On Friday, around 2:24pm EST, an asteroid called 2012 DA14 will pass within about 17,200 miles of Earth's surface, closer than the communication satellites that will be broadcasting the news of its arrival.
The asteroid is about 150 feet in diameter and has a mass estimated at about 143,000 tons.
While there is a zero percent chance of an earth impact, there is a very small possibility that it could collide with one of more than 100 telecommunication and weather satellites in fixed orbits above the Earth.
Should an object of that size ever hit Earth, it would cause a blast with the energy equivalent of about 2.4 million tons (or 2.4 megatons) of TNT explosives, more than 180 times the power of the atomic blast that leveled Hiroshima.
Scientists have a good idea of what the effect of such an impact would be because a similar sized meteor devastated a remote region of Siberia in 1908: http://en.wikipedia.org/wiki/Tunguska_event
Even though this asteroid is half the size of a football field and passes very close to earth, it will be very difficult to see without a telescope. If you have one, the best viewing will be between 7pm - 10pm.
NASA will be broadcasting the flyby live at NASA's Marshall Space Flight Center in Huntsville, AL here: http://www.ustream.tv/channel/nasa-msfc