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Lets assume that we have magic users that often like to hurt each other.

Now the simplest option is to try to blast the other mage directly, but it is not without drawbacks. First the enemy will be often magically protected making our work harder. Second they will try to magick us back making the whole endeavor risky.

The idea of hurling non-magical projectile at target using magic is known - it allows to attack from distance - and avoid or overcomes many forms of magical protection if the object is big or fast enough.

Dropping object from great height is next logical step that occurred to the mages - spells to magick a mass high enough exists and are reasonably well known.

Now my question is what could prevent escalation to the extreme - orbital bombardment?

Assuming that spells to put a house sized object into orbit are available to reasonably powerful mage - and the cost of them does not increase proportionally to the destructive energy of object dropped.

To clarify more there would be a two main ways to put the payload in orbit. First one is to levitate it up - if a mage can levitate a given object at all they can move it up as high as he wants until spell runs out - but it is slow - you can only get about 3-4 kilometers in hour. Second one is to teleport it instantly - again if you can teleport object of given size and mass at all you can teleport it anywhere - and as one can teleport payload to the other side of the planet without issues we have to assume that magic somehow takes care of rotation of the planet. Both spells are reasonably common and there are a lot of mages able to perform them.

Now as the spells are common there is a very little chance that the target can detect them and know that this time they are used to make a surprise for them instead of simply moving goods and people around.

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    $\begingroup$ Being in orbit isn’t the same as dropping from a great height. If you’re in orbit you’re going sideways very very fast, so if your mages have the ability to ‘drop things from orbit’ then they already have the ability to accelerate arbitrarily sized objects up to hypersonic speeds. That in turn means a better question might be ‘Why aren’t my mages all magical super-snipers?’ $\endgroup$ – Joe Bloggs Mar 31 at 21:07
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    $\begingroup$ To be clear: do you actually mean dropping from a great height (with no added sideways velocity) or do you mean dropping at orbital speeds? $\endgroup$ – Joe Bloggs Mar 31 at 21:10
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    $\begingroup$ being able to drop something from great height is not the same as hitting something from great height, try to hit a human with a water balloon from on top of a high bridge and you will see the problem, and that is when you have an ideal vantage point, from the ground they will be lucky to even be in the right vicinity. $\endgroup$ – John Apr 1 at 2:12
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    $\begingroup$ This seems like it's highly dependent upon the culture, magic system, and context of conflict in your world. As is this question needs more clarification before it's a good fit for this site. $\endgroup$ – sphennings Apr 1 at 3:29
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    $\begingroup$ Please read this: what-if.xkcd.com/58 - Getting higher than the ISS is easy, staying there is really hard $\endgroup$ – Falco Apr 1 at 11:48

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Putting stuff in orbit cannot be done on the sly

Sure you can put a house (full of dog poop!) up in orbit, intending to magically drop it on me. But orbit is a long way up there, and the magical process of putting something there is super obvious to anyone with any magical sense. Not only does everyone know that the poop house is going up, but they see where you are too as you magically lift it.

So now there is a house full of poop up there, and everyone with any magic knows you put it there and where you are. We don't know where you intend to drop it. But we know where you don't intend to drop it - back on top of yourself. And that is where we all think it should be dropped.

Every wart witch, two bit warlock and lich at loose ends can afford a little magic umph to make sure that what goes up comes back down right on top of whomever sent it up. You are a doughty magician to be sure, with extra dought even, but you are no match for the combined spare magical change of the entire magical world. Poop house coming home!

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    $\begingroup$ This is beautiful. $\endgroup$ – Daniel B Apr 1 at 9:33
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    $\begingroup$ Even if you don't know who's lifting it, when people can drop things from orbit, they can also defend against those things. The energy required to lift an object of a certain mass to a certain height is the same as the energy the object releases when it impacts the ground. Factor in the air resistance that is going to slow the fall, and stopping the object in midair is easier than lifting it into orbit. $\endgroup$ – Nolonar Apr 1 at 18:00
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    $\begingroup$ @Nolonar - stopping the object in midair is easier than lifting it into orbit? Lifting something slow does not imply the ability to catch something coming back fast. Imagine you have hauled a life sized bronze statue of yourself onto your roof. It takes you a while but you can do it because you work out. You did not secure it and now it is falling off the roof. Can you catch it in midair? Measuring force in newtons includes a time term (1kg / 1 m/s<sup>2</sup>. ). $\endgroup$ – Willk Apr 1 at 18:46
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    $\begingroup$ @Willk You could catch the falling object, but since your power output is limited, you'd have to slow it down over the same time period as it went up. As soon as someone "lets go" of the object, the other person has to start "catching" it, and spend a few hours bringing it back down slowly. You can't catch the bronze statue after it's fallen off your roof, but you can lower it back down the same way you got it up there. $\endgroup$ – Nuclear Hoagie Apr 1 at 18:52
  • $\begingroup$ Not trying hard enough. Would you like some relativistic pebbles? $\endgroup$ – Joshua Apr 2 at 3:57
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Demigan already mentioned this in his(?) answer but accuracy would be a major issue. There are some reasons for this.

Air exists

Unless your mages have a solid understanding of aerodynamics and ballistics and can form the projectile very accurately despite the great distance it will tumble chaotically when going thru the air. This will totally tank accuracy even before taking things like wind to account. Generally if you are shooting another mage you can see, you should be roughly as likely to hit yourself as the target.

Rotating sphere

This is not an issue when forming things a reasonable distance above you so your mages might not know this but increasing the height also increases the radius.

If the angular velocity is kept constant the linear velocity will vary based on your geographical location and the height. This will make the projectile move in relation to the target. It will also affect how the projectile tumbles thru the atmosphere.

Alternately, if you try to keep linear velocity constant, which might be reasonable way for magic to work, trying to materialize things above certain height that varies based on latitude might simply fail because the location you try to materialize it in moves in relation to you.

Angular precision

When you are doing something at a distance, you are really targeting it based on angle and distance. How precisely you can control the angle would have some limit. The error in where the projectile drops caused by this would be proportional to the distance to the point of materialization.

Note that error in the distance also hurts accuracy because of the sphere thing.

Time to hit

Longer the fall, longer the time your target has to move out of the way. This is a problem if your target is another magician who knows you have such an attack. Especially if they can sense your use of what should be a large amount of magic and recognize what you are doing.

Distance to hit

The longer the distance the projectile needs to travel before the hit more precise the initial targeting needs to be. This is a major issue if working well beyond any visual range that humans can conveniently manage.

Conclusion

Such orbital or high altitude bombardment would be only effective against large immobile targets. You would probably be able to hit a large city. Or an ocean to create a tsunami. Hitting things that are trying to kill you right now would not really be practical. Being able to hit things that are trying to kill you right now is pretty big deal as far as combat utility goes. So these spells would not be relevant to actual fighting.

Maybe it could be used for demonstrations of force to convince cities to open their gates after they have no army or mages to fight back. Cities might have little fortifications and be assumed open cities by convention.

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    $\begingroup$ to add to your bit about Time to Hit, it takes minutes to drop from orbital altitudes, unless the projectile has given additional velocity towards the planet (rather than the deceleration to lose orbit) it will definitely take between 3 and 15 minutes to hit its target... In a fight, that's essentially an eternity. Good against static targets like forts, but not so good against a mobile target who may well know it's coming. $\endgroup$ – Ruadhan Apr 1 at 15:25
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To quote HP Lovecraft (in an unconventionally literal sense)

Do not call up that which you cannot put down

A wizard can put a boulder into orbit. So what? That doesn't suddenly make it his boulder. It's up there with a whole lot of potential energy, yes, but that potential energy can be harnessed by any other spellcaster. If you do all the work of lifting it into space, you still might have to fight with your enemy over steering it, or he might just teleport it back at you.

At that point it's probably just a magical arm wrestle: which wizard can teleport the rock better/faster/more? I don't know exactly how your magic system works, but I would say the wizard who just spent a lot of effort lifting the rock into orbit would be at a disadvantage in such things.

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There's no magic in space, so you can't lift things into orbit like that.

In the tabletop RPG Shadowrun, magic is created by living things. As a result, magic surrounds the Earth, but ends at the edge of Earth's atmosphere, and using magic in space is nigh-impossible. Sure, your wizard might be able to cast a Levitate spell to pick up a car and start floating it up into the air - but once they start reaching up towards orbit, the mana field thins out and the spell will fail, dropping it right back down.

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    $\begingroup$ In A Certain Magical Index, magic doesn't work in space because there aren't any cardinal directions up there. (So, rather, it still "exists", but is incompatible with the sum total of human magical knowledge). $\endgroup$ – Sora2455 Apr 2 at 2:35
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1: Accuracy. You need to know where your enemy is and where the object is. Throwing something from orbit on top of something moving on the planet is difficult as both are moving at different speeds. If you try to bring your projectile in geosyncronous orbit you both need to know the exact position of your enemy and where the geosynchronous position above him is, then drop it on top of them. That is some next-level calculation in your head, or a very very complicated spell!

2: object lifespan. Dropping something from orbit means it'll hit the atmosphere and heat up. Many objects will evaporate or break apart long before they reach your target.

3: collatoral damage. You dont want to be murdering entire cityblocks I hope? Especially if it takes a few tries the death toll could be catastrophic.

4: magical counters. If you can generate enough energy to lift something in orbit, you can generate enough energy to hold something inbetween you and the projectile and protect yourself. Or since your opponent has the potential to locate you, hang a projectile above you and then launch it down on you there should be the potential to locate random things going in orbit above you and launching something directly into it to protect yourself. In fact maybe they could take control of your projectile as you are trying to get it in orbit and rip a part off to drop on you while you are busy.

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X Marks the Spot

As others have suggested, aiming an object from orbit to the ground with any kind of accuracy is going to take a lot of computational power. Either via actual computers, or very complex mental calculations on the part of your wizard. And it gets even worse if we don't assume your target is stationary. Dropping a bowling ball on a house from orbit is going to be tough. Dropping a bowling ball on a moving vehicle from orbit sounds like something you would need a specialized super computer to manager.

Air is Harder Than You Think

The next issue with your plan is that air resistance ends up being way more important than you would expect. We know from the various objects that have been put into space that entering the atmosphere is hard, if you want your object to survive until the ground. Friction is going to cause whatever you send up to space to heat up and potentially fracture apart before it makes it to your target. A solid metal sphere like a cannonball would probably be alright, but try to drop something like a house and all you are going to end up with is a bunch of debris landing over a wide area.

Horseshoes and Hand Grenades

Lets assume that you have sufficient calculating power to aim at a target on the ground, and a tungsten rod that you can teleport into orbit. Good news, you have a KKV now! Bad news, using it would make you an international terrorist.

The issue is that there is no way to have a small orbital bombardment. Even if you manage to hit your target, the blast radius is sure to be big enough to cause collateral damage. Even just having something hurtle through the atmosphere and not hit the ground can cause widespread damage. Actual impact is going to be so much worse than that. Imagine the Tunguksa Event, where a whole forest gets flattened, and then the same thing happening in a city. Yikes.

The fact that creating a weapon of mass destruction is so easy for mages means that it is going to be heavily monitored and its use punished. The entire world is going to agree that this is a Thing That Isn't Cool and that anyone who actually use does it is a Really Bad Guy. You basically end up with a mutually assured destruction scenario where everyone has access to a weapon but agree not to use it. Otherwise your story is very quickly going to turn post-apocalyptic.

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Terminal velocity

I don't know why no one else has mentioned this, although it's been hinted at in comments:

Being in orbit isn’t the same as dropping from a great height. If you’re in orbit you’re going sideways very very fast, so if your mages have the ability to ‘drop things from orbit’ then they already have the ability to accelerate arbitrarily sized objects up to hypersonic speeds. That in turn means a better question might be ‘Why aren’t my mages all magical super-snipers?’

Let's look at three options:

"Controllable" orbital velocity

If your mages can accelerate things, either "directly" or through "teleportation compensation", then why bother using orbit in the first place? Just take shots at each other with hypervelocity projectiles.

Orbital height, surface velocity

If lifting something into "orbit" just means lifting it really high without also giving it orbital velocity, it's going to slow down as it falls through the atmosphere. In fact, it's probably going to slow to terminal velocity, at which point you might as well drop it from exactly high enough to hit terminal velocity prior to impact, which is far short of "orbit".

Uncontrollable orbital velocity

Okay, lets say you are dropping something from orbital speeds, but for whatever reason you can't just arbitrarily impart those sorts of velocities without the object actually being in orbit. Well... now you have to drop the object. The problem is, this is actually a lot harder than you might think. The whole point of orbit is that things don't just fall out, or if they do, they fall slowly, are hard to target, and tend to be destroyed by atmospheric friction on the way down. This just doesn't strike me as practical.

If, on the other hand, you can teleport something into orbit, with orbital velocity, and then teleport it back without velocity compensation... well, now you're back to the first case of essentially being able to impart arbitrary velocity to an object. Even better, in this case you can teleport something from orbit to a spot right next to your target.

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    $\begingroup$ I think you might be surprised how high the terminal velocity of large rocks is. I've done a calculation on the terminal velocity for a spherical rock a few meters in diameter before and gotten a value equal to a significant percentage of the speed of light - though I expect that you probably start using different equations for air resistance once you get up to supersonic and hypersonic velocities that would reduce the terminal velocity accordingly. $\endgroup$ – nick012000 Apr 2 at 1:09
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    $\begingroup$ Teleportation (TP) throws out a lot of consistency we take for granted. Since there is obviously matter at the TP destination (except in orbit), why not teleport something to the literal same location as the victim and let the suddenly-coincident atoms rip them apart. If there's a mechanism to prevent TP into a mage's person, just teleport the ground underneath them to be on top of them instead. $\endgroup$ – Michael Apr 2 at 13:15
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Kinetic energy is easy to harvest for mages. They feel it coming. It powers them. A projectile would shatter as a small portion of the energy it generated would be turned upon itself, while the rest would increase your opponent's energy reserve. Since it also costs energy to throw stuff at people, overall, it's not a wise or economical choice. At least against other mages.

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Control of Magical energy decreases with distance. This means that a mage, if he or she is powerful enough, could get a house sized object into orbit even though the energy cost to do so is pretty staggering, but by sending it at a mage hundreds of miles away you are essentially giving your enemy a gift. The target mage can now take control of the object you put into space and send it right back at you using only a fraction of the energy it took you to push it into orbit in the first place.

This problem is further exasperated by the fact that the enemy mage can convert the heat generated by the reentry of the object into the atmosphere and convert it into kinetic energy. So by sacrificing some of the altitude of the object, the enemy mage can take the energy the attacking mage spent in putting the object into orbit, and combine it with their own energy turning it into a standard kinetic thrown object in atmosphere but with a higher energy output than the enemy mage could use on his or her own, then throw it back at the attacking mage while he/she is still low on energy after getting the object into space in the first place.

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The easiest one would probably be magical density. As you go higher the density of magic goes down, and thus it becomes harder and harder to maintain the power you need. In fact, link it to matter, so it's like neutrons except it provides magical pressure rather than gravitational. That way it's pretty much unchanged unless you've got a scientifically advanced society.

This also lends itself to a magic as technology world, where instead of technological advances they make magical ones, with similar scientific underpinnings.

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(Some numbers in the following are made up. Particular enchanters adjust the numbers to suit their purposes.)

You see this ring? It's enchanted to provide a small lateral force to any object of density greater than 10 mg/cm^3 in the entire cone of space above me with cone apex at the center of the Earth and radius 30 m at the surface. The force's acceleration is radially outward from the cone axis at about 1 m/s^2 -- not even enough to levitate an object.

You see this scarf? It is enchanted to detect objects out to a range of 10 miles approaching me at a speed greater than a running horse. It transports all their electrons 1 m backwards along their trajectory. They instantly disintegrate, with a great deal of lightning.

You see this shoe? It is enchanted to detect objects carrying more that 1 J of energy and more than 1 kg m/s of momentum that pass within 10 cm of my body. It instantly turns the object insubstantial and leaves it that way. (Now I'm imagining a planet with billions of translucent immaterial boulders diving into and popping out of the ground constantly...)

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Insubstantiality

This sounds like the "Rod from God" concept Kinetic Bombardment which is a fun read, but I would imagine the mages have an arsenal of magicks, including things like triggered spells, and the ability to become somehow insubstantial. So one could create such a trigger, "upon any mass coming within 2 inches of me, and moving faster than a common housefly, I shall become insubstantial". D&D calls this Contingency.

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It could be because it happened once, and now that mages have seen the horrifying results, such a thing is now considered "beyond the pale."

Something like this happened in the book "Newton's Cannon" (I think that's the one). An alternate history book where very mathematical magic worked. A fairly young Ben Franklin proved some sort of mathematical proof and was horrified to learn that this had been used to destroy a European city. I believe that an asteroid was used to bombard the city. It took a while, but cities don't move.

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