Is rocket propellant meaningfully a form of "space junk" I imagine that most people are familiar with the concept of space junk (or space debris), which is a hazard to space exploration that grows with every item that humans leave in space.  Wikipedia lists the following types of space junk:


*

*spent rocket stages 

*defunct satellites 

*erosion, explosion and collision fragments


I'm extremely curious, how much sense would it make to add ejected rocket propellant to this list?  To the extent of my understanding, for at least a fraction of the flight the products of the reactions that power a rocket will be left in space and maybe in orbit (?).  In the case of LOX/LH2 fuel, for instance, I believe this ejected material would be water vapor.  Normally this wouldn't be a problem, but if that gas is occupying an orbit, or is suspended as a part of the upper-atmosphere I wonder about detrimental effects such as:


*

*Increasing the gas content of near-space would cause greater drag on spacecraft

*In the case of high specific impulse thrusters, the propellant wouldn't be following the normal thermal distribution... so maybe that could lead to radiation damage

*Space could be polluted by activities on the ground, due to use of Helium, which escapes the atmosphere (and can thus contribute to increased orbit drag)

*Couldn't it increase the effective temperature of space?  If you assume a constant growth rate in energy use humans will inevitably cook themselves.


It might be easy to dismiss this is negligible, but I find the outcome non-obvious for several (basic physics) reasons.  Firstly, the mass of rocket propellent would probably exceed the mass of other space junk, since it is not reusable and because the ratio of payload to propellant is so poor for many trips.  Although Earth's atmosphere is large, the orbits that this propellant could affect only have an extremely sparse population of gas molecules.  Adding a few drops to a small bucket can still make a difference.
Some velocities:


*

*LOX / LH2 fuel: $v_e = 4,400 m/s$

*Ion thruster: $v_e = 29,000$

*LEO orbit speed: $V = 7,800 m/s$

*Earth and moon escape velocities: $11,200 m/s$, $2,400 m/s$


Similar to how anthropogenic CO2 emissions slowly affected the climate of Earth, could a scale-up of space traffic ultimately lead to problems by stretching out the upper edges of our atmosphere, injecting weird thermal distributions into space, and giving the moon a man-made atmosphere?  Even though the moon can't "hold" an atmosphere, a quick glance at the velocities above reveals that many plausible ejection vectors could leave it bouncing around for a good long time.
Just so that I don't leave this untouched, Project Orion (using nuclear bombs for propellant) seems like it could do the same thing for space that atmospheric testing did for the radiological environment we live in.  The further out you go, the more space you have, but also the higher speed propellant you need, so it seems like this is a potential problem at several stages of space development.
 A: I don't know enough about this topic to give a comprehensive answer, but I can point to some references.

Even though the moon can't "hold" an atmosphere, a quick glance at the
  velocities above reveals that many plausible ejection vectors could
  leave it bouncing around for a good long time.

Geoffrey Landis wrote an interesting essay about the possible effects of future human activities on the lunar environment: "Air Pollution on the Moon".

Similar to how anthropogenic CO2 emissions slowly affected the climate
  of Earth, could a scale-up of space traffic ultimately lead to
  problems by stretching out the upper edges of our atmosphere, [...]

There is a 1964 paper exploring exactly this question: "Pollution of the Upper Atmosphere by Rockets".

Just so that I don't leave this untouched, Project Orion (using
  nuclear bombs for propellant) seems like it could do the same thing
  for space that atmospheric testing did for the radiological
  environment we live in.

I haven't found technical studies about the residence time of fission products in the Solar System. But, with respect to the Earth's atmosphere, there is a brief analysis starting in page 81 of "Nuclear Pulse Space Vehicle Study, Volume III".
Edit 8/28
The "Nuclear Pulse Space Vehicle Study" landing page doesn't seem accessible at this moment, but the PDF (warning: 8 MB) can be downloaded from here.
