There is a presumption amongst many sci-fi writers that if you own the space over a planet, you own the planet. The reasoning is something so simple, an 8 year old could figure it out, yet the people writing Star Wars kinda missed it.
See, kinetic objects of large size that are accelerated to high velocities are detrimental to planets. No need to build a giant planet-killing death star, no, you just need a big rock and something to push/pull it into place.
A nice dense object like a mile-wide piece of nickel would be all it takes to End Life As We Know It on whatever planet you hate enough to smash it. Once the Large Dense Object (LDO) smacks into the planet, bad things happen to the atmosphere, to the crust of the planet, and the poor folk who survive the initial mega-explosion.
Why spend billions of galactic credits on a huge planet killer when you could hire a tramp freighter to tug a large rock into a high speed smackdown? This proves that the Empire are Republicans: Massively wasteful “defense” spending.
So, yes, a nice mag-accelerator to throw projectiles is cool, but a ship with an engine and the ability to catapult a large rock is just as powerful, if not more so.
Jerry Pournelle used this in his book, “Footfall,” where an alien species drops a rock (the “foot” of the title) on Earth. It’s been done right in a few other books, as well.
So now we get to part two of rock throwing. Suppose you arrive at a planet and want to do some close space support for friendlies on the ground. Cool. It’s easy, right? Computers and stuff will figure that out.
Not so fast, Fredolf. A lot of planets won’t have a magnetic pole system like Earth. Wait, did I say a lot? I meant to say no other planet has a magnetic pole system like Earth. We get lovely things like the magnetosphere around Jupiter, or Saturn, which are these enormous teardrops of radiation that would cook you if you tried to fly through it without a whole lotta lead. So the ship arriving at planet x must either use existing maps and landmarks, or they need to start taking pictures and plotting using active sensors. Pictures are passive, but your camera can only cover so much ground so fast to be accurate. Then you have to sequence all those photographs with some sort of arbitrary grid system so everyone has a point of reference. Once that is complete, you’re all ready to drop some rocks on enemy heads.
Except for one little detail. See, the guys asking for close air support, they don’t want to die. And this is where the system ought to be subtle. Remember above we said that the LDO must be a certain mass and velocity to obtain the right size explosion? Here’s where it gets tricky. If you fire something too small at the planet, you won’t kill the bad guys. If you fire something too big, or too fast, it’ll roast friendlies, too. So there’s that end of things.
And further, assuming you are using a visual reference system of cameras to hold your position -and- to figure out where the rock is going. Your guy on the ground has a map and he says to hit grid 14528, 93855 and he asks for a 100 meter explosion. That’s doable, but it is completely dependent on your cameras and computers to be sure the ship is in the right place to throw a slug out of the correct size at the correct velocity. Otherwise, poor Joe Infantry down there is going to get a heckuva surprise.
Also, remember that survey you did at the beginning? The one that you probably did on a widescan with lower detail so you could get it done quickly? That’s going to determine your grid accuracy, because down to a certain point, it’s just a pixel in your system and you cannot accurately drop your rocks in with a +/- error rate of, say, 5000 meters.
Of course, you can use radar, lidar, sonar in some locations to bolster or even be the first method of determining where you are and avoid the whole “sorry, can’t shoot, weather in the way” problem. This is SCIENCE FICTION, man, figure out a way to fix that end of things. A little rain will never stop us!
So there’s some of the things that can screw up the whole “we have the high orbitals” device. So what? If you have no boots on the ground, yeah, you have nothing to lose pitching rocks at the planet, but if you have any stake at all, you’ll want more precision and it’ll take quite some time for a new ship to index the planet and prepare a right and proper targeting grid system.
There’s further problems which are that even if you have the best cameras in the universe and they can see the scrotum on a flea from high orbitals, you still have to know where to aim that camera to see stuff to shoot at. You don’t just indiscriminately pitch rocks at a planet… unless you’re the Empire. And then you just build a death star (used once, awesome condition, good for destroying planets at great expense. Needs some sort of cover over the exhaust port to prevent that 1 in a million shot by a farmer’s kid.