Asteroid 2012 DA14 is almost certainly not going to hit the Earth next February

[pokerfan]

6. I don't believe it's necessarily as simple as simple proximity

as the object's velocity, which determines its momentum and energy, is also a factor. Gravity extends to infinity so it's not like the Earth suddenly reaches out and grabs an object that just because it's close. A very fast moving object could pass close by and hardly be influenced by the increased gravitational force where as a slower object could certainly be affected but it's more likely that it would manifest itself in the form of a change to its orbit and not getting sucked in.

That said, there is a phenomenon called a gravitational keyhole: A small region of space that could alter the course of a passing asteroid due to a planet's gravity in a way that could cause such an asteroid to collide with that planet on its next orbital pass. The word "keyhole" contrasts the vastness of space with the relatively tiny region through which a body might pass, and is not a reference to its shape.
http://en.wikipedia.org/wiki/Gravitational_keyhole