When a little drone built with the backing of Facebook founder Mark Zuckerberg touched down in May at Yuma Proving Ground, an Army testing facility in southwestern Arizona, no one hardly heard it. It didn’t even make the news until two months later.

But that drone, and a fleet of others like it, may one day fly around the world bringing internet connectivity to about 4 billion people who currently live without it. The billionaire has dubbed the project “Aquila.”

The solar powered “planes” will use lasers accurate enough to “hit a dime more than 11 miles away while in motion” to “transfer data 10 times faster than existing systems,” Zuckerberg said in an article on Facebook last July.

That article was written after the first Aquila test flight, which took place on June 28, 2016, ending in a crash landing. In the post, Zuckerberg lists a number of unique engineering quandaries with which the team behind Aquila has been confronted.

Perhaps the greatest of those challenges arise from the necessity that Aquila planes stay in the air for months at a time, so that they can provide consistent and reliable internet coverage. If Aquila is successful in remaining airborne for such a timespan, it will shatter the record for the longest unmanned aircraft flight, Zuckerberg says.

In order to stay in flight so long, Aquila planes must be as light as possible. Therefore, the aircraft are constructed from carbon fiber composite. They weigh less than 1,000 pounds—“about as much as a grand piano,” Zuckerberg says—despite a wingspan exceeding that of a Boeing 747.

Zuckerberg and his team have ironed out enough of the kinks to get Aquila off the ground, in the air, and back down safely. The drone tested this May took off successfully, rose to an altitude of 9,000 feet (ultimately, Zuckerberg envisions the planes operating at 60,000 feet), flew for about an hour and 45 minutes, and finally landed in the hot, desert sand, still in one piece.

The engineering team, most of whom monitored the flight using a feed from a camera mounted on a helicopter following the drone, was pleased with the progress.

“The improvements we implemented based on Aquila’s performance during its first test flight made a significant difference in this flight,” said Martin Luis Gomez, director of aeronautical platforms at Facebook, Inc. (you thought it was just a social networking site).

Those improvements include more sensors and a new design intended to help the craft land better.

The drones currently need about 5,000 watts of power (a mere three hairdryers’ worth), but they must save enough power during the day to keep themselves aloft throughout the night. A typical solar panel measures 39 x 65 inches and produces 265 watts of power, according to solarpowerrocks.com. Aquila would need about 19 such panels if the sun shined 24 hours a day, but because the planes must store power, they will need significantly more panels. Zuckerberg does not offer much detail as to how his drones’ solar panels are configured.

On-board batteries, which account for almost half of the craft’s weight, will provide additional power; it is not clear exactly how much.

Naturally, Zuckerberg and his team would like to reduce Aquila’s power usage as much as possible. The aircraft’s top speed is about 80 mph, because for Aquila, energy efficiency is far more important than speed.

Aquila is the stuff of science fiction: an aircraft that weighs about as much as a horse flying around at 60,000 feet transmitting data via lasers. And although the craft that took off and landed at Yuma Proving Ground in May must have been quite real, much of Aquila remains fiction.

Zuckerberg, who knows a thing or two about realizing a dream, expects Aquila to encounter its share of unforeseen obstacles.

“No one has ever built an unmanned airplane that will fly for months at a time,” he points out, “so we need to tune every detail to get this right.”