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Nuclear rockets, or as I like to think of them, "half way between a continuous small atomic explosion and reactor whose front fell off". There are many variants, and my aliens are using relative simple nuclear salt water rockets.

The Wikipedia page on EMPs shows that the strength of the EMP from a nuclear bomb is significantly affected by the environment, so a detonation in near-Earth space (400km) creates a much more powerful EMP than the same size detonation within the atmosphere.

How would the exhaust plume of a nuclear rocket interact with Earth's atmosphere, magnetosphere, and ionosphere? Is there any risk of inducing significant currents on or near the surface (effectively, if not literally, an EMP) when the engine is switched on?

On the assumption that engine size has some effect on the answer:

  • Scout: About the same size as a space shuttle, give or take, but with a max delta-V of ~400 km per second. Can sustain 55.88 m/s² for ~30 minutes. This one goes into Earth orbit at $some_altitude. How far away does the ship have to be to not cause problems?
  • Colony ship: I'm not sure exactly how large, and I suspect I will be vague when describing it, but large. 130,000 colonists (each about half the weight as a human, and crammed in like… perhaps not literally sardines, but they're not having much fun). Max delta-V 40 km per second. This vessel lands on Earth (well, on water). This ship is designed to land directly without waiting in orbit. How will the engines affect electronics in the surrounding continent, if at all? Assuming the engines must be turned off at some altitude (and they switch to chemical rockets for the remainder of the descent), what altitude would that be?

Edit:

On the recommendation of comments, I'll accept [science-based] answers as well as [hard-science] answers. [hard-science] still preferable, if it's possible.

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    $\begingroup$ A shuttle sized spaceship, I'd imagine, would use some pretty teensy volumes on fuel and might not actually be a serious issue, but the colony ship is definitely going to wind up spraying everything in nuclear waste as nuclear reactors generate radioactive by-products and the local inhabitants might not appreciate being dowsed in unstable actinides. $\endgroup$
    – Samwise
    Feb 25, 2017 at 23:44
  • $\begingroup$ They absolutely don't appreciate it, but they don't get any say in the matter either. :) $\endgroup$
    – BenRW
    Feb 26, 2017 at 11:21
  • $\begingroup$ I doubt the question can get good hard-science answer in the way as it is described in its about section. It seems more appropriate would be science-based tag for the case. The problem is in first place - which type of nuclear engine do they use. $\endgroup$
    – MolbOrg
    Feb 26, 2017 at 11:40
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    $\begingroup$ I would recommend Winchell Chung's Atomic Rockets website: projectrho.com/public_html/rocket/…. There's a lot more info there than on Wikipedia. $\endgroup$
    – praosylen
    Feb 26, 2017 at 16:06
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    $\begingroup$ A suggestion: drop the hard-science tag as it requires equations, research citations, and detailed answers. As it stands no-one can answer your question. However, a nuclear rocket isn't the same thing as a nuclear explosion, continuous or otherwise, and there won't be any EMP. Nuclear pulse propulsion vehicles might generate EMP, but nuclear salt water rockets. Landing your Colony Ship would be a nightmare. Too impracticable to be possible for reasons I can't answer because of your hard-science tag. $\endgroup$
    – a4android
    Feb 27, 2017 at 5:47

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Interestingly, the problems area of the page for Project Orion at wikipedia indicates that one of the most serious problems would be the flooding of the Van Allen belts (wiki too) with ionising radiation! Most of the data in the problems section for Orion do not apply to your issue directly but that one definitely does - the citations contain more information that you might be able to use too. No safety or environmental impact study has been done for that type of propulsion as of yet but it is a cool idea.

In order to address the more direct question of how far you'd have to go to be safe, realistically, we could deduce from the numbers pulled before the CTBT if you supplied a number in petajoules p/s of energy output for your rocket, I think.

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  • $\begingroup$ An excellent point, I wish I'd seen it earlier in the day so I could comment properly and not sleepily. Back-of-the-envelope calculations look like the scout's engines are ~10TW (assuming similar mass to space shuttle and scaling from the projectrho.com link in the comments above), so 18 petajoules total for arrival and departure. That exhaust is going to be moving against the solar wind at between 1/3rd and 1/6th the speed of the solar wind as the ship slows down… my gut feeling says that means the exhaust will be trapped in the Van Allen belts, not escape to the wider system. $\endgroup$
    – BenRW
    Feb 28, 2017 at 22:58

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