In a mission to promote clean energy, the entirely solar-powered Solar Impulse 2 plane arrived in San Francisco Friday and took a couple hours to circle around the city and get some photo opportunities over the Golden Gate Bridge. As ABC 7 reports, the plane is being piloted by Swiss explorer Bertrand Piccard and another man, and they're actually halfway through an attempt to circumnavigate the entire globe without the use of a single ounce of fuel.

The plane's batteries store solar energy to keep it running at night, and its cruising speed is only 28 miles per hour — thus the trip from Hawaii to here took three days. It can, as the Chron explains, pick up some speed in the middle of the day when the sun's rays are at their strongest.

Piccard and co-pilot André Borschberg took off on their journey from Abu Dhabi on March 9, 2015 and since then have made stops in Oman, India, Myanmar, China, Japan, Hawaii, and now here where they landed at Moffett Field in Silicon Valley last night.

Piccard said the stop in Silicon Valley was meant symbolically to link the Impulse 2's journey to the spirit of innovation here.

Reportedly the pilots stay awake when flying over populated areas, but over oceans they take multiple 20-minute naps per day. This must mean that Piccard is running on fumes himself, having not gotten a good night's sleep since early March, but perhaps he got one last night after landing.

From here, the plane will make a stop at an undisclosed location in the central US, and then again in New York before crossing the Atlantic.