Solar Impulse 2 begins world’s longest flight: Six days, five nights non-stop

5100 miles from China to Hawaii, powered entirely by lithium-ion batteries and the sun.

by Sebastian Anthony

Solar Impulse 2, which is attempting the world's first solar-powered circumnavigation of Earth, has begun the longest leg of its journey: a single, non-stop flight of about 5078 miles (8172km) from China to Hawaii. The plane, and pilot André Borschberg, will be aloft for six days and five nights, with Borschberg attempting to stay awake for much of that time.
The solar-powered aircraft, which has a larger wingspan than a 747, began its round-the-world trip in March. It departed from Abu Dhabi in the United Arab Emirates, headed down the Persian Gulf to Oman, hopped over to India, then Myanmar, and finally into Nanjing in China at the end of April. You can watch the current flight live on the Solar Impulse website. (The website is pretty cool; you should check it out.)
This flight, from Nanjing to Hawaii, is the first true test of Solar Impulse's capabilities, and Borschberg's endurance. The previous six legs were all fairly short hops; this, the seventh leg, will be longer than the previous six flights combined. The flight to Hawaii will finally showcase whether a solar airplane can reliably stay aloft at night, with lithium-ion batteries providing all of the necessary juice.

The Solar Impulse website shows the current state of the batteries (pictured above). At the time of publishing, the sun has just sunk below the horizon, and the batteries are starting to discharge. The plane is outfitted with four 41 kWh lithium-ion batteries, each capable of providing 17.4 horsepower (13 kW) to four electric motors. During the day, 269.5 square meters of photovoltaic solar panels on the topside of the aircraft provide the electricity needed to drive the motors, while also charging the batteries in preparation for the evening.

The plane's cruise speed is around 49 knots (90kph) during the day, and a rather placid 33 knots (60kph) at night to save power—which is why it takes so long to fly the 5100 miles from China to Hawaii. If Solar Impulse 2 is successful in reaching Hawaii, it will be the longest flight (in duration) ever for a single-pilot airplane