now there's a global race to get clean aviation off the ground. and some companies think they're close to cracking it. pretty soon, passengers will be flying in zero—emission aircraft. but how did we get here? archive: the new machine is called, optimistically, - the flyer. we've come a long way since the wright brothers' first flight in 1903. that day, they lifted the world into a new dimension. then we achieved bigger, faster planes. great, but that's also how we ended up with all these emissions. jet engines burn kerosene, which releases carbon dioxide, one of the main greenhouse gases causing climate change. the industry faces an enormous challenge to clean up its act, and that means coming