[Happy Rain]

Any asteroid,comet or meteor that comes close enough to a large mass (like a planet) to have it's trajectory altered by the large mass’s gravitational influence—can be slung hurtling towards the earth.

Such a body will be undetectable as it is coming directly towards us and when it closes enough to present a discernible image to our telescopes,it will be too late to do anything about it.

Like in war—the bullet with your name on it you never see coming.

[Lonesome in Massachussets]

Now that this pup has been detected, it will be carefully tracked for the foreseeable future. We should have several decades of warning before it ever becomes a threat. 18 Million miles is a long way from Earth and the effects of earth on its orbit will be quite minor. 99942 Apophis will pass within 5.6 earth radii in 2029, about 22,000 miles or 800 times closer than 18,000,000. Since there is nothing massive that passes anything like that close to this asteroid, its orbit should be quite easy to predict for centuries into the future.