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There is a common sci-fi trope, that if there is a war in space, you should have planetary defense in place to protect you from intruders from space. But: is it plausible?

Let's give you this scenario: There is a second Earth (Earth B) which is currently at the same level of technological development as we are now. Earth B does not like having our similarly advanced civilization near them, so they want to attack us. For a while now, they have been spending all their money and science in order to attack us.

Earth B is in a similar position to Mars, so given current tech, it will take them about half a year to approach us.

We, Earth A, know that they will have to stay in orbit for at least a day before they can come down and start attacking us. Their type of attack will be: Gather forces on orbit, land and attack us on the ground. So no orbital bombarding

Given that everyone is willing and wanting to cooperate with planetary defense system, what would be good ideas to protect us from the attack from space?

Is it planetary defense? Or is it actually better to wait for them to come down?

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    $\begingroup$ They totally would use orbital or even longer rang bombardment to attack the planetary defense system. $\endgroup$ – Ville Niemi Mar 26 '15 at 8:05
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    $\begingroup$ @VilleNiemi I sandboxed this question and specifying type of attack is one of things which narrows it down from "too broad" question. At this time, accept their type of attack as a fact $\endgroup$ – Pavel Janicek Mar 26 '15 at 8:19
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    $\begingroup$ LOL. But sadly true. It is much more plausible that people responsible for starting a major war are stupid than anyone would like from people that have the ability to start wars. $\endgroup$ – Ville Niemi Mar 26 '15 at 8:41
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    $\begingroup$ If they're currently at the level of technological development that we are now, then they would pose no threat at all - getting here alive would be a big enough accomplishment. $\endgroup$ – coburne Mar 26 '15 at 13:34
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    $\begingroup$ N.B. for space writing questions, see this website. In this case, there's an entire section on this. $\endgroup$ – imallett Mar 27 '15 at 4:46

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I would definitely place my bet on Earth B because we are human and they are inhuman.

Assuming both parties do not have any technological and biological advantage over each other and the inhabitant of Earth B had already parked their space-readied rubbish truck next to our moon landing site and vandalized the flag already. I believe depending on the threat level posed by our neighbor we can take necessary counter-measures to defend Earth A. (wait since when did the name changed?)

Threat Levels and Countermeasures of a effectively planetary defense system:

Level 0 - Inhabitant of Earth B uses radio wave signals to spam us

Counter 0 - Reciprocate

......

Level 1 - Our earth bound and deep space telescopes detected UFO travelling at a fast velocity heading toward us in tight formation and spell "Earth A sucks" on our radar screen.

Counter 1 - Firstly we need to find out their intention by hailing them the Star Trek style and find out the type of propulsion mechanism their vessels used for the entire journey. (is it a one-way trip like a homing missile) Secondly we can intercept them mid-space with our trump card... the space junks good luck to them trying to stay in one piece. Thirdly we must establish a committee on Earth and reveal our top secrets weapons between countries... united we stand.

......

Level 2 - The UFO are within hours away from us and they are most probably launching projectiles in our direction.

Counter 2 - The committee on Earth A must be saying something like this "why aren't we the attacker and where the hell are Mars and Curiosity?". We need to consider if their projectiles are capable of penetrating the Earth A several layers of atmosphere, most crucial is what will happens if the content inside the projectile is dispersed in our atmosphere will it spread and block out the sunlight and cut off potential energy supply and turn our weather against us. We must deploy rockets to intercept them to mitigate the impact caused to Earth A.

......

Level 3 - Dogfight

Counter 3 - Hopefully only a insignificant amount of UFO managed to get past our defense by now and they are turning tails. Suppose in the worst case scenario these aliens penetrated our atmosphere and I believe they must be targeting our power source and supplies first. Most of us on Earth A will be located underground to seek shelter from nuclear holocaust, else we will most likely engage in a awe-inspiring dogfight ever in the history of mankind.

......

Level 4 - Land Invasion a.k.a catfight

Counter 4 - where are our storm troopers? our choices are scarce but not limited, our weapons should have not problem granting the alien death wish. Most probably we must drag the invasion as long as our resources last so using guerrilla tactics if we are outgunned (I remember you mentioned they are only as advanced as us therefore just deploy the seal teams or drones in covert mission to sabotage their ration and power source) I don't recommend using biological weapon even though they may be inhuman. Once they are wipe out we must send fleets to harass them on their planet and force them into negotiation, else the future of mankind and alien alike will be less promising.

......

Level 5 - Planetary Destruction eminent

Counter 5 - [fall silent]

Verdict: Planetary defense is absolutely fun to ensure the survival our species, so pass the bill already!

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  • $\begingroup$ Excellent answer. Funny, but pragmatic. +1 for 'where the hell are Mars and Curiosity [probes]' $\endgroup$ – nothingisnecessary Mar 26 '15 at 20:18
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I see no reason not to start shooting when they get in range. Why give them the initiative? I do not think there has been a war in history where the defender said "awww, don't shoot them while they are still in the transport, that's not fair."

(If there was such a war, the defenders probably lost, because they were idiots)

Honestly, why even wait until they get into orbit? Start launching guided missiles as soon as you detect them heading your way. Any kind of hit is going to take a transport out of commission. If they're flying close together, the debris might even take out more. And you'll have plenty of time to work on other defenses while they close in.

In order of defense, I would expect this:

1) Long range bombardment while the transports are still in space. Probably few hits, but any hit counts. And you have plenty of time anyway

2) Bombardment while the ships are in orbit; preferable from space based platforms. Everything you launch from ground to orbit has to fight against gravity, but once you are in orbit, cannons will work just fine.

3) Ground to air bombardment when they try to land. At this point the ships will be effectively unable to divert from their course, so it's shooting sitting ducks.

4) If anyone is still alive at this point (they must've sent a lot of attackers), this is where you send in the aircraft and carpet bomb the crap out of any landing zones. It should be a piece of cake to determine where the ships came down and your bombers should be already in the air. Any artillery in range can also open fire, of course.

5) I doubt any attackers are still interested in fighting at this point, but if so, now is the time to do actual ground combat.

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    $\begingroup$ I do not think there has been a war in history where the defender said "awww, don't shoot them while they are still in the transport, that's not fair." How about "Don't fire until you see the whites of their eyes"? $\endgroup$ – Mason Wheeler Mar 26 '15 at 14:06
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    $\begingroup$ That's not about it being unfair, that's usually either about conversing ammo or only getting one shot in and making it count. If earth's defense force's resources are limited, definately start skipping the steps that have the highest $/kill ratio first. I was assuming no budget restrictions, this being a full-on planetary invasion and all. $\endgroup$ – Erik Mar 26 '15 at 14:15
  • $\begingroup$ So you've not read The Homecoming of Beorhtnoth Beorhthelm's Son? $\endgroup$ – TRiG Mar 26 '15 at 14:59
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    $\begingroup$ Don't fire until you see the whites of their eyes - they were using muskets and black powder weapons that got off one shot and took about 15-60 seconds to reload. The equivalent in this scenario could be don't fire until you get a lock on their engine signature (since missiles are limited and you want to wait until you can deliver maximum effect / guarantee a hit) $\endgroup$ – nothingisnecessary Mar 26 '15 at 15:19
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    $\begingroup$ 3b) Air to air attack as they try to enter/land. Orbiting things move really fast. Unless you have 100% ground coverage, it's going to be hard to know exactly where they're coming down until they start (and they'd probably try to avoid heavily defended areas). Aircraft can generally get to the target quicker and patrol a wide area. This is probably not a necessary step, but why leave the fighter jocks out of the fun? $\endgroup$ – Geobits Mar 26 '15 at 20:25
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It is definitely a better idea to attack them in space. And the reason is simple: Each of their ships will hold several (most likely hundreds or thousands of) soldiers. You can neutralize each of these groups with a single hit. It does not even need to be a good hit, it is enough to damage the ship badly enough to prevent it from entering the atmosphere.
Once they are landed, you have a larger number of smaller targets. Plus, on the ground they have additional options, like taking cover, evasive actions, they will be able to breathe without technical assistance, and all that.
So: "convince" them to abort their attacks while they are easy targets. A few well aimed missiles while they approach, some more while they prepare for descent, and the rest should not be much of a threat any more.

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  • $\begingroup$ >> "It is definitely a better idea to attack them in space. (...)" Also, you do not want ship wrecks falling on your planet, and possibly cities. $\endgroup$ – Trang Oul Mar 26 '15 at 7:39
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    $\begingroup$ @TrangOul One of the handy features of our planet is it's atmosphere :-) That would handle a lot of debris. And a shipwreck hitting a major city may be inconvenient, but an invading army can make your day even worse (that is, unless you are one of the people actually being hit by the wreck, of course :-) ) $\endgroup$ – Burki Mar 26 '15 at 8:48
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You start with the sci-fi trope idea of planetary defenses, but then you restrict the problem immensely. I doubt you'll get meaningful answers for "is it plausible[having planetary defenses]?"

In sci-fi that has planetary defenses, the reason is due to FTL travel capabilities - usually justified through warp drives, sub space, hyper space, etc. The problem with such spaceships in such sci-fi is that it usually comes with the caveat that you can't attack someone in hyper space, sub space, etc. That is to say, you can't have solar-system level defenses(missiles, rail guns, nukes, space mines, etc) that can destroy enemies before they show up on your doorstep. They just pop out of FTL in orbit around your planet. That is why you need planetary defenses in those pieces of sci-fi(typically, this is very generalized of course). You need planetary defenses when your enemy is able to transport a formidable military presence to your planet.

But you restrict your problem to sub-light speeds...not even relativistic speeds...I don't know how fast we travel to Mars but I think it's somewhere around 20,000-40,000 mph....not very fast at all. You also require meatbags to be in the payload. If Earth B is interested in wiping out intelligent life on Earth A, and Earth B is in a higher orbital than Earth A, then the simplest approach is to de-orbit tons and tons of 'stuff' into Earth A's orbital. With our current tech, NASA is already planning to move a 4 meter asteroid into lunar orbit. Earth B, given the same tech, could move a much larger asteroid and just try to ensure it impacts Earth A. Earth A might be able to nuke an asteroid or two - if they even detect them, which is not guaranteed with current tech - so you just throw a couple hundred down at them. By moving down in orbit, these asteroids are effectively converting potential energy to kinetic energy just like a ball falling off of a table. Earth A would have no hope of stopping these things...and they would cause impacts far more powerful than nuclear weapons.

The problem with current tech space-wars as you've defined is that establishing a beach head on another planet is almost impossible with what we have available to us. You're going to expend a huge amount of your GDP just building the rockets, fuel, and containers to carry your meatbag soldiers, and getting them in orbit around another planet. Then you have to not die while in transit and while in orbit, then land on a teeny tiny chunk of land, and start killing billions of Earth A natives on their home field with a force that's likely in the thousands or millions at best. There's no hope for victory in that kind of scenario...literally impossible. Earth B would lose even if Earth A didn't have planetary defenses, because Earth A could carpet bomb Earth B's landing site(s). It actually makes things easy for Earth A because there's no ethical/moral dilemma to wiping out the entire enemy population - they're all soldiers...no civilians whatsoever...blow the whole thing up. Given current tech there's no way Earth B has also transported anti-air, tanks, helicopters, fighter jets, etc - those things are way too heavy - so they have no way of obtaining or maintaining air superiority. The only thing they'll have(given your spec) is infantry. It would be a massacre.

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  • $\begingroup$ There still is a reason for planetary defenses even if you could defend the star system (for example, the Honor Harrington universe where the hyperdrive can't be used where the gravity gradient is too steep.) The issue is one of density--you can mount a much thicker defense around your high value targets than you can mount across a star system. $\endgroup$ – Loren Pechtel Mar 27 '15 at 1:54
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With anything remotely resembling current technology (i.e. rocket-based propulsion), their attack is doomed almost no matter what the defenders do. They simply would not be able to transport enough troops and weapons to fight a war once they got here. Their best bet would be to bring a load of Earth-Bian bacteria and viruses and hope to start a pandemic. But regarding planetary defense, we already have all the weapons we would need to destroy them.

The first line of defense would be launching weapons at them during their transit. We already have the ability to send several tons of equipment (orbiters and rovers) all the way to Mars. That means that we have the capability to send weapons to intercept them. A ship trying to fly between planets using rockets has very little ability to maneuver because any attempt to evade will mess up its trajectory making them risk missing their orbit and flying right past the Earth.

The second line of defense would be while they were in orbit. If their orbit was very high, this would be a poor place to engage because large rockets would be needed and their maneuvering capability would be much greater than during their transit phase. But if they are in a low-orbit that brings much smaller, cheaper and more numerous missiles into play. China has successfully destroyed a satellite with a missile launched from a ground vehicle, while the US has destroyed them with air and naval launched missiles.

The third line of defense would be while they were landing. Current technology requires that a vehicle de-orbiting lands with quite slow speed. This is generally accomplished through the use of parachutes. It would be trivial for regular air defense (fighters and land and sea based missiles) to shoot down these craft. If they want to come in faster, they would require the use of a shuttle-like landing craft. They would still be within the normal speed range of aircraft, though, which means that air defenses would still be effective against them. Additionally, an aerodynamic landing vehicle would likely force them to try and land at an existing airport, making post-landing defense easier.

The final line of defense would be once they landed. Even if the defenders chose to completely forgo attacking them until now, they would still have little trouble defeating them. With current technology getting 1000 soldiers and small arms to another planet would be a huge accomplishment. By comparison, the combined militaries of the world have millions of soldiers, tens of thousands of tanks, artillery pieces and aircraft, tens of millions of police, reservists and paramilitaries, local knowledge and great intelligence provided by the billions of civilians, and a huge logistical base.

In short, a near-current tech civilization wouldn't stand a chance at an interplanetary attack unless they came up with some breakthrough propulsion system, or were willing to resort to orbital bombardment. And even then it would be extremely difficult.

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Main problem I see is that we don't know where they would land, so we can't entrench, set up an ambush or concentrate our forces. I guess at worst they'd stay in orbit, rapidly spinning around earth until they decide to enter in the last spin before the point of no return to give us the least time to react. They could also try to spread, but that would give us the advantage of not needing as many forces in a single location.

Their orbiting ships would have a lot of extra features to help with their space attack. They would be able to go into space (airtight) and to descend into orbit (either a lot of fuel or good heat shields). This is going to make the ships fairly expensive, not too maneuverable and limited firepower compared to simple jet fighters. So just having low atmosphere air superiority should be enough to take them out. It's not insanely hard to build that up rather quickly (if we all work together we might already have enough) and we'd spend significantly less resources than they would. We could have bombers and ground troops as backup should some of their ships manage to land (goal is to avoid this).

It would be mainly an organizational issue of making sure the fighters can be at the correct place at the correct time. I have no clue how fast they could potentially land. This would dictate how we'd have to spread our forces for maximum coverage. We can allow for somewhat more risk for barren places or deserted places like the Sahara. We only need to make sure to arrive there with bombers just as they're landing (before they've had time to set up decent air defenses).

I'm also very in favor salvaging the wreckage. This way we start out with extra resources for fending of the next attack or a counterstrike.

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Ok, if we're assuming they would NOT fire cluster-nuclear bombs from space and totally wipe the planet clean... hmmmm...

I think it would make more sense to have planetary defense. You could just fire smart missiles from the ground into space and they could seek out ships and take them out before anyone had to deal with anything on the ground. The ships would burn up in the atmosphere. No collateral damage. If you wait for them to get on the ground there'd be explosions and gunfire. Civilians would get hurt; infrastructure would be damaged; resources would be damaged or stolen; people would be unhappy.

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The other answers have already explained why it would be easier and better to take them out while they are still in space.

However, I would like to point out that the best place to station your planetary defenses would be not on the surface but in satellites in orbit.

  1. Any rockets launched from the ground would need to leave the atmosphere and enter orbit, which requires quite a lot of fuel. A rocket launched from an orbital platform could have a much higher payload.
  2. Any direct-energy weapons (lasers, masers etc.) or kinetic weapons would have to pass through the atmosphere first, which would cause refraction and dissipation. When fired from orbit, these weapons would have a much higher efficiency and precision.
  3. When using beam- or kinetic weapons, a ground station needs an unobstructed line of fire, which means that each ground-station has 12 hours each day during which it is on the wrong side of the planet. Also, the aforementioned problems caused by the atmosphere would be worse when the target is low above the horizon, so it only has its peak efficiency once every 24 hours. A satellite, on the other hand, would either be on a low orbit with a lot shorter periods of being usable and unusable, or on a high orbit where the planet blocks a lot less of its field of fire.
  4. Satellites have a chance to evade incomming kinetic weapons through changing their trajectory. This could also help against beam weapons when those weapons require several seconds of constant exposure to do damage and are fired from several light-seconds away. Ground bases, on the other hand, are sitting ducks.

There is, however, one argument for ground-based defenses: They can be concealed. An orbital weapon platform is impossible to hide and would be the first target when launching a surprise attack. But a ground base can be placed underground where it is protected and hard to detect until used. Also, the atmosphere does not just acts as an obstacle to them, but also to the enemy who try to take them out from orbit.

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Is it planetary defense? Or is it actually better to wait for them to come down?

Planet defense is better. Too many risks and collateral damage if they land, and predicting the enemy's LZ may not be possible until the last minute (though you could be computing their trajectories with RADAR or something). And if possible a counter attack should be prepared.

what would be good ideas to protect us from the attack from space?

  • An array of satellites with solar-powered space lasers (and/or ion cannons, but they could shield against this). Throw a new 'targeting' lens on the Hubble Space Telescope and turn it into a super-space-laser focusing device.
  • Send up rockets full of daycare kids with runny noses, kamikaze style, to crash through their hulls and infect them with our viruses, War of the Worlds style. (Or pretend to be their friend and offer a gift of smallpox blankets.)
  • Repurpose the Star Wars project like they did in the 1979 film Meteor
  • Create a defensive 'net' of explosive-laced satellites with proximity friend-or-foe detectors; a space minefield.
  • Retrofit rockets and the international space station with armaments from fighters, bombers, and gunships. (Orbital defense fleet)
  • Avoid large scale nuclear weapons detonating in proximity to the Earth's atmosphere, but we could possibly develop smaller tactical nukes. You only need to disable one of the following systems: navigation, engine, or life support.
  • Have AA guns and typical surface-to-air defenses ready to blast any large debris that may cause collateral damage.
  • Introduce them to alcohol and legal marijuana. Maybe they don't have recreational drugs on their planet which is why they are being such uptight ***holes.
  • Share our love of Jesus Chris and the Book of Mormon with them; perhaps we can persuade them to a path of peace.

Avoidance

If at all possible, avoid conflict. It is arguably in our best interests not to wipe each other out, but to work together and share technology so that we can ultimately leave our solar system to colonize other systems since we know that our Sun has a finite lifespan. A political approach should be attempted, and some compromise reached.

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I concur with all the rest of these answers that the best option is to attack while they are still in orbit, but would like to add an extra reason why. Planetary defense is not always ideal, or even workable (space being really big and all), but in this case it is actually pretty easy.

Per your description, the invading force is near modern tech levels. This makes preventing an orbital landing painfully easy. Re-entry is freaking dangerous, and without more advanced technology (like some kind of deflector screen) they are reliant on the same heat shielding that we use when entering the atmosphere.

A piece of foam striking the heat shielding on the wing of the Space Shuttle Columbia was all it took to compromise the craft such that it broke up and burned on re-entry. I assume an invading force would have thicker heat shields, but it would still be comparatively fragile. You don't have to hit them hard enough to destroy their ships, just compromise their heat shields and re-entry mechanics will do the rest.

Simply put, re-entry unleashes absurd amounts of energy onto your ship as you slam into the atmosphere at orbital speeds. Without an intact heat shield, all that energy goes into the ship. Boom, Shred, Burn. You get small bits of shrapnel falling to Earth instead of a landing army.

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It is definitely a planetary defense situation:

  • I think it's a fairly bad idea to Nuke them because of radiation fallout in our own atmosphere. If it's done beyond Lunar Orbit, then we have a fair chance of avoiding nuclear fallout on Earth and our atmosphere.

  • Since it's said that Earth B is roughly in same place as Mars, I think we can use brighter sun at Earth A to our advantage. Such as using Large mirrors placed in stationary point around earth, perhaps in L2 position to take advantage of large amount of solar radiation and concentrating in a very narrow beam to possibly melt the incoming ships. Given our current technology and similar development of Earth B the ships wouldn't be made of a very thick material with a high melting point like Tungsten but rather something light like carbon fiber or perhaps Aluminum, which can surely be compromised using a very very focused solar beam, or using this energy to generate a high energy Laser beam (theoretically possible, look at Boeing YAL-1)

  • It's fairly straight-forward to assume that if you breach their spaceships then the occupants won't survive for too long, so with sufficient number of solar concentrators we can fend off any invasion far before they reach any closer to earth.

  • In any case, I think any alien species would be stupid to attack Earth A in person (and thus deserve to die to be brutally honest) The smarter approach would be to send biological/chemical agents and disperse them in target planet's atmosphere with the help of small probes which will completely burn up at about 1000 meter above surface and then disperse the contents into atmosphere. If the probes are comparatively small, then they would be too small to detect by telescopes until they have entered our atmosphere and started burning, at which point we won't be able to do anything at all. Plus given usually high number of small meteoroids hitting Earth surface, we will most likely ignore them. Although this will certainly make the planet inhospitable until you sterile entire atmosphere of your dispersed agents, a really tall order to perform with our current state of technology

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  • $\begingroup$ Considering all the atmospheric nuclear tests, and rather minimal effect of those (globally and in the grand scheme of things), I don't think radiation is something to worry about when defending against space invaders. Also we can make pretty clean bombs today, I think. $\endgroup$ – hyde Mar 26 '15 at 19:40
  • $\begingroup$ @hyde I think what you may not have considered is, since NTBT was signed, we haven't tested any Nuclear weapons above ground. Also when we are defending against planet invaders, we will use colossal amount of Nukes to damage all the incoming ships. Plus radiation fallout from these nuclear tests has already been reported to cause more than usual number of cancer cases near the test areas. So in short, Nuking our own atmosphere seems like a bad idea at least in 21st Century knowing all the side effects. $\endgroup$ – Mellon En Eina Mar 27 '15 at 9:35

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