There's no need to panic; NASA and FEMA have a plan to respond to a potential asteroid collision [space.com]:
It's a scary scenario: an asteroid headed for Earth, just four years away from slamming into our home planet. It may be too short a span to plan an asteroid-deflection mission [space.com], but it's long enough to present very different challenges from those of a more typical crisis, like a hurricane or earthquake.
NASA and the Federal Emergency Management Agency (FEMA) came together Oct. 25 to plan a response to such a hypothetical event. In a "tabletop exercise," a kind of ongoing simulation, the two agencies tested how they would work together to evaluate the threat, prevent panic and protect as many people as possible from the deadly collision.
"It's not a matter of if, but when, we will deal with such a situation," Thomas Zurbuchen, NASA's Science Mission Directorate's new associate administrator [space.com], said in a statement. "But unlike any other time in our history, we now have the ability to respond to an impact threat through continued observations, predictions, response planning and mitigation."
Also at JPL [nasa.gov].