Asteroid 2003 QQ47 is about a kilometer wide and the impact on March 21, 2014 would have the effect of 20 Million Hiroshima atomic bombs. But we’re not sure it will hit at all.
The margin of error is still quite large because it has only been under observation for a week or so. Astronomers will be tracking QQ47 over the coming months to more accurately predict its trajectory.
As it stands, the chances of it hitting earth are 1 in 909,000, and those chances are expected to get smaller as more information is gathered.
Further observations have confirmed that QQ47 will not hit Earth. You are free to go about your lives. Carry on.