Are orbital defense plateform possible?

MAC cannon from Halo 1, 2, 3, 4 in other words? Maybe we can blow some Grey alien or reptoid mother ships

info
A standard ship-based MAC fires slugs of either ferric Tungsten or depleted Uranium and aproximately 9.1 meter's long at around 30, 000 meters per second.[2] The high muzzle speed gives the 600-ton slug the kinetic energy and momentum necessary to damage a target and partially mitigates the unguided nature of the slug and its lack of maneuverability. Orbital Defense Platforms fire a 3, 000-ton slug at point four-tenths, or 4% of,[3] the speed of light, around 12, 000 kilometers per second.

Added (1). One thing to add those things gain there energy from a wireless source on the ground kind of like the new tech there researching on wireless recharging.look it up.

The issue governing this weapons system is the same that governs the A10 tank killer (aircraft.)

the A10 is basically a bathtub for the pilot sitting atop a huge cannon that makes up the body of the plane that has wings filled with fuel to keep it flying -- it's basically a huge gun with wings that fires huge, heavy slugs at high velocity to kill armored vehicles on the ground.

the problem is Newton 3 ("for every action, there's an equal and opposite reaction.")

when the plane fires its weapon, it has to make sure it has enough forward momentum to make sure that the recoil from the weapon will not cause its airspeed to drop to the point where the wings no longer provide enough lift to keep the plane aloft.

the space cannon will suffer the same problem.

the platform will obtain a momentum equal and opposite that provided to the slug.

since the slug is directed away from earth, that recoil will send the platform moving towards the earth.

it will cost as much energy spent firing the slug to overcome the earthbound momentum gained by the orbiting platform and place it back into fixed orbit.

that's why directed energy weapons are preferred to momentum transfer weapons in space. Chemistry.thermodynamics and quantum physics often proves more effective than classical physics in the world of space weapons.