WASHINGTON (Reuters) - Aerospace and defense company Northrop Grumman is working with SpaceX, the space venture of billionaire entrepreneur Elon Musk, on a classified spy satellite project already capturing high-resolution imagery of the Earth, according to people familiar with the program.

The program, details of which were first reported by Reuters last month, is meant to enhance the U.S. government's ability to track military and intelligence targets from low-Earth orbits, providing high-resolution imagery of a kind that had traditionally been captured mostly by drones and reconnaissance aircraft.