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Traveling through the Interstellar Medium

  1. Mar 25, 2017 #1
    I understand that the interstellar medium (ISM) consists mostly of neutral hydrogen gas, plus a smaller fraction of helium gas. Furthermore, I gather that between 0.1 and 1.0 atoms per cm3 correspond to the average density values pertaining in our "Local Bubble". The above, however, refers exclusively to the gas content of the ISM; that's to say it doesn't include dust particles (or "presolar grains") - never mind more massive objects, like micrometeoroids. True, these objects comprise a tiny percentage of the ISM, but they are in individual terms disproportionately more massive when compared to the interstellar gas. How much more massive, I simply don't know. All that can be said that their average distribution in the ISM is claimed to be around 10-6/m3 (0.00001/m3).

    None of this would be an issue were it not for a desire to work out how the average density of the ISM (as it applies to our Local Bubble) compares with conditions here on Earth. Given that Earth's air density at sea level is about 1.29x10-3 g/cm3, I've calculated that this density could be as much as 300 million, trillion times greater than that pertaining to the ISM.

    The above is significent from an SF writing point of view. It might be expected that the interstellar gas itself wouldn't prove too hazardous for a sufficiently shielded spacecraft travelling at 3%c. Encountering a dust particle at this speed, however, could spell instant doom - e.g. a single 30mg dust speck would have a kinetic energy of around 0.3 kg TNT (a little under that of a modern hand grenade). There are SF workarounds even here, of course. Nevertheless, I would like to have a clearer idea what the "average" density of the ISM is (or could be) - a density figure too that includes material objects like dust particles etc. More than anything else, though, any comments or criticisms about my calculations would be extremely welcome.

    Many thanks.
  2. jcsd
  3. Mar 25, 2017 #2
    Hi !!
    IIRC, we're currently in a 'bubble' of 'lower than average' density. https://en.wikipedia.org/wiki/Local_Bubble

    Also, IIRC, fast interstellar craft that lack an ST 'Deflector Array' or Alcubierre Bubble would have to deploy *and renew* a cloud of fine dust as a shield. Unfortunately, that could consume possible payload...
  4. Mar 26, 2017 #3


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    The average density of dust is small compared to the average density of gas. You just have to spread out the impact. A thin layer far ahead of the spacecraft should do the job. A grain of dust will make a grain-of-dust-sized hole in it, and the debris hits the spacecraft distributed over a large area at ~400 MeV/nucleus, which is easily absorbed by 1 meter of water shielding for example.
  5. Mar 26, 2017 #4
    Hi @Dr Wu:

    You may want to look at the discussion earlier in this thread about the novel Tau Zero. That novel describes a technological solution regarding dust and larger stuff. The technology used seems to me to be compatible with current science knowledge, even though the novel was published in 1970.

    At the time the space ship traveled at a relatively small fraction of c (I forget now the actual value), a dust cloud caused an accident.
    The possibility of a recurrence was later avoided by traveling very much faster.

  6. Mar 26, 2017 #5


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    A couple of comments. First, I think most of the Hydrogen gas in the vicinity of the sun is ionized, not neutral. This Wikipedia page gives some details. Second, the total mass in the dust is about 1% of the total mass in the gas. Third, if you look at the attached paper, you will see that most of the dust particles are 10 microns in size or smaller. A 10 micron particle with a density of 2-3 g/cm^3 will have a mass on the order of 10^-9 g, not 30 mg. I think the incidence of 30 mg particles will be very very low, and as mfb says, even these can be shielded against fairly easily.

    Attached Files:

  7. Mar 30, 2017 #6
    An embarrassing admission: I now feel I've been a tad obsessive about the above issue. A little knowledge (i.e. not enough) can be a disabling thing in my case. The underlying point, which I was reluctant to reveal at the time, centred on the possibility - or otherwise - of an astronaut performing an emergency EVA whilst coasting along high above the plane of the solar system at 3% light-speed. I didn't stop to consider various SF standbys - like forward "droplet" shields, adjustable whipple shields, ionising UV lasers. . . and the like.

    I've since noted that the vacuum inside the detectors of the Ligo gravity wave interferometer in the USA is several hundred million times denser (softer?) than that claimed to exist throughout the local interstellar medium. Such 'mind-expanding' notions never fails to bring home to me just how inconceivably unearthly outer space truly is.

    Once again, many thanks for bailing me out.
  8. Mar 30, 2017 #7


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    The BASE experiment created a vacuum that should be similar to interstellar space. It is so good that they couldn't measure any remaining gas, and it is quite possible that they did not have any atoms in the vacuum chamber.

    A space suit could be designed to shield against the gas atoms. Micrometeorite damage would be a different issue - I guess you just have to hope that nothing hits the astronaut. Or work in areas shielded by the spacecraft only.
  9. Apr 11, 2017 #8
    Well, ideally you wouldn't go venturing out of your spacecraft while it's moving at that kind of velocity for exactly the reasons you have suspected. If you did have to perform an EVA while the ship was moving, you would want to adjust the orientation of the ship so that the damage is facing away from the direction of travel, putting the ship itself in front of the astronaut.
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