The team that's attempting to make the first solar-powered flight around the world completed another milestone Sunday, when its craft landed at Moffett Airfield in California's San Francisco Bay area. Having taken off from Hawaii, this was the ninth leg of the trip and represented the team's first return to the air since last July, after weather and repairs to its plane led to a delay in the project.

At the helm of the Solar Impulse 2 on this leg was Swiss adventurer Bertrand Piccard. The solo pilot had taken off from Kalaeloa, Hawaii, shortly after dawn on Thursday, April 21, completing a journey of 2,200 nautical miles (4,000 kilometers).

The experimental craft gets all its energy needs from the 17,000 solar panels lining its top, meaning it requires no other fuel. The cells power propellers and charge up batteries that are used for flying during the night. (Learn more about the project.)

Route of the Si2 The completed and planned legs of the around-the-world flight of the Solar Impulse 2, which resumed its journey after a nine-month hiatus

The team had been waiting for favorable weather, since the plane is highly sensitive to winds. A high pressure system previously made the Pacific crossing possible.

On Earth Day, April 22, Piccard spoke with United Nations Secretary-General Ban Ki-moon from the cockpit, after 175 nations signed the Paris Agreement on Climate Change.

“If an airplane like Solar Impulse 2 can fly day and night without fuel, the world can be much cleaner,” Piccard said.

Piccard has been alternating times at the controls with Swiss pilot André Borschberg. After several aborted attempts, Borschberg brought the solar plane into Hawaii last July, finishing the most dangerous leg of the whole journey.

That trip earned a record for the longest non-stop, solo airplane flight ever. The effort caused damage to the craft's batteries, leading to lengthy repairs in Hawaii.

On that grueling 116-hour voyage, the Solar Impulse crossed more than 4,000 miles (6,400 kilometers) of ocean, the same desolate region where Amelia Earhart disappeared 78 years ago. Borschberg battled a malfunctioning warning system and turbulence and cold fronts, which buffeted the lightweight plane. (Read the secrets of surviving the flights.)

The pilots must also battle exhaustion, because they can only get about three hours of rest per day, broken up into 20-minute sessions while the craft is flown by autopilot.

Yet their accomplishments prove that “energy efficiency, solar power, and modern technology can achieve the impossible,” Piccard says.

The Solar Impulse 2 first took off from Abu Dhabi in March 2015 and is scheduled to complete its voyage there in the coming months.