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In the 22nd century C.E. human already colonised the entire Solar system, and Earth becomes a dangerous place to live because of the spread of deadly synthetic virus that kills every animals and screws even electronic circuitry by emitting EMP.

Right now Earth is being quarantined and I need a kind of reliable and cost effective barrier to block spacecraft from entering or exiting the Earth's atmosphere. How do I erect such a physical barrier to imprison the left for dead planet?

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  • $\begingroup$ You wouldn't. Satellites with space-to-space missiles would be far more cost effective. With solid barrier, you still need something able to shoot down any spaceship that tries to cut or ram barrier. If you're shooting anyway, barrier has no purpose. $\endgroup$
    – Mołot
    Oct 29, 2016 at 9:46
  • $\begingroup$ The purpose of a quarantine isn't to destroy ships trying to get in. It is there to stop the spread of the virus by keeping people away. $\endgroup$
    – Bellerophon
    Oct 29, 2016 at 10:25
  • $\begingroup$ What technology is available in your time period? Would a net of carbon nanotubes be plausible? $\endgroup$
    – Bellerophon
    Oct 29, 2016 at 10:26
  • $\begingroup$ @Bellerophon That's why all modern quarantine plans incorporate shooting people who try to get out... $\endgroup$
    – Mołot
    Oct 29, 2016 at 10:27
  • $\begingroup$ @Belleropbon: I guess by mid 22nd century we should be able to mass produced it in vast quantities. $\endgroup$
    – user6760
    Oct 29, 2016 at 10:28

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Build a kessler field. It is basically millions of small debris orbiting the earth. With enough of these no one will dare to get in or out.

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    $\begingroup$ Whipple shields are already universally present in current-gen orbital spacecraft. It wouldn't be much of a challenge to overcome for those determined enough to go to Earth despite the obvious dangers. It would also create problems on the defensive side as any interceptor craft/missiles would have to face it too and because it would probably be quite large in radius it would increase the border area to be guarded. $\endgroup$
    – Vaesper
    Oct 29, 2016 at 13:17
  • $\begingroup$ Probably it would be massive, but it is a passive defense without any need for maintenance. If the debris is large enough I am quite sure whipple shields will break before they are through. $\endgroup$
    – Cem Kalyoncu
    Oct 29, 2016 at 14:01
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    $\begingroup$ That's a good point, but we are talking current-gen vs 100-200 years later. If you look at how much spacecraft has developed in the past 100 years, and how much focus there probably will be on dealing with micrometeorites it would not surprise me if shield quality developed a lot during that time. $\endgroup$
    – Vaesper
    Oct 29, 2016 at 15:05
  • $\begingroup$ Well that could be the case, however, it is also possible for regular nextgen shielding would be against few strikes every now and then and would slowly repair the damage. A shower of debris could easily overload them. Obviously this would depend on the particular story, but this interpretation would be within the realm of possibility. $\endgroup$
    – Cem Kalyoncu
    Oct 29, 2016 at 15:12
  • $\begingroup$ Fair point, it definitely is a plausible solution, and clearly the most economical one. Perhaps the fact that it leaves a possible workaround is actually what OP is looking for in the first place. $\endgroup$
    – Vaesper
    Oct 29, 2016 at 15:50
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It kind of depends on how dark you want it to be, but this kind of thing tends to get, historically speaking, very ugly. You can draw inspiration from various fortification techniques, as basically what you're doing is building a fortification around Earth that prevents unlawful entry and exit.

Some concepts to consider:

  • Create a "moat" in functional sense - an area the purpose of which is to make it difficult to traverse quickly. This slows any intruders down and allows your sentries ample time to take them out. This can be implemented with:
  • Orbital-space-mine field. Powerful enough to pose a serious threat even to ships with strong plating, distributed enough to be nigh-unavoidable even with good detection/pattern prediction capabilities except maybe by the smallest of spacecraft.
  • Of course you're going to want sentries as well, to take out any ships nimble enough to avoid the mines/reduce the amount of space mines you will have to redeploy to replenish the minefield. A system of monitor satellites and several strategically located carriers with interceptor craft could work very well for this. Or perhaps, you can use @Molot's idea and have stationary weapons platforms that "snipe" intruders with missiles, lasers or whatever weapon type works in your setting.
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There is one sad truth about quarantine:

People will do a lot to escape death

You are leaving them for almost certain death. The only thing that will keep them down are:

  1. Time

  2. Resources

  3. Fear

Exactly the same three things works against you.

Physical barrier probably makes little sense

If material is common and readily available, tools to cut it are pretty common and readily available. Carbon nanotubes? "CNTs are in fact very soft in the radial direction." Kessler Syndrome particle field? There are Whipple shield. And so on.

For everything you can have with little time, they will have countermeasures in short time. For everything you prepared long time, crime syndicates prepared just as long. No way around it.

So what to use if time and resources give us a tie? The only thing left:

Fear

The last outbreak of smallpox in the 1972 in Yugoslavia required a martial law. Literally soldiers on the streets, roadblocks. Order to shoot anyone moving without lawful reason.

Earlier, if quarantined ship was trying to get to harbor anyway, it was not uncommon to burn it. Or, in some cases when it was sure there was a plague, ships might be burnt and survivors quarantined.

What kept people from moving? From escaping probable death? Only one thing. rulers was prepared to kill some and save nation. Smallpox kills only 30% of it's victims. What's the chance that soldier, afraid to get it and ordered to shoot, will? Probably much higher than that. That's why it worked.

You need weapons. Mine field, satellites with auto targeting systems, and soldiers / police / militia with superior firepower - because automats can be fooled (hacked, EMPed etc) sometimes and you can't risk that.

If virus kills 95% of it's victims, you must go for 99% of quarantine-breakers. Oh, and don't forget to make it a fireworks show. Really. Big, bright, loud and colorful. The only way to stop future breakers is to show them how suicidal it is. For peoples' own good you need to make a show of it.

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I suggest a net of carbon nanotubes plated with some form of immensely tough material in the gaps stretching right round the planet. The best bet would be to string the nanotubes between low orbit satellites with plates attached on both sides of the net. To keep costs low you could reuse old communication satellites as the points to hold up the net. One downside is that this does block out the sun but since Earth is dead anyway that doesn't really matter. If you need light then fix some bulbs to the underside of the inner panels.

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  • $\begingroup$ You know that the sphere around the planet will not hold itself "centered"? What would prevent your net sphere from crashing into the atmosphere? OK, over the equator "orbiting" may work, but further from it weird things would start to happen, I doubt nanotubes would be strong enough to withstand the forces. And nanotubes are hard to breal by tensile force, but "CNTs are in fact very soft in the radial direction." - so they are pretty easy to cut and are not likely to keep anyone in. $\endgroup$
    – Mołot
    Oct 29, 2016 at 10:50
  • $\begingroup$ Why does it need to stay centred? Just let the earth orbit within it. $\endgroup$
    – Bellerophon
    Oct 29, 2016 at 11:47
  • $\begingroup$ The net grav effect between planet and enclosing rigid sphere is 0. No "orbit within" possible. You need to actively correct every disturbance or they'll crash. For net, it's more complicated, but in the "only worse" way, I'm afraid. $\endgroup$
    – Mołot
    Oct 29, 2016 at 12:07
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You'd have to use lasers. A civilization so advanced as to be able to build an effective shield around a planet, or a totally enveloping minefield will have the ability to solve the virus problem too.

Use x-ray or gama ray lasers fired from a constellation of satellites. Orders of magnitude more economical (and effective) than shields.

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