  The Breakthrough Initiatives Project Starshot proposes to send a gram-scale
laser driven spacecraft to the Alpha Centauri system in a 20 year mission
travelling at v=0.2c. One of the challenges of this mission as the spacecraft
moves through the interstellar medium is the presence of dust and gas (mostly
hydrogen). The dust has a typical matter-density of 2.57*10^-27 g/cm3 with
typical particle mass being 3*10^-13 g although some of the largest particles
may be 5*10^-9 g in mass. These dust particle will deposit 10^12-10^16 MeV onto
the spacecraft with an energy flux of order 0.3 J/sm2. We consider the erosion
of the spacecraft frontal area due to dust and also heating effects. We attempt
to characterise the likely environment for the starshot mission and estimate
the particle bombardment shielding requirements in terms of mass and thickness
of material. Current analysis estimates that the likely erosion rates are of
order 10^-11-10^-8 g/s and that the frontal area temperature for the models
examined in this paper will be 135.2 K depending on the ratio of frontal area
to radiating area. For an assumed shielding material with atomic number range
3-13 (Lithium to Aluminium), and for spacecraft geometries with radii ~1 mm and
cylindrical length 5 mm, over a 21.5 year mission duration, this would suggest
a shielding thickness of 1.4 - 3 mm. This would also suggest a shielding mass
in the range 0.01 - 0.05 g; depending on the material choice, spacecraft size
and chosen geometry. This would represent between 1 - 5% of the total mass,
assuming a spacecraft mass of 1g (driven by a 100 GW laser power). We also
examine the additional effects of charged particles and estimate the stopping
power and penetration range for different materials.
