To many people, the thought of pilotless vehicles whizzing overhead delivering everything from clothes to groceries to skateboards seems like something out of a science fiction novel. But some of the world's biggest e-commerce companies are in a fierce race for early control of the drone delivery market, especially in the United States. They are spending hundreds of millions of dollars to design and develop drone delivery systems that are cost-effective, energy-efficient, environmentally-friendly, unobtrusive, and most of all, safe to operate.