A solar-powered plane travelling round the world is about to take on its most challenging leg: the Pacific Ocean.

Solar Impulse 2 has so far made it from Abu Dhabi in the Middle East to Nanjing in Eastern China over six legs, all without using fossil-derived fuels. But its next leg, from Nanjing to Hawaii, will take at least five days without landing; the longest leg to date was just 20 hours.

As we covered earlier this year, the plane has a 72-meter wingspan — making it wider than a 747 — housing 17,000 solar cells. These power four electric propellers as well as charging four batteries for nighttime use.

On paper there’s no reason why the plane can’t make the next leg as in theory it could run virtually continuously at this time of year, given the sunlight hours. In practice, both the weather and human frailty could pose challenges.

Excessive cloud could  mean the plane doesn’t pick up enough charge during the day to power it throughout the night. Mission staff believe the plane needs to be at around 93 percent charge at the start of the night to be certain of getting through to daylight again. The big problem there is that for a five day journey, there’s only so much reliance you can place in even expert meteorological forecasts.

The other possible challenge is that the pilot for this leg, Andre Borschberg, will need to be strapped to his seat for the entire five days. He plans to only ever sleep for 20 minutes at a time, during which vibrating armbands will wake him if the plane heads off course and requires immediate attention.

If the plane runs into difficulties anywhere other than the start or end of the route, the plan is for Borschberg to jump out with a dinghy, leaving the plane to ditch unmanned. The BBC explains that attempting to control the plane’s descent into the water would be too risky because of the possibility of electrocution.

Assuming Solar Impulse 2 makes it to Hawaii, fellow pilot Bertrand Piccard will take over for a four-day leg to Phoenix. After two shorter hops in the US, the challenge is set to conclude with two more five-day legs running from New York to Abu Dhabi with a break somewhere in Europe or North Africa, though the precise itinerary has yet to be set.