.

Besides the change in altitude daily, the pilot will also experience temperature changes of 55C in the unpressurised, Solar Impulse 2 cockpit. Nap times will be short.

The light weight craft weighing a bit over two tonnes travels at a maximum speed of 140km/h.

The Solar Impulse epic journey began on 8 March in Abu Dhabi with the craft touching down in Oman, India, Myanmar, China and Japan so far. The founders seek to demonstrate capabilities of clean energy technologies.