Gas to deorbit debris

[theridane]

Suppose a tanker and a cloud of orbital debris are orbiting with a relative inclination close to 180° (i.e. going for a head-on collision). The tanker releases its payload and clears the scene. How long would the gas cloud remain in orbit and would it remain dense enough to have any significant effect on the orbital velocity of the debris field?

The gas released would have to be heavy in order to achieve maximum kinetic energy (e.g. LOX, water, but not LH2). It would also have to be heavily refrigerated in order to minimize its rate of diffusion/dispersion.

Is this feasible?

 

[theridane]

I've done some rough calculations. An imaginary tanker has two cylindrical tanks (100 m length, 10 m diameter) with 15700 m³ of LOX at 60 K. The payload is released 10 minutes before the expected collision and the tanker pushes off into a safer orbit.

Ten minutes later the gas cloud expanding at 220 m/s in each directions now occupies a sphere with a radius of 132 km (tidal effects omitted). It now occupies a volume of 7.3e10 m³ and has a density of 246 mg/m³, roughly the density of air at 60 km AGL. Since both the debris and gas clouds are going against each other at LEO speeds the braking effect would be equivalent to passing through the upper atmosphere at about 15 km/s.

Dynamic pressure would be somewhere around 25 kPa. A 1 kg steel ball bearing (something you seriously don't want to see in LEO going against you) would be decelerated at a rate of around 75 m/s². It would pass through the cloud in roughly 20 seconds and lost about 1.5 km/s of its prograde velocity (more than enough to bring it down) and (guessing) a significant amount of its mass.

So, is it even remotely plausible or is it just weapon grade bupkis?

 

[Shpaget]

15700 m³ of LOX is almost 18000 tons. How do you propose to get such mass up there?

How did you get the expansion speed of 220 m/s?

Don't you think that if you could haul 18000 tons of useful cargo to LEO that a simpler and more effective method could be devised? Probably even one that can be used more than just once.

 

[theridane]

How do you propose to get such mass up there?

I don't.

How did you get the expansion speed of 220 m/s?

Root mean square speed of ideal gas under ideal conditions. That figure is wrong though, it should be somewhere around 265 m/s for molecular oxygen at 90 K.

Don't you think that if you could haul 18000 tons of useful cargo to LEO that a simpler and more effective method could be devised? Probably even one that can be used more than just once.

Definitely. I was only trying to get some feedback on this thought experiment (a future assignment for my physics seminar) to verify whether my assumptions and methods are correct, or at least conceptually correct (e.g. approximating real gas physics away with ideal gas, because that's all I can cram down a herd of seventeen-year-olds in the three weeks I got for that, etc.).

 

[Shpaget]

Well, 15000 m³ is large volume. I don't think the LOX would expand (boil) uniformly. I guess the part in the middle would still be liquid while the outermost regions would evaporate and expand beyond useful density.
The thing is I don't think this can be tested on Earth, there isn't enough vacuum down here, even for a small scale version of the experiment.
It could probably be calculated using some advanced thermodynamics software and a great deal of computing power, but we don't have that at our disposal, do we?

 

[theridane]

The evaporation bit has to be handwaved, because just the energy required is in the vicinity of 4 TJ (1/10 of "hiroshima") and (as you said) it would never boil uniformly.

As a side note, many consumer-level GPUs have more than enough firepower to simulate fluid dynamics, but that would be an overkill for me :lol:

 

[kelvinzero]

How do you propose to get such mass up there?

I don't.

This is a bit similar to an idea I proposed a while ago. In my case the scheme was not to aim the cloud at anything, just to polute the orbits with tiny (eg gas) particles that would drag on small particles much more than satellites, due to the greater surface area-to-momentum ratio of smaller particles.

In my case the mass you got up there was simply part of your station keeping propellant mass, which has to be expelled at about 16km/s to put it into a stable reverse orbit.

 

[theridane]

Gases don't do stable orbit though. All it takes is a couple photons* (e.g. sunlight) to drastically change the velocity vector of a free molecule. Given enough time (anyone knows how long?) all of the orbital gas sinks back to the planet and a significantly smaller amount escapes the planetary system altogether.

* for example an O[sub]2[/sub] molecule going 7500 m/s has an energy of around 10 eV. UV photons have energies anywhere between a couple eV and well over a hundred. Visible light has up to 3 eV. Each individual particle of the gas could get knocked right off with as little as a single UV photon.

I'm starting to think that the gas cloud wouldn't last long enough even in my 10-minute scenario. Darn

 

[kelvinzero]

Who is this attempting to bring science into our physics forum?

Maybe you could chose something with much larger sized molecules?

 

[theridane]

Or maybe dump the gas idea altogether, since even the heaviest gas is still pretty light, still easily knocked down with UV.

With some kind of a nano-powder we'd both get the kinetic action needed to bring space junk down, reasonable stability of the "gas cloud" we're deploying and as a bonus the whole evaporation deal is a thing of the past :]

So once again a problem can be solved with a glorified shotgun :]