why can't satellites be launched through a super gun rather than using slower rockets?

Answer 1

It's not that it couldn't. It's just not proactical:
(1) The impulse a "super cannon" would impart on a satellite would likely destroy the vehicle.
(2) The recoil force would need to be managed so as not to destroy the "super cannon"
(3) The heat generated would likely destroy the satellite, and cannon.

Answer 2

Gerald Vincent Bull was working on that problem in the 1960's and 1970's, with the HARP project, among others.
One of the restriction is that a gun would impart an acceleration of several 1000's of g to the payload, and that the satellite would have to be made extremely strong, therefore very heavy, for the intended mission. That could be OK for military charge, but not so good for scientific or communication payload.
It is actually a lot cheaper to use rockets, which are more gentle with their accelerations.

Answer 3

The problem with railguns for launching satellites is that the acceleration needed would be so great, it would be like dropping the spacecraft onto concrete from ten stories up.

If you want to figure it out for yourself, the fundamental equations are:

d = 1/2 a t^2
v = a t

Answer 4

Because we have an atmosphere on Earth! It would drag and cook the cargo. Shielding would make the sucker so heavy that it'd probably be impractical.

Heinlein proposed building the linear accelerators on the highest mountains, to reduce the amount of atmosphere the cargo pod would have to go through. Dreamer, he was. You can read about it in his novel The Moon Is A Harsh Mistress.

Answer 5

The idea has been considered for centuries. The main problem is building the satellite to withstand the shock of launch.

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

Answer 6

So called super guns do not provide enough thrust for long enough to accelerate the satellite to escape velocity!

Answer 7

They are working on it. The most promising technology so far seems to be electromagnetic rail guns.