Why do high-orbit space stations fall once destroyed?

Question

The Empire in this world has plenty of space stations - they make up a good 2% of the total livable space of the Empire, and there’s at minimum one per colonized planet but usually many more. Most are civilian habitation and commercial, so that miners can live near the mines even if there isn’t an actual habitable planet or moon nearby (turns a 12-hour commute by FTL busing to a 30-minute flight from the station to the ground). These civilian stations are usually equipped with spin-gravity and placed in low orbits, with constant nuclear-thermal propulsion systems running to avoid orbital decay, with the intent that both a) every spot on the celestial primary is swept over by the station every so often so that people from all over the world can reach the station by space-bus easily and b) people from the station can enjoy easy and fast access to supplies from the surface.

Military installations also exist; these are usually equipped with orbital surveillance and weapons systems so that threats from the planet’s surface can be identified and dealt with quickly, and so that threats arriving from interplanetary space can be intercepted and destroyed before they threaten the planet below (with civilian stations also having limited planetary defense capability if a small asteroid or simple thief stops by, but not enough to repel a larger moonlet or invasion fleet like the military outposts above).

For plot reasons, one of the main aspects of the military installations is that, once they sustain enough damage, they fall towards the planet and are destroyed for good. These are massive stations, the size of small cities and equipped with plenty of railcannons and astronaut-boiling lasers, and which can call on the support of the Imperial fleet in a matter of minutes, but if a decisive blow is dealt to a station before it can launch a counterattack or otherwise stop itself from being destroyed, it falls down to the planet after being destroyed, in a matter of hours to weeks or perhaps months.

This is plot-consistent throughout the world and story; the question is, in the context of a space station built in a very high orbit around a planet, why this would happen. Things in high orbits tend to stay in high orbits with nothing to slow them down, and while these military stations are heavily armored, they can certainly be destroyed without imparting enough momentum to physically stop them in their tracks around the planet.

Why do high-orbit military installations fall down after being destroyed?

Answer 1

It's a safety feature to avoid Kessler Syndrome.

Early in the days of space warfare, several planets got crippled by the cascading destruction of their space stations. Not only did the debris destroy all of the orbital habitats, causing mass casualties, these planets were also completely isolated from the rest of the empire from the destruction of their communication satellites and from the inability of space ships to land safely. It took years to clean up the debris, and several of these planets went from among the most affluent to impoverished worlds. One planet was so badly affected that it's orbit still hasn't been cleared hundreds of years later, and it stands as a warning to the galaxy, its billion inhabitants imprisoned in their cage of floating trash.

All satellites (military, civilian, and utility) now come with damage monitoring and active de-orbiting systems. When a satellite detects that it has severe structural failure, its deoribiting systems engage. These systems substantially decrease the orbital velocity of the station. Sufficiently large satellites (like these military space stations) have secondary systems that can be engaged when their orbit has degraded sufficiently to steer their crash site to a non-populated area. (Note that splitting them into small pieces probably doesn't help with impact: https://space.stackexchange.com/q/5617/894).

Worth noting: city-sized space stations are going to do a lot of damage on hitting the ground (again, splitting them into pieces doesn't help here). Depending on the mass of these space stations, de-orbiting one of these into an ocean could still cause megatsunamis and other massive natural disasters. You may want to run some numbers on the mass of these space stations to make sure they won't wipe out all life on the planet below from being de-orbited.

Answer 2

If the station is far below orbital velocity, hovering on that nuclear-thermal propulsion you mentioned in the first paragraph, then how high it hovers doesn't matter (much). Knock out the lift jet in a low orbit station and it'll fall into the atmosphere in a matter of minutes; do the same for one in high orbit and it'll take many hours to come down -- but come down it will.

No special properties (other than unlimited supply of reaction mass for the hover jet) are needed; simple physics dictates that an object not moving fast enough to fall around the primary will fall into it.

Answer 3

To pull this off I think you need to justify two things:

1. Why a self destruct mechanism isn't used.
2. And "it's standard operating procedure."

You could techno-babble your way to a physical explanation, but to me that just makes the story too predictable. This ship sinks. Every. Time. Boring!

No, instead as a captain of a super weapon, I'm thinking through a myriad possible outcomes for a battle. War is complex, so I imagine their missions are equally complex. Captains will be given primary and secondary objectives. The theaters in which these craft operate likely have civilian and military craft to be concerned about, both friend and foe alike.

Why not blow the silly thing up?

• A city-sized space station will generate an enormous cloud of high velocity debris. Great for destroying enemy ships. No so great at not destroying friendly ships and civilians.
• Can the crew escape fast enough to avoid the self-destruct debris cloud? Maybe the escape pods don't have fancy FTL drives and acceleration dampeners. Now you are constrained by what the meat bags can survive. No sense in abandoning ship only to be killed by said ship after it self-destructs.

While you don't want the space station to fall into enemy hands, you don't want to make matters worse for the planet you are protecting by showering your remaining defenders with bits of their former savior. Self-destruct mechanisms are just too risky and the stakes are too high.

Besides, this thing is a super weapon. Why just blow it up?

This leads to the next key reason:

It's Standard Operating Procedure

Sending a crippled space station into a death plunge is literally in the manual, for crying out loud! No physics techno-babble necessary. This is a purposeful maneuver for a doomed ship so its demise won't result in friendly casualties, civilian deaths, and provides a means to safely destroy the ship to prevent those pesky rebels from learning about your super weapon.

What exactly happens next and why will depend on the mission objectives and battle conditions, but the end result is the same: space station fall down and go boom.

Planned De-orbit to Avoid Allied and Civilian Casualties

Theater of Operation: defending allies.

Battle conditions: all weapons systems down. No high value targets between you and the planet or moon.

Objectives:

1. Keep space station out of enemy hands.
2. Don't hinder remaining defenses.
3. Avoid civilian casualties.

SOP: chart a course to safely de-orbit the space station in an uninhabited sector.

Disengage To Allow Time For Possible Repairs

Theater of Operation: any.

Battle conditions: damage seems critical, but repair crews report a Hail Mary shot to restore critical systems, which might put you back in the fight.

Objectives:

1. Hedge your bets and plan to de-orbit so the space station doesn't fall into enemy hands.
2. Disengage from the fight to give repair crews a narrow window of time to make necessary repairs.

SOP:

• If repairs can be made in time, change course to avoid the planet and re-engage.
• If repairs are made, but too late, or the damage is too great, change course to avoid the planet and retreat.
• If repairs can't be made in time, abandon ship and prepare for de-orbit.

If you can, by some miracle, get this hunk of metal back in the fight, you could make this death plunge more than once. First time for repairs, re-engage, take more damage, de-orbit. Oh, the drama!

Go Out Guns Blazing

Theater of Operation: defending allies or on offense to take out critical enemy targets.

Battle conditions: critical damage, but some weapons systems are still operational. High value targets exist between the space station and the ground/high value target is on the ground.

Objectives:

1. Keep the space station from falling into enemy hands.
2. Take out primary or secondary targets through alternative means.
3. Take out as many enemies as possible to inflict maximum damage.

SOP:

1. Chart a course to de-orbit into something critical on the ground.¹
2. Shoot everything on the way down.
3. Screw civilian casualties.²

¹ Or de-orbit safely to avoid friendly casualties.

² Or not because you are chivalrous or you want to avoid friendly casualties.

Many Reasons, Many Techniques, One End Result

Crashing a doomed space station into a planet becomes another strategy in a less-than-ideal fight. This should get you many plot reasons and twists to weave into your story. It's not just "because gravity". It's because it allows your characters to achieve their goals through other dramatic means. Oh, and it's in the manual, too. So, get creative.

Answer 4

You put your station in the L1 Lagrange point.

L1 is unstable (which suits your needs), but can be maintained with minimal effort.

See this question for more information about L1 (and the others): https://space.stackexchange.com/questions/10144/are-there-really-just-5-lagrange-points

Lagrange-Points visualization:

L1 is unstable towards the planet and the star, so once your station gets destroyed, your station will fall either into the planet or the star. You can also orbit L1, so you are able to have multiple independent stations there.

As @Justin Thyme the Second pointed out, I might be more suitable to your story to use the L1 point between the planet and any of the moons. Doing so, you could have more military stations, observing more of the interplanetary space independently. Debris would still fall either into the planet or the respective moon.

Your only problem with your plot might be that uncontrolled L1 deorbiting will take a considerable amount of time. And you might still get Kessler Syndrome, should any part of the deorbiting station hit any LEO object.

Answer 5

Nuclear propulsion

Between the lines I feel the problem needs some haste to de-orbit. The station has a wealth of military knowledge and hardware on board, so with enough structural damage you want to keep it out of reach. Simply having the nuclear propulsion turning off will take too long. It is also not clear if many military stations even need the propulsion to begin with. So how would you move a city sized station into the atmosphere of the closest planet? Nuclear propulsion.

There are many types of nuclear propulsion. One of them is simply detonating a (small) nuclear device in such a way that it propels the space craft. Often it involves not the direct shock wave of the explosion, but the vaporising of other material thanks to the explosion, which in turn pushes on the spacecraft. The lay-out of your station can help. As the danger away from the planet is more difficult to track, it stands to reason it is better armoured at that side against first strike attacks. You can place small nuclear devices all over this armoured area. The nuclear devices detonate in the event of too much structural damage, propelling the ship at high speeds to the planet. The blast can also spray debris to attackers.

They could be seen as a liability when attacking the armoured side. However, the armour is at least strong enough to survive a relatively close nuclear detonation. Nuclear devices might explode by other ordnance, though this results in explosions that are a fraction of the original power of the bomb. Nuclear bombs are very particular in their detonation methods. Moreover, the attacks on the heavily armoured side can already propell the military craft towards the planet, which can be reversed if the attack is defeated.

Nuclear ecplosions give a great way to both attack the enemy, propell the spacecraft away at high speeds, and can possibly start the destruction process of the military hardware early. An effective solution for unscheduled rapid destruction of the hardware.

Answer 6

It's a function of the weapons used against the station, not the station itself

A big enough space station is not easy to simply "destroy" in one fell swoop. It's not like a battleship that will sink when the hull is breached, but more like a fortress where each individual artillery battery will keep firing at you autonomously until you do something about it.

So there are two ways to take down a station: you can gradually try to destroy each individual weapons emplacement to "de-fang" it, or you can sink the whole thing with one powerful de-orbiting strike. Naturally, such a weapon would need you to degrade the station's shields first, or get close enough, or whatever causes the right degree of fun and exciting space combat.

The nature of the de-orbiting weapon is really up to you. Perhaps a reverse Orion kind of idea where well-timed nuclear detonations in just the right places nudge the station onto a doomed orbit (in which case the preceding combat involves the degradation of the station's engines, making it unable to course-correct). If your SF is softer, it can even be a dark matter weapon that degrades the orbit by messing with the gravity.

Answer 7

The attacks used deliver enough impulse to destabilize the orbit

A slight push on one side of the orbit results in a strong effect on the other side. Anyone playing kerbal learns this quickly. If you slow down on one side of the orbit, it pulls the other side down.

Whatever weapons you use, if it's not outright disintegrators they will deliver some sort of impulse - whether it's the mass and velocity of railgun rounds, the push of explosives, or lasers vaporizing material which is basically like a small explosion in effect.

If you want to destroy something in an orbit, direct damage is good - but if you can also destabilize its orbit as a bonus, doing so would be the obvious choice. So when attacking a space station, it would be smart to attack it from the "front" (as it goes around on its orbit), so your attacks deal direct damage and destabilize its orbit.

Of course if the station is still intact enough, or help arrives in time, the station's thrusters (or the help) may be able to stabilize the orbit again. But a strong enough strike will take out the thrusters and deliver enough of a push to make sure the other side of the orbit ends up inside the atmosphere (or the lithosphere), sending the station down.

Answer 8

Stations are in oprbit but planet scale height (and suitable handwavium) causes enough drag to require small but ongoing thrust to maintain orbit. Stations are duly streamlines to minimise drag in mormal configurations.

Fragments are not streamlines, have no stationkeeping thrust and usually have high area to mass ratios - so rapid orbital decay.

Answer 9

Once damage appears by the kessler syndrom, the shotgun pellets of debris, puncture the soft upper side- domes and landscapes. There- water exists, that is kept from freezing by the still functioning internals systems. In fact, they heat it up - and it boils- and escapes as steam with the air, howling through the holes. Forming basically thrust vectors, shoving the system downwards. Once the beginning of the deorbitting starts, the internal energy system shuts down- and emergency systems deploy- which are solarpanels. Unfolding, they - involuntarily, increase the drag, sticking into the thin atmosphere, like the blades of a water wheels. They spin up the falling collosus, fast and ever faster- until the internal structure groans and gives, breaking up what once was the pride of a empire. Sometimes, the falling cities are used as weapons, there fall accelerated, halted and guided gy rail guns and lasers from below, to park them over enemy citys. For nothing is sacred, but the sky falling on those heads who want to go back to barbarianism.

Answer 10

Skyhooks.

For some reason, true space elevators did not catch on. Perhaps the planets have high gravity, slow rotation rate, heavy winds... either you can't bring them all the way to the ground, or at least, you keep most of the important functions at a base suspended up out of the atmosphere.

Rotating orbital tethers are also appealing, rotating steadily in space to help ships move up and down, but they don't play well in crowded company.

So your stations are made of robust carbon cables, linking a Lower Base and an Upper Base which are both in geosynchronous orbit, despite being respectively below and above the right orbit for that, due to constant acceleration from the cables. The Upper Base comes together at a counterweight with a single long strand of cable going further, meant to take an elevator as high up in orbit as possible before releasing it. The Lower Base contains all the complex habitats, with many cables linking up to the Upper Base. Damaging some, even most, of the station may not bring it down, but at some point the cables start snapping and that's all she wrote.

The bases need to be geosynchronous so that very small rockets and orbital catapults (like Spinlaunch) or even continuing space elevator cables (if you want to go that way) can target their vicinity with people and packages. Assuming they are just a short distance past the Karman line, it is little more distance around the planet at that height as on the surface, but now people can ride a 'monorail' of sorts sideways at extremely high speed in vacuum, which is suspended from polygonal links between all the different stations around the planet. (The monorail doesn't have to be a continuous physical cable if you're willing to throw the elevators ballistically and catch them at each station or midway point going around the planet...) Basically, think "Global Wonkavator" with perhaps some rocket thrust at the beginning and end. If it does have rocket thrust, then it could also detach in the middle and land in off-grid destinations, though the fuel to rise again would be more expensive and it might have to refuel before moving on the grid again.

Answer 11

Nuclear propulsion deactivation

Each of those stations is located somewhere lower than planet-stationary orbit and as described slowly traverses around the planet to allow "bus hopping", therefore each of them is located on a decently low orbit. But since the station's inhabitants need gravity to operate in vast majority of situations, the developers went with a very simple and elegant solution - they support the station with constant nuclear propulsion so that the station behaves like having less weight but the same inertial mass, this causes the station's orbital speed be a lot lower than what's required by Kepler's laws, AND provides artificial gravity on the station with "down" direction towards the planet.

Once the station has suffered enough damage, it might just lose power to supply those suspenders, which cause its orbit to stop being circular; therefore the station's now astronomical orbit happens to intersect with the planet's ground, and the station goes boom.

Answer 12

Reduce, Reuse, Recycle

There is a limited amount of vital resources for military spacecrafts available. By returning the materials planet-side, they can be reused for the next launch. As a side effect, it also makes it harder for enemies to claim the resources.

While trowing the satellites on the planet to scrap them is less efficient than keeping them in space and repairing, it also ensures that hostile forces can't harvest the satellites for the vital resources. Making their hit-and-steal tactics useless. It would require an enemy setting down on the planet, building defences, and collecting the coveted materials. Which will make them vulnerable to a large attack & orbital bombardment.