= Solar Wind =
 * 1.3e36 particles per second, presumed monatomic hydrogen
  . [[ https://en.wikipedia.org/wiki/Solar_wind#cite_note-kallenrode-28 | WP ref ]]
  . Area at 1 AU = 4π ( 150e9m )² = 2.8e23 m², hence flux = 1.3e36 H/s / 2.8e23 m² ≅ 5e12 H/(m²-s)
  . Hydrogen mass 1.67e-27 kg, mass flux - 8e-15 kg/(m²-s)
 * slow wind 300–500 km/s, temperature ~1e5 K, twice as dense → 5e-15 kg/(m²-s)
  . RMS isotropic thermal velocity ~50 km/s
   . RMS isotropic thermal velocity = sqrt( 3 k T / m ) ≈ 0.157 km/s * sqrt( T )
 * fast wind 750 km/s, temperature  ~ 8e5 K, half as dense → 3e-15 kg/(m²-s)
  . RMS isotropic thermal velocity ~140 km/s
  . [[ https://web.archive.org/web/20080610125820/http://solarscience.msfc.nasa.gov/suess/SolarProbe/Page1.htm | WP ref ]]
 * estimated H cross section ≈ 0.5 Ų = 5e-21m²; 5e12 H/(m²-s) * 5e-21m² → 25e-9 H collisions per second → 0.8 collisions per year
  . That's against a zero radius target - for a hydrogen target 3 collisions per year, for an oxygen target many per year