It's all coming down to this. Well over a year after beginning its round-the-world trip, Solar Impulse 2 has embarked on the final leg of its journey. The solar-powered aircraft left Cairo early on the morning of July 24th and should reach its original starting point, Abu Dhabi, within 2 to 3 days. This certainly isn't the most arduous part of the adventure (the Pacific crossing was far more challenging). However, it'll likely be the one that everybody remembers -- it'll be the definitive proof that clean energy can be used to accomplish impressive feats.