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During the launch, when the rocket is itself yet to attain a stable orbit, the exhaust from the launch vehicle is directed approximately in the opposite direction of its velocity. So, there is no chance for the exhaust particles to attain stable orbit around Earth.

But once orbit is attained by a satellite, it uses its onboard propulsion systems for altitude control and/or to maintain proper orientation. Further, during deorbit burns, the exhaust particles are fired prograde whereas the spacecraft's motion is retrograde. In these cases, there is a high possibility that the exhaust particles attain a stable orbit around Earth. Evidence for this can be inferred from the fact that Soyuz spacecraft after undocking fires its engines for the orbit lowering burn only when it is at a large distance from the International Space Station (ISS) in order to avoid contamination of ISS's components by exhaust particles. There is also a possibility that during retrograde propulsion manoeuvers, the particles being fired prograde are being given additional velocity i.e., orbital velocity plus the velocity due to the burn. So eventually, they might enter a higher orbit.

What will happen to the rocket exhaust particles fired by spacecrafts? Will they attain a stable orbit around the Earth as I suggested, or will they eventually re-enter Earth's atmosphere? Are there any current studies on orbital pollution due to chemical rocket exhausts (and not space-debris)? What are the steps taken to reduce this contamination?

Even though the size of particles is small, I think we must take these into consideration as they might have huge impacts on the future of space travel (For example, contamination of solar arrays, optical surfaces, etc.)

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  • $\begingroup$ Exhaust of a rocket using liquid oxygen and hydrogen is only water vapor but no particles. If rocket petrol and liquid oxygen is used, exhaust is mostly gaseous, maybe few soot particles. $\endgroup$ – Uwe Oct 23 '19 at 20:10
  • $\begingroup$ similar question with four answers: Does “What happens beyond Kármán, stay beyond Kármán”? $\endgroup$ – uhoh Oct 23 '19 at 23:18
  • $\begingroup$ @Uwe, Thanks for the valuable point. Further, if we use hypergolic propellants, then they are extremely toxic (carcinogenic). They may contain particles which I think will contaminate very sensitive spacecraft components. Is there any model (like Kinetic theory of gases) which determines the concentration of particles varying due to the extreme conditions of space? $\endgroup$ – Guru Vishnu Oct 24 '19 at 5:41
  • $\begingroup$ @uhoh, Thanks for sharing that question. Now, I have got some insight into what happens based on the first answer there. Interestingly, my question and this answer is similar in many ways. It would be great if you could give possible steps to reduce this contamination, if possible. $\endgroup$ – Guru Vishnu Oct 24 '19 at 5:46
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Hopefully someone comes along with a sourced answer, but until then here's one that's just reasoned out:

When exhaust first leaves a spacecraft, it's hot and relatively dense. As it gets further away it expands, both because the particles have varied starting directions and because they'll hit each other and go their merry ways. It'll also cool down over time through radiative heat transfer, which tends to reduce chemical reactivity.

The expansion means that the more time passes, the fewer particles you will encounter at any given time. Craft in low orbit already plan to encounter some amount of particles from the atmosphere - the ISS for example periodically needs to fire engines to boost itself. The small amount of atmosphere is always slowing it down, jut a little bit.

It's my understanding that the exhaust of modern rockets is mostly molecules, rather than clumps of matter. Even at orbital velocities, individual molecules aren't much of a concern. In fact, there are individual gas molecules impacting your eyeballs at hundreds of miles/kilometers per hour right now.

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  • $\begingroup$ very similar question with four answers: Does “What happens beyond Kármán, stay beyond Kármán”? $\endgroup$ – uhoh Oct 23 '19 at 23:20
  • $\begingroup$ @CarlKevinson Could you explain what phenomenon you are referring to in this phrase: "In fact, there are individual gas molecules impacting your eyeballs at hundreds of miles/kilometers per hour right now."? $\endgroup$ – Sergiy Lenzion Dec 20 '19 at 8:46
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    $\begingroup$ @LeoS We often think of the air in a room as one item, but in reality it's like a ton of bouncy balls colliding with every surface and each other. Those collisions are what we are measuring when we measure "air pressure". Each molecule is very small, but there are a ton of them and they are moving quite fast. Some are faster and some are slower, but at room temperature, the average speed is on the order of 350 m/s. (780 mph, 1260 km/hr) $\endgroup$ – Carl Kevinson Dec 20 '19 at 14:30

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