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= Star Travel and the Interstellar Medium, particle effects =

The [[ https://en.wikipedia.org/wiki/Interstellar_medium | interstellar medium ]] - the coronal gas - forms a column between locations in space.  The sun is 4 light years inside one edge of a [[ https://en.wikipedia.org/wiki/Local_Interstellar_Cloud | 60 light-year-across Local Cloud]] with a density of [[ http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html | 0.3 hydrogens per cubic centimeter ]], or 3e5 H/m³, encountering the sun as it moves towards Scorpio at 25 km/s.   Around that cloud is the [[https://en.wikipedia.org/wiki/Local_Bubble | Local Bubble]], 300 LY across with a density of 50 H/m³.   A light year is 9.5e15 meters, so for the first 4 light years in one direction, and more than 50 LY in the opposite direction, we encounter a column of hydrogen atoms with a density of about 1e21 H/m²-LY.   That is the density of a layer of liquid water 30 nm thick per light year.  100 light years, 3 micrometers.

These protons will implant themselves in the surface of any object passing through them.  At a mere 25 km/s, the energy per proton is 3.6 electron volts - many will bounce off.   Approaching the speed of light, the energy approaches multiples of the mass energy of the proton, 938 MeV/c².  That will damage solid structure to the implant depth.  The table below (from Janni 1966) shows stopping range in silicon extrapolated by gram density to glass and water - perhaps poorly.

The numbers below are estimates - real measurements needed!

||<-9>'''Conditions in Local Cloud, 3e5 H/m³, 5e-22 kg/m³ as perceived from moving frame'''||
|| speed || v/c ||$1\over{c²-v²}$||energy||flux||<-2>Stopping Range est mm||Pressure|| ||
||   m/s ||        ||      ||         ||1/m^2^-s||!SiO₂ || H₂O  || Pascals || ||
||   25k ||   8e-5 ||    1 ||    4 eV ||    8e9 ||      ||      ||         || ||
||   30k ||   1e-4 ||    1 ||    5 eV ||    9e9 ||      ||      ||         || ||
||  100k || 3.3e-4 ||    1 ||   50 eV ||   3e10 ||1.1e-3||2.8e-3||         || ||
||  300k ||   1e-3 ||    1 ||  500 eV ||   9e10 ||3.3e-3||8.5e-3||  14&mu; || ||
||    1M || 3.3e-3 ||    1 ||   5 keV ||   3e11 ||1.6e-2||4.1e-2|| 500&mu; || ||
||  1.4M ||   5e-3 ||    1 ||  10 keV ||   4e11 ||2.5e-2||6.5e-2||    1.4m || low energy ion implantation                   ||
||    3M ||   1e-2 ||    1 ||  50 keV ||   9e11 || 0.08 ||  0.2 ||     14m || low energy van Allen belt                     ||
||  4.2M ||  0.014 ||    1 || 100 keV || 1.3e12 || 0.15 ||  0.4 ||     37m || 100 keV van Allen belt, 6000x more flux       ||
||   10M ||  0.033 ||    1 || 500 keV ||   3e12 ||  0.6 ||  1.6 ||     0.5 || high energy ion implantation                  ||
||   14M ||  0.042 ||    1 ||   1 MeV ||   4e12 ||  1.2 ||  3.1 ||     1.4 || 1 MeV van Allen belt, 2e5 more flux           ||
||   30M ||   0.10 || 1.01 ||   5 MeV ||   9e12 ||  4.4 ||  1.2 ||      14 || 7 W/m^2^ particle power                       ||
||  100M ||   0.33 || 1.13 ||  50 MeV ||   3e13 ||   37 ||   95 ||     570 ||                                               ||
||  200M ||   0.67 || 1.80 || 380 MeV ||   8e13 ||      ||      ||      7K || highest energy van Allen belt, 1e12 more flux ||
||  250M ||   0.83 || 3.28 || 500 MeV || 1.4e14 ||      ||      ||     26K || 11 KW/m^2^ particle power                     ||

{{https://upload.wikimedia.org/wikipedia/commons/7/74/Local_Interstellar_Clouds_with_motion_arrows.jpg | | width=600 }}{{https://www.spenvis.oma.be/help/background/soldam/sprcp.gif| |width=350}}
 . [[https://upload.wikimedia.org/wikipedia/commons/7/74/Local_Interstellar_Clouds_with_motion_arrows.jpg]]
         . grams/cm² &#8594;  10mm/density [[https://www.spenvis.oma.be/help/background/soldam/sprcp.gif]]