SALT LAKE CITY — It sounds like a technological advancement ripped from the pages of a sci-fi novel: a plane, powered completely by the sun, making a three-day flight across the Pacific.

“It's a new era. It's not science fiction. It's today,” Swiss explorer and psychiatrist Bertrand Piccard told CNN. He co-pilots such a plane with André Borschberg, a Swiss engineer and former Swiss Air Force pilot.

In 2009, Piccard and Borschberg unveiled Solar Impulse, the first completely solar-powered plane. Solar Impulse made history by flying through the night in 2010, then across the continental U.S. in 2013. During a round-the-world trip in 2015, the lithium-ion batteries overheated and the trip had to be stalled for 10 months — after a record-breaking five-day and five-night journey from Japan to Hawaii. Now, Solar Impulse 2 will continue making that around-the-world flight, promoting clean energy as it travels to Abu Dhabi by late summer.

The scientific wonder has made headlines all over the world as an environmentally-friendly technological advancement to air travel. But are solar planes really the future of flying?

How Does Si2 Work?

Si2 is covered in 17,248 solar cells, which work a lot like a home’s solar cells: they capture energy, store it, and then distribute it. Those solar cells then feed the plane’s four lithium polymer batteries. The batteries allow the plane to fly for longer periods, and it helps the plane fly in the dark. The four electrical motors are completely powered by this renewable energy, without using any fuel.

What Are the Pros and Cons of Solar Flight?

Solar-powered flight is a fascinating possibility. But is it a commercially viable flying option? Let’s consider the pros and cons.

Pros of solar-powered flight:

• Perpetual flight: Solar panels produce their own energy on site, so the plane could, in theory, fly forever in the right weather conditions. Solar Impulse’s longest flight so far was from Japan to Hawaii — a world record at 117 hours and 52 minutes.
• No harmful pollution: Because solar-powered planes don’t need fuel, they emit no harmful exhaust.
• Cost of solar panels would decrease: If solar planes go into mass production, solar panels of all types — even for residential use — would decrease in cost.

Cons of solar-powered flight:- Slow speeds: With a cruising speed of 43 miles per hour, Si2 travels at about the same speed as a car. While it takes a regular jet roughly five hours to travel from Hawaii to California, Si2 took over 62 hours to complete the journey.

• Perfect weather required: Solar-powered planes need practically perfect weather conditions to fly: it needs fair, sunny conditions to power its solar cells. Plus, because of the plane’s light weight, it is sensitive to high winds, and rain can also cause the plane to be grounded. Currently, meteorologists must work closely with the Solar Impulse team to create flight plans.
• Awkward size: The aircraft is large but light. It has a wingspan comparable to a Boeing 747 — 236 feet — but the weight of a Jeep Grand Cherokee. Lighter planes consume less energy to fly, but transporting people requires additional weight.
• Pilot life is difficult: The hours and days spent on a solar-powered plane are rough. The cockpit is tiny, so the pilots cannot stand up, walk, or shower. There is a small hole in their seats for a toilet. Because weight has to be kept to a minimum, they cannot bring any unnecessary items, like razors. Their diets are strictly organized, and the food they consume on the plane is mostly mashed, nutrient-dense food. Pilots train to sleep in short, 20-minute intervals by reclining their chair — only when the weather is clear, of course.

Are Solar-Powered Planes the Future of Travel?

The possible benefits of solar-powered planes are encouraging, but the current conditions of the Si2 prototype are prohibitive for commercial flying, at least for now. Is it, then, paving the way for future solar-powered air travel?

The Swiss Broadcast Corporation says the Si2 technology, as it is, could not replace commercial aviation — and that it’s not meant to. As Piccard said, “Solar Impulse was not built to carry passengers, but to carry messages.” It’s meant to encourage questioning and innovation to spur new solutions that Solar Impulse cannot provide. The route around the world will make several destination stops, where the Solar Impulse team will introduce the plane and promote renewable energy.

Wired magazine reported that mathematically, it was impossible the Solar Impulse could even fly in its 2013 state, based on its reported weight, average speed, and glide ratio. Solar planes, based on these numbers, are not scalable for commercial flight: they simply do not have enough power to carry numerous passengers. Unless there are huge scientific and technological advancements, no one will be flying commercially on a solar-powered plane.

Piccard told EE Times that the main challenge for solar technology is to create lighter planes that don’t need an actual pilot. Large commuter aircrafts, on the other hand, could benefit from further advances in clean energy — including chargeable batteries, which he believes will be a viable option within 10 years.

Solar Impulse has been a landmark event for science, technology, and the world. Though we’re still far away from booking commercial flights in planes powered by the sun, this clean energy still offers abundant opportunities.

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Brooke Nally is native to Utah but likes to see other parts of the world as often as she can. You can contact her at brookelynnally@gmail.com.