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If a ship is moving through a nebula, would it slow down due to the gases and dust in the cloud, and would it prevent things like radar from having as much range as they usually could?

I want to create a setting where it would make sense for space warships to have a maximum speed, like regular ships on water. I also want ships to have a limited range on their sensors so ships cant easily detect each other from incredible distances.

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  • $\begingroup$ "slow down" or "reduce range" by how much? Even the difference between the vacuum in interstellar and intergalactic space will have some measurable (to a sufficiently advanced technology) effect, so the key point is what you consider to be significant. $\endgroup$ Jul 11, 2023 at 11:05
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    $\begingroup$ If you want interstellar space warships you need a solution for fast space travel. This will have some kind of fiction, since we currently don't know any way how such a ship would be viable (relevant fraction of light speed) or if it is even theoretically possible (faster than light). When you design this "magic" means of travel, you can add why it has a maximum speed and why sensor have limited range while this space-engine is active. $\endgroup$
    – Falco
    Jul 11, 2023 at 12:52
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    $\begingroup$ If you were talking sub-light, then you could set an energy absorption maximum for the "forward deflector shields." This would apply even in interstellar space, and is one of the major challenges to interstellar flight at "normal" speeds. A nebula would shift your maximum speed 2-4 orders of magnitude lower. $\endgroup$ Jul 11, 2023 at 16:45
  • $\begingroup$ @Falco - ... otherwise I guess he does have a built-in maximum speed - c $\endgroup$
    – T.E.D.
    Jul 11, 2023 at 19:59
  • $\begingroup$ Star Trek: The Wrath of Khan thinks that nebula do everything you want of a nebula, and everyone (including hard SF nerds) think it's a great movie. $\endgroup$
    – RonJohn
    Jul 11, 2023 at 20:08

6 Answers 6

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Use a protoplanetary disk

It's much denser than a nebula in deep space. It gets denser and hotter the closer in you get, and gas drag is important for planet building. According to this study, a disk around a young sun can have 1 Pa of pressure at 300K (closer in heat becomes a big problem). This is enough to make significant drag and heating at orbital speeds. Around a Jupiter like planet, the study gives a pressure of 1/3 bar at 300K, enough to not need a space suit.

This is about 45 times less dense than air since the gas is mostly hydrogen. But it is still enough to be looking at fighter-jet velocities instead of space-shuttle velocities. The disk will be in orbit around the star/planet, so velocities will be low relative to a circular equatorial orbit.

Say your ship is pressurized to pure oxygen at 1/5 bar, which is the same amount of oxygen at sea level. Not including nitrogen saves precious mass for the pressure hull.

The 1/3 bar of outside gas is at a higher pressure than the ships default atmosphere, which adds to the excitement for ill-prepared crews. The pressure could crush the delicate walls (it's easy to crumple a soda can). Or the hydrogen could enter through the tiniest leaks and mix with the oxygen.

The caveat is that this 1993 study is a rough estimate of the pressure-temperature curve, but it is at least plausible. I can't find newer numbers.

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  • $\begingroup$ It doesnt technically answer the question, but it does provide a solution for the environment i was looking for, as I got my answer to the original question from all the other answers which was pretty resounding "no, they wouldnt." $\endgroup$
    – Chuck
    Jul 11, 2023 at 20:58
  • $\begingroup$ And I bet it'd also make for some very stunning visuals indeed. Not necessarily the "fog bank" of pop sci-fi but you'd be able to very easily see reflection/scattering on it just like the sky, particularly given it is far larger than a few tens of km wide. $\endgroup$ Jul 12, 2023 at 19:51
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A nebula is less empty vacuum than the regular interstellar space, but it is still a very good vacuum, which would be considered absolutely good enough for almost any physics experiment. With numbers, a very dense nebula may have something like 100,000 particles per cubic centimeter, about one hundred trillion times less dense than ordinary air; and most nebulas are very much less dense than that.

I have no idea what you mean by a "maximum speed" for "space warships". At ordinary spacecraft speeds, crossing a nebula would have no effect whatsoever. But, obviously, those space warships must be able to travel at speeds very very much higher than our primitive spacecraft, or else they would take forever to get to the nebula in the first place. If the warship is travelling sufficiently fast, hitting even one lonely molecule could be problematic...

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    $\begingroup$ +1 interesting how the public images of a nebula and an asteroid belt are so different to the reality. I blame Star Trek - The Wrath of Khan and The Empire Strikes Back respectively for popularising the "wrong" ideas. $\endgroup$ Jul 11, 2023 at 5:16
  • $\begingroup$ @KerrAvon2055 I can't blame you for your observation, but I wonder at what particle density a nebula would become fairly opaque at 10-100 kilometer distances. My knee-jerk reaction is that you could still fly a traditional space ship through it with no problems - but at scifi speeds it would make a remarkable ion trail. $\endgroup$
    – JBH
    Jul 11, 2023 at 5:51
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    $\begingroup$ @JBH a nebula at the thickness you propose would last for a miniscule amount of time before aggregating through gravitational attraction into a larger body or bodies. Which is basically the story of the universe, first the primordial soup formed into planets and stars, then the stars went nova/supernova and spread particles everywhere, then those particles aggregated into new stars and planets... Concentrations of matter never stay evenly distributed. $\endgroup$ Jul 11, 2023 at 6:45
  • $\begingroup$ @JBH The Earth's atmosphere isn't even opaque at 10-100 kilometer distances. $\endgroup$ Jul 11, 2023 at 14:36
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    $\begingroup$ It may be difficult to argue how a space war(!)ship can withstand enemy attacks, but has problems navigating through a nebula ... $\endgroup$ Jul 11, 2023 at 14:36
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Let's talk about aerodynamics. Our fastest vehicles can sustain speeds of about 1x10^3 meters per second (mach 3, rounded for easy math). At airliner altitudes, the density of air is roughly 5 x 10^-1 kg/m3. This means a mach3 vehicle has to push 500kg of air out of the way every second to keep moving.

Obviously aerodynamics makes a difference in this, but let's just use those numbers for comparison.

If we're bopping about the solar system, a high-end density is around 1x10^-25. In a nebula, we can guestimate around 1x10^-18.

Back-of-the-napkin math says that, for the equivalent of mach3 air resistance, you'd have to be moving at 1x10^20 m/s.

Considering that the speed of light is only 3x10^9 m/s, you'd hit relativistic issues long before you had to worry about air resistance in a nebula.

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Fog is comparatively dense by exponential orders of magnitude compared to a nebula, and yet there's so little moisture in it that we easily breathe. Visibility may only be a few feet or meters.

Nebula are enormous. The size of a solar system or much larger in many cases. Yet for most nebula (with the exception of some star birthing nebula) star light still shines through it. This indicates there isn't a lot there to create any meaningful resistance.

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A ship designed for stealth would have a really low radar cross-section (recommended: Lincoln Laboratory: Introduction to Radar Systems, Lecture 4: Radar cross-section), maybe comparable to a smaller asteroid. Hiding near an asteroid field, or moving with similar speed would make it quasi undetectable.

Also: radars have side-lobes (see the course above Lecture 7 - Radar Clutter and Chaff, or here on quora), which act as they were the main beam, but the direction is very different. The side-lobes could make a target with a low radar cross-section, maybe in the distance, undistinguishable from the asteroids/particles nearby.

All-in-all: in order to detect a very distant target with radar you would need a

  • very well directed antenna, to have a good spatial resolution in the distance (in this case it would take a lot of time to scan all the space with a small beam)
  • receiver array, to increase the spatial resolution (but in this case you would need a very large space-ship)

Plus: a lot of energy, which would in turn make your ship detectable.

So I think asteroids are the solution (don't know how common they are in nebulae).

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  • $\begingroup$ They are quite common in our Solar system: tens of thousands are catalogued and tracked as potential danger to our planet and spacecraft. Yet it is a complex task to actually approach or hit one when it comes to research (or hiding a warship "among" them). Mean distance of a million kilometers (roughly 3 Earth-Moon distances) between sufficiently sized chunks (1m or more IIRC) may play a role in this :) $\endgroup$
    – Jim Klimov
    Jul 12, 2023 at 15:29
  • $\begingroup$ Not sure about the exact quote right now, but someone put it like "If you travel from Earth to Jupiter and worry about randomly hitting something dangerous, consider flying across America on a plane and randomly hitting a bird... If there are just 3 birds on the continent, overall". $\endgroup$
    – Jim Klimov
    Jul 12, 2023 at 15:30
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There may be a problem even with 10,000 particles per cc. Suppose you are travelling at 0.1% of the speed of light. Relativistic effects are small, but the kinetic energy of a hydrogen atom striking you is about 2 MeV. If you are travelling at 300,000 m/sec, every square cm of the front of your craft will be struck by $3 \times 10^9$ 2 MeV particles. This will erode it and make it radioactive. The drag on the craft is probably negligible if you have an engine that can get you to those speeds. The radioactivity is probably not a problem if you have a thick plate at the front of your craft, and a good space between it and the crew. The plate will get sputtered and eroded eventually.

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  • $\begingroup$ This is why Star Trek retconned "navigation deflectors". $\endgroup$
    – RonJohn
    Jul 11, 2023 at 20:12

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