In 2016, Solar Impulse 2, a solar-powered all-electric plane, completed an epic round-the-world flight, a landmark in the long pursuit of an alternative to fossil fuel-powered aviation. NOVA looks at the array of promising new electric planes on the horizon and takes you for a ride in some impressive prototypes that are already in the air-such as Burt Rutan's Long-ESA that can fly at over 200 mph and climb to 3,000 feet in under six minutes. Boeing, Airbus, Raytheon and other major companies are currently racing to develop practical short-haul passenger planes that promise to slash the operating costs and emissions of regular flights. But first, significant hurdles of battery weight, energy storage and cooling must be overcome. How long will it be before the dream of super-quiet, super-efficient airliners becomes a reality?