Solar Impulse 2 began the continental American leg of its round-the-world flight on Monday morning — part of an attempt to become the first solar-powered plane to circumnavigate the globe.

Swiss pilots Bertrand Piccard and Andre Borschberg have been taking turns flying the plane since it took off from Abu Dhabi last spring.

The Phoenix-bound flight took off from Moffett Airfield in Mountain View, California. The plane has a huge wingspan, stretching wider than that of a Boeing 747. The top of the wings are equipped with 17,000 solar cells that power propellers and charge batteries onboard the aircraft.

At night, the plane runs on stored energy.

The technology behind the solar-powered plane is groundbreaking, and its creators hope it will be a global example of the power of alternate and sustainable energy sources.

According to a mission statement on the Solar Impulse 2 website:

"The two record-breaking solo flights of André Borschberg from Nagoya to Hawaii and Bertrand Piccard from Hawaii to San Francisco give a clear message: everybody could use the same technologies on the ground to halve our world’s energy consumption, save natural resources and improve our quality of life."

Borschberg tweeted video of his final preparations before the flight took off on its 16-hour journey to Phoenix. He was also at the controls during the longest stretch of the journey so far: last summer's five-day trip from Japan to Hawaii. Borschberg said he did yoga and meditated to keep himself alert.

This sunshine reminds me of the good old days when I relied on #Si2 solar power for 117 hours last year pic.twitter.com/vcee7tKxp6 — André Borschberg (@andreborschberg) May 2, 2016

"We have demonstrated it is feasible to fly many days, many nights, that the technology works," said Borschberg at the time.

The Solar Impulse 2 will make two more stops in the United States after Phoenix, before crossing the Atlantic Ocean to Europe or northern Africa, according to the team's website.

Performing the final checks of #Si2's electrical systems with the #Monaco Mission Control Center before takeoff pic.twitter.com/pajP2kzsqA — André Borschberg (@andreborschberg) May 2, 2016

The plane restarted its round-the-world journey last month, flying from Hawaii to California in a risky, three-day, nonstop flight.

The flight was initially going well last year, making stops in Oman, Myanmar, China, Japan and Hawaii. But it was unexpectedly halted last summer due to technical difficulties. When the plane landed in Hawaii in July, it was forced to stay in the islands after the plane's battery system sustained heat damage on its trip from Japan. The team was delayed in Asia, as well.

When first attempting to fly from Nanjing, China to Hawaii, the crew had to divert to Japan because of unfavorable weather and a damaged wing.

Additional information from the Associated Press.