Solar Impulse 2 took to the skies once again early Friday morning for the first time since July.

The solar powered aircraft, which launched its round-the-world journey in Abu Dhabi last March and captured global attention, took off from Hawaii at 6:15 a.m. local time. Grounded last July for repairs, the Solar Impulse team decided to continue the plane's epic journey as a fitting tribute for Earth Day.

Using only solar energy for power and manned by only one pilot, the aircraft is expected to land in California on April 23. The relatively slow speed of the aircraft and the long journey present unique challenges, as the pilot must remain alert for extended periods of time without sleep, reportedly relying on yoga and self-hypnosis.

Swiss natives Bertrand Piccard and André Borschberg are the founders and the pilots of Solar Impulse. The two men have been alternating piloting the aircraft each leg of the journey. The current leg is expected to take about 62 hours with Piccard in the cockpit.

Talking about flying the aircraft, Piccard said, "Every morning you have the suspense of knowing how much energy is left in your batteries. Then, with the sunrise comes the virtuous circle of perpetual flight."

While this isn’t the first solar plane, nor the first around-the-world journey by a plane, it is the first combination of the two. The plane carries no fuel and its 17,000 solar cells are the only source of energy for the four engines. The solar cells charge 633 kilograms of lithium batteries, which allows night-time flying.

Originally, the aircraft's epic journey was scheduled to end last July. However, due to unpredictable weather conditions and technical issues, the various legs of the journey were plagued with delays. Upon reaching Hawaii, the team announced that the aircraft would be grounded until the next spring as repairs were necessary.

However, the plane's journey from Japan to Hawaii set two new records. Not only did the flight break the world record for distance and duration for solar aviation, but also for the longest solo flight ever. In total the flight lasted lasted 117 hours and 52 minutes with Borschberg in the cockpit.

According to the BBC, the record-breaking flight was a controversial one for the Solar Impulse team. A system that monitors the plane's performance while the pilot rests quit working during the leg of the journey. Ground crew members wanted Borschberg to abort the journey and return to Japan, but he insisted on continuing.

While several team members threatened to resign after Borschberg ignored their advice, the team made it through the hurdle and is now set to finish the journey that began more than a year ago. Although Solar Impulse has already come a long way and broken records, the voyage is far from over.