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)

  1. michael.million@sky.com:

    If the sail is thin these particles will go straight through without doing much damage unless they hit a nucleus. Edge on and the particle is also likely to be knocked to the side as to carry on going through material.

  2. Breakthrough Initiatives :

    Apr 14, 2016 18:16 mgs@lbl.gov Posted on: Breakthrough Initiatives
    "A 0.1g projectile made of silicon will have an area of 0.1sq.cm. 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. "


    For 20mW power in the 1 cm2 chip (assuming 100% energy transfer to chip), black body radiation will keep the chip at 240 K for =1; definitely a good potential source of power.
    The 15nm sail will not stop the protons. Sail surface density (for about 3g/cm2) is about 5g/cm2. Taking 20MeVcm2/g for 20 MeV protons we get 100 eV of power deposition by the protons.
    The dose on the chip is high but new devices, GaN/AlGaN UV-LEDs for example, hold promise: see 4).

    The 10keV electrons loose about 75 eV in an Al film of 150 A. For this energy and material (see 3) below:


    Answer:

    The sail stops significantly less than 1% of electrons.
    Further analysis is needed for this subject.

    1)http://www.kip.uni-heidelberg.de/~coulon/Lectures/Detectors/Free_PDFs/Lecture2.pdf

    2)http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640020596.pdf

    3) http://www.agiati.it/UploadDocs/5006_art_03_dapor.pdf

    4)http://iopscience.iop.org/article/10.1088/1742-6596/154/1/012028/pdf

    – Prof Sasha Buchman, Breakthrough Initiatives

  3. Robert Clark:

    It has been theorized planetary-scale lasers might be detectable at galactic distances:

    http://www.popularmechanics.com/space/a25609/fast-radio-bursts-alien-space-travel/

    I wonder if the effects of lightsail propulsion through the interstellar medium might be detectable in our own galaxy. The lightsail moving at relativistic speeds should accelerate the interstellar plasma. This should generate EM waves that might be detectable by us.

    Bob Clark

  4. Breakthrough Initiatives:

    RE:
    Jun 12, 2017 14:52 Robert Clark Posted on: Breakthrough Initiatives

    Answer:
    The simplest way to detect lightsail propulsion is through leakage of the laser light beyond the sail boundaries. This leakage is inevitable and was calculated by J. Guillochon and A. Loeb in the following paper, http://iopscience.iop.org/article/10.1088/2041-8205/811/2/L20/pdf

    The possible detection of such signals as Fast Radio Bursts was discussed by M. Lingam and A. Loeb in https://arxiv.org/pdf/1701.01109.pdf

    The power emitted due to the interaction of the spacecraft with the interstellar medium is negligible compared to the leakage in laser power (or else the spacecraft would have slowed down rapidly).

    - Avi Loeb, Breakthrough Starshot

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