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Some Context

I'm creating a universe where humanity has reached the stars. They've colonized a few star systems and, obviously, FTL drives are a common thing.

The part of the story I'm currently working on features a ship trying not to be captured by "space police". As a desperate move, they end up jumping into a nebula, but not before being damaged by the policemen. They can't jump away until repairs are complete, but they also can't stay very long inside the nebula - and the reason for this is what I want to know.

The conflict in this scene must be time-pressing, like "either we get out of here in x hours/days or we're screwed". The nebula is the bad guy here.

Maybe it's composition is toxic and, as soon as the ship's force field is down, the crew is doomed - or something like that. However, as soon as I took a better look at nebulae, I found out that a nebula's density is really low - something between 100 to 10.000 particles/cm³. According to my research, this amount is practically irrelevant. But is it enough to be dangerous?

The Actual Question

The ship is stranded in a nebula. The force field is damaged and will fade... soon.

Once the shields fail, what kind of danger is the crew in and how long do they have to get out?


Stuff you might want to know for a better answer

Composition: I don't really know what kinds of elements compose a nebula, so the ideal answer should enlighten me a bit about it and propose something that could be harmful to the crew. Remember that the whole point of this question is making nebulae the bad guys.

Threat: You've estabilished what it's made of, now tell me why is it bad. Is it a radioactive element? Corrosive to the ship's hull? Bad for human lungs? Give me something that is dangerous to the level of true concern. It has to be dreadful, but there must be enough time for the crew to try and figure out a way to solve this issue. Note that you can also choose ignore the force field's protection, if necessary.

Time Sensitivity: Like I said earlier, this scene has a time sensitivity to it, but it's all a matter of perspective. The ideal answer should be something capable of killing the poor castaways in a few hours after initial exposure ('cause the ship's shields integrity would be the countdown, the thing between them and death). But if you can find a way to do it in a few days, that's okay as well. I'm setting the maximum DEADline to a week.

The VIP: There's a genius inside this ship. In the narrative, she's an engineer that will help fix the ship's engine, but I will give you freedom to make her a doctor or a biologist or... something you need - if that helps keeping the crew alive for the time needed. Example: The shields are useless and everyone is already exposed in the moment they pop in the middle of the nebula. Knowing the effects of that particular element to the human body, Dr. Tina Awesome warns the crew to get into the vacuum suits to prevent further exposure - which will buy them a few days to make the needed repairs.

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    $\begingroup$ "I don't really know what kinds of elements compose a nebula". en.wikipedia.org/wiki/Nebula $\endgroup$ – RonJohn Sep 4 '18 at 16:50
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    $\begingroup$ "Is it dangerous for a ship's crew to get stranded in a nebula?" It is if your plot says it it. Star Trek mined this a number of times. $\endgroup$ – RonJohn Sep 4 '18 at 16:51
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    $\begingroup$ Certainly related, possible duplicate: What are the effects of a planet staying long-term inside of a nebula? Full disclosure: The accepted answer is my own. $\endgroup$ – a CVn Sep 4 '18 at 17:07
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    $\begingroup$ @M.A.Golding you're ignoring drama. Unless he's writing hard SF, "nature as menace" is a perfectly good plot. $\endgroup$ – RonJohn Sep 4 '18 at 17:07
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    $\begingroup$ I'd expect being stranded in space anywhere would be pretty dangerous. The ship's propulsion is busted, but maybe some life support systems were damaged, too. Is the nebula a requirement, or is the boring, empty, merciless vacuum of space sufficient to threaten our protagonists with certain death? $\endgroup$ – Nuclear Wang Sep 4 '18 at 20:38

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The density of a stellar nebula is likely to be much too low to pose any threat to a human-scaled spacecraft.

They are very sparse. Typical densities are in the range of 100 to 10,000 particles per cm3.

These structures appear dense from a distance, but they are only difficult to see through because of their depth. Any particular "slice" of the nebula doesn't contain that much mass.

If you were within a nebula, it is hard to say what it would look like. But nebulae are so large that the optical depth of the cloud would actually probably be quite high, and I would guess that it would look like you were surrounded by glowing green and red gas in the far distance - instead of space looking black and dark, it would be colored all over. But this would only be an effect caused by the fact that you are looking through so much gas - even if your spaceship were a thousand kilometers away, it probably wouldn't look much different if you were inside a nebula versus outside of it.

https://physics.stackexchange.com/questions/26326/how-dense-are-nebulae

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    $\begingroup$ I should note that 100 particles per cm^3 could be dangerous to pass through at extremely high speeds; it's a lot more dense than normal empty space, and ablative shielding will be worn down much faster than usual. $\endgroup$ – Fund Monica's Lawsuit Sep 4 '18 at 19:52
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    $\begingroup$ @NicHartley, so basically, "is the OP's combination of activity and vehicle unsuited to a density between one quadrillion and a hundred quadrillion times less than air?" (and around the order of one million less dense than the air around the ISS). Maybe yes, maybe no, spaceships aren't planes and apparently they're meant to go really fast. $\endgroup$ – Nathan Cooper Sep 5 '18 at 9:18
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    $\begingroup$ So, basically Space Fog® $\endgroup$ – Martijn Sep 5 '18 at 11:26
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    $\begingroup$ Hmmm. I didn't actually intend any sarcasm. I agree with you. If I was building a space ship for crossing the cosmic void, I would probably not build it to deal with travelling at max speed through atmospheric level (or maybe nebula level) particle densities, just as I wouldn't build a plane to handle going mach 1 underwater. The numbers just sound a bit silly, but that's fine, this is space. $\endgroup$ – Nathan Cooper Sep 5 '18 at 17:39
  • $\begingroup$ I think the question really is whether the particle density of a nebula is THAT much higher than that of "empty space". If you are designing a faster than light ship for travel through, say, any given system's Oort Cloud, it has to survive micro-impacts of a certain frequency. Would a ship traveling into a nebula and then remaining at rest for a given period of time experience noticeably more impacts than that design limit? It doesn't appear that it would. $\endgroup$ – tbrookside Sep 5 '18 at 19:21
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Hiding in a nebula is usually a bad idea. Space is transparent. Very, very, very, very transparent. A typical nebula is a little thicker than normal interstellar space but still a hard vacuum much thinner than any vacuum scientists on Earth can make, and thus very, very, very transparent. Nebulae look very bright and opaque in photographs because those are long exposure photos.

In order to hide in a nebula it has to be vastly denser than most nebulae. Maybe it is a nebula collapsing to form a solar system, in which case there might be clouds of dust and rocks that hide and also pound the ship.

Maybe they need to stay hidden in a dense cloud of space dust and space rocks within the collapsing proto stellar nebula until they think that the space police give up looking for them and go away.

And the slight gravitational attraction of their ship is attracting the space dust, so that eventually instead of a cloud of space dust there will be a big, ship-shaped clump of space dust revealing the position of the ship. And tiny sand grains, pebbles, rocks, and boulders are also being attracted to the ship and can damage or destroy it if they don't get out soon enough.

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  • $\begingroup$ I like this idea of a collapsing nebula. Is this what the so called "planet factories" should look like? $\endgroup$ – Magus Sep 4 '18 at 17:24
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    $\begingroup$ Nebulae are definitely less transparent than most interstellar space. Some of them are even visible to the naked eye from Earth, and of the ones that aren't visible, in many cases it has more to do with them being dark than being transparent. So depending on the distances involved, a nebula could be a reasonable place to hide. You'd just have to have probably a few light-years of separation from whoever's trying to find you. It also relies on your pursuer using a method of detection that gets blocked by hydrogen (or anything else that there may be significant amounts of in the nebula). $\endgroup$ – David Z Sep 4 '18 at 20:35
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    $\begingroup$ it'd probably take a few thousand to a few million years to attract any meaningfully large amount of nebulae dust. We're still talking a near perfect vacum here $\endgroup$ – Baldrickk Sep 5 '18 at 9:58
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    $\begingroup$ It might also take millions of years for the light from your ship to even reach the space police $\endgroup$ – Ummdustry Sep 5 '18 at 15:37
  • $\begingroup$ @DavidZ - the problem with that approach is that detecting a ship-sized object at that kind of range that doesn't want to be found (and therefore has turned off any equipment that broadcasts at high energy levels) is essentially impossible anyway. A nebula won't make the job any harder. In order for there to be a need to hide beyond simply turning off your navigation beacons and maintaining radio silence, the pursuer must be reasonably close, say within a few million kilometres. Over that range the effects of a nebula are negligable. $\endgroup$ – Jules Sep 6 '18 at 11:56
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The nebula is not the problem, the problem is inside the nebula because the nebula is actually a remnant of a supernova like the crab nebula and it is hiding an unsociable neutron star. You ship is currently there because nobody would think that anybody would park the ship there.

Please look intensely at the neutron star page to understand that even on cosmic distances the power of a neutron star is so dangerous that it will fry electronics and bombard the crew with radiation. Your ship is so dangerously close that nobody search for it there (and the power output of the star will camouflage your ship perfectly) because it is deadly. Your crew must be in utter despair to even try such a maneuver.

As HDE 226868 in the comment is suspicious of the seriousness of neutron star radiation: The crab pulsar has here on earth a flux density of $2.4 \cdot 10^{-8} \, Wm^{-2}$ in the X-ray band. To kill a human in 36 hours we need a dose of 36 gray (Cecil Kelley who was killed after 35 hours, for purists 1 Gy = 1 Sv for X-rays), let's say it's a human with 100 kg and a skin area of 2 $m^2$.

A deadly dose of 36 gray means an energy of $36 \; Gy \cdot 100 \; kg = 3600\;J$.

To reach this threshold in 36 hours, we need a power of

$\frac{3600\; J}{36 \cdot 3600  \; s} = \frac{1}{36}W \approx 0.03 \; W $

The total power output of the crab pulsar is the flux density with the area of the surface sphere. The problem with a pulsar is that it is in effect a giant lighthouse, we assume that the ship is not in the beam. In the case of the crab pulsar the background radiation is a factor of 2000 smaller than the main beam:

Total power in X-ray band: $ \frac{1}{2000} \cdot 2.4 \cdot 10^{-8} \, Wm^{-2} \cdot (4 \cdot \pi \cdot (2200 \cdot 3 \cdot 10^{16} \; m)^2) = 6.5 \cdot 10^{29} \; W $

As our human has $2 \; m^2$ surface area, $0.03 \; W / 2 \; m^2 = 0.015 \; Wm^{-2}$ is sufficient.

So the deadly distance from the crab pulsar is:

$d = \sqrt{\frac{6.5 \cdot 10^{29} \; W}{4\pi \; 0.015 \; Wm^{-2}}} = 1.9 \cdot 10^{15} \; m \approx 0.2 \; ly \approx 12 \, 600 \; AU $

0.2 light years is far, far beyond our solar system and approximately one twelth of the distance from our sun to the next star, so I stand by "cosmic" distance. The beam is even worse, it can kill at a distance of 8 light years.

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    $\begingroup$ I think you need a citation here that neutron stars are dangerous at "cosmic distances". Nebulae can easily be many light-years in diameter; neutron stars are absolutely not going to be problematic at such a distance. $\endgroup$ – HDE 226868 Sep 6 '18 at 13:25
  • $\begingroup$ @HDE226868 The crab pulsar can kill unprotected people with its beam at 8 light years distance, so they are problematic at such a distance. As you are interested in x-ray spectroscopy, you may check the calculation for yourself. $\endgroup$ – Thorsten S. Sep 6 '18 at 16:56
  • $\begingroup$ Ah, but you're missing something key. The beam isn't stationary; the pulsar rotates, and so the beam spends only a small amount of time aimed at the person. Therefore, you've massively overestimated the amount of exposure. Plus, 0.2 light-years is tiny on astronomical scales; nebulae can be hundreds of light-years in diameter. $\endgroup$ – HDE 226868 Sep 6 '18 at 19:06
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There's nothing especially dangerous about the nebula specifically, deep space will kill your crew in time, if their ship is damaged. They need to escape this nebula to get to somewhere they can do long-term repairs on their ship.

Deep space can kill you in many ways: radiation, leaking air, freezing to death after your reactor shuts down and the list goes on.

In this scenario they simply happen to be in a nebula, while the real killer is plain old space.

If you really must have the specific area they are in be the hazard, then i'd advise saying they simply jumped too close to a star, That sort of immense heat & radiation is bound to kill them in as many hours as the plot needs.

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  • $\begingroup$ I've started considering that a while ago. Only difference is that I thought of a BH. The main reason why I'm asking this is to see if someone can give me a reasonable enough answer to put my mind at ease. If I no one can, I will change it - probably to a star, now that you mentioned it. It's way more ellegant and would also solve another issue that I decided not to include in this question. $\endgroup$ – Magus Sep 4 '18 at 16:59
  • $\begingroup$ "They need to escape this nebula to get to somewhere they can do long-term repairs on their ship." Why so? You say that as if it's the most natural thing in the world, which I'm not at all sure it is; would you care to clarify why the crew couldn't make repairs to their spacecraft while inside a nebula? $\endgroup$ – a CVn Sep 4 '18 at 17:10
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    $\begingroup$ the nebula is presumably remote, since its a good place to hide from the space police. It's for the same reason you can't fix a busted car in the middle of the desert, you often simply don't have the part or tool you need. $\endgroup$ – Ummdustry Sep 4 '18 at 17:16
  • $\begingroup$ "freezing to death after your reactor shuts down." I think a more realistic concern is boiling to death if you don't shut your reactor down. Space is a near perfect insulator, after all. $\endgroup$ – UIDAlexD Sep 5 '18 at 19:07
  • $\begingroup$ I'm assuming any spaceship is designed to take this into account, with large radiators and high surface area to deal with the reactors waste heat in normal operation, which might prove detrimental without tour reactor online. But your definately right baking to death is just as if not more realistic $\endgroup$ – Ummdustry Sep 5 '18 at 20:45
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As stated by the other answers, space is empty. Really, really empty. Nebulae are marginally less empty, but from the perspective of someone inside one, they might as well be regular (empty) space with a nicer color palette.

That said, there can still be things inside a nebula that threaten your ship and crew. Rogue comets, asteroid fields, stars, planets and more can still be found inside of a nebula, which can act as more mundane dangers to your ship. yawn
We don't want mundane dangers, that's what the rest of space is for. We want something unique. Something cool. Something exotic. Something... dangerous.

So let's do that. Nebulae are made up of a wide spread of tiny particles, single or small globs of molecules floating through space. These molecules can be anything, really, so long as it's moderately stable under the influence of icy cold and stellar radiation. Things like small flecks of iron or clouds of hydrogen exist here. Sure, they might ping off the ship's shields or cause a nasty short when they build up static electricity when they pass but relatively speaking, they're fairly safe. And those would be the things your crew had to deal with if they were lucky.

They are not.

The nebula they jumped into is the graveyard of a civilization. A long time ago, in a star-system deep within this nebula, this dead civilization mastered fire, then electricity and then the atom. They built great works on a huge scale before turning inward and focusing their efforts on nano-technology. A mistake here and an oversight there and now they were dealing with a grey goo scenario. Rather quickly, the entirety of their planetary mass was converted into more nano-machines, killing off anything and anyone on the planet.
Thankfully, the combination of gravity and a lack of actual driving intelligence stopped the nano-plague from reaching other planets and multiplying across the stars. That is, until their star went supernova. Most of the nanites were cooked into non-functionality but the rest were flung out into the nebula where they occasionally gobble up nearby atoms and build more of themselves, slowly converting the entire nebula into more nano-machines.

The good news is that thanks to the incredibly low density of the nebula and relatively low velocity of the nanites, it'll probably take a few trillion years for this to happen. The bad news is that a lovely chunk of raw materials just arrived in their feeding zone. The ship's minute, though noticable, gravitational pull and magnetic field will soon attract nearby nanites who will gladly eat through the hull to build more and more of themselves. And while one or two nanites aren't a problem, with this much raw material, there will soon be more than you can count.
The clock is ticking, better get that engine back online...

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    $\begingroup$ … and carry the pest throughout the galaxy afterwards … $\endgroup$ – celtschk Sep 5 '18 at 15:07
  • $\begingroup$ If it wasn't horrifyingly dangerous, it wouldn't be fun. $\endgroup$ – Valthek Sep 6 '18 at 6:07
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Electrostatic discharge

Particles from a nebula can charge the hull of a spaceship, especially if it's in moving [Citation needed.].
For example, you can read in Quora that static is a real concern in the ISS. If the spaceship gets charged it can be dangerous for the crew to get outside the hull to fix the damage produced by the attack.
Also with some sci-fi, massive amounts of static could produce: blindness in sensors, lack of communication radio, damage in external circuits, failure in weapons, batteries, thruster, life support system, and in the most severe cases death of crew produced electrics arcs inside the corridors...
Ah, just for curiosity. I've also read [Citation needed.] that gamma rays (that are always floating on the space) can also produce this effect, at a lower scale, without a nebula, just a radiative zone.

How much time? I would say at least a few days in a normal nebula in order to gather some charge. But it's your story, maybe the nebula was especially dense and that is they choose it to hide them. Maybe there was a pulsar (which produce gamma rays) near and that would help hiding the ship from sensors.
If you are looking for some information, read this question Storms in space.

Friction

At higher speed (some percent of lightspeed?), friction becomes a huge problem travelling on nebulas.
Friction can damage the hull of a ship, make holes in it (remember that exist micrometres of a few centimetres travelling really quickly), produce static charge and even overheat the ship.
In space heat and cold are huge problems. A ship powerless can freeze because space is really cold, but an active ship can overheat. That is because in space you can't cold down nor heat stuff using convection with the atmosphere like in Earth we do. In space it's only available thermal radiation.

In Earth, if a something is hotter or colder than the environment it will be cold down or heat up in order to archive the same temperature. In space your ship cold down always producing thermal radiation and never recover it (unless you are close to a star or doing massive friction). Here, in your problem, the friction (due to the speed) is supposed to be greater than the cold down.

Depending on your speed the damages or heat could take from weeks to hours, I don't know even if they are moving. You say that the ship is stranded, but I guess that is about the FTL drive and not the sublight drives.

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  • $\begingroup$ It's generally "citation needed", not "quotation needed". $\endgroup$ – Kat Sep 4 '18 at 22:20
  • $\begingroup$ @Kat, ups. I'll fix that $\endgroup$ – Ender Look Sep 5 '18 at 0:22
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The nebula contains FOOF. Not much, not very much at all, but it doesn't take a lot at all. Drifting through a nebula with even a tiny amount of FOOF will cause incremental but extensive damage to the vessel, FOOF will react with almost everything. If the crew don't jump out of there post-haste the ship will be dissolved from around them.

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    $\begingroup$ Dioxygen difluoride would definitely not exist in a nebula. There is far too much hydrogen, far too little oxygen and fluorine, and both oxygen and fluorine greatly prefer to react to hydrogen than to each other. -- Plus, interplanetary gasses are highly ionized; creating far too much energy to keep the FOOF together. $\endgroup$ – Ghedipunk Sep 4 '18 at 18:57
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    $\begingroup$ In short, FOOF is one of the most reactive chemicals anyone has been both smart and stupid enough to make, precisely because both oxygen and fluorine would rather react to everything else, including fluorine with some noble gasses. $\endgroup$ – Ghedipunk Sep 4 '18 at 19:03
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    $\begingroup$ @Ghedipunk One should probably take landing in such a nebula as a sign that they really need to settle down on a nice safe planet somewhere because space clearly hates them and wants them dead but that's a given if you land in any nebula dangerous enough to constitute a threat. $\endgroup$ – Ash Sep 4 '18 at 19:12
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    $\begingroup$ Looked it up, because that would be super cool and defy all expectations of astronomy and chemistry... The "pure" ethanol nebula designated W3(OH) is anything but pure. It is, like every nebula, 98% elemental hydrogen, 1% helium, with the remaining including a diverse range of chemicals, with chemicals split into filaments by masers, much the way a centrifuge separates compounds by weight. A MentalFloss article speaks of another such ethanol bearing nebula in the constellation Aquila, but I can't find more information about it. Unless something is intentionally making FOOF... well... $\endgroup$ – Ghedipunk Sep 4 '18 at 19:24
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    $\begingroup$ @Ghedipunk "Dioxygen difluoride would definitely not exist in a nebula. There is far too much hydrogen, far too little oxygen yadda yadda yadda." Yeah, but it sounds plausible to everyone but a few chemists and physicists. Thus, +1 for thinking of a technobabble reason for the drama. $\endgroup$ – RonJohn Sep 5 '18 at 1:58
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What if the nebula, despite being unimaginably thin, is still too dense for your extremely sensitive FTL drive, and most of the damage was done by plowing through against safety protocols?

Probably the space patrol doesn't want to follow you since their bigger ship would be affected even worse and you've detonated a missile or something to make it look like you died as expected, so they won't stick around waiting to capture you when you emerge.

Your VIP FTL physicist could then note that the rough trip into the nebula will have carved out a temporary corridor of safe space to travel precisely back along, but either the safe tunnel is an FTL effect that fades over time, or the nebula gasses are diffusing back into a really wide corridor.

Waiting too long would thus mean that the departure would do further and increasingly certain fatal damage to the ship, with the exact deadline being as vague and threatening as you like. With potential to push the deadline back by moving all the remaining armor to the front of the ship or other stupid engineering tricks like that. (The option of taking hundreds of years to leave the nebula at sublight speeds is obviously not on the table)

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Probably not.

Nebulae are not as dense as you'd think. This article states that the density of your average nebula is only around 10,000 atoms per cubic meter (which is orders of magnitude less than earthly air). 10,000 may seem a lot for an average person, but on an atomic scale, 10,000 atoms is nearly nothing.

If you're going extremely fast, this could present a maintenance issue, because this is still much denser than intergalactic space, which can be as sparse as just one lonely atom per cubic meter. Nebulae are pretty big, and losses can add up.

A Loophole

Being stranded anywhere in space is not good. Food supplies are finite, and even a large ship with few people will run out of supplies eventually. This is not isolated to the nebula but rather to anywhere far out in space, especially if your crew cannot communicate.

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Is it dangerous for a ship's crew to get stranded in a nebula?

Yes it is, if the ship has one of those clunky first-generation FTL drives that spews a certain exhaust byproduct even when they're idle.

The material in the nebula interacts with your FTL drive's exhaust

Your FTL engine, even when idling, produces a nasty byproduct (you pick: anti-matter, microscopic black holes, gravitational dipoles, colimated dark energy beams, polarized gravitons, proto-wormholes, etc). As a result:

The ship's FTL drive needs large expanses of empty space to vent into

For the same reason nuclear power plants on Earth are built near oceans, your ship must be operated in interstellar empty space. The drive can store a certain amount of exhaust, but after a while it must be vented into space, diluted into the vastness.

Unfortunately the low-density material in the nebula, compounded by the somewhat stationary position of the damaged ship, when exposed to the exhaust stuff, is just dense enough to start interacting and gravitating towards you, accelerating planet and star formation in your vicinity by several orders of magnitude.

Completely shutting down an FTL engine is not practical in such an environment. It takes years to shutdown a nuclear power-plant on Earth. An FTL drive would be much worse.

There's a genius inside this ship.

They should have known not to go into a nebula. Perhaps the nasty exhaust only occurs when the FTL drive is damaged, and they didn't know it. Or the on-board genius knew it but was over-ruled by the rest of the crew. The genius' warnings can be a good opportunity to weave the technical explanation into the story and introduce the character's superior mental abilities.

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    $\begingroup$ "It takes years to shut down.." No, it doesn't. It takes about a quarter of a second to do a full emergency shutdown, by dropping the control rods. Even aside from that, stopping it safely doesn't take years; most of that is, quite simply, politics. Things like finding places to move the nuclear waste, getting new power sources online, and thorough inspections to make sure things are properly decommissioned. If there's an imminent catastrophe, a nuclear power plant can be shut down extremely quickly. $\endgroup$ – Fund Monica's Lawsuit Sep 5 '18 at 20:11
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  1. fast moving particles.
  2. Small hand sized rocks
  3. Asteroids

These threats can be naturally present in the nebula or come from outer space. Maybe a giant meteoroid field is just happens to be passing through the area when they arrive.

In your situation the size of the particles you need depends on their mass. Sand moving at 20%-40% the speed of light would be dangerous. 60-80% would be catastrophic especially with out shields.

Small hand size rocks moving at 40k-50k mph would be terrible without shields. These rocks exist all over even within our solar system. This is one of many reason it has taken NASA and etc so long to develop a suitable hull. They still don't have anything that would handle a sustained bombardment.

Your shields would normally absorb or deflect them, but apparently your shields were damaged when fleeing the police.

Even if your clever scientist/engineer used the shields to put a bunch of matter together to form a rock shield, debris would eventually break through it.

If impacts with your shield generate heat then you could naturally fuse rocks together to get a make shift rock shield. If the process doesn't generate heat you could use a laser or etc to generate heat to allow the rocks to fuse together. However, with persistent bombardment your rock shield will eventually be destroyed.

  1. You have make a rock shield
  2. The shields are down
  3. The ship has to be repaired before the rock shield is destroyed.
  4. Every second/minute you wait risks a hole be blasted through the rock shield.

Space is vast and random there are a 1000's reason why you might enter a nebula or any part of space and find large moving fields of rocks, asteroids, or etc.

  1. Gravity of nearby planets,suns,stars, or etc might be causing the rocks to clump together.
  2. Two asteroids or any arrangement of matter could collide in sufficient quantity to start planet formation. Now tons of matter is being dragged towards the core of the new planet. You enter the nebula anywhere near this, and large quantities of matter will definitely be coming your way. Your engines are failing or power source needs repair and boom your ship is stranded there until repairs are made. Now you only have enough power to maintain your shields and maybe some really slow maneuvering.
  3. Your plain unlucky and a giant fields of debris just happens to be coming your way.
  4. Nearby explosion of nova or anything and debris from it is coming directly toward your nebula. (Even if its just 2 big rocks colliding at 30% speed of light. Say 10-100 miles in diameter).
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The nebula is full of fast-moving particles

This particular nebula happens to be full of radioactively decaying heavy elements like uranium. As these elements break down, they release high-speed particles in all directions. You are essentially in a nuclear reactor.

These particles are constantly pinging off of your force field, draining its power. Soon the shields will go down. Not only does your crew risk exposure to the radiation which could cause genetic mutations and give them any of a bunch of frightening disorders, but those particles are also a danger to the hull. Because this ship has a force field, the hull is minimally reinforced. After your encounter with the police, your hull is pretty badly damaged already. A single fast-moving particle could punch a hole in the hull, damaging equipment, venting the atmosphere, and otherwise wrecking your ship. Of course, the resident genius knows this and has started dumping electrons out of the ship. Now the ship has a positive charge which should repel the slower particles that are draining the shields and keep them running a bit longer. If you want to keep the vip occupied, she could be busy getting rid of electrons or (this is a bit of a stretch) sending out repeated electromagnetic pulses to hopefully repel the faster-moving particles.

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Being stranded in a nebula at rest (relative to the nebula at least) isn't dangerous in the least. Your problem isn't really with anything in the nebula, but that there are things at all in the nebula. See, when you're traveling through a nebula -- which solar winds push out of solar systems -- you're traveling between stars, which means you want to go fast. But in a starship, you don't come to a stop by default; there's barely any friction in space, so if you happen to try to go really fast, your ship is going to get worn down by friction.

See, the engine didn't cut out while you were in the nebula, leaving you adrift. It cut out when you were about to do your final burn to drop into a solar system, leaving you shooting through empty space, out of control. At a quarter the speed of light. In normal, empty space (which has roughly one atom per centimeter cubed) this isn't a huge deal; your unobtanium adaptive ablative shielding can handle it easily (its maximum capacity for atom impacts per second is well over double what you'd normally encounter in empty space). In a nebula, though, traveling at empty-space speeds, it gets worn down anywhere between a hundred and ten thousand times faster -- and you're heading straight for one.

The issue then becomes less "oh no we're stranded in a toxic environment", and more "oh no we're about to crash into the space equivalent of a brick wall". You may even have quite a while in the nebula itself before the shielding goes, but as fast as your ship is traveling, it won't slow to safe speeds until well after the ship has been reduced to a cloud of scrap metal with some interesting carbon stains where people used to be.

You need the engines back online so you can turn the ship around and slow down before that happens.

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Why this won't work

Hiding in a nebula just plain doesn't work. They're too transparent to hide anything from any scientifically plausible method of detection. See Phil Plait talking about how nebulae look when you get close to them. A nebula would only produce enough radiation to hide a ship over a very long range, which would probably be measured in terms of at the least light months, but more likely light years.

Unfortunately, if your crew are trying to hide from a pursuer who is light years away, there are some easier ways of avoiding detection that are going to end up being plot holes:

First, if they simply stop emitting any concentrated radiation (i.e. they turn off their damned radios and not use any high energy equipment, e.g. FTL drives, then they simply aren't going to be detectable at that kind of range. As far as we know, the highest resolution, best detection method available is to look for signs of the object you're trying to find in electromagnetic radiation images (e.g. emissions, reflections, or shadows caused by them blocking radiation coming from behind them). There are other plausible detection methods (e.g. gravity-based detection) but all of them are likely to have lower resolution limits than EM-based detection.

The limits of resolution of EM-based detection are discussed in this answer on astronomy stack exchange, and depend on the size of your detection base and the wavelength you're working in. The answer describes this in terms of using a mirror in a traditional telescope, but you can also use interferometry to get the same result using two smaller detectors and a connection between them. Plausibly, a pursuer who has time to stop and look in detail could send out probes to extend their interferometry base, which might let them use a D of say 1,000km. Searching for a non-emitting object means you're looking for either reflections (which you'd have to be lucky to line up correctly) or occlusion (which means you need something behind the object).
The shortest wavelength radiation that's likely to be behind the object and is strong enough to easily detect is the cosmic microwave background radiation, whose wavelength is ~1mm. This means the smallest plausible angular size which a pursuer could detect is around 1/1,000,000,000 radians, which approximately means that they can detect an object at a distance of around 1,000,000,000 times its size. If your ship is 100m long, and they're lucky enough to be looking at it from its broadside, the closest they can plausibly detect it from is a range of 100,000,000km.

Second, and perhaps more critically, if the pursuer really is multiple light years away, all they actually have to do is not hanging around waiting for multiple years, which is of course how long any EM radiation they emit will take to reach the pursuers.

What will work

Fortunately, you don't need a nebula for your ship to hide in. There are a few other options that would work well:

  • Just switch off the ship's systems. Your ship produces a variety of EM signals that make it relatively easy to detect to any nearby pursuer, but if you turn them off (preferably at a point in time when they can't see you, e.g. because you're temporarily behind them and they can't see you because they were using thrusters and the glare from the thrusters would hide you, and don't know where you're going to be, e.g. because you've just executed a change of direction) the pursuer won't be able to find you unless they're close (see above discussion of detection ranges). In this case your time limit is the fact that you've had to turn off all of your ship's systems (because chances are you're using some fancy power supply mechanism, e.g. fusion power, and that's bound to produce a lot of detectable emissions) and your air is slowly clogging up with CO2. Or maybe the direction you're heading in isn't safe and you'll need to change direction soon.

  • Hide behind a convenient asteroid/piece of debris/etc. This isn't likely to work for long, because if the pursuer moves to a different vantage point they'll be able to see you. But if the repairs are going to be quick it might be enough.

  • Hide in the atmosphere of a gas giant. These are much denser than nebulae and therefore provide much better protection against detection, but the density is your issue here: you'll need to maintain speed to prevent yourself falling into the planet, and the atmosphere will cause drag. You'll start heating up due to atmospheric warming, and you'll be burning up fuel. Perhaps the ship has an ablative heat shield, and when it all burns up the crew will be dead. Or perhaps they're already short on fuel. The deeper into the atmosphere you go, the worse it'll be, but the safer from detection you'll be.

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The nebula has reached peak density

The nebula is beginning to collapse into itself and form a star, the pull of the slowly increasing mass at the center of the collapse will eventually be able to prevent the spacecraft from escaping.

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  • $\begingroup$ This gives you a time limit that would realistically be measured using the unit "My" (i.e. millions of years). $\endgroup$ – Jules Sep 6 '18 at 11:58
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They got in the wrong zone of the wrong nebula, at almost the wrong time.

The nebula is a type C emission nebula with secondary coherent discharges. The emission factor is what helps to hide the ship from FTL scanners (waves hand) somehow. The coherent discharge detail wasn't known at the time of jump, and is only discovered when inspecting the ionization and emission patterns of the nearby gases (or when noticing that the core star is actually a binary system), leading to a classical "WE'RE SO F*CKED" moment and much finger-pointing.

The downside of this kind of nebula is that when the twin stars that are at the nebula's core are aligned just so, which, given their orbit, they do every (waves hand) some days or hours, bad things are going to happen. Or rather, the bad thing is going to happen where the ship is, instead of the other side of the nebula.

The shields will defend the ship against such an event, if it was driven by another ship - or even, for some (shortish) time, from a planetary defense battery. But this cannon here is powered by a star, and the pulses won't last milliseconds but whole minutes. Diluted as they will be, just remember that it's still enough to be detectable at five hundred light-years' distance; being somewhere inside the barrel of this gun might be survivable, or might not be. The crew obviously doesn't want to verify.

Enters Tina Awesome

The ship cannot move, but it does have FTL missiles - they work by disappearing into hyperspace and dropping inside the enemy's shields. They are never used at any great distance because accuracy gets shot to Hell, but they would be able to pop a couple of light-days coreward.

If the warheads are reprogrammed for maximum dispersal instead of maximum yield on target, the explosions will disrupt a small part of the cosmic laser assembly through random ionization(*), thereby carving a tiny sliver of incoherent, reasonably innocuous discharge inside the bolt that's coming their way. Reprogramming, targeting and dispersal patterns are (waves hand) difficult, but dr. Tina Awesome is not any engineer - she's an ex weapon designer as well as the great-great-nephew of Marylin vos Savant, by way of a certain Scottish Starfleet engineer on her father's side.

(*) something like this happens in Fred Hoyle's The Black Cloud.

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