Update: Solar flight forced into unscheduled stopover in Japan

Update:The Si2 aircraft has been forced to touch down this morning in Japan as worsening weather conditions made the Pacific Ocean crossing too treacherous today.

Solar Impulse said on its website: "Yesterday we had the possibility to cross the weather front just before Hawaii on day 5. However, with the forecasts we now have, we don’t see this possibility anymore, which means that for the moment the road to Hawaii is blocked. We need all the data from the next weather forecasts, so that our weather experts can analyze what’s going to happen in the next 4-5 days."

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The longest and most arduous leg of Solar Impulse 2’s (Si2) round-the-world attempt may have already run into difficulties as poor and unexpected weather over the Pacific Ocean threatens to hamper the attempt.

Having taken off from Nanjing, China, at 2:39 am on Sunday (18:39 GMT Saturday), the flight across the Pacific Ocean to Hawaii is expected to last for six days and five nights, with Swiss pilot Andre Borschberg, 62, alone at the controls for the duration.

However, a little over 24-hours into the crossing meteorologists working with the Si2 ground team have warned that the forecast has worsened. Borschberg has been instructed to stay in a holding pattern while the support team assesses the situation. The pilot may be forced to turn back towards land should conditions prove too unsuitable or unpredictable.

The current leg is the seventh and most challenging part of the circumnavigation attempt, and this weekend’s take-off follows a month-long delay in China as the team waited patiently for the most clement and favorable weather conditions.

Fitted with more than 17,000 solar cells, the Si2 gets all of its power from the sun, storing the energy in four lithium-ion batteries fitted in the wings to enable it to continue flying at night.

The journey began in March in Abu Dhabi, and has already seen the aircraft and fellow Swiss pilot Bertrand Piccard touch down in Oman, India, Myanmar and China.

All being well, there will be 12 legs in all, and Borschberg himself admitted before take-off on Saturday that this current leg would define the record-breaking attempt. "This is the moment of truth," he said.

The ground team is assessing the weather situation this Monday morning, and will inform Borschberg of the best course of action later today.

"We have asked Andre [Borschberg] to stay where he is: it’s fine, the weather is good and the battery is charging," said Piccard, who is working with the ground-based monitoring team.

Any delays at this stage threaten to hamper the U.S. leg of the journey, which Piccard revealed had been calculated to avoid Hurricane season, which begins in August.