Any possible setbacks in deorbiting larger space junk and let it burn up upon reentry into atmosphere?

Question

Space junk, orbital debris, or space waste pose a risk on functional satellites and space laboratories / stations in orbit around Earth. According to Wikipedia:

Currently about 19,000 pieces of debris larger than 5 cm are tracked, with another 300,000 pieces smaller than 1 cm below 2000 km altitude. For comparison, ISS orbits in the 300–400 km range and both the 2009 collision and 2007 antisat test events occurred at between 800–900 km.

It is a nuisance at best, a potential threat to working equipment, extravehicular activities of astronauts working on or repairing satellites, telescopes, space stations, e.t.c., even space stations themselves. They serve absolutely no purpose, so my question is, are there any safety related, environmental, or political reservations to prevent us from simply deorbiting larger pieces of such defunct debris into Earth's atmosphere and let them burn up upon reentry?

This is assuming, that we do have means of deorbiting larger orbital debris, and is not a question of technical, or economical feasibility. Let's, for the sake of argument, assume we can build and launch into orbit above targeted debris a railgun, that is shooting high-precision soft body projectiles at these larger debris that we're tracking, and knock them off orbit into the atmosphere, where they hopefully burn on re-entry.

What, besides financing and technical challenges, is stopping us?

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I'm editing to re-emphasise the request to describe the non-technical and non-economical nature, or challenges such deorbiting of space junk might come with. We already have a good answer on which methods have been proposed to do so, at what stage of development they are, how much they'd cost, e.t.c.. I'm explicitly seeking more information on possible safety, environmental, or political reservations, maybe even legal? For example:

• How hazardous materials are we talking of?
• Can we expect all of such materials to burn up completely in the atmosphere on re-entry?
• Are there any legal repercussions we should consider, such as this junk being a property of some foreign body?
• What political challenges we need to address, has this been discussed by leaders of the Earth in any detail before, e.t.c.?
• How much threat there is any attempt at deorbiting debris will only end up with them orbiting at even less convenient altitudes concerning safety of future launches?
• e.t.c.?

Answer 1

Reentry can be hazardous, for several reasons:

• The debris might not burn up completely. Whether a given object will be consumed in the upper atmosphere depends a lot on its materials, shape, velocity, reentry trajectory, and even direction (to take an extreme example, a Gemini spacecraft had a heat shield on one side, so that the way the craft was pointed made all the difference between a safe landing and astronaut BBQ). A non-burned debris can make some damage on land. It seems that a debris from a failed Thor rocket launch on November 30th, 1960, killed a cow in Cuba (referenced from here, but the link appears to be dead); granted, this was not a debris "from orbit" but this still highlights the issue.

  For a more recent example: in late 2011, a metallic, obviously man-made object fell in a field in Namibia, most probably from orbit. It seems to have been an hydrazine tank from an old satellite, which survived re-entry.

• If the debris burns up, then what the debris was made of becomes spread out in upper atmosphere. This can be problematic, especially if the debris included a RTG. People, generally speaking, are not very keen on the idea of spreading out radioactive material over their head; indeed, this was what really prompted the ban on atmospheric nuclear tests.

• A country who develops a way to drop debris on Earth is a country who can drop "debris" on "Earth", where "debris" can be "heavy pieces of hard materials with a lot of kinetic energy" and "Earth" can be more specific, like "the Kremlin". That's not necessarily the kind of technology that we want to be seen deployed and employed. Diplomatic consequences are probable.

A more important point, however, might be that large debris are not a big issue because their position and trajectory is known and predicted. The really scary debris are the one in the 1 cm size range, because at orbital speed they have enough kinetic energy to destroy a satellite or spacecraft, and yet they are small enough to evade detection. And there are a lot of them; a lot more than large debris.

If financial and technical aspects are "not a problem" then instead of dropping the debris, we should push them outside of Earth orbit, and then drop them into the Sun. In the long term, that would be much safer.

Answer 2

First off, the things we put into space, even the things we use to put people and other things into space, are not really friendly when you get right up close. Keep in mind that in order for the Space Shuttles to be decommissioned and turned into aerospace museum exhibits, they had to go through an extensive decontamination process to remove all the volatile, toxic chemicals used as propellants, fuels and even lubricants on the orbiter, replacing them with substances that only have to do the job well in Earth's atmosphere and ambient temperatures, instead of at +-200*C in a vacuum.

One example, a common fuel used for lift, deorbit and station-keeping/orbit-modification rockets, is hydrazine, N₂H₄. Ideally, one molecule of hydrazine and one molecule of diatomic oxygen combine to produce two molecules of water vapor, one molecule of diatomic nitrogen, and about 592 kJ/mol. For the size of the molecules involved, that is a lot of heat released, which is why it's used in rocket fuel; pound for pound, it's better than the well-known explosive C4 (C4's primary active ingredient, RDX, gives you a little over twice as much energy per mole at about 1120 kJ/mol, but weighs almost 4 times as much at 222g/mol to the hydrazine and oxygen's combined 64g/mol). With a rocket being little more than a long, controlled explosion, the rocket's fuel tank is not something I'd like to see entering the oxygen-rich atmosphere at hypersonic speeds; that's a one-way ticket to a short, uncontrolled explosion, and a powerful one (especially with the LOX tank entering right beside it bringing lots of concentrated oxygen goodness to the party).

Hydrazine is also acutely toxic; the NFPA, which rates chemical hazards for use in industrial settings, gives it the maximum rating of 4 as a health hazard, placing it in the same category as substances we have developed specifically to kill each other, like hydrogen cyanide, sarin, and VX nerve gas. The substance will also spontaneously ignite at ambient Earth temperatures and the presence of oxygen (another reason it's used as fuel; no ignition source needed), and it will go ahead and spontaneously detonate given a good physical shock (like, say, the impact with the ground after re-entry if it makes it that far). With this overall NFPA 704 rating of 4-4-3, it's among the nastiest substances our chemical industry produces in bulk (there are nastier, but they're typically not sold and shipped by the tanker-load).

Lastly, the ideal reaction between hydrazine and oxygen isn't the only possibility; given various mixtures of the two, you'll end up with ammonia gas, various oxides of nitrogen, and other not-nice things (though almost anything's better than hydrazine itself). And that is if LOX is used as the oxidizer; there are things known to modern science that oxidize better than liquid oxygen, like the halo-trihalides (chlorine trifluoride, chlorine tribromide and bromine trifluoride), perchlorates, peroxides etc, all of which could be in the satellite's oxidizer tank and would combine with hydrazine in new and exciting ways. A common one is nitric acid (in its white fuming form, often "inhibited" by adding hydrofluoric acid which creates a protective layer of metal fluoride, to prevent the nitric acid eating through the tank), because it doesn't require cryogenic handling; the ideal reaction would be one part fuel to two parts oxidizer producing nitrogen gas, water and oxygen, but nitric acid doesn't like to act "ideally", and you're much more likely to get a (un)healthy dose of toxic nitrogen dioxide instead of the clean nitrogen and oxygen.

This is all just from one example, with "interesting" consequences from a chemistry perspective. More glaring is the simple physics; a spacecraft in geostationary orbit is travelling at about 11,068km/h, or about 3,074 m/s. It's an interesting contradiction in projectile physics and orbital motion, that "you speed up to slow down". The opposite holds; by slowing this spacecraft's angular velocity down to de-orbit, you end up going faster relative to the earth's surface. By the time you begin re-entry, you're actually going faster than 8,000m/s, or faster than Mach 25; 10 times faster than a .223 Remington round fired from an M-16. And the satellite weighs a hell of a lot more than a .223 slug.

Just as a rough estimate, let's say the spacecraft weighed a modest 5 tons (~22000 kg), which is the current average for a communications satellite. At escape (or reentry) velocity, the craft would have a kinetic energy of .5 * 22000 * (8000)^2 = 7.04E11 J. In perspective, that's a little more energy than you'd get by detonating 10 MOABs simultaneously. Adding back in the hydrazine goodness, let's say 400kg of the craft's weight was hydrazine fuel. At 32g/mol, and 592 kJ/mol, the potential chemical energy of the hydrazine fuel is 7.4E9 J, one one-hundredth the energy of the spacecraft itself, but still about the energy inherent in the average lightning bolt.

All figures from Wikipedia.

Answer 3

The upper atmosphere is a sensitive environment. Unlike compounds released in the troposphere, compounds that end up in the stratosphere or mesosphere have a long lifetime, so stuff accumulates there. This issue has not been adressed very much, but unlike @JeremyKemball's answer, I do not think it is safe to assume that none of it matters.

That being said, the true reason that it's not being done routinely are not environmental. A lot of things are being deorbited on purpose, and so far the impact on the upper atmosphere has never been a reason to look for different solutions. So, possible environmental setbacks — yes. But what's stopping us in practice are not the possible environmental setbacks — rather, the technical and financial challenges are.

Answer 4

The big challenges are financial and technical. Above a certain size big space junk could land on people, but I think that's more or less the only danger. About a hundred tons of space dust burns up in the atmosphere every day(most of it microscopic, so estimates vary widely) so it would be safe to assume the addition of a few more tons of satellites wouldn't impact upper atmosphere chemistry too heavily.

Apparently one of the challenges in satellite senescence is that it is much cheaper(financially, in rocket fuel, timewise) to park a dead satellite in a 'graveyard orbit' than it is to pull it down into a decaying orbit.