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The fleet is in orbit around the planet. The enemy is in a similar orbit, and they vastly outnumber the allies. The allies have a plan, ‘invisible’ bombs, incapable of being seen will float in orbit to collide with the enemy fleet, crippling them.

Just how ‘realistic’ is this? Could I theoretically conceal them (EM Radiation, etc.) so that against cosmic standard levels, the bombs don’t show up?

Said bombs are about 4m in radius and a sphere.

Never mind how I’m ‘floating’ the bombs over, orbital dynamics isn’t relevant, they just will be on a crash course.

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  • $\begingroup$ Do you mean actually invisible, ie. cannot be seen with a naked eye, or hard-to-detect bombs that make use of stealth technology? en.wikipedia.org/wiki/Stealth_technology $\endgroup$ Commented Aug 29, 2023 at 22:14
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    $\begingroup$ In that case, it's totally feasible. Check the link in my comment above. We don't have orbital stealth bombs as of now, but the idea is nonetheless the same. $\endgroup$ Commented Aug 29, 2023 at 22:18
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    $\begingroup$ That "never mind" part at the end is, IMHO, the biggest weakness of it all. Stealth technology is a thing, but the acceleration needed to get the bombs on a crash course requires energy expenditure, and that is much easier to detect than an inert body. Also, depending on the way they were accelerated, they might still be "hot" somewhere in the EM spectrum. $\endgroup$
    – DevSolar
    Commented Aug 30, 2023 at 11:22
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    $\begingroup$ Large orbital debris (> 10 cm) is tracked routinely by the U.S. Space Surveillance Network. Objects as small as 3 mm can be detected by ground-based radars. source $\endgroup$
    – Wyck
    Commented Aug 30, 2023 at 15:26
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    $\begingroup$ The technology levels of the ships matter a lot. If this is hard science fiction interstellar, the presumed capabilities are way different than interplanetary, or even single-planetary that (somehow?) has a fleet. Second, you rarely get a large fleet without prior conflicts; from those conflicts, both sides will have a great idea of what kinds of weapons work. The naval aircraft in WW2 was one of the few cases where a revolutionary new tactic was tried live, and even it had lots of precedence before it was proven dominant. If this tactic works, both sides will expect it. $\endgroup$
    – Yakk
    Commented Aug 31, 2023 at 13:49

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Velocitas Eradico

You don't even need to make them into bombs. Just put 4m spheres in an eccentric orbit that intersects with the expected locations of the enemy ships at the correct time (fortunately, orbits are very predictable).

A solid stainless steel sphere 4m in diameter masses about 268000 kg, if I did my math right. At a LEO velocity of 7.8 km/s, that's 8,152,560,000 kilojoules, which is around two kilotons of TNT. That'll make a pretty nasty dent in anything made out of matter.

As for detection, just coat them in stealth-bomber paint and let 'em fly. They're small, incredibly fast moving objects. They're pretty naturally stealthy.

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    $\begingroup$ Also, because of the speed differentials in LEO, it is very unlikely that they would detect the thing (even if it were a radio beacon with flashing lights) in time to react. "...A space collision obviously will seldom be the gentle sideswipe of the earthly highway which you can 'see coming' but will more likely strike completely without warning--producing a lightninglike disintegration from explosive friction and heat, with death mercifully instantaneous to all." (Guy Murchie, "The Music of the Spheres V. 1") $\endgroup$
    – Conrado
    Commented Aug 30, 2023 at 0:42
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    $\begingroup$ Wouldn't successful space fleets have the ability to detect large, fast-moving objects? And/or shields? $\endgroup$
    – jdunlop
    Commented Aug 30, 2023 at 3:04
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    $\begingroup$ There's a reasonable argument that suggests any ship capable of interstellar flight must be capable of somehow shrugging off any simple impact even at massive speeds. It's not necessarily true, but it fits in a sci-fi scenario $\endgroup$
    – Separatrix
    Commented Aug 30, 2023 at 13:45
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    $\begingroup$ Space is really, really big - you're very unlikely to hit anything with ballistic projectiles, especially ones taking a long roundabout path that intersect the enemy's orbit at a single point. This will be like shooting a gun into the air and hoping that your enemy happens to walk under the bullet as it lands. You're describing something like an artificial Kessler syndrome, but that only precludes the orbit occupied by the debris. Passing through the debris field can usually be accomplished unscathed. $\endgroup$ Commented Aug 30, 2023 at 16:21
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    $\begingroup$ Space is too big for this to work unless the enemy is completely predictable in their orbit. You have to know exactly where the enemy is going to be. How dumb would the enemy have to be, to let you do that? $\endgroup$
    – causative
    Commented Aug 31, 2023 at 3:22
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The problem is actually scoring a hit in the vastness of space, more than stealth

Stealth is not hard to achieve. As others have mentioned, have the bomb exterior consist of flat, angular surfaces, and the enemy won't pick it up on radar or from reflected sunlight. Also make sure the object is fairly cold, so that it does not have much of an IR signature.

What is harder to achieve is actually making contact with the enemy ship. As long as your bomb is stealthy, it is in free fall and cannot maneuver.

Space is freakin' huge, even in orbit around a planet. If the enemy ships are moving around even a little, then you don't know exactly where they're going to be when the bomb gets there. What enemy fleet would sit there motionless in their orbits, just waiting for you to blow them up? They'd have to be pretty dumb.

So, you should not plan on having the enemy ship actually physically make contact with your bomb. At best, the bomb might pass within a few kilometers of the enemy.

But! The bomb can have its own (low-power, passive) scanners, and can detect when it is about to be near the enemy. From there you have a few options.

  • The bomb suddenly shoots thousands of hypervelocity rounds at the enemy, using perhaps a railgun or some other type of gun.
  • The bomb blows itself up weakly, creating a cloud of shrapnel. If it does this late enough, and the enemy's drives are not powerful enough, the enemy may not have enough time to get out of the way of the cloud, and some of the fragments might hit them. (Remember this shrapnel needs to be spread over an area of multiple square kilometers, so you still have to get lucky.)
  • The bomb was "pre-blown up" and was already a cloud of shrapnel. The problem here is that it's not so stealthy. Some of the shrapnel is going to be angled to reflect the sunlight or radar, and the enemy could see it.
  • The bomb is a thermonuclear device, and blows itself up to hit the enemy with mostly radiation. This can work over fairly large ranges. A 100 MT bomb will deliver 7 tons TNT equivalent per square meter, at a range of 1 km, which is probably a kill depending on how thick the enemy armor is. At a range of 10 km it drops to 70 kg TNT per square meter, still probably a kill. This would be almost entirely in the form of radiation, not shrapnel or a shock wave, so it's not super clear how much damage it would truly do. Based on this article, thermonuclear weapons in space might give lethal doses of radiation to unshielded humans at a range of hundreds of km.
  • The bomb shoots a bomb-pumped laser, like Project Excalibur. This might help focus the energy of the nuke to be effective at greater ranges. A nuclear shaped charge is a similar concept. The explosion might also be used to propel a hypervelocity bullet at 100 km/s. However, note that this concept would only work with relatively low fission yields, not thermonuclear.
  • The bomb has a powerful short-burn rocket in it, and when it's about to pass within 10 or 20 km of the enemy, it suddenly accelerates closer to get in 1 km range before detonating with one of the above methods. The enemy, of course, sees the bomb as soon as it moves, and can try to shoot it down before it gets too close.
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  • $\begingroup$ The bombs knows when it's time for it to be near the enemy, and does late stage correction with its engine when it is too late for the target for evasive action or aiming to take it down. $\endgroup$
    – alamar
    Commented Sep 1, 2023 at 13:48
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If the bomb is perfect pyramid in shape, radar will almost never detect it (similar to stealth bomber but much simpler shape due to no aerodynamics needed). Such bomb may be indeed launched on a collision course and would likely hit the target unless it moves. It can even have engine for late stage trajectory correction.

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    $\begingroup$ Spray on some Vanta Black, and they might be so thoroughly undetectable that even the people who deployed them would have a hard time finding them. $\endgroup$ Commented Aug 30, 2023 at 4:56
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    $\begingroup$ There is still an infrared signature though. $\endgroup$ Commented Aug 31, 2023 at 5:48
  • $\begingroup$ This is only true 1) If the target is the only one whose radar is looking for the missile; AND 2) the tip of the missile is always perpendicular to the radar antenna. Any radar looking at the missile from any other angle will see it, so even if it's invisible to the target, the target's buddies will absolutely see it (and tell them) $\endgroup$
    – automaton
    Commented Aug 31, 2023 at 21:41
  • $\begingroup$ No, radars do not work that way. It will be invisible in reflected waves to almost every radar. Radars only really see curved or diffuse shapes. $\endgroup$
    – alamar
    Commented Aug 31, 2023 at 22:52
  • $\begingroup$ Radars will see flat surfaces if this surface OR any of its boundary lines are perpendicular to radar, with margin of allowed error from strict orthogonality dependent on wavelength and object size. $\endgroup$ Commented Sep 1, 2023 at 8:57
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To be stealthy means to not be detected.
To be that you would need to know the enemies detection methods, and outplay them.
You are the author and you only know which sensor systems are used onboard the enemy ship.
So here is the general gist of, which i can think of right away.

Telescope | Camera | Luxmeter
uses visible light.

  • If manned by a human a black color, against the general black of the universe could suffice
  • If manned by good machine it will detect lights going black, cause while the universe seems to be black, there is always still something very marginally lit there. Hubble Deep Field is the very definition of that.
    Avoidance is later written

Other detection 'meters which i cant think about
Coming from Space the radiation either:

  1. passes through the bombs body (like neutrinos, although they are a bad example) and is of rather no concern

or

  1. is stopped by your bombs body.
    In which case you:

    a. channel the radiation around your bombs body (hard and being tested around the world)

    b. replicate the radiation. even harder under normal conditions, but the bomb is being looked at from one direction, which is almost like a 2D plane => a picture.
    With a couple (thousand?) radiation emitters the bombs can act like the starry sky behind them.

So what about Radiation coming from the enemy ship e.g. lights, radar?
typically (Maybe your world has cooler non-realistic stuff) detection methods send out something to receive that exact something back. We send light to reflect back into our eyes, or send radio waves for bounces of surfaces.
typical methods for avoidance are:

  • send the radiation further
    by not interacting the radiation or channeling it around you, like that invisibility point a. from before.
    (by having a special form, like stealth bombers)
  • cancel the radiation.
    (you know in those stuffy wavy wall stuff in recording studios? afaik that cancels sound waves (radiation) to not create a hall effect)

I have written 'radiation' synonymous with 'emission', which may not be entirely accurate

Edit: Know what you need to outplay: Sensors which want to detect cosmic radiation, can sometimes be very satisfied with seeing an enormous amount of background radiation. Every human would be weirded out by that and notice it. Humans will in general look for something seeming right: not noticing small changes, only huge ones. Maybe you can confuse the radar by sending same waved emissions back at their sensor, confusing the enemy humans, thinking the radar might be bugged out. Be creative, thats what people like to see!

Hope this helps you ^^

TLDR:

  1. identify detection methods, by which radiation they pick up.
  2. Channel radiation around you or replicate a normal radiation
    (or confuse enemy detection methods, by overloading them with radiation of that type, also called jamming)
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There is no stealth in space

Anyone capable of building a fleet of ships is capable of detecting your space mines. If nothing else by detecting the black body radiation those mines will emit (like literally every other object in existance). You don't need active detection like radar to detect them, so proposed shapes/stealth-bomber paint or other nonsense will make no difference whatsoever.

One other thing you have to keep in mind. Even if you magically make your mines invisible, unless this is the first usage of this weapon (with nothing similar being deployed before) the enemy WILL have countermeasures. A screen of small drones would counter that strategy as an example of cheap low tech solution.

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    $\begingroup$ Black-body radiation of a reasonably cool object in a low orbit is actually quite difficult to detect, because there's so much other IR radiation around. If these things only need to stay hidden for a couple of hours, it should be feasible to keep them cold by simply filling with frozen ethanol, or something like that. $\endgroup$ Commented Aug 30, 2023 at 10:05
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    $\begingroup$ I despise this sentiment as it completely bypasses the reality: there is no INVISIBILITY in space. Stealth however can be used to make you show up far later. Especially if there is a background (say a planet) or if the projectile is small and without propulsion so you can cool it down beforehand. $\endgroup$
    – Demigan
    Commented Aug 30, 2023 at 10:48
  • $\begingroup$ And what usage is of having mines in a LOW orbit? What reason would the invading fleet have of giving up their high ground advantage? Sure, they could be sending troops on the ground. But you do that AFTER there is no more defending fleet. $\endgroup$
    – Negdo
    Commented Aug 30, 2023 at 10:48
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    $\begingroup$ "There is no stealth in space" and "space is big, really big." It will be hard to break contact once a sensor is tracking, but it will also be hard to make contact in the first place unless the target is radiating strongly. $\endgroup$
    – o.m.
    Commented Aug 30, 2023 at 17:48
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    $\begingroup$ There's tons of stealth in space. Astronomers concerned with finding NEOs (near earth objects) keep discovering hundreds of previously undetected ones every year, and that's just for the ones big enough to pose a danger if they hit. There must be millions of small boulders out there that we haven't noticed. $\endgroup$
    – causative
    Commented Aug 31, 2023 at 3:27
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Radar-absorbing bombs would be very hard to detect. Your idea is feasible. Also consider a similar missile launcher instead of a single bomb. The missiles could be actual rockets, homing in on nearby ships, or they could be a missile in the alternate sense - anything flying through "the air". In the latter case, the launcher could start flinging out stealthy .5 kilogram kinetic penetrators by the hundreds.

If the enemy ships are using fuel for maneuvering, versus, say, a magic force, then making them expend their maneuvering fuel yields tremendous tactical advantages. Eventually, they'll have to leave orbit or risk destruction.

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Even if your "stealthy bombs" were not suspected, opposing fleets orbiting a single planet will be constantly adjusting their positions to avoid being hit by space junk (sand, paint flecks, bolts, etc) being deliberately released by the opposing force. Both sides will be constantly calculating the possible location of "chaff" released by opposing spacecraft and avoiding those lanes. That your "space junk" happens to be invisible bombs wouldn't change the strategy to avoid such dumb ammunition.

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Forget about bombs, use sand.

Even if you are using invisible tsar bombas, space is big and you'd need a fleet of bombs to maybe cause some splash damage. Enemy ships will be far enough from each other that they're not visible to each other without powerful telescopes, so even powerful nukes would take at most one ship each. Also notice that if an enemy ship changes speed by even a fraction of a meter per second, non-guided bombs will miss them.

A kinetic weapon as suggested by Daniel B is preferrable, but here is the deal: a collision in space against an object of mass $x$ is the same as two collisions against of two objects of mass $\frac{x}{2}$, or four collisions against objects of mass $\frac{x}{4}$ and so on. If you move your ships to a higher or lower orbit, and spam the original orbit with powder going retrograde... anyone crossing paths with that powder is doomed.

Supposing a grain of your powder has a mass of 4 grams, and its coming at the enemy at 16 km/s (remember, you set the grain in retrograde motion), then each grain will have a kinetic energy relative to the enemy of about:

$$\frac{mv^2}{2} = \frac{0.004kg \times 16000^2 m/s}{2} = 512000j$$

That's about the detonation of 100 grams of TNT. It may look like a very small value, but consider that enemy ships will be bathed in this. If a billion grains of powder hit a ship in a second, it would be like all the electric energy that the US consumes in about forty eight minutes compressed into the surface area of the ship and in an instant (napkin calculation using a table from this wiki and some estimates, i.e.: 512kj approximates to 0.1kWh). With this energy amount, even an apartment building sized ship made of steel should be vaporized.

You can then make the powder as fine as you want to make it hard to detect.

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