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In the world I'm building, a nuclear war and subsequent repressive chaos have led all satellites to be destroyed by the powers that be, around 10 years from now. A few years later, a small spacecraft, which doesn't have a heat resistant cover to allow it to enter the atmosphere, reaches Earth orbit and proceeds to gather space debris. Of course, with all the destroyed satellites, it has a lot of material, and it has a laser which it can use to change its course. It may also have other things but it has come from Mars so would need to have either made it there or have reused or re-purposed man made materials there.

Also it can use solar power to recharge said laser and keep things going as it floats along. It has as much time as is needed as the solar power can keep it going almost indefinitely. So the questions are: How much space debris would I have to collect, and how would I collect it, so that I'd have enough cover around my space ship to leave orbit without burning up, and so I can use my last bit of fuel to attempt a descent? Would this resulting debris cause a lot of havoc once I detached it from my ship before landing?

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    $\begingroup$ Seems plausible. But if you are looking for more reliable information about the math behind ablative heat shields for atmospheric reentry, you might get better answers on space.stackexchange.com $\endgroup$ – Philipp May 5 '18 at 13:50
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    $\begingroup$ We think there's a lot of space junk, and videos show that there's a lot of space junk, but those videos are lying to you, since -- in order for you to see each bit of junk -- they make each bit of junk the size of Manhattan. There is not much junk up there, because space -- even in LEO is big. $\endgroup$ – RonJohn May 5 '18 at 13:50
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    $\begingroup$ Or more to the point, most space debris is actually the size of paint chips, small nuts and bolts and other assorted detritus. It is still very dangerous moving at 7km/sec, however. $\endgroup$ – Thucydides May 5 '18 at 14:09
  • $\begingroup$ @Thucydides right. You won't get hit immediately, but -- given enough time -- eventually you will be. $\endgroup$ – RonJohn May 5 '18 at 14:37
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    $\begingroup$ Welcome to WorldBuilding, Ale Fernandez! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Have fun! $\endgroup$ – FoxElemental May 5 '18 at 15:55
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The problem you are trying to solve is similar to the one asked here: what is the minimum size of an asteroid so that it impact Earth?

In your case you don't want to impact, but you want to go through the atmosphere with no damages, which is the same for the sake of your question.

Based on approximating the problem by assuming that the matter in the path of the object is being pushed at the same velocity of the object, so as soon as the object has swiped out path containing the same mass as its own mass, it will have lost all of its initial momentum. All its kinetic energy will then have dissipated there, so if this happens in the atmosphere it will have burned up before reaching the ground.

This is, of course, a gross oversimplification, but it will yield correct order of magnitude estimates. We can then calculate the critical diameter as follows. The mass of the atmosphere per unit area equals the atmospheric pressure at sea level divided by the gravitational acceleration, so this is about $10^4\ \ kg/m^2$. If an asteroid of diameter $D$ and density $ρ$ is to penetrate the atmosphere, it's mass of $1/6πρD^3$ should be larger than the mass of the atmosphere it will encounter on its way to the ground, which is $5π10^3D^2\ \ kg/m^2$. Therefore:

$D>3×10^4ρ\ \ kg/m^2$

If we take the density $ρ$ to be that of a typical rock of $3×10^3$, then we see that $D>10\ \ m$, which is reasonably close to the correct answer.

The correct answer is mentioned in a NASA article, which states

Space rocks smaller than about $25\ \ m$ (about 82 feet) will most likely burn up as they enter the Earth's atmosphere and cause little or no damage.

To make a sphere of $25 \ \ m$ radius your crew needs to harvest at least $65449.8 \ \ m^3$ of space garbage. In mass, assuming again a density of $3000\ \ kg/m^3$ (aluminum is 2700) it would mean $ 210 × 10^6\ \ kg$.

This will protect the ship from the ablation damage of the reentry. Getting insulation from the resulting tremendous heat will be another story (aluminum and other metal are good heat conductors...)

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    $\begingroup$ I would imagine that the radius of a penetrating meteorite would be dependent on velocity. Reentry from LEO is going to be much slower than most meteorite reentries. $\endgroup$ – bendl May 7 '18 at 13:24
  • $\begingroup$ @bendl, OP states the spaceship is reaching Earth from outer space, not from LEO. Therefore the two velocities are comparable. $\endgroup$ – L.Dutch May 7 '18 at 13:49
  • $\begingroup$ Yes, but you've got to spend time in LEO gathering space junk. Unless you don't. If you don't then this works just fine, I'm just saying it could take less time $\endgroup$ – bendl May 7 '18 at 13:56
  • $\begingroup$ Thanks for this answer! I think it's got some brilliant calculations so I can tell how long I want my space ship to be hanging out scavenging. It assumes it's going to be bits of space rock though, so I think I can shorten the time needed by gathering up some missile casings, doing previous scans from Mars to know what is available and plot its course before leaving. As a result I looked up info on various satellites and space flights about how they have various types of shield to avoid smaller debris, and how they actually change their course to avoid the larger ones in the case of the ISS. $\endgroup$ – Ale Fernandez May 7 '18 at 23:35
  • $\begingroup$ I doubt you will survive with this design. While riding behind the mass will shield you from the frontal shockwave you're still going to be inside a tube of white-hot plasma. You will pick up heat by radiation from it, if you have no shield you'll fry. Look at returning spacecraft--the shields are the thickest on the forward faces but all sides are shielded. $\endgroup$ – Loren Pechtel Jun 11 at 4:51
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No.

(but with a little caveat)

Space junk is, well, junk. It's essentially just the contents of some factory's dumpster. Most of it is very small, and the stuff that isn't very small is not designed for this purpose. Making matters worse, it sounds like your satellite is not designed for reentry. This means that it's not made for structural integrity. Or really anything besides sitting in microgravity. Even if you manage to collect a large cloud of space junk around you, how do you make it stay around you? The small stuff is going to have much less momentum and will fall behind you as soon as you hit the atmosphere. The large stuff has too much momentum, and any shifts in the junk heap will likely destroy your craft.

Even putting aside the ability to keep the junk steady, reentry is a chaotic procedure, only made less so by careful geometry in our reentry capsules. There is almost zero chance that you don't end up tumbling throughout reentry and destroying your satellite. If you do make a stable shape, it's probably stable in the wrong direction and you'll end up burning your satellite.

Finally, the engines on satellites are made for long, slow, efficient burns. Even if you manage to do everything right up until landing, that engine will do little besides lower your terminal velocity from, say 200 kph to 199 kph.

But you said there was a caveat!

Okay, so this satellite went to Mars. Let's say that it has a more powerful engine for some reason (it really wouldn't, but let's roll with it). In addition, it has devices that are purpose built for grabbing and trapping space junk around the craft. Moreover, it can retract its solar panels so that they don't get snapped during reentry. And finally, there is some really really perfect space junk out there. Like your space junk grabber was designed to grab these exact pieces of space junk. Then yes, in this miracle of coincidences, you could potentially ride this space junk heap down to the surface, and maybe, maybe survive it. I just don't buy it.

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  • $\begingroup$ According to what you say, I think the whole ship would have to have been built with the debris problem in mind before launch. Maybe having a way of tethering on to larger bits of debris, welding them on to an outer casing, and even using the tether functionality for transport would make it more probable. As it happens, in the story, the recycled debris cone conveniently ends up detaching and raining down on a bunch of bad guys so it's all good... In this case it's been designed by a very good rocket scientist, but yes it's all supposed to be a bit unlikely or it wouldn't be much of a story... $\endgroup$ – Ale Fernandez May 7 '18 at 23:46
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Use big space debris.

skylab chunk https://www.space.com/21122-skylab-space-station-remains-museum.html

Depicted: a 1 ton oxygen tank from Skylab which survived re-entry and looks pretty good here in its museum.

I assert that if this bug chunk survived re-entry, you could survive re-entry behind it. It would be your heat shield.

The trick would be finding chunks this big still in orbit. I think people are more careful about this sort of thing now than they were when Skylab went up.

But maybe big stuff still does go up, just with explosives or other means to break it up before re-entry. Or you could make it so in your world. If there were a space arms race maybe there would be proton cannons or other space weapons platforms up there. Maybe some stuff already has heat shields you could scavenge - consider if you were going to fire a missile down from orbit it would need to survive re-entry. A phalanx of missile heads would make a badass MacGyvered heat shield.

My question for the comments - if during the OP's endeavor you found a full Skylab oxygen tank, would it be better to drain the oxygen into space before using it as a heat shield, or leave it full?

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  • $\begingroup$ Leave it full, and vent it during descent to dump some heat. Of course, vent it at a sufficient distance behind that you don't risk turning it into an oxy thermal lance. $\endgroup$ – LSerni May 6 '18 at 15:45
  • $\begingroup$ Apparently when satellites and other things are retired, they go into a higher orbit area where there aren't so many ships. In this case though, lower earth orbit is now a big mess too. The difference would be that older things are in higher orbit. There wouldn't be space cannons, as it's a 2030s world war / coup situation that leads repressive regimes to destroy all satellites so as to impede communications. This should allow for enough heat shields floating around for a single vessel to use. I'm hoping as well as oxygen tanks there's some old tv broadcasts in those bits of satellite! $\endgroup$ – Ale Fernandez May 7 '18 at 23:58

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