Photo-ablative pellets to clean space junk

[farmerben]

 

One solution to the space junk problem is to target debris with a pellet gun.  This is far more economical than capture solutions because the relative speed of the target can be very large.  

If a piece of debris is large enough to see, its orbit can be tracked.  From this we derive the set of all impulses which would lower the perigee of the debris to reach atmosphere.  A subset of those will match the set of what we call a good shot or a safe shot.  

Our cleaning satellite has many pellets to shoot at debris.  The satellite will have a high inclination low altitude orbit.  It will not maintain a fixed orbit, but must guarantee that a high proportion of its shots move the satellite into a higher orbit or change its inclination.  A good shot also requires that we know where our misses will end up, and we cannot add to the debris problem.  

In very low orbits a set of good shots will have our misses burn up in the atmosphere.  At higher orbits this becomes less feasible.  

Collisions between pellets and debris have the possibility of fragmenting into smaller debris.  Very flimsy debris may make poor targets for this countermeasure.   However lots of space junk would be suitable if the projectile is much flimsier.  This gun is designed to impart momentum, which is different than the goal of almost every other gun.  

The safest thing then is to make the projectiles out of a phase changing material, such that solid pellets become gas some hours or days after they are fired.  We can also design such that pellets will instantly vaporize (or at least pulverize) on impact with the target.  This will reduce fragmentation of targets.  

An example of suitable pellet material is dry ice CO2.  Most other types of ice, and perhaps some types of wax or plastic could be used.   It could be advantageous (lighter) to have electrical conductors in each pellet so that they can be launched electrically.  What materials could be used as conductors so that they dissipate or otherwise become safe as debris hazards for other spacecraft?  

 

[StrandedonEarth]

This sounds like a good idea. I’m guessing an ideal “bullet” would be  frozen mercury (Hgun?), if you don’t mind the toxicity. Electromagnetically launching it could (almost) melt it which should reduce target fragmentation. If it’s almost melted, the impact pressure could liquefy it, hopefully reducing splatter. 

Edit: Not sure how the bullet fragments would behave, however. On the night side they would freeze, to melt again on the day side. Would they vaporize completely?

Not sure how much of a threat molten globs of mercury would be to other spacecraft. 

Edited August 23, 2018 by StrandedonEarth

[Terwin]

3 hours ago, StrandedonEarth said:

Not sure how much of a threat molten globs of mercury would be to other spacecraft. 

At orbital velocities even space-dust requires mitigation to avoid serious damage: https://en.wikipedia.org/wiki/Whipple_shield

If a screw passing within 1 km of the space station requires the astronauts retreat to shielded areas, I imagine the thought of strewing small globules of mercury into orbit would cause all sorts of alarm at NASA.

 

[ment18]

You could go for a gun that shoots really short projectiles, like plates.  They are super good for guns since you have a very high surface area to mass ratio, allowing higher muzzle velocities.  The plate would also transfer its energy much less precisely to target, reducing fragmentation.  They are obviously garbage in atmosphere and would reenter very quickly in event of a miss.

[kerbiloid]

Why not just lasers?

[farmerben]

Maybe mercury or gallium or something would work.  It would have to fragment to micron size every time or be used only on trajectories which will aerobrake most of the projectile.  The atmospheric pollution is minimal... but the globule threat is bad.  

Lasers are good for cleaning up debris as part of a hunter killer network.  Lasers might be the best way to clean up loose bolts and things like that.  One problem is that it may not be possible to fully deorbit junk in a single pass.  Pellets offer a much stronger and well timed impulse.  If you shoot junk with lasers and don't bring it down before losing line of sight, then you have a lot of paperwork to do...

[StrandedonEarth]

But wouldn't a mercury globule evaporate in the +200^oC of direct sunlight?

[Cunjo Carl]

I like the idea! Using a subliming/evaporating pellet sounds like a great way to prevent further build up of orbital debris. It would also be a perfect projectile for a 10-cent whodoneit novel!

Let's put some some numbers to the idea and see how it pans out... The shuttle used ~100m/s deorbit burns, so let's assume our debris will be similar (LEO). Perhaps the debris is sputnik sized (10kg) and our pellet is large pellet sized (.01kg). How fast would our pellet need to go to to accomplish the deorbit?

Vpellet = DeltaV*MassDebris/MassPellet   (for a pellet that would splat on impact = "inelastic collision", also for large debris / small pellet)
100000m/s = 100*10/.01

Well, that's pretty fast. So perhaps we can use a quite large pellet (.1kg) That means we'd only need 10km/s, but that's still really fast! How could we design a system to impart that velocity? There's one approach that would require a tricky rocket launch but would be quite easy for our pellet launching satellite.

Spoiler

Retrograde orbit!   ... For practical reasons I like the polar orbit, but it's hard not to like the retrograde option.

 There's plenty of interesting design problems involved, but I think this is a fun idea to tackle. I suppose the next question is what the effects of such a collision would be . From there maybe we could place an upper bound on the debris size we'd be aiming to deorbit. For the lower bound, perhaps someone has hard data on this, but I think the very smallest tracked debris is in the ~100g range.

Edited August 24, 2018 by Cunjo Carl

[YNM]

I say leave it as is. Otherwise Hactar would wake up again.

[kerbiloid]

A chain of orbital scavenger crafts equipped with rear lasers and front shields. 
(Snotshields. Catches dust particles and partially evaporates, periodically gets refilled by a liquid agent which gets solid and doesnt't leak, i.e. like snots.).

Moves in prograde direction, to increase the fire contact duration.
Always shoots back, so always hits a target in retrograde direction.

Shares the current target highlight with the next in chain. When one hides below horizon or gets out of hit distance, the next one continues the target deorbiting.

Pushes it back either by the light pressure, or by evaporating its outer layer causing a retropropulsion.

Solar or nuke-powered, gyroscope-oriented.

Spoiler

A planetary protection laser network managed by the best persons of the mankind.

Because who manages it - is the best by default. Doubt? Argue? Discuss?

 

Edited August 24, 2018 by kerbiloid

[wumpus]

17 hours ago, kerbiloid said:

Why not just lasers?

Lasers are an inefficient means of increasing or removing momentum, although I can't understand why the OP wants to increase orbital height (and thus keep junk in space much longer).

I'd recommend an ion-powered device capable of rendezvous (more or less ahead of the device) and firing whatever (preferably something sticky, possibly magnetic if sufficiently ferrous) at the junk to lower its orbital velocity (only needs a 100m/s or so lower to de-orbit junk at ISS orbits).  Lasers might work as well, but I suspect that any space junk at such heights might naturally decay before the laser removes enough delta-v.  At sufficiently low orbits, it might be enough that the 'pellet' would be a parachute made of an extremely fine film, which would be expanded (presumably by a gas) after the line adhered to the spacejunk.  A parachute might work slow in orbit, but when it only has to reduce velocity by 100m/s, it should work well enough (and once it reduces velocity a little, the parachute should get more and more effective.  It might even let you chose where to land the thing).

Increasing orbital height is the idea for GSO and similar satellites.  For anything between LEO and GSO, things aren't terribly obvious (I'm guessing capture and de-orbit, so things have to be extremely large and/or dangerous).

[StrandedonEarth]

28 minutes ago, wumpus said:

although I can't understand why the OP wants to increase orbital height 

The reference to increasing height was referring to the cleaning sat, not the debris. You don’t want to deorvit the cleaning sat when it shoots

[farmerben]

17 hours ago, Cunjo Carl said:

I like the idea! Using a subliming/evaporating pellet sounds like a great way to prevent further build up of orbital debris. It would also be a perfect projectile for a 10-cent whodoneit novel!

Let's put some some numbers to the idea and see how it pans out... The shuttle used ~100m/s deorbit burns, so let's assume our debris will be similar (LEO). Perhaps the debris is sputnik sized (10kg) and our pellet is large pellet sized (.01kg). How fast would our pellet need to go to to accomplish the deorbit?

Vpellet = DeltaV*MassDebris/MassPellet   (for a pellet that would splat on impact = "inelastic collision", also for large debris / small pellet)
100000m/s = 100*10/.01

Well, that's pretty fast. So perhaps we can use a quite large pellet (.1kg) That means we'd only need 10km/s, but that's still really fast! How could we design a system to impart that velocity? There's one approach that would require a tricky rocket launch but would be quite easy for our pellet launching satellite.

  Reveal hidden contents

Retrograde orbit!   ... For practical reasons I like the polar orbit, but it's hard not to like the retrograde option.

 There's plenty of interesting design problems involved, but I think this is a fun idea to tackle. I suppose the next question is what the effects of such a collision would be . From there maybe we could place an upper bound on the debris size we'd be aiming to deorbit. For the lower bound, perhaps someone has hard data on this, but I think the very smallest tracked debris is in the ~100g range.

The figure of ~100 m/s is overkill.   As long as the debris touches atmosphere we don't mind if it skips off dozens of times.  

A simple gun with ice pellets should be expected to shoot 1 km/s or less with respect to its launcher.  But that doesn't matter much.  Orbital velocity is 30 km/s.  So our cleaner will pass debris anywhere up to 60 km/s.

In other words a .01 kg pellet hits sputnik 10 kg at 60 km/s.  Sputnik changes velocity 60 m/s and grazes the atmosphere.

[farmerben]

Rendezvous with debris sounds like the slow and expensive solution.  But it may be key for certain applications.  

The simplest lightest way to deorbit very large debris is to attach an electro-magnet with a solar panel.  The electo-magnet must be capable of changing the direction of its field according to program.  It can pump against the Earth's magnetic field.  If necessary it can increase inclination toward the Earth's poles, and then retrograde.  

[kerbiloid]

8 minutes ago, farmerben said:

.01 kg pellet hits sputnik 10 kg at 60 km/s. 

E = 0.01 * 60000² / 2 = 18*10⁶ J.

P = 0.01 * 60000 = 600 kg*m/s

dV = P / M = 600 / 10 = 60 m/s.

E/M = 1.8*10⁶ / 10 = 1.8*10⁵ J/kg.

dV_debris = sqrt(1.8*10⁵ * 2) = 600 m/s.

I.e. the satellite will be destroyed, its debris will be spread around at several hundred m/s speed.
The cloud of debris will be decelerated by 60 m/s relative to orbital speed.
So, ~2/3 of debris will enter atmosphere, 1/3 of debris will be spread in various orbits.

[farmerben]

When a very light thing collides inelastically with a very heavy thing, the light thing rebounds with nearly equal and opposite velocity.  A pellet that vaporizes is about as close as you can get to an inelastic collision.  Of the energy gained by the debris only a fraction of it will be kinetic energy of fragments.  Most of it will become heat.  Part of the target will vaporize at those energies.  So most of the stuff moving at high speed is gas.  

[kerbiloid]

7 hours ago, farmerben said:

Of the energy gained by the debris only a fraction of it will be kinetic energy of fragments.  Most of it will become heat. 

A 10 kg piece made usually of aluminium is ~20 cm big.
At 60 km/s collision speed their collision will last for 0.2 / 60000 ~= 3.5*10^-6 s.

Thermal diffusivity of aluminium ~= 8.5*10^-5 m²/s.
So, the heating depth (?) = sqrt(8.5*10^-5 * 3.5*10^-6) ~= 2*10^-5 m = 0.02 mm.

I.e., almost all energy will be released as heat in the cracks and cause the target destruction and pieces spreading by the plasma in the cracks.
But 0.18 MJ/kg is much less than 12 MJ/kg required for the aluminium vaporization or ~0.5 MJ/kg for melting.
So, the target will burst into pieces.

Edited August 25, 2018 by kerbiloid

[farmerben]

Suppose all the energy went into vaporizing aluminum.  
How much aluminum would be vaporized?  

m = E/S = 1.8MJ  / 12 MJ/kg = 0.15 kg of aluminum vaporized.

volume of cavity = 5*10^5 m3.  Which corresponds to a cavity 20cm long and only a couple of millimeters across.  

Since the target is large compared to the cavity it is likely to penetrate all the way through before the target cracks all the way across.  

 

This plasma does some work on debris fragments scattering them radially out from the point of impact.  About 1/3 of this work is done along vectors undesirable to us.  

I'm not saying the problem is negligible, I just think the work done on fragments is a small fraction of the total energy.  1/10 seems high to me.  So more like 1/30 of the debris cloud remains a hazard.   That said, these massive energy collisions can create problems.  So the ice pellet gun is probably more suitable for applications where the relative velocity is much smaller.  Going for shots with half the momentum change, gives us a quarter the energy...

Edited August 25, 2018 by farmerben

[kerbiloid]

2 hours ago, farmerben said:

1.8MJ

I'm afraid, 0.18 MJ/kg. 
So, 0.015 kg.
And as the contact duration is short, there will be less aluminium vaporized, but more vapourized aluminium overheated.
So, the overheated gas should tear the target apart before it vaporizes. (And also it's still insufficient to vaporize the whole thing)

Edited August 25, 2018 by kerbiloid

[farmerben]

37 minutes ago, kerbiloid said:

I'm afraid, 0.18 MJ/kg. 
So, 0.015 kg.
And as the contact duration is short, there will be less aluminium vaporized, but more vapourized aluminium overheated.
So, the overheated gas should tear the target apart before it vaporizes. (And also it's still insufficient to vaporize the whole thing)

Check your units.  I was supposing all the energy is concentrated at the point of impact.  Thus neglecting the total mass of the target.

[kerbiloid]

On 8/25/2018 at 7:32 PM, farmerben said:

Check your units. 

Yes, my fault. Lost a period, sorry.

[farmerben]

And I meant to say the maximum amount of aluminum vaporized is 5*10^-5 m3 not 5*10^5 m3.  

 

[Gargamel]

This sounds a lot like fighting fire with fire. 

But in this case, the fire just gets bigger.