What happens to the gas/propellant released by spacecraft?

[dryer_lint]

So, if you have a spacecraft in orbit around Earth and it fires its engines, it will release some sort of gas/exhaust in that direction, right? What would happen to this gas, would it keep somehow... orbiting or what? Is it possible for satellites to eventually (over lots of time) fall to earth because the gas slowed them down? Did I just realize this post is entirely made up of questions??

[shynung]

The exhaust gas molecules will either escape from the Earth's local system and venture out into space, or it will smack into something (moons and satellites, but the effects are manageable at that point) before it can do so.

Either way, rocket engineers usually doesn't really care where the exhaust gas ended up. Except when said spacecraft is still on the booster, launching from Earth; at that point, they have to make sure whatever the rocket is spewing out the nozzle doesn't wreck the immediate environment.

[Seret]

There's a fair amount of atmospheric gas at lower orbital altitudes anyway. The upper levels of the atmosphere do cause drag on spacecraft in orbit, and will eventually deorbit them if not corrected for. Any tiny amount of gas released by engines would have a negligible effect compared to something the size of Earth's atmosphere.

[Superfluous J]

According to Wikipedia's Specific Impulse page the velocity of rocket exhaust is 4.4km/s. Low Earth Orbit's orbital velocity is 7.8km/s. Escape Velocity is somewhere near 11km/s depending on how high you are orbiting.

So if you're in LEO and speed up (to go to Mars or Venus or the Moon or whatever) then your exhaust ends up going about 3.4km/s and deorbits. If you burn to deorbit, your exhaust speeds up to 12.2m/s and escapes.

In theory some of it won't escape and will go into an elliptical orbit. However its periapsis will be within Earth's atmosphere and it will eventually degrade just like the ISS would if we didn't keep boosting it.

However, all the rocket exhaust from all the missions ever flown is a drop in the ocean that is Earth's atmosphere. It's almost literally nothing.

[bs1110101]

Something i've been wondering, could you safely use a gas core open cycle NTR in low orbit (or for that mater whatever orbit where the fine mist of radioactive death would fall back to the planet from, seeing that the exhaust velocity is way faster) without worrying about where the exhaust goes, or would it stay together enough it could cause problems even after it dispersed over flying several km?

[Skyler4856]

Something i've been wondering, could you safely use a gas core open cycle NTR in low orbit (or for that mater whatever orbit where the fine mist of radioactive death would fall back to the planet from, seeing that the exhaust velocity is way faster) without worrying about where the exhaust goes, or would it stay together enough it could cause problems even after it dispersed over flying several km?

It should disperse over a large enough distance, and that design isn't realistic.

[Camacha]

It should disperse over a large enough distance, and that design isn't realistic.

Dispersed radioactivity is still radioactivity. Besides the legal issues, since I think there are treaties that ban this.

[andrewas]

A gas core NTR has at least a 30km/s exhaust velocity, so unless you point it directly at the planet the exhaust will escape harmlessly.

[lajoswinkler]

Exhaust velocity is not very important because it's miniscule compared to the speeds of the molecules in the exhaust. They behave like ideal gas, and they disperse in every direction.

The distribution of speeds for molecules follow Maxwell-Boltzmann distribution, meaning there are a lot of very fast molecules at exhaust temperatures.

What happens to them? They go everywhere, like little cannon balls, each on its orbit. A lot of them have enough speed to escape Earth's SOI, even Sun's.

[Nibb31]

Burn residuals are a real problem that are taken into account in space operations. The ISS for example is actually enveloped in a "cloud" of burnt residual fuel from its own thrusters and those of visiting vehicles. These residuals are corrosive and can damage or interfere with instruments and equipment that is located outside. Visiting vehicles must follow strict rules for using thrusters in close vicinity of the station.

[SlyReaper]

A gas core NTR has at least a 30km/s exhaust velocity, so unless you point it directly at the planet the exhaust will escape harmlessly.

Gas core NTRs are also extremely difficult to build. As in, actually beyond our technology. Solid core rockets are the ones which are realistic, and their exhaust velocity will be much slower. To push an orbiting craft to a higher orbit, that exhaust will fall out of orbit and re-enter the atmosphere. But it'll be so diffuse, it'll be barely detectable.