I assumed an interstellar medium density of 10 H atoms/cm^3, which is 1e7 atoms/m^3, given 1e6 cm^3 per m^3. I think the volume swept out by the spacecraft is

I assumed an interstellar medium density of 10 H atoms/cm^3, which is 1e7 atoms/m^3, given 1e6 cm^3 per m^3. I think the volume swept out by the spacecraft is about right: (3e8m/sec)(.2)6.45e-4m^2 is about 3.9e4m^3/sec. Then, 1e7 H atoms/m^3 * 3.9e4m^3/sec gives about 3.9e11 atoms/sec hitting the (1 inch square) spacecraft.