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Solar Impulse landing
The Solar Impulse 2 plane lands in Tulsa, Okla.,, after an 18-hour flight. (Credit: Solar Impulse)

After crossing the Himalayas and the Pacific, the fuel-free Solar Impulse 2 plane overcame the Rockies on Thursday during the Arizona-to-Oklahoma leg of its round-the-world odyssey.

“As you can imagine, flying over the Rocky Mountains is a challenge for an aircraft like Si2,” the Solar Impulse team said in a blog post. “But perhaps not for the reasons you would expect.”

The altitude wasn’t the biggest concern, although pilot Bertrand Piccard used an oxygen mask to cope with altitudes ranging up to 22,000 feet. Rather, it was the weather. Solar Impulse 2 is designed to soak up enough sunlight during the day to keep flying during the night, but it doesn’t do well during cloudy and stormy weather. That’s just the sort of weather that tends to build up during this time of year in the Rockies.

Thursday provided a window of opportunity for Piccard to make his way over the mountains in northern New Mexico and head eastward. Until this week, the plan was to stop over in Kansas City, Mo., but the Solar Impulse team said “we had to find a different solution” due to difficult weather conditions over the Kansas plains. So Piccard targeted Oklahoma’s Tulsa International Airport instead.

The flight plan required an early start from Phoenix Goodyear Airport, at 3:05 a.m. MST/PDT. Piccard landed in Tulsa more than 18 hours later, at 11:15 p.m. CT.

“Good morning, Tulsa!” he told the crowd that turned out to greet him.

Piccard could have made the 975-mile trip in a little over two hours on a commercial jet, but that’s not the point. Instead, the $150 million, Swiss-led Solar Impulse project is aimed at demonstrating how advanced technologies can power a trip around the world without using a single drop of jet fuel.

Thanks to composite materials, the plane has a wingspan that’s wider than a 747’s, but it weighs only about as much as an SUV (5,000 pounds). More than 17,000 solar cells feed electricity to 1,400 pounds of lithium polymer batteries, to power four scooter-sized electric motors.

The plane started out from Abu Dhabi in March 2015 and made stops in Oman, India, Myanmar, China and Japan. The five-day Pacific crossing to Hawaii overtaxed the batteries so much that the team needed nine months to make repairs and wait for favorable weather. Last month, the odyssey resumed with a 2.5-day flight from Hawaii to California, and then a daylong hop to Phoenix.

The round-the-world flight is a follow-up to Solar Impulse 1’s two-month-long trip across America in 2013.

Piccard and Solar Impulse’s other founder, Andre Borschberg, have been alternating stints in the plane’s solo cockpit. This was the 11th leg of the trip, and future flights are expected to touch down in locales that include New York and southern Europe. “Our goal now is to reach New York as soon as possible in order to have enough time to find a clear weather window to cross the Atlantic,” the Solar Impulse team said.

The 22,000-mile journey should wind up in Abu Dhabi later this year.

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