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. 
 
 Follow Rebecca on Twitter. ||||| Editor's Note: Four weeks after the original planned departure, Solar Impulse 2 took off from Nanjing on Saturday, May 30, bound for Honolulu. A dangerous cold front forced pilot André Borschberg to land at Japan's Nagoya Airfield, where the team will wait for better weather conditions to resume the journey across the Pacific. 
 
 On Saturday, a single-seat, solar-powered plane with a wingspan longer than that of a Boeing 747 took off from Nanjing, China, headed for Honolulu. For a normal passenger jet, that's about a 12-hour flight. Solar Impulse 2 , the 5,000-pound plane powered by nothing but sunshine, was supposed to take five days, before the a nasty cold front forced it to abort the flight and land in Japan.^1^ 
 
 This is by far the hardest part of the plane's journey around the world, which started in Abu Dhabi last month, and should finish there in August. Swiss pilots Bertrand Piccard and André Borschberg have been working up to this for 12 years, and they're fully aware of how trying it will be. 
 
 It's very interesting how the human mind can adapt to this type of new situation. Pilot Bertrand Piccard 
 
 "If we are optimistic, we will say that we've done six legs out of 12," Piccard says. "And if we are pessimistic, we will say have have traveled 8,000 kilometers out of 35,000." 
 
 Which is to say, things are going according to plan, but there's much left to do, including the flight to Honolulu, then a 3,000-mile leap over the rest of the Pacific to Phoenix, Arizona. 
 
 The trick to staying aloft for days at a time is straightforward. The solar panels that cover the wings and fuselage of Solar Impulse 2 charge four extra-efficient batteries, which power the 17.4-horsepower motors. You charge up when the sun's out and cruise at up to 28,000 feet. At night, you drop to about 5,000 feet, converting altitude into distance. 
 
 There are two factors that make the Pacific crossing especially challenging: The ocean's size and the pokey speed of the plane (try 20 to 90 mph) mean each pilot will need to spend four or five days and nights aloft to reach land, in a cockpit that resembles a tube hotel in miniature. The second problem is the weather: Solar Impulse 2 needs pretty specific conditions to takeoff, cruise, and land—and that all needs to be planned out five days in advance. 
 
 Borschberg is scheduled to take off from Nanjin on May 7, at the earliest. If the flight goes as expected, he will take five days to make the trip to Honolulu. Then Piccard will make the four-day trip to Phoenix. 
 
 André Borschberg has spent 72 hours at a time in a simulator to prepare for this flight. Niels Ackermann/Rezo.ch/Solar Impulse 
 
 Pilot Preparation 
 
 It doesn't sound like much fun: There's no walking around, or even standing up in, the 135-cubic foot cockpit. The cabin is neither heated nor pressurized, though it is insulated. 
 
 To get used to the cramped conditions, the pilots have spent long stretches in a simulator. They use meditation, breathing exercises, and whatever yoga they can manage to keep their bodies and minds feeling as fresh as possible. 
 
 Piccard and Borschberg will sleep in 20 minute stretches (the aircraft has autopilot functions and there's not much to collide with over the Pacific), six to eight times a day. It's hardly a good night's rest, but it's enough to get by, and the seat fully reclines. They have an alarm set to wake them up, but their bodies have gotten used to the routine, and don't really need it, Piccard says. "It's very interesting how the human mind can adapt to this type of new situation." 
 
 The grub sounds pretty good, especially for air travel: Nestlé made special meals that can survive temperatures from -4 to 95 degrees Fahrenheit. There's mushroom risotto, chicken with rice, and potatoes with cream and cheese. "It's very nice," Piccard says. The toilet, FYI, is built into the seat. 
 
 It may seem difficult to stay focused when you've seen nothing but ocean for days on end, but it's not a major concern to the pilots. "There's quite a lot of things to do," Borschberg says. More importantly, they've been working up to this for more than a decade. They're jazzed. 
 
 Of course, they've got to be ready for things to go wrong. In the event of a sudden catastrophe, like an engine or battery fire, they've trained for bailing. If cloudy weather stops the panels from charging the batteries adequately, Piccard and Borschberg will take their time putting on the dry suit, preparing the parachute, alerting mission control, and switching on an emergency beacon. "You get out very peacefully," Piccard says. 
 
 They key in any situation is getting away from the plane—there's a serious risk of electrocution when you fly a pile of electronics and batteries into the ocean. Then you settle into your life raft, because you're thousands of miles from land, and major shipping lanes, it may take two or three days to be picked up. 
 
 The cockpit of Solar Impulse 2 resembles a tube hotel, in miniature. Solar Impulse/Pizzolante 
 
 The Weather Game 
 
 There's a lot to take into account, says team meteorologist Luc Trullemans. Routes are decided using radar and satellite data, and flight simulations. Equipment from engineering consultancy Altran and a team mathematician lend a hand. Cloudy skies mean the solar panels can't recharge the batteries. Wind conditions are crucial: Tailwinds are best, and the team will tolerate cross tailwinds up to 45 degrees (90 degrees would be blow fully sideways). Headwinds are a problem for a plane with limited power: Between Myanmar and Chongqing, China, Piccard found himself flying backwards at one point. 
 
 The six legs of the Solar Impulse 2 journey already completed have been relatively short affairs, between 15 and 20 hours. That's not so tricky to plan, because at the time of takeoff, you have an excellent idea of what the weather will look like for the whole flight. 
 
 All that gets way more difficult now, because the team can't predict the weather with the accuracy it wants more than three days in advance. "We must be 100 percent certain with our weather forecasts for the first three days of the flight and of course, the takeoff conditions," says Trullemans. After that, the route can always be changed ... on the fly ... but major deviations are best avoided. The plan is to keep a sharp eye on coming conditions. The pilot can fly to the north or south to avoid a cold front, for example. 
 
 Beyond that, you hope for the best: Tailwinds, clear skies, and no need to inflate that life raft. 
 
 ^1^Story updated Monday, June 1 at 12:25 EST to include Solar Impulse 2′ s early landing in Japan.
– 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.)