I've just replied to your other other post! You need a telescope rather than a lens for that or at least a 300mm+ high specification lens. Something like the Canon 100-400 L, Sigma 50-500 or a prime.
EDIT: I forgot to add, very clear sky as well.
[Edited 2012-01-05 07:49:23]
Equipment: 2x Canon EOS 50D; Sigma 10-20 EX DC HSM, 50-500 EX APO DG, Canon 24-105 f/4 L, Speedlite 430EX