Scientists located an asteroid with a similar orbit to Earth in 2004. Since then, they have been monitoring it to try and work out its precise size which is estimated to be between 270 and 325 meters in diameter. Thankfully, the Russians are planning to send a robotic mission to the asteroid to plant a radio transmitter on it. That will give us more precise information about its path. It won’t prevent its collision with our planet though which I imagine will be pretty dramatic.
The latest prediction is that Aphophis (named after the Egyptian god of destruction) will hit our planet in 2036. I’ll be 89 by then so maybe it won’t matter much to me!