In honour of those who asked for some calculations:
The planet Mercury has a mass of 238,500,000,000,000,000,000,000 kg.
To accelerate it to 1 km/h would require a force of 3,975,000,000,000,000,000,000,000 Newtons, applied for one hour.
If Mercury was at its closest (77,000,000 km) it would then take more than 8,789 years to traverse the distance from its current orbit, to Earth's orbit. If the relative positions of Earth & Mercury were at their furthest extent (222,000,000 km) it would take more than 25,342 years to get to the Earth's orbit.
Ignoring the fact that the Earth is also most likely to be in another position when Mercury would cross its orbit (Earth's diameter is approx 12,756 km but the diameter of the orbit of the Earth is about 300,000,000 km, not to mention the likelihood that Mercury may not stay on the orbital plane of the Earth and because Mercury's diameter is only a small fraction of the diameter of the Earth, Mercury's diameter can be ignored), the whole premise in the OP is ludicrous.
The numbers are extremely huge and the likelihood therefore inversely infinitessimal.
edit on 12/2/2013 by chr0naut because: I made a oops!