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I'm writing a setting where superEarths abound, frequently with intelligent life, but Earth-size habitable planets are rare, with the focus being on the five intelligent species from Earth-size planets functioning as traders. As per Space.com "No Way Out? Aliens on 'Super-Earth' Planets May Be Trapped by Gravity, the many intelligent species on these superEarths are functionally confined out of space. (That's about relatively heavy superEarths, though the setting says the number of intelligent races trapped on their planets far exceeds those that can build a space program and construct interstellar spaceships.)

This is a fairly hard-science setting; there's subspace handwavium where ships can get from point A to point B through empty-enough space at light speed for a reasonable energy cost, and there probably is limited artificial gravity to avoid rotational issues on small ships or issues of life without gravity. Otherwise, it should all be accord with physics as we know it. Specifically, objects in subspace crossing 1 millibar of atmosphere will immediately self-destruct and exit into real space in the form of subatomic particles, and around the microbar level, would introduce around 1 Gray/microbar to the objects, quickly fatal to computers or living creatures.

If they came across a hostile species, what could that species do to them? I'm trying to be realistic here, but optimally there'd be some danger, but not too much. Eyeballing XKCD on laser pointers a laser weapon might be a risk; thicker atmospheres combine with a much smaller target make taking out a single ship conceivable; anti-laser armor would develop as a defense, to the extent that's possible. It seems unlikely a missile could escape the atmosphere with enough maneuvering fuel to threaten a mobile ship, but possibly if one was clearly negligent in threat awareness. Even without advancement in computer technology, 2020's AI should be enough to give you plenty of warning. And there's Trojan horses—launching objects that are only dangerous if the spacefarers pick them up and open them without precautions. Anything else?

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    $\begingroup$ It is unclear, what do you want. What threat? They can beat them, or stab them, or annihilate them, or whatever. $\endgroup$
    – Anixx
    Oct 7, 2021 at 10:01
  • $\begingroup$ The five spacefaring species can't launch from a superEarth, at least not living creatures, so aren't going to land on a superEarth unless they're colonizing it, and the fiction isn't primarily about that. If they roll up to a superEarth with a new species and start trying to barter knowledge for GPS or the like, what could the locals potentially do (within modern physics) to hurt the traders, given the traders aren't going into the gravity well and the gravity well is too deep for the aliens to practically launch stuff from? $\endgroup$
    – prosfilaes
    Oct 7, 2021 at 10:13
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    $\begingroup$ Aaah, I understand now, you are asking, how they can harm objects in an orbit. $\endgroup$
    – Anixx
    Oct 7, 2021 at 10:19
  • $\begingroup$ Anti-laser armor is a mirror. $\endgroup$
    – Anixx
    Oct 7, 2021 at 10:21
  • $\begingroup$ I wonder if a railgun can be strong enough be able to launch projectiles as some sort of anti-orbital cannon... Apparently not. Railguns go mach 6, escape velocity of our normal earth is mach 33 :| $\endgroup$
    – Hearsay
    Oct 7, 2021 at 12:01

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There's a difference between space travel being uneconomically impractical due to the expense and number of expendable stages required, and it being actually impossible, and it's far more likely for super-Earths capable of supporting technological civilizations to fit in the former category. It may be that the presence of alien spacecraft in the system is seen to outweigh the factors (economic, environmental, etc) that have prevented them from doing what is necessary to get off the planet. Perhaps they get desperate enough to try an Orion or something.

Though as far as just posing a threat goes, the hard part of getting to orbit isn't getting above the atmosphere, it's reaching orbital velocity once you've done so. It takes far less propulsive capability to just launch to the same altitude of an object in orbit, so a civilization that can't actually reach orbit can still launch missiles at orbiting ships. I wouldn't bet on them being less maneuverable, coming from a higher gravity environment and the propulsive needs of the interception stage being a smaller fraction of their overall capabilities.

You've also given them a potential way to escape their planet once they see your "subspace handwavium" in action. Once they've duplicated it, they'll be able to skip most of the hard part of propulsively escaping their planet's gravity well: they just need to reach a high enough altitude for the subspace drive to be usable.

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  • $\begingroup$ So good old-fashioned missiles are probably the greatest risk. $\endgroup$
    – prosfilaes
    Oct 8, 2021 at 0:44
  • $\begingroup$ The subspace drive would make it easier to escape the gravity well. I've not fully fleshed out the subspace drive, and nor included details here beyond trying to exclude a direct launch from the planet. In final tweak, it will be very hard; perhaps for humans from Earth, with reasonable survivability, it would require launching from beyond the thermosphere. But I don't need the setting to be static in that fashion, so this giving some ground-bound aliens the impetus and skills needed to leave their planet is a plot point, not a problem. $\endgroup$
    – prosfilaes
    Oct 8, 2021 at 1:16
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Buoyancy supported launching platforms - balloons well above the turbulent zone of the atmosphere, say at an altitude corresponding to 50km in Earth conditions (mesosphere).

Will work for rocket based weaponry, platforms for direct energy defenses. Likely will work for space exploration rocket launches (bollocks, I don't think an air breathing intelligent species would be stopped just by a puny deeper gravity well).

We don't use them on Earth (for now) because of the inherent squabbles between nations on the air-space control - no self-respecting Earthling would like to see a platform capable of carrying 1Mt of... well... anything that can be carried (rods from God included) drifting from the "trust but verify, beloved" neighbor's airspace over one's capital city.

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    $\begingroup$ Apparently balloon-launched rockets are called Rockoons ; en.wikipedia.org/wiki/Rockoon , though that's not the world's most helpful page. $\endgroup$
    – prosfilaes
    Oct 7, 2021 at 12:37
  • $\begingroup$ @prosfilaes TIL rockoon. Thanks. $\endgroup$ Oct 7, 2021 at 12:42
  • $\begingroup$ Look at JP Aerospace (specifically, airship to orbit). They are trying to do just that. jpaerospace.com $\endgroup$
    – ShadoCat
    Oct 7, 2021 at 22:20
  • $\begingroup$ Buoyant platforms will help with attaining altitude, and on a planet with exceptionally high surface atmospheric pressure would be almost necessary for efficient rocketry (though that might require pressures too high for life to be feasible), but would otherwise do little for reaching orbit. Launch vehicles actually lose surprisingly little to the atmosphere...they are moving their slowest when they're in the densest regions, and quickly exit it. $\endgroup$ Oct 8, 2021 at 4:14
  • $\begingroup$ @ChristopherJamesHuff "Buoyant platforms will help with attaining altitude... but would otherwise do little for reaching orbit" Do you have a citation for that "do little for reaching orbit"? 'Cause, the way I know, the longer you stay in high gravity, the exponentially more fuel you need to burn to get to the same final velocity (one of the reasons for which the rockets have boosters, to clear Earth proximity as fast as possible. The other reason is the efficiency of the nozzle/expansion cones at different ambient pressure) $\endgroup$ Oct 8, 2021 at 12:47
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Super Earth life may not be able to launch lifeforms easily, but perhaps those Super Earths have multiple moons or other forms of long-lived attracted debris of different sizes/masses. Sufficiently advanced super earth lifeforms could manipulate those objects to crash or interfere with visiting craft or their communications systems.

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  • $\begingroup$ This doesn't strike me as a hard science answer; if you're not already in space, how do you move a moon? Lasers are a possibility, but not an effective one versus shooting the ship. $\endgroup$
    – prosfilaes
    Oct 7, 2021 at 11:23

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