Bad weather was hindering the round-the-world mission of Solar Impulse 2 for a second day Tuesday, with a support team unable to join the plane after its impromptu stop in Japan.

The record-breaking aircraft was on the tarmac in Nagoya waiting for a mobile hangar to arrive to protect its delicate technology and 72-metre (236-foot) wings from the elements.

But a delayed commercial flight from rain-sodden Shanghai meant many members of the ground support crew were unable to join pilot Andre Borschberg in Japan.

"Fighting against the clock. Waiting to build the mobile hangar to protect @solarimpulse against bad weather," tweeted Borschberg.

"To my good friends from Chinese ATC (air traffic control), @solarimpulse team is on (the runway at Shanghai). No (take off) slot: can you help? Big XieXie," he added, using the Mandarin word for "thanks".

According to aircraft tracking website Flightradar24.com, flight MU529 had been scheduled to take off at 9.30 am (0130 GMT), but by mid-afternoon, its status was listed as "unknown".

Heavy rain was pounding the Shanghai area, resulting in dozens of flight delays, Shanghai Airport Authority said.

Borschberg's landing in Nagoya, in central Japan, concluded what had been the aircraft's longest non-stop period of flight.

But it nevertheless cut short what was due to have been a marathon trip across the Pacific Ocean, the longest single leg of an effort to raise awareness of green energy by circumnavigating the globe using only the power of the sun.

Solar Impulse 2 was trying to fly continuously from Nanjing to Hawaii, and planners had expected the 8,500 kilometres (5,250 miles) to take six days and six nights of non-stop flight, with onboard batteries charging up during the day.

But a developing cold front in the Pacific Ocean that weather forecasters said Borschberg would encounter as he neared Hawaii made the crossing risky, mission controllers decided, ordering the pilot to divert to Japan instead.

- 'Great pleasure' -

At a press conference after he touched down, Borschberg told reporters the diversion was no problem for the success of the mission.

"I would say it has no impact," he said, adding it was a "great pleasure" to be in Japan, a place he lived 30 years earlier.

And he vowed his circumnavigation would go on.

"Looking forward to continuing this adventure with @bertrandpiccard," he tweeted early Tuesday, referring to the mission initiator and fellow pilot Bertrand Piccard.

Curious locals gathered in a park near the airport Tuesday, hoping to get a glimpse of the plane, which has 17,000 solar cells and weighs just 2,300 kilograms (5,000 pounds).

LEDs that festoon the huge wingspan gave the plane an ethereal look as it glided in to land on Monday night -- even sparking rumours of an extra-terrestrial invasion.

"People were tweeting that a UFO was coming," Shigeru Akoshima, 56, told AFP, while 67-year-old Mieko Murayama said she had been baffled by the lights.

"My husband even called the police," she said.

The landing was live streamed on the project's website, with viewers treated to scenes of jubilation and relief from the Monaco mission control room as the plane touched down.

Despite having been cut short by several days, the flight from China notched up at least one first -- Solar Impulse 2 managed to fly day and night powered only by sunshine for the first time.

The round-the-world attempt began in Abu Dhabi in March and was originally intended to be completed in 12 legs, with a total flight time of around 25 days.

It was not supposed to include a stop in Japan, but, as the last bit of land before the vast stretch of the open Pacific, it had always been a possible backup destination.

The mission's well-oiled PR operation wasted no time after the unexpected landing, posting messages on their Twitter feed in Japanese thanking the nation for its support.

The plane is the successor to Solar Impulse, which managed a 26-hour flight in 2010, proving its ability to store enough power in lithium batteries during the day to keep flying at night.

Ridiculed by the aviation industry when it was first unveiled, the venture has since been hailed around the world, including by UN chief Ban Ki-moon.