Imagine an airplane without fuel and powered only by the rays of the sun as it flies across the world's largest body of water. Seems unbelievable? Well, better believe it, because this airplane already exists.

A single-seater and solar-powered plane called Solar Impulse 2 took off from Nanjing, China, on Sunday with a mission to cross the expansive Pacific Ocean and reach Hawaii.

This ambitious journey will cover a total of 4,971 miles in a span of five days and five nights.

Raymond Clerc, director of Solar Impulse 2's Pacific mission, said the solar-powered aircraft will fly inside South Korean and Japanese airspace before heading out across the ocean.

Two co-pilots will be alternately maneuvering Solar Impulse 2: former fighter pilot Andre Borschberg and adventurer Bertrand Piccard.

In his Twitter account, Borschberg could not contain his excitement about Solar Impulse 2's Pacific mission after the flight was pushed backs several times due to unfavourable weather over the ocean.

"So great to consider tomorrow, Sunday, a go for this fabulous flight to Hawaii to prove what @bertrandpiccard dreamed about many years ago," Borschberg said in a tweet, calling the Pacific mission the "flight of my life."

Borschberg was also the one who piloted Solar Impulse 2 in the first-leg of its flight across the world. The leg saw the solar-powered plan flying from Abu Dhabi to Oman.

Piccard, for his part, pointed out how historic Solar Impulse's Pacific flight will be.

"Solar Impulse must accomplish what no other airplane has achieved in the history of aviation," he said in a separate tweet.

Piccard is also scheduled to lead the Atlantic leg of Solar Impulse's journey later this year.

The flight was packed with food, water and sports drinks enough to sustain the pilot for a week. It also has oxygen bottles, a parachute and a life raft for emergency situations.