28

Suppose we establish a moon base. This base would have some population, with some need for supplies, and needs for machinery in the early stages.

If we establish shipping lanes in space for these daily deliveries, would rocket exhaust build up in the shipping lane and cause "turbulence" (for lack of a better word) for future spacecraft using that shipping lane?

It seems like a very bad idea to have a rocket moving at Mach 10 hit a pocket of gas.

mcnnowak
  • 383
  • 3
  • 7

6 Answers6

55

Interplanetary space is a pretty good vacuum. Any gas introduced into that vacuum will be at a higher pressure than that vacuum. Gas expands until its outward pressure is in equilibrium with the inward pressure exerted on it by its surroundings, so rocket exhaust will rapidly dissipate until there's no noticeable trace left.

The gas leaving the rocket nozzle will expand in all available directions. The pressure along the rocket's trajectory will be higher than the pressure radially outward from the trajectory, so you end up with a column of gas that epxands outward.

This photo shows a SpaceX Falcon 9 launch. You can see how quickly the plume expands, even in very low orbit - this is a timelapse photo that covers a few minutes.

enter image description here

Hobbes
  • 127,529
  • 4
  • 396
  • 565
  • In the long term the gas is most likely going to be attracted to and fuse to the nearest (and largest) mass-- except for certain propellants expelled at greater than escape velocity, right? This kind of only answers the "could [this gas] hinder further space exploration?" not the "what happens to gas once its introduced into space?" The question doesn't explicitly ask the latter, but it seems to be where his over-all confusion is. How does gas "dissipate" in a vacuum? Should I ask the latter in another question? I have interest in that too. – Magic Octopus Urn Oct 30 '19 at 20:22
  • His comment, how could it potentially impact future craft and where does the exhaust go, prompted this. +1 – Magic Octopus Urn Oct 30 '19 at 20:23
  • @Magic Octopus Urn The exhaust has the same velocity as the spaceship, plus its ejection velocity. So it must remain on the same orbit with the spaceship, with a deviation that depends on the exhaust speed and the ship's attitude during the burn (lower orbit if the burn is prograde, higher if retrograde, etc...). If the orbit is lower enough compared to the ship's own, the gas molecules might fall back on earth's atmosphere but otherwise they will just stay in orbit. – armand Oct 31 '19 at 00:45
  • 6
    @armand Although your statement is true for the entire mass of gas molecules as a whole, like any other hot gas there is a distribution of velocities for the individual particles of the gas. They would vary in speed and also in direction. Upon initial expulsion from the craft, the particles will be in close enough proximity that their random collisions will cause them to eventually fly off in different directions. They might stay in orbit if they do not have enough energy to escape farther into space, but also not likely to stay "on the same orbit with the spaceship". – C Perkins Oct 31 '19 at 03:29
  • 1
    @MagicOctopusUrn As already mentioned in my other comment, gas particles will have various velocities and directions. Although they are likely to initially collide with each other, over time (likely within a very short time), their directions and speeds will be such that they no longer collide with each other. They will fly off in different directions and get farther and farther apart. That is how they dissipate. That really is the definition of dissipate... to scatter in different directions. – C Perkins Oct 31 '19 at 03:35
  • 1
    @MagicOctopusUrn Although a certain amount could fuse to other materials they collide with, the predominant direction and speed of the molecules will be away from the craft that is being propelled, so they will not immediately collide with any material to which they would fuse. I suppose that eventually some of the molecules will collide with other objects or migrate back toward the earth or moon, but out in space a significant distance from an external boundary to provide a restoring pressure force, the gas molecules will not bunch and group to cause pockets that hinder other spacecraft. – C Perkins Oct 31 '19 at 03:40
  • @C Perkins this is interesting, thank you. I don't know much about gas physics. Is the variation in speed and direction due to collision susceptible to make the course of individual molecules vary that much compared to the spacecraft ? We are talking about orbital velocities in the order of 10km/s. (yet, the gas is very hot so maybe collisions are very fierce ?) – armand Oct 31 '19 at 04:38
  • 2
    @armand gas in rocket exhausts can be thought of as having two components: the useful part which is directed away from the rocket, and the random velocity that comes from having a temperature above absolute zero. Assuming LH2/LOX giving H2O exhaust at 3500 K, the rms thermal velocity ([3RT/M]^0.5) is about 2.2 km/s. So each exhaust molecule has, roughly, 10 km/s of velocity in the direction of the rocket and 2.2 km/s in some random direction. This is enough to quickly spread out far from the path of the rocket – llama Oct 31 '19 at 15:31
  • @llama Excellent explanation of the two velocity components, the one in the direction of the rocket and the thermal random direction component. Thanks a lot. – Uwe Oct 31 '19 at 20:31
  • The image does seem to depict a beautiful "pocket" though. So we need to show how dense the gas is after expanding for however many years with 2 km/s (and multiplied by the number of flights which will hopefully be huge ;-) ), how it distributes in the gravity of the moon, and whether that density would be a problem. I think the notion of "pocket" cannot be easily dismissed; there will be some kind of expanding "pocket", similar to the heliosphere, even in free space. – Peter - Reinstate Monica Nov 01 '19 at 09:24
  • 1
    This photo is taken in the few minutes after launch. Of course it's visible then. Think about airplane contrails: a fine line right behind the aircraft, a broad cloud 10 minutes later and completely dissipated in an hour. – Hobbes Nov 01 '19 at 09:32
  • Although I agree with dissipation, a contrail is not good evidence, for the simple reason that it is within the atmosphere, not in vacuum. – WGroleau Nov 02 '19 at 22:32
24

The Sun ejects as Solar wind about a million metric tonnes per second.

The ejection of some tonnes per minute occasionaly from a second stage rocket engine above the atmosphere of Earth is neglible small compared to Solar wind.

A planet like Earth needs a huge mass to hold an atmosphere by its own gravity. There is no force that would hold such a gas pocket together. The gas would just spread out in the huge available space and its concentration would be incredible small when compared to the Solar wind. The Solar wind is indeed a nearly perfect vacuum at the distance of Earth from the Sun.

Gas pockets under the surface of Earth need a solid enclosure to exist. Spacecrafts need high pressure tanks to store gas or a very low temperature to store liquified gases. But in a vacuum under zero gravity gas pockets without a solid enclosure could not exist.

Uwe
  • 48,975
  • 4
  • 121
  • 206
  • 5
    Solar wind in all directions is 1 point something million metric tonnes per second. However, the fraction that arrives at a single place at the Moon is more like 0 point many zeroes grams per minute, negligible compared to the nearby rocket exhaust. Perhaps you could compute the distance (from the rocket) at which the Solar wind and the rocket exhaust wind will approximately match (at being nearly perfect vacuums), and then I'd start understanding why you mentioned Solar wind at all. – Jirka Hanika Oct 31 '19 at 22:21
  • 1
    @JirkaHanika If the Solar wind could not create a pollution of space, how should the very small mass of rocket exhaust cause one? – Uwe Oct 31 '19 at 22:53
  • @Uwe I think the distinction might be that the exhaust gas is collecting in certain spaces instead of being spread through the solar system. – Paŭlo Ebermann Oct 31 '19 at 23:08
  • The Earth's magnetosphere diverts most of the solar wind. It'll still have a minor effect, but definitely not as much as it would in low orbit around Mars for example. – Corey Oct 31 '19 at 23:13
  • 4
    @PaŭloEbermann except the rocket exhaust won't collect. In space it will quickly dissipate, and even if it does briefly collect anywhere the solar wind will blow it away.

    Case in point, if there were any chance of gasses collecting around the moon then it would already have an atmosphere.

     – Turksarama Nov 01 '19 at 01:46
  • @Uwe The key word is pocket. Rocket exhaust will expand (and hence lose density) -- but at a very low speed in space terms. So the resulting expanding pocket of gas will be distinguishable from its surroundings for probably millennia. Whether the fairly low and degrading density poses a problem for space flight is a different question. I would say, only for relativistic flight which will probably never occur inside the solar system because acceleration of all conceivable relativistic drives is too low. (And correct: For relativistic flight solar wind will be a problem, too.) – Peter - Reinstate Monica Nov 01 '19 at 09:09
  • 1
    Comets are careering regularly through the inner solar system, too and dumping heavily via outgassing (for Halley's comet, apparently "about 3 tons of matter every second"). No-one even notices. – David Tonhofer Nov 01 '19 at 10:03
  • @DavidTonhofer "ion tails [of comets] have been observed to extend 3.8 astronomical units" (https://en.wikipedia.org/wiki/Comet_tail). Quite noticeable. Hundreds or thousands of spaceflights would (locally) also emit substantial quantities of gas. – Peter - Reinstate Monica Nov 01 '19 at 14:15
  • 1
    But when the comet is on its way to the outer solar system again, the tail vanishes very fast. So the gas released by the comet is invisble because it is spread so much. – Uwe Nov 01 '19 at 16:21
  • @Uwe - You are right, Solar wind doesn't collect on the Moon and so rocket exhaust doesn't. I still don't know how the total Solar wind rate is a relevant number - if you are measuring it all around the Sun, it's in the wrong place, you should rather measure it on the Moon. (I would hate having to disagree with you whether Solar wind tends to collect on/near the Sun, in the form, of, er, the Sun.) – Jirka Hanika Nov 01 '19 at 23:55
8

The short answer is no.

In space the rocket exhaust isn't really a gas it's more like a molecular dust, where all of the molecules (CO, CO2, H2O, etc) are travelling on their own divergent trajectories. In order for a 'gas' pocket to form those molecules would need to be held together in some way, and there's just nothing acting on them to do this. Instead you have a variety of factors acting to spread them apart, including their own individual momentum.

And don't forget: Space is big. Really big.

This document gives details on one LEO flight of the Falcon 9S9 (Falcon Heavy). The second stage fuel load was 107,500 kg of fuel (LOX + RP-1). 100.5 seconds of return burn accounts for the bulk of the fuel, 15.6 seconds for the landing burn used most of the remainder. Approximately half of the return burn took place above the Karman Line, and from the acceleration curve it looks like it accounts for about 45% of the expended fuel. Let's say that's on the order of 50,000 kg of combustion products spread over a couple of hundred km of trajectory in under a minute.

From the picture in Hobbes's answer you can see that the exhaust spreads out pretty quickly, and with very little to impede it that spread is going to continue. You'll end up with a curved plume of molecular 'dust', most of which will either fall back into the Earth's atmosphere or escape orbit on its own momentum. At low orbits there are enough other gas particles that most of the exhaust probably won't make it out of the magnetosphere to be swept away by the solar wind.

So yes there is a slight increase in the density of matter in low orbit, but it's extremely thin and relatively short lived. It would take some hypersensitive apparatus to detect it at all by the time of the next launch.

On the other hand, a handful of molecules may be ejected with just the right momentum to enter an elliptical orbit around the Earth. Come back in a few centuries (if we're still using combustion to get to orbit) and you might notice an effect. At worst it is probably going to require more orbital corrections per year for LEO satellites, which already have problems with the density of molecules so close to Earth's atmosphere.

Corey
  • 181
  • 3
  • The second stage fuel load ->did you mean 'first stage'? That's the one doing the return and landing burns. – Hobbes Nov 01 '19 at 07:43
  • @Hobbes Possibly, yes. In which case the fuel numbers are wrong. First stage carries ~411,000 kg, 13% of which was not used. Looks like roughly 110,000 kg of propellant used by the core return burn. The S2 reentry burn was 2 seconds, so likely on the order of a thousand kg of propellant (I don't have the Merlin 1D-V fuel usage data unfortunately). Do those numbers look better? I'll amend the answer if so. – Corey Nov 03 '19 at 23:33
4

I would think it would be pushed out of our solar system by the sun's radiation just as the atmosphere of mars (or earth would be if we didn't have a magnetic field). In regards to distances further away from our sun, I believe it would act as described above. In a vacuum gas molecules dissipate.
Lets be extreme and assume the hwy gets used ALOT - would the gas clump together in the areas void of any other objects with mass?

Sarcoise
  • 41
  • 1
0

The International Space Station leaks a little air, and enough stays in the orbit to limit the experiments that can be done there. My former colleagues in Harwell make vacuum better than that to support high power laser experiments etc. So it is possible to pollute space.

chrishmorris
  • 101
  • 1
  • 1
    The decay of low orbits caused by the very thin natural atmosphere is well known from the beginning of satellites in orbit, long before space stations have been used. – Uwe Nov 01 '19 at 16:35
  • 4
    What do you mean by "enough [..] to limit the experiments that can be done there"? What can't be done there because of leaks? – BlueCoder Nov 01 '19 at 16:43
  • 2
    No air leaking is necessary for the folks at Harwell to make better vacuums than outside the ISS. The ISS orbits at about 400km which is still well within the thermosphere. Air pressure is about 1.5e-6 Pa which is a vacuum, but not a particularly good one. – Schwern Nov 01 '19 at 20:30
  • There was a dedicated Shuttle subsatellite built to create high vacuum in its wake exactly for the reason that LEO is not that hard of a vacuum. https://en.wikipedia.org/wiki/Wake_Shield_Facility – Organic Marble Nov 01 '19 at 23:09
-1

Well now, I think that is an interesting question. The key to the answer is that nature is a complex and messy place. We represent that complexity with a mental image that represents it, but is always a compromise.

The reality is that the space above us is extremely energetic, as has been pointed out by others, contains a constant wind of high energy particles we call the Cosmic Wind, and a powerful set of magnetic fields, not just from Earth, but an even more powerful field emanating from the sun.

For example, did you know the sun exerts 6 times the gravitational force on our moon as the Earth does? Stunning, I know! I learned that in my Physics class one day applying Newton’s Gravitation equation. Now that was counterintuitive. Still, it shows the power of that mighty orb even clear out here in our neighborhood.

Add to that complexity the Earth and Moon orbiting each other showing a constantly changing aspect to the mighty forces of the sun’s gravity and solar winds. So, combining just these forces, and considering other points made by other contributors, as well as others not discussed, like Jupiter’s influences, we can conclude that any gasses expelled would be quickly dissipated.

However, if we want to address a more imminent threat, how about the tons of space garbage we’ve left in orbit. That could be a growing issue we should shine a bit of sunlight on.

user579980
  • 1