Let’s say there’s a Human Space Navy, called the Terran Defense Force. They’re currently in a war with their interstellar neighbors. They’ve reached one of their enemies colonies at the very outskirts of known space. My question is, given all their weapons, what would be a logical reason for them not to use orbital bombardment, but instead send ground forces?
Imagine you want to seize a mine. It’s a good mine. Lots of equipment, easily accessible tunnels, resources untainted by pollutants.
Would you do this by bombing the hell out of it with nuclear weapons? No. That would destroy equipment, collapse tunnels, and irradiate everything in the nearby area. You’d send in troops.
If there are any resources to be claimed on the colony (IE you want the territory, not just the utter genocide of your enemies) then indiscriminately bombing things will only make life harder for you in the long run. Instead of taking over an industrialised world (potentially with a viable workforce if you’re not too totalitarian) you’d be taking over a ruined hellscape that you’d have to then build back up.
If your aim is total genocide (you monster) then the other answers have you covered.
The Gliese 667 convention on inter-stellar warfare of 2283 (Earth reckoning) has explicitly banned orbital bombardment and flare-inducing as war crimes and against good galactic order. Other space-capable civilizations will take note of their use, and condemn them at the United Space Civilisations in Upsilon Andromedae.
Ok, on Earth, certain acts, such as carpet-bombing civilian areas of cities are explicitly banned by the Fourth Geneva Convention, and most countries try to abide by this. There may well be a galactic version of the Geneva Conventions.
Because the technological balance has swung from offence to defence
In general, there has always been a competition between offence and defence, for most of history it's been fairly strongly in favour of defence, a crossbow bolt is stopped by a shield, a sword-blow is deflected by plate-armour or even leather plate, a stab can be stopped by chainmail.
Today, a bullet is effective at normal ranges against almost every personal armour ever created. The balance has swung strongly in favour of offensive technology. An RPG will defeat most passive defences, a tank-round will hurl straight through almost anything.. but that's changing, active defences can swat aside RPGs or detonate them early. Tank armour gets better every year.
In the future though, the advent of active forcefield defences has taken this to its logical conclusion. With enough power and heat-dissipation, any attack can be resisted.
A defensive emplacement on the surface of a planet has access to petawatts of power and the heat-dissipation of entire oceans. No orbital bombardment can mean a thing against a well established planetary forcefield. No ship can sustain the firepower required to overwhelm it.
There is a weakness though. the shield cannot cover the whole planet, only a region.
Regardless, pounding the shield down is not a viable tactic, an army must instead be landed outside the range of the bubble and wage a conventional ground war to win the planet.
For the same reason we don't today: they don't want to cause a humanitarian crisis.
Orbital bombardment will have a measure of inaccuracy and overkill. Even with precision weapons, are you really sure you can tell the difference between a hospital and a defense building? Are you really sure your intelligence is correct?
Why wouldn't they want to cause a humanitarian crisis?
They're The Good Guys and don't want to harm civilians nor their infrastructure.
Or they're pragmatists and know collateral damage will leave them with an angry, hungry populace they must now support and more willing to take up arms against them.
Or they want to avoid escalating the war, knowing the enemy is capable of doing the same.
Or they are held to treaties banning the use of orbital weapons and do not want to be denounced by other signatories.
Orbital bombardment with KE weapons is over-hyped in many SF settings. Causing a rock or a metal rod to impact with the force of a nuclear weapon requires lots of delta-V.
Orbital bombardment with nukes requires, well, lots of nukes. Nuclear weapons are not efficient at attacking dispersed enemies. Assume that the planet is habitable, and that the settlement patterns are small homesteads. It is cheaper to land an infantry squad which travels from one farm to the next than to expend one nuke per farm.
There's lots of reasons, all surrounding a single theme: you dont want to destroy everything.
You might want to convince the colonists to become your citizens eventually. Having more taxpayers and a larger workforce is always nice.
Murdering everyone in a solar system brings baggage. Your own population might not like it, the people pulling the triggers might not like it. What is even worse is that now your enemy isnt going to hold back either and annihilate entire planetary populations as well. Preferably the one that contains the ruling body, including you.
Resources are nice, and nothing contains as many refined and ready resources as the buildings and equipment themselves. Destroying it is a waste.
The planet is so nice this time of year. Using kinetic impactors on a large scale makes it rather hard to inhabit afterwards. Maybe invade instead?
Or how about one that I've never seen anyone use: invading solar systems isnt as easy as "warp in and nuke it". It requires you to get the materials in, but you can only carry so much so you need to manufacture a lot of the equipment in the solar system you are trying to take over. Your opponents are going to dig in and make it a type of trench warfare. Every time you use an orbital bombardment they just hide in deep bunkers, then dig themselves out and assault your landingcraft as you try to gain control. Why waste the time, the resources and the planets environment with OS's if you'll end up in ground battles anyway? Better use it to build up your forces and have only a light bombardment to get the space for building staging area's on the planet.
Realistic orbital mechanics put severe restrictions on orbital bombardment. Depending on the technology level of your setting, unless your troop transport vessels and space dreadnought have cloaking devices, energy shields or weapons with infinite ammo like very poweful lasers, they probably want to spend the least amount of time in orbit around the planet they attack.
1 Orbits are predictable
Just like it is possible to know exactly when the ISS will pass above a given point on Earth, your enemy on the ground knows exactly were your vessels will be at any point in time.
Your vessels can bomb only points of the surface their orbits take them over, with very little deviation. Sure, with enough orbital inclination and the rotation of the planet, you will end up covering most of the ground, but you lack initiative.
As soon as you cut FTL and enter normal space, they can compute your trajectory and start speculating about your target, evacuate or shield up, or mass their anti-orbital countermeasures on your path.
Orbital plane change is a very costly maneuver, realistic space ships with reaction drives can't perform it as much as necessary to catch the enemy by surprise.
2 There is no stealth in space
There is no realistic way to hide something in space. Your orbital ships will at least emit an infra red signature easily detectable against the cold background of empty space. Detection can be done by cheap, passive sensors the enemy will probably have hidden scattered all around the planet. You will have a very hard time detecting those and making your enemy blind.
If your ships fire their engine to modify their orbits, especially if they are big and require powerful engines, you can bet the enemy will detect it and keep track of your position more easily than you can change it.
The enemy on the other hand, can burry his installations on the surface and hide from you. Which leads to my third point.
3 Space ships in orbit are (counter intuitively) sitting ducks
The enemy is hidden and shielded. They know where you are and where you will be. In fact, you are in serious danger that they bombard you. They might have to climb the gravity well, but they have many advantages over you.
they are hidden, you are visible.
they are shielded, but your ships can never offer the protection that a few meters of solid rock can offer.
they have nigh infinite resources and stuff to throw at you, when you have only the ammo you brought with you from home. Even your defensive anti missile counter measures will run out faster than their orbital railguns.
In fact, in a somewhat realistic setting there can be no way to invade a planet from space unless you have a significant technology advantage, utter and complete surprise or sabotage missions like informatics warfare already crippled their reaction, if possible all 3 at the same time. Even the landing pods for your troops are sitting ducks for the several minutes that atmospheric entry will last. Expect 90% loss even before reaching ground.
TL;DR: use spies and hackers before you risk any ship in their solar system.
You can bomb as much as you want, but if you want to conquer a place, you need your troops to set foot on that place.
This has been a constant in all wars: bombing just prepares the ground for the invading forces, if the ground needs to be prepared.
If there are no surface structures to be destroyed, bombing is even more useless.
Just as your ship’s precision missiles and bombardments are very accurate, planetary defenses — rail guns rounds and interceptor missiles — are more accurate, faster, and cheaper.
The only other weapons available to your space side forces are KE attacks using asteroids too large for the planetary defenses to destroy or Assault Troops (Mobile Infantry).
One asteroid would obliterate the habitable surface for thousands of years, at least.
This makes boots on the ground — Mobile Infantry — the most effective means to prosecute the war because they can hit planet side outside the range of defenses protecting cities, military bases, and manufacturing enclaves from bombardment, and then advance on the ground to their objective to destroy or capture them.
David Webers Honorverse uses the Eridani Edict for this purpose: Mankind warring factions once bombed Epsilon Eridani into oblivion, with billions of casualties. Afterwards, all Major factions agreed to NOT do that again and the sole superpower of that time decided to back that treaty up. If you violate the Eridani Edict your faction will get destroyed, your leadership shot. It's a hell of an incentive to not do that.
The flip side is that if you conquer the orbitals, a Planet should surrender to your faction and give up active resistance. Therefore most army action are more like anti insurgency operations.
If your setting does not have such a superpower available your factions could still agree to not bombard each other. Since bombarding a Planet is relativly easy, it is something like MAD: If you bombard my colonies, my fleet will destroy your worlds too. This only works if both sides value life in some form. If your opponent things you are disgusting space vermin, this is not viable.
Besides the reasons everyone already provided, because of cost.
In ideal scenarios, the energy cost to send a piece of whatever from orbit to land is the same cost to send it from land to orbit. If an atmosphere is involved, then the cost of landing is much reduced due to aerobraking, but it gets harder to aim due to winds.
Also remember Newton's third. Firing an orbital weapon will change your orbit. That means whomever is firing is breaking formation really fast relative to those who are not firing, or firing at different targets.
Don't trust me just because I am saying it. Open up Kerbal Space Program and do some orbital bombardments yourself (make some rockets wich can break up into smaller rockets, then burn retrograde from low Kerbin orbit). Try to aim for the space center or some other feature, see how well it goes.
Orbital KE bombardment costs more than it's worth. Asteroid bombardment is easier than you think to defend against.
Rods from God - dropping tungsten telephone poles from orbit - is actually a dud. The planet's gravity does not help you. It takes more energy to move them into a low enough orbit that they start falling than they deliver to the target. The explosion is pathetic compared to a nuke, one ton of tungsten delivers KE equivalent to ~11 tons of TNT. The only reason to consider this is speed: from launch to delivery is only a few minutes, it was meant for a Cold War scenario where you want to blow up the Kremlin faster than the enemy can get a missile from Russia to Washington.
Nukes work better than inert rods, but the enemy can just as easily nuke you, and you don't have a bunker to hide in, so the defenders have the advantage there.
Asteroid bombardment means you have to lower the solar orbit of an asteroid until it intersects with the orbit of the target planet. You want a pretty big asteroid, or it will burn up in the air without killing anybody and nobody will be impressed. In 2013 a 13,000 ton asteroid hit the atmosphere over a Russian city and didn't kill anybody. So you start with an extremely heavy asteroid (the dinosaur-killer was ~ a million tons!), and you have to move it, which could take you years if your engines aren't superweapons already.
During that time the enemy will notice, and turn their entire planetary economic output towards nuking your little engine off the side of that big asteroid before you can get the asteroid's perihelion low enough for it to hit the planet.
Because you don't want to get mass-drivered yourself
The French used to have a red flag they would hoist in battle. It meant "we will take no prisoners today". It was meant to strike fear in the enemy, particularly the nobles, who were generally captured, held courteously and traded back. However it also meant that if the French attack was repulsed, the enemy would slaughter any Frenchman captured. "No prisoners" swings both ways.
So. Every spacefaring species has lots of history with war before space travel. They certainly develop nuclear weapons on their way to interstellar travel, and have been forced to navigate that balance of terror. So even if they have never formally developed Geneva Conventions, they certainly understand chivalry, taboos and "what we do, gets done to us".
You are in a neighborhood where interstellar foes don't habitually mass-driver each others' cities. You know that because wars get talked about. It isn't a written rule, but nobody does it.
You can do it anytime. Anyone can. But you know... the day you do it, the enemy will feel license to do it to you.
But not just this enemy this war. You are sticking up a billboard at the edge of your space that invites any future combatant to do it to you. "Please, come mass-driver us, we think that's A-OK in warfare.”
The problem with this is, it is irrevocable. Unlike the French where they can decide on a battle-by-battle basis whether to hoist the red flag, your choice will be treated by others as a long-term choice. It would take your diplomats decades to walk it back and convince the other powers of your renewed commitment to chivalry. The victim party would have to accept your apology, reparations and other concessions. They would have to see a fundamental change in the government, e.g. A flip from dictatorship to democracy.
Given the staggering cost of that, you will keep your field commanders on a tight leash regarding such tactics.
Friendlies or neutrals at the enemy colony: Maybe the terrans aren’t at war with all their neighbors, and some of those neutrals or friendlies are at the colony.
Maybe the friendly or neutral neighbors would get angry if you killed all their traders, diplomats, observers, tourists, scientists, hostages etc currently at the enemy colony.
Or maybe they have taken human hostages.
They don't have bombs.
Or lasers, or particle beams, or photon torpedos. The space ships of the Terran Navy are troop ships because that is the tech Terra has in the year 2022. And is lucky to have that: they are powered by FTL drives salvaged from captured / wrecked enemy spacecraft and welded onto whatever Terra had left that they could seal against space. The initial assault by the enemy was devastating and the consequent destruction prevented development of new tech. Maybe the humans could push a nuclear warhead or two out of their flying shipping containers, but after the first one went off the response would clear the Terrans from orbit.
The hope of the humans is to show up unexpectedly, get to the surface as fast as possible, get out of their "ships", and show them how humans make war in person.
Here are a few:
Treaty. Even if we haven't been on friendly terms, we may not have always been at war, so there may be agreements in place.
We don't want to open that door. They haven't started throwing rocks at us yet, we don't want to give them reason to.
Bigger, badder interstellar neighbor. There may be a bigger, badder power in the local neighborhood who doesn't care what we do to each other, as long as we don't cross a certain line, and this crosses it.
There are some lines you just don't cross. Earth is a coalition of governments, and there's just not the political will in place to do this. Besides, this would be a war crime by our own laws.
Rigid rules of warfare. It's relatively easy to bomb a world back to the stone-age. In fact, once that particular Pandora's box is open, it wouldn't be long until all the desirable worlds in a particular neighborhood were rendered unusable. The difficulty lies not in defeating an enemy, but in bending him to your will or in having usable real estate, once you're done. I suspect use of nukes on planetary targets would face a similar moratorium.
What if it's just pointless.
If your enemy can prepare to defend against bombardment then they can potentially defend against it very well. If they've been at war for a while then they'll have an idea of your tactics, strengths and weaknesses.
As an example, the Battle of the Somme started with a 7-day artillery bombardment against the German trenches. 1.5 million shells actually achieved very little - the barbed wire wasn't cut, the German artillery largely survived and they used the wrong type of shell to get through the concrete in the trench bunkers.
Or it could go another way. What if the enemy lives wholly underground? Massive cities powered by highly efficient geothermal energy. Of course, over the centuries they've had to go deeper and deeper into the crust to both expand and extract more energy and most of their cities are now around a mile or two deep. In that case, the most sensible option is to find the entrances and go in with troops.
Because colonists are expensive and you'd rather not kill them.
Consider the cost of finding willing colonists, getting them into orbit, then FTL to a new place, then dropping them and getting them set up. These people are difficult to replace and you don't want to be responsible for the bill when it comes due.
They do, but it's not enough.
With all the potential targets on the surface, it is very expensive to bring along enough mass to hit them all, plus enough propellant to accelerate them to impact velocity.
Instead, it makes sense to land perfectly reusable troops, which can potentially handle so many targets for the same mass+propellant. And they can adapt to changing requirements.
Besides, they would be needed anyway even if more bombardment was plausible. No operation is complete without boots on the ground.
The navy does provide limited surgical strike support for the marine force, eliminating a few critical targets that would otherwise become a tough obstacle for the ground forces.
Launching such a strike isn't a simple affair though. There is a limited time window when the navy is in the right vector to launch projectiles at a specific target. Unless the target is a static installation, it could easily avoid the kill zone on the slightest suspicion that a strike was coming. Even a static target can brace for impact and mitigate most of the damage.
The best tactic is to have a very mobile and stealthy scout force approach the unsuspecting target right before the launch window, mark a few critical targets for the navy, and do most of the work after the strike eliminates those but before reinforcements arrive.
Also, don't forget that any orbital maneuver that would align vectors for a strike is a dead giveaway to what your intentions are. It gives the enemy time to prepare, and exposes your strategy. Layers upon layers of ruses and deceptions is the only way to put your navy in a firing position at a still-unsuspecting target. This takes time, and is expensive in terms of delta-v spent on fake maneuvers.
Naturally such tactics are reserved for what command believes to be decisive blows. Regular operations are handled by the resources of the ground force.
On top of that, the navy has a number of better missions for its kinetic kill weapons: Space superiority, strategic orbital asset elimination, launch denial, interdiction. They would be much more helpful handling things that would be very hard (if not impossible) to reach from the ground.
So, unfortunately, Operation Mjölnir is a no-go.
[Some of this overlaps a bit with what “Grand Phason” has written. I wrote the following before reading that, so I am just going to post it as it is (with this apology).]
For the TDF to not do orbital bombardment, they must be either • unable or • unwilling to do it.
I take it from the question that • they are immediately able to (i.e. they have O.B. weapons), and that • their purpose is simply to destroy the aliens.
Thus, we need either • a non-immediate reason why they are not able to, or • a reason why they are unwilling to.
Of course, whatever the reason is, it can not count against sending in ground forces.
Not Willing To — my favourite
• I like the idea that there is some information that they need, that is on the planet. Any vital or very useful or important information would do. Perhaps it looks from orbit as though the aliens may be the long-lost Second Colonisation Wave (somewhat evolved, of course)… or have such as slaves… or might know something about them. Perhaps there is a (plausible) claim that the aliens have kidnapped a loved and important Terran personality, or a religious figure. Perhaps there is a spy with vital information (stored in some physical object), who failed to escape as planned. Perhaps there is some vital military technology, or there is some hope of discovering a piece of medical technology that is useful for some reason. Perhaps there is a discovery (or suggestion) that there is some information about the aliens, that would be important to the war effort (if correct), that requires going down there. Perhaps the aliens are in intensive communication with some unknown party, using an ansible — that is, they can tell this from orbit, but can not tell where (nor what) the other party is from orbit. Perhaps there is a discovered possibility that there is another planet of the aliens somewhere else, and they want to (try to) find out if that is true, and if so where it is. Perhaps there is something that the aliens have been doing, or might be able to do, or said, that they want to know more about.
Not Able To
• It is a water (or viscous goop) planet, and the TDF“s O.B. weaponry is unsuitable, because the aliens hide under-water.
• One possibility here is that they initially believe that they can, but find out that they can not. [I do not think I have any good ideas here; maybe someone else can help?] • Maybe the atmosphere is thick, and is corrosive to orbital weapons in some fatal way. • Maybe they are using nuclear weapons but the aliens have some sort of technology that causes them to detonate too early or not at all (or any parallel technological superiority in the arms race)… or they have some amazing and exotic military technology, and it just happens to not work in this alien environment (because of ionisation or no metal core or no atmosphere or no oxygen or who-knows-what)… or maybe it [the nuclear part, or some imaginary/future technology] does not work and they just have no idea why.
• They want to do both an orbital assault and then a ground assault… but it turns out that this does not work — perhaps because the aliens inform them that they [the aliens] have planted canisters of gas that is poisonous to human beings… such that blowing stuff up would have the result that their ground troops would die if and when they send them down. (Either they commence the O.B. to check or they are able to see for themselves… or they are not sure but can not take the risk.) Possibly some sort of biological agent would also work here. On second thoughts… if they can do this, then presumably they would release said agent regardless. To make this work, there would have to be some significant cost in releasing this agent, for the aliens, such that they only do this if forced to — perhaps it is also somewhat destructive for them [which they would not tell the Terrans (but maybe the Terrans already know)], or it is incredibly expensive… or perhaps it is just that it only lasts for a little while. [This needs some polishing.]
• Possibly the humans are simply not certain that this is the alien planet — it might be a decoy, or there might be another one — and they have enough O.B.s for only one full-scale planetary destruction — not to mention the element of surprise, or fighting on two fronts or what-have-you. (Perhaps their instruments tell them that there is — meaning that there might (or might not) be — a sizeable population in the next star system.)
• The enemy has some sort of technology that means that they can not see what to bombard, or what the results of bombarding are (along the lines of a hologram, or some sort of jamming, or an engineered atmospheric layer). They can bombard all they want, but they have no idea what the effect is. For the OP’s purposes, this would require that they do a bit of O.B. and then give up.
• It turns out that there is a saboteur on board (or back at base)… and all the nuclear safety codes have been changed, or the uranium is depleted, or the AI that is the sole authority to launch an orbital bombardment has developed a conscience… or all the O.B.s are in one ship and there is a terrible virus on board that ship… or there was an accident that released gallium [(?) - the corrosive stuff] into the tungsten rod storage area… or the targeting computer clock is wrong, and it insists on launching the O.B.s towards where the planet was 49 minutes ago… or the advanced AI in the nukes has developed a conscience, or… .
• It might suit the OP’s story for the TDF to do the whole invasion and then find out that there is no one there. Perhaps they just fled, or perhaps they are all in a massive attack fleet on the way to Earth.