How scientists find the big asteroids that can threaten Earth

When you're fast asleep at night, telescopes atop lofty mountains continually sleuth out unknown space rocks that might fly close to Earth, or even potentially hit us. Congress directs NASA to find and track the asteroids and comets that swoop into our cosmic neighborhood, meaning some 30 million miles from Earth's orbit around the sun. They're aptly named "near-Earth objects," or NEOs, and thousands of sizable ones are thought to remain undiscovered. Specialized telescopes in Hawaii, Arizona, a...