Under pressure, plane industry vows cleaner flight—someday

Battery-powered planes, solar planes, hydrogen planes — jet makers are working on myriad ways to make flying less damaging to the planet. Yet clean flying on a mass scale remains decades away. That's despite growing pressure from regulators and from a blossoming environmental movement to shun air travel altogether. The problem, argue aviation powerhouses gathered this week at the Paris Air Show, is that growing world populations and economies mean that the number of planes in the sky could double in the next 20 years. And today's clean-aviation technologies aren't ready to keep up. Story covers some options already out there, and what's on the horizon.<br/>