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Imagine a spacecraft travelling at very high speeds, say 0.3c. While a crew member could theoretically suit up and perform spacewalking maintenance on the outer body of the ship, will they be exposed to the risk of a few stray atoms floating in space and hitting them at 0.3c and shredding their bodies? How big does such a chunk of matter need to be to present a serious risk to such an astronaut?

Thanks!

[As stated in my profile, I am working on a novel titled "Generations". Please consider I may use the information provided to help in building the story's world properly. Thanks!]

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    $\begingroup$ It all depends on the density of atoms / molecules / particles in the specific part of the universe... Where is the spacecraft travelling at such great speed? In intergalatic space, in interstellar space in the outer parts of a galaxy, in intestellar space in the inner parts of a galaxy, inside a molecular cloud, or inside a solar system? In intergalactic space, or possibly even in interstellar space in the outer parts of a galaxy, maybe OK. Inside a solar system, definitely not OK. $\endgroup$
    – AlexP
    May 8 '20 at 19:43
  • $\begingroup$ @AlexP Thanks Alex. The ship is travelling in interstellar space in our beloved MIlky way galaxy. It is a Generation ship on a multi-century journey to inhabit another system a few dozen light years away, so we're not far from neighbourhood :) The trajectory was well planned to avoid known dust clouds or dense parts of the area, but a stray atom or molecule can always occur... so in this case, is outer maintenance of the ship totally out of the question (or reserved for emergencies)? $\endgroup$ May 8 '20 at 19:50
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    $\begingroup$ Let's say that there is an average of 1 particle per cubic centimeter. The cross section of a human is about 1 square meter. At 100,000 km/sec, the human sweeps a volume of about 100,000,000 cubic meters per second, thus encountering an average of 100,000,000,000,000 (one hundred trillion) high-energy particles every second. Health insurance would be very expensive. The HR department will insist very strongly that any exterior maintenance be performed by robots. $\endgroup$
    – AlexP
    May 8 '20 at 20:45
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    $\begingroup$ "Please consider I may use the information provided to help in building the story's world properly." ...Isn't that sort of the point of this SE? 😉 I would rather hope that disclaimer is unnecessary. $\endgroup$
    – Matthew
    May 10 '20 at 1:58
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Your astronaut would be protected in the same way as the ship.

It is bad to be hit by a very fast piece of stuff. Very bad if you are alive, but also bad if you are not alive and you get hit over and over all the time. The latter is the case for your ship. Pieces of space dust and particles are plowing into it and ablating it.

And going right thru it. An atom at 0.3c is a cosmic ray. Your passengers would not be able to sleep at night, because of the flashing lights inside their eyes from the cosmic ray visual phenomena. Cosmic rays going thru you are are bad in several other ways as well.

Your ship needs protection too. Maybe a magnetic field to deflect charged particles from all angles and a dispensible physical mass in front (asteroid, chunk of ice) to absorb larger impactors. Maybe a railgun or guided rockets to shoot larger incoming masses detected by radar.

Your spacewalker would hunker down, protected by the above just like the ship. If the spacewalker strayed outside of the protections, it would be bad.

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    $\begingroup$ "Maybe a railgun or guided rockets to shoot larger incoming masses detected by radar.". That is a great way to reduce a large, detectable and thus avoidable object into lots of small shrapnel you can no longer detect but will still hit you. Furthermore, at 0.3c radar will have a negligible forwarn time unless you put lots of energy into it. $\endgroup$
    – Polygnome
    May 9 '20 at 18:48
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    $\begingroup$ Polygnome - I admit to being heavily influenced by the star destroyer of the Empire chasing the Falcon thru the asteroid field, and using its guns to clear a path. $\endgroup$
    – Willk
    May 9 '20 at 19:26
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    $\begingroup$ Yeah that scene is utterly ridiculous on so many levels. $\endgroup$
    – Polygnome
    May 9 '20 at 21:57
  • $\begingroup$ Yup, thus the almighty term "suspension of disbelief" - when the story's good, you let such things slide :) :) $\endgroup$ May 12 '20 at 15:54
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    $\begingroup$ Collisions with macroscopic particles are a very manageable problem even for very small relativistic spacecraft: arxiv.org/abs/1608.05284 $\endgroup$
    – mic_e
    May 13 '20 at 17:27
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Talking about any speed is meaningless unless a reference frame is used. Since you cannot sense your own speed, you must use objects as reference points. For the sake of the question, lets assume that your heroes started on a planet, which is located in galaxy not much different than our own. To accelerate to 0.3c relative to the star you started on would pose a significant rise, not only for spacewalking, but also for the ship itself.

Objects in a galaxy typically move around 10s km/s relative to one another and range to the extremes of about 1000 km/s relative to each other. 0.3c is close to 100000 km/s. So hopping star systems at these speeds in a galaxy would be ridiculously risky to say the least. If a dust particle were to hit you at these speeds (that is in your reference frame), yes the damage would be catastrophic to say the least. At these speeds, the amount of energy a particle with a mass of about a microgram is around $10^{9}$-$10^{8}$ joules. This is around the kinetic energy of a plane landing, concentrated into the scale of a dust particle.


Addition from the comments

At these speeds, atoms would be too small to be too terribly damaging individually, but they add up pretty quick. For instance a helium atom would have around 10^-9 Joules of energy, this is around the energy in the original CERN collider which converted to atomic energy units is about 50 GeV. To see what happens when hit with a beam a bit larger (76 GeV if I remember correctly) look up the story of Dr. Anatoli Bugorski, a Russian scientist who was struck in the face with a proton beam of this energy while working on a particle accelerator. Thus the density of atoms in the space in which the ship is traveling will be an important factor.

Thanks to @HDE 226868 for providing a useful number density, of say $10^4$ hydrogen atoms per cubic centimeter, which is high but not impossible. Hitting this many protons at these speeds for any length of time would be quite dangerous to one's health.

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    $\begingroup$ @NoamJosephides At these speeds, atoms would be too small to be too terribly damaging individually, but they add up pretty quick. For instance a helium atom would have around 10^-9 Joules of energy, this is around the energy in the original CERN collider which converted to atomic energy units is about 50 GeV. To see what happens when hit with a beam a bit larger (76 GeV if I remember correctly) look up the story of Dr. Anatoli Bugorski, a russian scientist who was struck in the face with a proton beam of this energy while working on a particle accelerator. $\endgroup$
    – user110866
    May 8 '20 at 19:54
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    $\begingroup$ I have to quibble with the numbers here. Dust grains in the interstellar medium are unlikely to be more than a micron across, which means that each grain would have much less energy than the figures you're quoting here - if the astronaut encountered any dust at all, rather than gas. $\endgroup$
    – HDE 226868
    May 8 '20 at 20:05
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    $\begingroup$ @HDE226868 Thanks I added your numeric to my answer with credit. $\endgroup$
    – user110866
    May 8 '20 at 20:30
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    $\begingroup$ @fuenfundachtzig Thats sounds like a bumper sticker made by some cynical astronomer or cosmologist. $\endgroup$ May 9 '20 at 15:24
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    $\begingroup$ At 0,3c, it is meaningless to talk about a reference frame. All matter has practically a speed of 0 relative to 0.3c. This is because a) the speed of anything macroscopic (above nanograms) does not exceed escape velocity + orbital verlocity. That's a rounding error compared to 0.3c. And b) anything moving with a significant fraction of c will still do so, e.g. particles. Should this not apply, you're in black hole accretion disc territory and spacewalking is a no-no. $\endgroup$
    – Jens
    May 9 '20 at 19:39
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You might be interested in reading up on the Bereakthrough Starshoot program. The concept is to send a thousand nano light sail spacecraft each with a camera, to Alpha Centauri, at 0.2c taking approximately 50 years, propelled by a laser beam from earth. One way only, of course. They would be protected by a thin, light ablative coating to protect them from atomic particle impact. The hazard and probability of bigger particles hitting these tiny ships has been discussed extensively, and has been dismissed as being any kind of relevant threat, once out of the solar system.

Protective coating: A coating, possibly made of beryllium copper, is planned to protect the nanocraft from dust collisions and atomic particle erosion.[37][46]

That is, an ablative coating that will withstand particles hitting it. I saw a scientific article, that I can not put my hands on, that did the engineering math that proved such a coating would protect these miniature sails from damage.

It turns out that all of the sci-fi gobbledygook paranoia about small particles the size of atoms hitting a ship at speeds of 0.2c or faster totally obliterating a spacecraft are just simply bogus. The damage of such a high speed impact is very local, and easily protected against. A small atomic sized particle hitting an astronaut space walker that is protected by a thin ablative surface that is able to conduct heat very effectively would do no damage. There are an abundance of articles that describe how beryllium copper protects against high heat and mechanical impact.

Larger particles, of course, are another problem. But once outside our solar system, the chances of being hit by a lightning bolt from earth would be greater than confronting space debris. The voyager series have addressed that issue quite successfully.

Here is one that looks at the problem from several perspectives.

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  • $\begingroup$ thank you, extremely interesting read! $\endgroup$ May 9 '20 at 4:55
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You'll be scooping up interstellar medium at significant kinetic energies.

There's two problems: The atoms that hit you will actually accelerate you away from your position (pressure measured in Pa), and of course they'll impose a radiation hazard as you absorb them (radiation dose measured in Gy).

Let's do some math. Let's assume an interstellar medium consisting of one hydrogen atom per cubic centimeter.

This gives a density ρ = m/V = 1.7e-21 kg/m³.

You'll scoop up the ISM at dm/dt/A = ρ·v = 1.5e-13 kg/s/m². Note that this depends on the area of your body that is facing into the stream; you might want to go head-first or feet-first depending on if you're still planning to procreate.

The pressure that the ISM exerts on you is p = F/A = dm/dt/A · v = 1.3e-5 Pa.

This is pretty negligible, you won't even feel it. At an exposed area of 1m² and a mass including the EVA suit of 100kg, the acceleration is just 1.3e-7m/s², enough to displace you by less than a meter in one hour.

The radiation hazard is another thing, though. The kinetic energy at relativistic speeds is calculated by multiplying the relativistic gamma factor, minus one, with mc². The relativistic gamma factor in your case is 1.04828.

The kinetic energy of the mass stream is calculated by P/A = (relativistic gamma factor - 1)·dm/dt·c^2/A = 653W/m². This is half the power of solar radiation as felt on earth. Which is a lot.

The radiation dose is calculated as energy absorbed per body weight: Assuming a body weight of 80kg and an exposed area of 0.5m^2, D/t = P/m = (P/A)·A/m = 4.08Gy/s. This handy table tells you what will happen to your spacewalker.

  • They'll pretty much immediately feel the extra heating
  • After 2 seconds, their biology is damaged beyond repair with a life expectancy of 2-4 weeks, and "rapid incapacitation" sets on
  • After 7 seconds, they'll experience "Seizures, tremor, ataxia and lethargy", and their life expectancy has dropped to 1-2 days

So... without some sort of shield there's no way a human could do a spacewalk.

however, these levels of radiation will not only affect spacewalkers, but everybody on the ship, so the ship will likely have massive leaden (or similar) shields at its front. As long as your space walkers would be in the "shadow" of the frontal shield, they have nothing to worry about... apart from accidentally drifting into the death zone...

Of course the radiation levels outside the ship would still be elevated because

  • the ship's hull provides some additional shielding
  • some particles can be scattered around the edges of the shield which would lead to a gradual increase in levels the closer you get to the "edge" of the shielded zone, especially near the rear end of the ship.

but if the inside of the ship is shielded well enough to allow humans to live a normal live and life, they should be able to survive outside for a few hours without problems.

P.S. I think the leaden shield is actually doable in practice. I'm not sure how to calculate its thickness, but it will be probably ~50 times the mean free path of 0.3c hydrogen atoms in lead, whatever that works out to. You could make the shield out of Unobtanium which I hear has excellent properties in that respect. Unobtainium shields may even be light enough to be worked into the fabric of the EVA suit... but note that you'll still need to cool them, and since your visor can't be made of it, don't you ever stare into the direction of oncoming death.

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    $\begingroup$ haha, you killed me with Unobtanium (which was always my prime grunt towards Avatar... so non-elegant, eh?) Thanks for the detailed math and explanation - yes there would be a shielding to the ship (not just proper coating of the hull itself, but also an ionizing beam shooting forward to disintegrate accidental space matter and "suck in" the ions to an exhaust tube of sorts. the spacewalker would be theoretically behind this umbrella but still - I'd like to make the walk risky and hazardous so it's a rough decision whether to do it - but not get him killed within 7 seconds either :) $\endgroup$ May 9 '20 at 14:35
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    $\begingroup$ well, if your ionizing beam captures 95% of the particles, the radiation dose get reduced by a factor of 20... you can tweak the numbers any way you want :) $\endgroup$
    – mic_e
    May 9 '20 at 14:40
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    $\begingroup$ also, I just assumed one particle per cubic centimeter... the actual density varies greatly: en.wikipedia.org/wiki/Interstellar_medium#Interstellar_matter $\endgroup$
    – mic_e
    May 9 '20 at 14:42
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    $\begingroup$ Radiation is definitely the killer in this scenario, thanks for the math $\endgroup$ May 9 '20 at 19:52
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Having 0.3c related to any known macro object in our university pretty much imposes having ~0.3c related to anything else (redshifted objects included, if/when you reach near them).

At 0.3c all those stray helium and hydrogen atoms will behave as both deeply penetrating and ionizing radiation. At 0.3c you will "collect" a lot of them, even in the space between galaxies where they are ~1 atom in 1m3 or less.

Not sure about spacewalking, but you will have hard time shielding the whole ship. Think muons.

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  • $\begingroup$ Thanks @fraxinus ! Actually for the protection of the whole ship during travel (we are speaking about a future scientific endeavour so some tech and science advancements from today's knowledge could be expected, although I don't want to solve narrative complexities by just saying it was magically solved in the future...) - I was reading a lot about "shooting" ionizing beams forward, thus collecting the ions into a king of "Exhaust" tube. The spacewalking person will be behind this umbrella but for drama's sake I would like for the spacewalk to carry some risk thus not be a trivial decision:) $\endgroup$ May 9 '20 at 14:08
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It is completely unfeasible for the reasons listed above. What's more, the ship itself would be exploded by stray particles hitting it at 0/.3C relative.

Suppose your ship is tiny, just a cross section of 100 square meters and the trip is 100 light years. The volume of that cylinder is enormous. Every piece of space dust in there impacts your ship at 0.3C relative speed, about 200,000,000mph.

100 meter cross section times... uh.. 9.461e+15 meters... is about 9.5*10^17 cubic meters. So you are pushing that (improbably tiny) ship through 950,000,000,000,000,000,000 cubic meters of space. Air is 1kg/m on Earth. Let's pretend there is only 1kg of mass per TRILLION cubic meters on a path between two stars. That means that your ship is being hit by 9,500,000 kg of matter at 0.3C. Even if it hits as one proton at a time, gamma rays from collisions kill everyone.

For a terrestrial example with a baseball at 0.9C, see Randal Monroe's book excerpt here: Here's what the author of the XKCD comic states: https://what-if.xkcd.com/1/ Protip: Don't be in the city on game day.

It is not going to work without magic^H super science. So just take your perpetual motion machine free energy generator (cough warp core cough) and make a magnetic shield to hand waive it all away.

Read Larry Niven's Ringworld or Man-Kzin novels for great examples of what you want to emulate.

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  • $\begingroup$ Thanks @Guest ! Actually for the protection of the whole ship during travel (we are speaking about a future scientific endeavour so some tech and science advancements from today's knowledge could be expected, although I don't want to solve narrative complexities by just saying it was magically solved in the future...) - I was reading a lot about "shooting" ionizing beams forward, thus collecting the ions into a king of "Exhaust" tube. The spacewalking person will be behind this umbrella but for drama's sake I would like for the spacewalk to carry some risk thus not be a trivial decision :) $\endgroup$ May 9 '20 at 14:09

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