The Solar Impulse 2 plane took off Monday from Nagoya, ending a frustrating four-week hiatus during its unprecedented bid to circle the world without fuel. 
 
 Flight engineers spotted a possible cloudless stretch over the Pacific and in a pre-dawn scramble got the solar-powered plane airborne from Nagoya Airport, where it has been grounded since June 1. 
 
 “We are very, very happy,” project spokeswoman Elke Neumann said by phone from the airport. “We have now packed the hangar up. We are just doing the last logistical operations.” 
 
 She said a cargo plane will ferry the inflatable hangar together with the support crew to Kalaeloa Airport near Honolulu, where Solar Impulse 2 is expected to land after a nonstop flight of roughly five days and nights. 
 
 Solar Impulse 2 is a lightweight aircraft driven by four electric propellers and banks of solar panels atop its fuselage and wings. It took shelter at the Nagoya city airport, also known as Komaki Airport, part way through a flight from Nanjing, China, to Hawaii. 
 
 A previous effort to resume the flight last Wednesday was canceled at the last minute as ranks of journalists looked on. This time, the project team announced plans to take off only an hour or so before departure. 
 
 “We just wanted to make sure that we are really safely on the way to Hawaii,” explained Neumann. “It created a lot of disappointment.” 
 
 The team issued a statement thanking their Japanese hosts for helping accommodate the aircraft and support staff during the unplanned visit. 
 
 “Solar Impulse extends its gratitude to all those in Japan who have worked very hard to accommodate us,” the statement said. 
 
 It said those who had helped during this “challenging” phase included Aichi Gov. Hideaki Omura, the Swiss Embassy and project sponsors Schindler Holding AB and ABB Ltd. 
 
 From Hawaii, the plane aims to reach the United States mainland and to cross the Atlantic while calm summer conditions persist. Its round-the-world voyage will end at its starting point, Abu Dhabi. 
 
 Swiss pilot Andre Borschberg said June 17 that the unplanned grounding in Nagoya was eating away at the time margin and that “question marks” hung over the project’s completion this year. 
 
 He said with every day of delay the effort comes closer to having to pause and resume next year. ||||| Image copyright Solar impulse Image caption The plane has a bigger wingspan than a jumbo jet but is incredibly light 
 
 A solar-powered plane has passed the "point of no return" in its second bid at making a record-breaking flight across the Pacific Ocean. 
 
 Solar Impulse took off from Japan's Nagoya Airfield at 18:03 GMT on Sunday. 
 
 The journey to Hawaii is expected to take approximately 120 hours. 
 
 The team spent nearly two months waiting for clear weather to cross the Pacific, and a developing cold front forced the plane to make an unscheduled landing in Japan earlier this month. 
 
 "Andre Borschberg has passed the point of no return and must now see this 5 days 5 nights flight through to the end," Solar Impulse said on its website. 
 
 The pilot now no longer has the option to turn around and return to Japan, if the weather forecast changes. 
 
 Weeks of delay 
 
 The first attempt to fly over the ocean was cut short after a change in the forecast forced an unscheduled landing. 
 
 And another attempt to take off last Tuesday was cancelled at the last moment because of concerns about the conditions. 
 
 If the pilot succeeds, it will be the longest-duration solo flight in aviation history, as well as the furthest distance flown by a craft that is powered only by the Sun. 
 
 The Pacific crossing is the eighth leg of Solar Impulse's journey around the world. 
 
 But this stage has proven to be the most difficult, and has been hit by weeks of delays. 
 
 Image copyright Reuters Image caption The plane had to make an unscheduled landing in Japan after its first attempt to cross the ocean 
 
 Swiss pilot and Solar Impulse co-founder Andre Borschberg, who is flying the experimental single-seater craft, was initially supposed to begin his journey to Hawaii from Nanjing in China. 
 
 But he spent weeks there, with his ground-support team, waiting for the right flying conditions to present themselves. 
 
 He finally took off on the 31 May, but a deterioration in the forecast a few hours into the mission meant that he had to divert to Japan. 
 
 The rainy season in Nagoya has meant another long wait there - but after the false start last week, meteorologists are now confident they have found a weather window to make the five-day, five-night crossing to Hawaii. 
 
 A spokesperson said that the plane would be heading straight out across the Pacific. 
 
 LEG 1: 9 March. Abu Dhabi (UAE) to Muscat (Oman) - 772km; in 13 hours and 1 minute 
 
 LEG 2: 10 March. Muscat (Oman) to Ahmedabad (India) - 1,593km; in 15 hours and 20 minutes 
 
 LEG 3: 18 March. Ahmedabad (India) to Varanasi (India) - 1,170km; in 13 hours and 15 minutes 
 
 LEG 4: 18 March. Varanasi (India) to Mandalay (Myanmar) - 1,536km; in 13 hours and 29 minutes 
 
 LEG 5: 29 March. Mandalay (Myanmar) to Chongqing (China) - 1,450km; in 20 hours and 29 minutes 
 
 LEG 6: 21 April. Chongqing (China) to Nanjing (China) - 1,241km; in 17 hours and 22 minutes 
 
 LEG 7: 30 May. Nanjing (China) to Nagoya (Japan) - 2,852km; in 44 hours and 9 minutes 
 
 Leg 8: 28 June. Nagoya (Japan) to Kalaeloa, Hawaii (USA) - 8,000km; 120 hours (estimated) 
 
 The experimental craft - which has 17,000 solar cells - is powered only by the Sun. 
 
 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. 
 
 Mr Borschberg has been trained for that eventuality. 
 
 He has a dinghy and enough supplies for several days while he waits for the team to identify a vessel to go pick him up. 
 
 But, of course, the team hopes none of this will be necessary. 
 
 Mr Borschberg's will spend the duration of the flight strapped into his seat in a cockpit that is about the same size as a telephone booth. 
 
 He will only be allowed to take 20-minute cat-naps, but says he will use yoga and meditation to make his journey more comfortable. 
 
 If this flight succeeds, the plane will continue its journey around the world, with Bertrand Piccard taking the controls for the next Pacific crossing from Hawaii to the US mainland. 
 
 The plane will then continue across North America, before attempting to fly over the Atlantic. 
 
 However, the build-up of delays could impact on the later stages. Ideally, the plane needs to cross the Atlantic before August, when the hurricane season reaches its peak. 
 
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– Round two of the Solar Impulse's toughest leg is under way. After the solar plane's first attempt to cross the Pacific was interrupted a month ago, flight engineers spotted what the Japan Times calls a "possible cloudless stretch." That allowed pilot Andre Borschberg to take off from Japan's Nagoya Airfield at 3:04am local time bound for Kalaeloa, Hawaii. The team announced the plane was to depart only an hour before it did so. Borschberg has since "passed the point of no return," meaning he can't return to Japan if he encounters bad weather, according to the project's website, per the BBC. If successful, the leg will be the longest-duration solo flight in aviation history at 120 hours and the furthest distance flown by a strictly-solar aircraft at 4,900 miles. Prior to starting this leg on a trip around the world, the Impulse 2 had flown about 5,000 miles. Co-pilot Bertrand Piccard is next set to fly the plane from Hawaii to Phoenix before the plane eventually arrives on the eastern seaboard. The Impulse 2 will then attempt an Atlantic crossing, though any further delays could seriously hamper the trip. The team optimally needs to traverse that ocean before the peak of hurricane season in August. The plane had been scheduled to take off from Japan last week, but had to cancel at the last moment due to weather. (Discover how Borschberg will cope during the lengthy Pacific flight.)