Cruise | Interstellar medium and cosmic rays

The mean free path and Larmor radius of interstellar plasma particles is far greater than the size of the nanocraft, meaning that they would impact the nanocraft walls independently rather than forming a bow shock.

Protons from the interstellar plasma would impact a nanocraft travelling at 20% of the speed of light with kinetic energies of 18MeV, and electrons would have kinetic energies of 10.2keV. Whether the protons and electrons are combined in a hydrogen atom or arrive separately is not very significant. There would be some erosion of the surface of the nanocraft by sputtering; the number of sputtered atoms per area would be of the order 1000 per cm2. The net loss of mass from the forward-facing surface would only amount to a few monolayers.

The 18MeV protons would travel several millimeters into the target before stopping. A protective layer sufficient to stop 18 MeV protons would be required to avoid damage to the electronics by proton implantation.

Cosmic rays are much rarer than interstellar protons and hence can be ignored. Impact by heavier elements would need to be mitigated by means of protective shielding: helium nuclei carry 72 MeV and are 10% as numerous as single protons. Hits by CNO group nuclei carry 200-300 MeV and are ~0.1% as common, and hits by iron nuclei carry ~1GeV of energy and are ~0.01% as common. The depth of damage per nucleus scales roughly as the total energy to the 1/3 power.

Laboratory experiments on ions traveling at 20% of the speed of light (from a modest particle accelerator) and impinging on a solid could study the impact of interstellar particles on the nanocraft walls.

Collisions with interstellar ions and electrons might conceivably also have a benefit: they could charge the nanocraft to a potential of up to 10kV (the kinetic energy per electron). The forward-facing surface of the nanocraft would be heated at a rate of 6mW per cm2, in principle providing a thermoelectric energy source during the cruising phase in the interstellar medium.

Knowledge and experience in the interstellar medium might be gained on a precursor mission to the ultimate Alpha Centauri one, venturing beyond hundreds of AU within the solar system.

Comments (14)


    Could direct charging of a capacitor, by charge separation rather than via thermoelectricity, be used with ions and electrons ?


    If I multiplied right (never a given), the areal density of gas from here to Alpha Centauri is about 1.5 g/cm^2, or about 1.5 cm WE--on the scale of, or beyond, the range of 20 MeV protons in water. This suggests the possibility that even if all of the impact damage is confined to a protective layer, the total momentum kick of all the impacts might be enough to stop the chip before it reaches AC. Is this addressed somewhere?

  3. Dmitry Novoseltsev:

    Please note the pictures of the Mira star in ultraviolet light, made in the GALEX project.
    Speed of the Mira –about 130 km/s, but before it is clearly visible the detached shock wave in the interstellar gas.
    With low weight and higher speed of our device (almost 600 time) the effect will be much stronger, which will lead, firstly, to intensive braking of the sail immediately after disconnection of the accelerating beam, and secondly, to its rapid destruction.
    In this connection I consider it expedient after stage laser acceleration to deploy the sail perpendicular to the direction of flight (edge of course). Then the braking and the wear will be minimal.
    If the sail is stabilized by rotation around the axis perpendicular to its plane, with a large enough frequency so that the canvas sails kept rigid and is not formed under the action of force from the oncoming flood, you can provide it with minimal even wear around the perimeter.
    In this case, the optimal, apparently, the round shape of the sail.
    If the sail of electrically charged, as I suggested earlier, this provides partial protection from positive ion colliding at a small angle to the plane of the sails – they will be leaving.
    On approaching the target it is expedient again to deploy the sail perpendicular to the direction of flight is then due to the resistance of the interplanetary gas and the oncoming solar wind can some what reduce the speed and increase the time of observation during the flight.

  4. Dmitry Novoseltsev:

    Explain the previous message. If the sail is not flat, but corrugated (volume) and charged (as, and is rotated, and the payload is located in the center, the sail is a shield from the particles of the interstellar medium, including charged. By the way, making the sail rotation can be ensured during acceleration due to the effect of "photon turbine" (for example, if you run radial corrugations with different reflectivity edges, whereby under the action of a laser beam will arise torque).


    A 0.1g projectile made of silicon will have an area of and will therefore sweep a volume of interstellar medium of about 4E17 cc in a distance of 4 light years. Assuming 1 hydrogen molecule per cc of ISM, that has a couple of serious implications.

    1) Assuming the 4E17 molecules in the path are just absorbed by the projectile, the total absorbed mass (AM) is 1 microgram = 1E-5M. From momentum conservation, the velocity is reduced by M/(M+AM). The change in Kinetic energy is (classical approximation) KE{ 1 - M/(M+AM) }. This lost energy is dissipated as heat. The initial KE was 1TJ, so the heating is 10MJ. Given that the trip lasts 20 years, the heating power is 16mW. That's a pretty big power for a tiny piece of silicon. The good news is that here's your energy source- don't need a battery. The bad news is that can't use a radiator to get rid of the heat, because more area will just sweep more volume and heat up even more.

    2) Radiation. The projectile will be bombarded with 4E18/cm^2 hydrogen molecules of 40MeV kinetic energy in the projectile rest frame. This is a huge radiation dose. For 18MeV protons the total ionizing dose would be 600 Grads. No idea how to make electronics that can work after such dose.

  6. Dmitry Novoseltsev:

    To make the CPU flat, and to deploy edge course: its midsection not 0.1, is 0.01 cm^2.

    For improving the protective shielding effect of the rotating circular corrugated sails with a central position of the payload flying on the edge first, his corrugations to perform better not radial, but curved as the blades of the impeller of a radial pump or fan. Better – with a significant curvature, in this case colliding particles towards the centre of the sail will have to overcome several layers of sail material.
    Perhaps before the main probe will need to run a protective probe – the "bulldozer" to clear the path.

  7. Micky Badgero:

    Some of the math here I am not following. Here is my math.

    4.370E+00, light-years to Alpha Centauri
    2.998E+10, centimeters/second (speed of light)
    3.156E+09, seconds/year

    4.134E+20, cc's /4.37 ly, also the number of atoms of H @ 1/cubic centimeter
    6.023E+23, atoms/mole

    6.919E-04, ~0.7 mg/cm^2
    1.000E+04, cm^2/m^2
    6.919E+00, ~7 g/m^2

    In regards to's question, the probe would not stop, but it would slow down. If the sail was used as a parachute, it would slow down enough to cruise through the system at close to planetary velocities.

    The frontal area of the probe could use a phosphor as a shield. The phosphor could be doped with a radioisotope for power with photovoltaics. The impact ions would supplement the power from the radioisotope.

    In regards to's concern about radiators, the radiators would be at a right angle to the travel and could be as large as necessary without increasing the frontal cross-section.

  8. Dmitry Novoseltsev:

    In regards to's concern about radiators:

    If the sail has the form of a thin corrugated disk, flying edge forward, its broad side surfaces themselves should serve as the radiator. Especially if it is made of material with high thermal conductivity of metal nanostructured films.


    If the spacecraft becomes charged, its path will be deflected by any interstellar magnetic field.
    Is this field vector known? What are the resulting constraints on the acceptable rate of charging during cruise?
    - will the spacecraft require a means of discharging itself?
    - is controlled charging a useful means of course correction?

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