Q:

Falling metal cylinders more devastating than a nuke?

I just saw G.I JOE - Retaliation. In the movie the bad guys (spoiler alert) 

 destroy London by dropping a large metal cylinder from space onto the city.

Could this device cause more destruction than a nuclear weapon? If so, how? 

A:

Also known as Rods From God, the idea may be somewhat practical in the far future, although they are significantly weaker than nuclear arms. For example, with rods situated at an orbital radius of 500 miles (twice that of the International Space Station), the energy density becomes (in Mathematica):
<< PhysicalConstants`
Convert[GravitationalConstant EarthMass (1/EarthRadius - 1/(
    EarthRadius + 500 Mile)), Joule/Gram]

(7003.49 Joule)/Gram

For comparison, the energy density of gasoline is 45,000 Joule/Gram, and most conventional nitrate explosives clock in at around 4,000 to 10,000 Joule/Gram.
At larger radii (10,000 miles) the energy becomes more significant, but still comparatively small:
Convert[GravitationalConstant EarthMass (1/EarthRadius - 1/(
    EarthRadius + 10000 Mile)), Joule/Gram]

(44771.9 Joule)/Gram

