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As ion thurster designs improve, the ions emitted could have a velocity (relative to the spacecraft) of well above 10^5 or even 10^6 m/s.

It the likelihood of any such ions ever hitting a human completely negligible? (I'd love to see some rough calculation considering divergence angles, number of ions, and the solar system's dimensions).

Also, what might be the effect of a single ion with such energy that hits a spacecraft or a human body, either on Earth (less likely I presume due to the atmospheric collisions) or in orbit?

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"ever hitting a human in space right behind the thruster" or "ever hitting a human on the surface of the Earth with the thruster in space"? – Ignacio Vazquez-Abrams May 16 '12 at 3:28
edited to clarify – GJ. May 16 '12 at 3:30

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Space is full of cosmic rays with vastly higher energies than this, and humans have so far been unaffected (though looking around at my co-workers I wonder sometimes).

Ions travelling towards the Earth get deflected by the Earth's magnetic field. The only ones energetic enough to make it through scatter off the atmosphere and generate a shower of less energetic particles, but the ions from an ion thruster aren't energetic enough for this. I suppose ions from an ion thruster could reach the surface if you turned the thruster on while still well within the magnetosphere, but ion thrusters wouldn't be used this way. They are a continuous low thrust engine and you'd only use them well away from the gravitational wells of the Earth or other planets.

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merely comparing their energy to cosmic rays (which is primarily protons) isn't convincing. xenon ions are 2 orders of magnitude more massive and occupy many orders of magnitude more space than a proton... – GJ. May 16 '12 at 10:21
@GJ.: so what? aren't their energies being compared here?? which takes into consideration their masses!! – Vineet Menon May 16 '12 at 12:08
According to en.wikipedia.org/wiki/Electrostatic_ion_thruster the energies used are a few keV. At these ion energies the magnetosphere would laugh in their face. – John Rennie May 16 '12 at 14:15
@Vineet Menon a neutrino with 1GeV would be harmless passing through a human body, while a 100keV photon could start a terminal cancer. in the same way, merely comparing the energies of a proton vs xenon ion doesn't seem sufficient. – GJ. May 16 '12 at 16:48
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@GJ. Ions are subject to the same deflecting, scattering and energy loss processes as cosmic rays (which neutrinos are not), so John's analysis is good. The big risk from an ion engine would be the total amount of energy if the beam played across you at close range. – dmckee♦ May 16 '12 at 19:17
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