On February 4, an asteroid called 2002 AJ129 is due to slip past Earth. It is between 1600 and 4000 feet across, according to NASA's Center for Near Earth Object Studies, but there's no chance it will make impact—NASA has calculated it will remain 2.6 million miles away. That still makes it what astronomers call a "potentially hazardous asteroid," thanks to its size being more than about 500 feet across and an orbital path that carries it within about 4,650,000 miles of Earth. But while they're confident we won't all go the way of the dinosaurs, scientists do want to keep an eye on the...