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In my world Mars is fighting Earth. the problem is what would they (Mars and Earth) use for orbital bombardment. Things to know:

  • set in the early 2100s
  • both worlds are inhabited by humans
  • both have advanced technology
  • Earth has about 11 billion people and Mars has 10-15 million

I have came up with a few ideas but I need to know if they seem realistic or if there is a better way. The ideas are

  • nukes
  • antimatter
  • redirecting asteroids

Are these viable ways for a space invasion, or is there a better way?

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    $\begingroup$ Why would you use infantry for orbital bombardment? $\endgroup$ – L.Dutch - Reinstate Monica Apr 5 at 15:29
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    $\begingroup$ The type of projectile depends upon the goals of the conflict and upon the resources available. Example: If the goal is limited, then maliciously scattering radionuclides across the enemy's biosphere may be counterproductive. However, if the goal is extermination, then nuke away! $\endgroup$ – user535733 Apr 5 at 18:14
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    $\begingroup$ "early 2100s" realistically is leaving us just with nukes and small-sized kinetic missiles. $\endgroup$ – Alexander Apr 5 at 20:12
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    $\begingroup$ What is the goal of the bombardment? Total destruction? Just destroy some cities/facilities? Demonstration of capability to force capitulation? The results required may determine the best method. $\endgroup$ – Turophile Apr 6 at 0:06
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    $\begingroup$ Nukes, antimatter, redirecting asteroids?? Wow, they must REALLY hate each other. If they are seriously considering these options, they have lost all rationality and are consumed by rage, wishing only for annihilation of valuable resources and possibly self-destruction. A war with only losers, no winners. For a more reasonable war, I would consider small "smart" bombs, lasers, and drone/troop deployment. $\endgroup$ – cowlinator Apr 6 at 22:34
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Check out Heinlein's "The Moon Is a Harsh Mistress", that deals with this problem from a lunar perspective.

Bombardment from Mars to Earth (or vice versa) has some unique requirements:

  • Sophisticated navigation systems: at interplanetary distances, even hitting the plane require precise targeting, much less hitting specific cities on a planet
  • Some sort of AI controller: assuming the opposing planet is mounting a defense, they will likely have weeks to detect and prepare for incoming bombardments, and transmission delays can be minutes long, making planet-side control unfeasible.

That leaves three likely approaches:

  1. Slow, heavy, 'dumb' missiles: basically large asteroids with attached engines, and a comparatively simple guidance system — meant to be juggernauts. They plow through defenses by being too massive to deflect easily and cause inertial impact damage like a typical meteorite.
  2. Agile 'smart' missiles: Missiles that are AI guided, and using some type of warhead. they try to actively evade defenses and deliver their payload to selected targets
  3. Small, extremely fast, 'dumb' missiles: This would imply accelerating a smallish asteroid to a significant fraction of the speed of light, perhaps by an immense orbiting linear accelerator. The small size and high rate of speed would make detection and interception difficult, and reduce some of the complexities of targeting (since you could fire it almost linearly). This approach would probably produce and air-bust like the 1908 Tunguska event.

Nuclear devices are reasonable; antimatter devices are somewhat out of reach for 21st century technology (we still don't know how to produce or contain antimatter in large quantities). I'm not sure why you're limiting the conflict to bombardment, though, when something like an Xray or gamma-ray laser (assuming the latter could be designed) might be more effective. A laser like that wouldn't disperse or attenuate much traveling through the vacuum, and could do some real damage (particularly on Mars, which lacks Earth's protective atmosphere).

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  • $\begingroup$ well mars is the planet that's supposed to win... so $\endgroup$ – Topcode Apr 5 at 16:41
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    $\begingroup$ Note, the OP specified early 22nd century, not 21st. $\endgroup$ – RBarryYoung Apr 6 at 20:12
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    $\begingroup$ @RBarryYoung: Duh, you're right, I misread. $\endgroup$ – Ted Wrigley Apr 6 at 20:14
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For pure and simple orbital bombardment, you want to cram the target with objects falling from the sky. Any rock big enough to survive the atmospheric impact will suffice, and will spare the attacking side from excessive investments.

If you are going to use nukes or antimatter, the vast part of the damage will be delivered by the explosion. In that case you actually want to be sure that the atmospheric reentry doesn't damage the device, complicating your design.

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  • $\begingroup$ I wonder if a nuke would still be effective if placed in the center of a giant rock... $\endgroup$ – Redwolf Programs Apr 6 at 14:44
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    $\begingroup$ This answer has the key – lots of impactors. Forget dropping explosions, as @peristence shows below, the impactor has plenty of energy. Unfortunately, their trajectories are easy to plot as they move between planets. Lots of impactors would make it harder to track them, and very hard to defend against. Don't worry about targeting cities. Even the idea of impactors would cause civil unrest, and just the dust etc from a big impact in an uninhabited area will alter the weather globally. $\endgroup$ – Dan W Apr 6 at 15:00
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Tungsten rods are the typical munition of choice in a "Rods from God" scenario.

Take a 10m Tungsten rod of diameter 1m.

It would have a volume of 10m^3.

A mass of about: 200000kg

Consequently, falling from a low earth orbit of 1000km, it would gain:

A kinetic energy of: 200000 * 9.81 * 1000000 = 2TJ.

Further to this, an impact of this magnitude would cause a huge nuclear reaction at the impact site releasing yet more energy.

The kinetic energy of this scales proportionally to the mass of the rod and the altitude it is launched from.

Tungsten is usually chosen because it has a melting point of 3,422 °C so a rod made of it would likely survive re-entry intact.


As an added aside, another benefit of using rods over meteroids is that the concentrated impact point will result in it being able to penetrate a target very deep in the ground, underground bunkers would not be safe.


Since the comments have exploded in a debate around the practicality of "dropping" something from orbit, the other option would be to slingshot it around the moon in such a way that the return path was a direct collision with your target. This would require about the same amount of energy as dropping it from orbit (it takes about whilst also raising your effective altitude to 400,000km.

This would give you an 800TJ impact from the aditional altitude alone.

You could also manufacture the rods in lunar orbit to further reduce the energy costs.

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    $\begingroup$ "Further to this, an impact of this magnitude would cause a huge nuclear reaction at the impact site releasing yet more energy." No, it wouldn't. Energy release akin to a nuclear weapon, yes. Actual nuclear reaction? No. $\endgroup$ – ceejayoz Apr 6 at 16:25
  • $\begingroup$ @ceejayoz I'd imagine at the energies involved there would be some nuclear reactions happening. Not a chain nuclear reaction like we think of with a nuclear bomb though. $\endgroup$ – Sdarb Apr 6 at 16:51
  • $\begingroup$ Tungsten's primary benefit for a rods from god weapon is its density. If a orbital bombardment weapon is heating up very much, it's shedding a lot of its potential damage on the way down. $\endgroup$ – Charles Bamford Apr 6 at 19:28
  • $\begingroup$ @ceejayoz actually, I think you'll find it absolutely would cause fusion at the impact site... The temperature and pressure at the impact site would be astronomical, fusion would definitely occur which would then increase the temperature and pressure further creating yet more fusion until the entire system exploded. $\endgroup$ – Persistence Apr 6 at 20:16
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    $\begingroup$ You can not just drop an object out of orbit. It takes an incredible amount of energy to take an object out of planetary orbit. In fact, it’s of a similar order of magnitude as the amount of energy it took to put it into orbit (which is approximately the same as its orbital potential energy). $\endgroup$ – RBarryYoung Apr 6 at 20:18
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While it seems that the kind of attack the nations of both planets would use are the same, i would argue that it is way more asymetrical.

In General

Every possible attack is more or less a balisitic object and have to follow newtonian physics. Asteroids and Nuclear rockets (anti-matter seems still way of to be realistic in 2100) will travel around 8 months before hitting their target. Enough time for counter measures if detected.

In this regard, asteroids are exceptional good. They are dark, extremly cold and dumb. But hey are also untargeted and its impossible to hit something precisly. And depending on the mass hard to destroy, even if detected early.
Rockets can have pinpoint accuracy, destruction comparable to asteroid impacts, but will need power and heat to work, so detectable by their emissioned heat. Also, EMP or simply shooting at it may destroy it.

Mars

Earths population density is way higher than mars. That means Mars could hit anything and would have a catastrophic effect on infrastucture and morale of the population. Additionally, Mars is closer to the asteroid belt that has some asteroids to throw at earth.
That would make asteroids a great weapon to use by mars, as even a big asteroid hitting empty land or even the ocean can have devastating effects. Biggest problem is the size and mass to move those objects on a trajectory hitting earth. Still seems like the best weapon

Earth

I assume that the terraforming of Mars has not been done yet, so people on mars will life in sheltered groups, maybe below the mars surface with only a few buildings and accesses on the surface. Every attack has to hit its target or is wasted. But every hit will also likely have more devastating effects. One domed city hit could mean 3 Million lives, with no chance of survival. Additionally, while moving an asteroid on the mars trajectory would need less fuel, reaching it would take longer than those estimated 8 months for an direct attack with rockets. Also impacts on the uninhabitated parts of Mars would have less of an effect, as the atmosphere is thinner and so does reduce the effect of greater explosions.
So earth would definitly would use something more precise like rockets.

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  • $\begingroup$ yes mars is not terraformed and most population would probably be in small 1k-5k size clusters around the planet aside from 3 or 4 big cities $\endgroup$ – Topcode Apr 7 at 4:09
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Phobos

The Earthlings slingshot rocks around the sun and Jupiter. They are not aiming for Mars. They are aiming for Phobos, the Martian moon and a rock bigger than anything practicably shot from Earth.

If Phobos' orbit is slowed, it will crash into the Martian surface. Once it starts coming down there will not be much the Martians can do about it except evacuate the premises.

https://www.space.com/20346-phobos-moon.html

Doomed

But Phobos won't zip around Mars forever. The doomed moon is spiraling inward at a rate of 1.8 centimeters (seven-tenths of an inch) per year, or 1.8 meters (about 6 feet) each century. Within 50 million years, the moon will either collide with its parent planet or be torn into rubble and scattered as a ring around Mars.

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    $\begingroup$ And of course, the Martians can do the same by just continually blowing chunks out of Luna. Blow enough chunks, and you get a Seveneves "White Sky" situation, for which--assuming Neal Stephenson did his math right--there's basically no going back. $\endgroup$ – A.E. Stephenson Apr 5 at 21:59
  • $\begingroup$ In Kim Stanley Robinson's Mars trilogy, Phobos is dropped on Mars (or at least the space elevator connected to it... been a while since I read it). Might be worth a read, as he wrote it pretty well. $\endgroup$ – Dan W Apr 6 at 14:57
  • $\begingroup$ @Willk fantastic answer but mars is supposed to win so i cant exactly use this idea $\endgroup$ – Topcode Apr 6 at 19:11
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    $\begingroup$ @Topcode - no, it is great. Earth brings down Phobos and thinks it has delivered a knockout punch. The dust obscures everything. But the Martians were ready for Phobos to come down. Up thru the dust comes their counterpunch... $\endgroup$ – Willk Apr 6 at 19:27
  • $\begingroup$ @willk well, i could do that $\endgroup$ – Topcode Apr 6 at 19:31
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They could use any & all of those things, apart from getting the stuff out of their own gravity well pretty much all the same factors apply as in answers to questions about how would the moon do it.

For both mars & earth staging their attacks from their respective moons is going to be easier.

Raw asteroids need to be big enough to have a worthwhile payload left over after reentry.

Kinetic bombardment with shaped metal (or perhaps rock) with a ceramic heat shield skin could be smaller & also relatively cheap depending on the materials used.

Its thinner atmosphere makes burnup considerations less of a concern for earth's attacks on mars.

You may find something useful in answers to this space exploration SE question with regards to launches from the moon.

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Antimatter does not work, it will react with the atmosphere on the way down.

For psychological impact, Martians could use Earth forces for orbital bombardment. Specifically, POWs and captured ships. The actual damage would be relatively small, but the demoralizing effect might be great.

Earth forces could do the same, but the outrage caused in such a small population might outweigh the demoralizing effect and the forces used as the bombardment might become instant martyrs.

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RODS!

You would need to place in orbit around the planet a satellite with a targeting system. It would be filled with dumb metal rods. The sheer kinetic energy created from just releasing the rods in low gravity would obliterate any target.

I see this as a useful solution if the point of the war is to keep the planet for resources. If you nuke it, then you limit the resources you can get.

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  • $\begingroup$ well this wouldnt work exactly how you thing, you would need propulsion on the rods and you would not be able to get good accuracy nor the effects you think because mass would be taken up by fuel. the point of the war is for mars to gain freedom from earth in order to gain profit. $\endgroup$ – Topcode Apr 6 at 19:09
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Mars Can Use Planetary Mirrors to Create "Death Rays"

Mars could deploy mirrors on it's surface to gather radiant energy from the Sun then beam it back at the Earth.

The principle is the same as found in research telescopes, satellite transmissions, and radar.

enter image description here

Each mirror would be a few thousand kilometers in size and made of an array of smaller mirrors. The place where the red "x" is would be where the energy is collected.

To vary the strength of the beam, the number of array mirrors "turned on" could be adjusted. A couple of really strong mirrors or prisms at the "x" would redirect the energy back out toward the Earth.

It could only be "fired" when the mirror was in Martian daylight, thus they would install several at different locations on the planet so they could fire more frequently.

It could also be strategic to have multiple firing locations. The Earth wouldn't know which from location on Mars would be firing. Also, it takes light a little more than three minutes to travel between Earth and Mars, so there would not be a lot of warning.

This tech would heavily favor Mars. A beam from Mars would be much more powerful than one from Earth for two reasons:

  1. Mars does not have an atmosphere to block collection of solar energy
  2. Mars wouldn't have many surface buildings or features to hamper construction of really large mirror arrays

The amount of solar energy collected would be massive. So massive, that they may only be able to fire each mirror array a couple of times a day because of the danger of overheating.

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  • $\begingroup$ while this is a great idea, its completely impractical for mars to get the needed supplies, $\endgroup$ – Topcode Apr 7 at 4:11
  • $\begingroup$ @Topcode I completely agree in terms of present day logistics and technology. But if Mars has a population of 10-15 million, can fire nukes, and isn't worried about re-supply from Earth (since they are going to war with them) they should be able to figure out how to create mirror arrays ;-) $\endgroup$ – Jesuisme Apr 7 at 14:02

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