Tungsten Rods / Orbital Weapon

[Into Darkness]

I'm an avid cinema goer and I recently watched GI Joe: Retaliation and in that film they had a weapon in orbit which basically dropped large tungsten rods towards the Earth, these rods only had gravity behind them and weren't rocket propelled in any way. One of these rods dropped and hit London and wiped it out.
How realistic would such a weapon be?

What speed would the rod reach in Earth gravity? and would a rod be capable of such devastation?

 

[Asbo Zaprudder]

http://en.wikipedia.org/wiki/Kinetic_bombardment

Jerry Pournelle thought up the concept, but he'd most certainly read the "Doc".

The ones mooted for use by the USAF were 11.5 ton TNT equivalent, but would have a mass of 8 tons. Impact speed was Mach 10. A conventional bomb could do as much damage for less cost as the article points out.

A city rather than bunker buster would have to mass many tens of thousands if not millions of tons -- as the Chelyabinsk meteor demonstrated. It would take project Orion to heft that much stuff around so why no just use the nukes themselves.