It began its 5,079-mile (8,174km) flight to Hawaii from Nanjing, China, early yesterday, but storms are said to have forced the plane to land in Nagoya.
The plane landed safely at 11:49pm local time (3:49pm BST).
A tweet from the project's official Twitter handle said earlier today: '#Si2 will be landing due to worsening weather conditions after an extraordinary flight!'
The plane took off from Nanjing, in eastern China early on Sunday morning with pilot Andre Borschberg, 62, from Switzerland, at the controls in the tiny, cramped cockpit.
But with conditions getting worse, the team made the call to head for Nagoya in Japan, rather than attempt the daunting journey to Hawaii across the Pacific.
Despite the unscheduled stop, Mr Borschberg still set the record for the ‘longest ever solar flight in both distance and duration,’ according to Solar Impulse.
They will now wait in Japan for conditions to improve before trying again.
To make the dangerous crossing from China to Hawaii in five days, which would have been the longest solo flight of an aircraft in history, the team required perfect weather.
They had waited for optimal conditions to make the flight, to make sure the chance of success was as high as possible.
But spotting a problem so early on was important - had the plane gone much further, with the conditions worsening, Mr Borschberg would have had to ditch the plane in the ocean and jumped out to await rescue.
Speaking to MailOnline before the latest flight, he said: ‘What we are always looking for is a weak spot [in the weather].
‘You work 12 years for this, so you don’t want to jeopardise everything you did by making a stupid decision.’
The danger arises because the plane is very fragile: it is as wide as a passenger jet but as light as a car.
This means that, in high winds or turbulence, the plane can struggle to stay aloft. It must travel to high altitudes in order to recharge its batteries during the day, before dipping down at night.
Its maximum altitude is 27,900ft (8,500m), before dropping to 3,280ft (1,000m), when the pilot is able to take short 20-minute catnaps.
But if the plane had been unable to return to an airport, in this case Japan, Mr Borschberg would have bailed out, using a parachute to land before inflating a life raft and awaiting rescue.
The plane, in this scenario, would be left to crash into the ocean by itself, as if the pilot went down with the plane, he would risk being electrocuted in the water.
‘If you manage to get in the life raft, then you are pretty fine,’ he said.
‘You can spend two days waiting for a ship to pass by, and an aircraft can drop you a larger life raft so that you are a little more comfortable.’
Speaking to the media before the flight, Mr Borschberg had said this was 'the moment of truth’ before embarking on the flight across the Pacific Ocean, expected to last six days and five nights or at least 130 hours.
Well-wishers shouted ‘we love you’ as the plane took off in the darkness and soared almost silently, but for a whirring sound, into the sky on a journey that would push both pilot and machine to the limits of endurance.
Mr Borschberg and his fellow Swiss pilot, Bertrand Piccard, 57, have been taking turns flying the single-seater, propeller-driven plane during its 12-leg, five-month journey to promote renewable energy use.
It is the first solar-powered aircraft that can fly both day and night.
It began its 22,000-mile (35,400km) round-the-world trip in March, taking off from Abu Dhabi and landing safely in Oman 12 hours and 250 miles (400km) later.
The multi-million-pound aircraft, funded by large corporate sponsors such as Google and Omega, is powered by 17,000 solar cells on wings larger than those of a Boeing 747 jet.
It carries 600kg (1,320lbs) of high-tech lithium batteries, making up a quarter of the craft’s total weight, to achieve this.
Overall it weighs just 2.3 tonnes, slightly more than a family car, has a top speed of 86mph (138km/h) and a wingspan of 263ft (80 metres).
The successful launch from Abu Dhabi in March capped 13 years of research and testing by the two pilots.
From Oman the plane flew to Ahmedabad and Varanasi in India, then to Mandalay in Burma, then on to Chongqing. After Nanjing in China, the next was a five-day flight to Honolulu in Hawaii, before this unscheduled stop in Japan.
When the plane eventually makes it to Hawaii, it will then attempt a crossing of the US.
That leg will take in Phoenix, Arizona, and JFK airport in New York, followed by a crossing of the Atlantic to either southern Europe or northern Africa.
The final leg will take the craft from either of those locations back to Abu Dhabi, where it will make its final landing. All this will happen without burning a single drop of fuel, and is seen as being a major revolution in solar power technology.
The two pilots, who fly the plane in alternating shifts, have to contend with some testing conditions aboard the plane as they fly alone for up to five days at a time. They use yoga and self-hypnosis to keep themselves occupied.
'It's a flying home in some ways,' Mr Borschberg told MailOnline in an interview from Myanmar in March.
The cockpit has no heating, and the pilots are not able to stand up or walk around - for up to five days on longer flights. Instead, they can only recline in their seats to get a bit of exercise or rest.
A 'visit' to the toilet is pretty uncomfortable too - they simply use a hole in their seat.
The pilots trained hard for the mission, during which they have faced temperatures dropping to -40°C (-40°F) in their cramped cockpit and have had to breathe oxygen from a tank to cope with the extreme altitudes.
If the mission is successful, the plane will represent a considerable achievement for solar power - as the plane uses only solar energy to power its four engines, and not a single drop of conventional fuel will be used as it crosses three continents and spans two oceans.
'I believe that the adventure of the 21st century is not to go back to the moon - it is to improve the quality of life,' Mr Piccard told MailOnline, adding that Solar Impulse 2 could be key in bringing solar power to the masses.
Mr Piccard wants to see solar power become one of five major sources of energy in the future - including wind, biomass, hydroelectricity and geothermal.
By 2050, it is estimated that solar power could become the world's dominant source of energy, and while Solar Impulse 2 is not bringing any new technologies to the table, it is proving just how far solar power can go.
(dailymail.co.uk)
