Put it this way: It's been calculated that it took a world roughly the mass of Mars striking the protoearth to create the Moon. To be sure, you wouldn't have wanted to be anywhere around at the time (assuming you went way, way back in your time machine) but even that just shaved a bit off the top of the planet.

Of course, there's nothing like that around these days that could hit Earth (sorry, bmpbmp) and if you want to do a good job shattering the world, it would take something either larger or faster. It takes a lot of energy to disrupt a planet the mass of Earth.

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