An asteroid about as big as a football stadium is headed toward Earth -- and it will be here by the end of the week. That's according to NASA officials, who have been tracking the massive rock as it hurtles through space. The asteroid, 2002 NN4, measures approximately 1,100 feet wide -- which is about the size of a football stadium, officials say. And it's expected to come uncomfortably close to the planet within the next few days, NASA reports. But before you start digging a bomb shelter in your backyard, it should be noted that NASA's definition of "close" differs than the one most people are familiar with. In this case, "close" means 3.16 million miles, which will be the distance between 2002 NN4 and Earth as it flies by, officials say. In fact, it's among three asteroids that are expected to do fly-bys in the next few days.