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In my novel, the many moons of Jupiter and Saturn have been colonized, but I'm running into some logistical problems - namely, waste disposal. My first instinct was to dump it on-moon; while this is fine on places like Io, for others, like Europa, it won't work due to environmental reasons. So, would colonists be able to launch their waste into the hearts of Jupiter and Saturn and let the immense gravity do all the dirty work?

Some clarification by request: For purposes of ease-of-story and narrative, sub-light drives are common, but not very cheap. Admittedly, the exact math and speeds elude me, but I would put it comparable, price-wise, to commercial shipping on Earth today (expensive, but cost-effective). The waste, by and large, would be the everyday waste of colonists - any place with radioactive or dangerous waste would have long ago stopped worrying about standards of living, similar to a coal mining operation today.

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    $\begingroup$ in a space colony there are very few things that actually are waste, those tend to be radioactive or toxic. Organic waste is very valuable and would be recycled. $\endgroup$
    – John
    Dec 4, 2018 at 16:17
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    $\begingroup$ Why is everyone using rockets in their answers? Won't the better approach be to wrap the waste in a touch skinned container with just enough metal so that a magnetic rail gun can shoot it down towards the gas giant's atmosphere. Yes, it would still have orbital velocity, but it would also have rail gun inspired inertia on a vector which intersects the atmosphere and once waste meets gas, the energies should work themselves out quite spectacularly. $\endgroup$
    – Henry Taylor
    Dec 4, 2018 at 16:26
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    $\begingroup$ Once we have the ability to settle other planets, it seems to me like space ceases to be a problem for a long while. Mars has 56 million suquare miles of surface area. Europa has 12 million square miles. There are hundreds of moons, dwarf planets, and oodles of asteroids. Here on Earth we have 7 billion people and we have so much space for garbage dumping we haven't even had to use Greenland or Antarctica as a landfill yet. So until we have many hundreds of billions of people living in space for centuries, I don't expect we'll ever have such a shortage of cheap real estate! $\endgroup$
    – workerjoe
    Dec 4, 2018 at 18:35
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    $\begingroup$ @Joe Why concern yourself with the surface area of planets? There is no lack of space in space (and you don't need to spend a lot of energy moving it around the system). $\endgroup$
    – pluckedkiwi
    Dec 4, 2018 at 21:32
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    $\begingroup$ @pluckedkiwi If you have space on the planet then why even bother wasting energy (and matter!) to put it in space? $\endgroup$
    – user253751
    Dec 5, 2018 at 1:00

12 Answers 12

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What you suggest is possible, but the solution is a major problem, for larger reasons.

We have sent probes to crash into Jupiter. It is physically possible to send waste into a gas giant. Just as it is theoretically possible to send waste now into the Sun.

However, the primary issues encountered commonly with waste is economic and logistical. Waste is not a desirable commodity, unlike other products it has negative value. Therefore people will often settle for spending the bare minimum to 'get it out of sight', in contrast to spending a lot to 'get it where it needs to be'.

Many people don't realise that waste produced in cities often go into landfill close by - waste companies simply do not want to spend millions transporting waste long distances. 'Out of sight, out of mind' is the primary rule here, no one will support the enormous fuel costs of sending waste to another planet or into the sun, unless it is actually cheaper to do this than to dump it in a hole nearby.

The other contradictory dichotomy is that waste is valuable. Not in the fact that it holds value, but in that it represents the 'cradle to grave' aspect of society, ie lost economic, social and labour investment in redundant items. In other words, it should be more efficient to re-use, recycle, reduce waste than to dispose of it because that investment is re-invested, otherwise the society will bleed this value. This is difficult to resolve, and requires pre-emptive planning, but some are attempting to tackle it.

So rockets to dispose of waste is possible in a physical sense, but not possible / preferable economically or societally.

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    $\begingroup$ Dumping all your future resources down the gravity well when they could be recycled is poor planning to say the least. $\endgroup$
    – Joe Bloggs
    Dec 4, 2018 at 16:23
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    $\begingroup$ @Joshua a nice incinerator would seem like a better answer for that kind of waste. $\endgroup$
    – Murphy
    Dec 4, 2018 at 17:32
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    $\begingroup$ @Joshua I work in the same building as a clinic so I know that waste like that is typically autoclaved with hot steam for half an hour and then sent to the landfill. $\endgroup$
    – WaterMolecule
    Dec 4, 2018 at 18:48
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    $\begingroup$ @Joshua Anything off-planet would have no easy way of replenishing basic nutrients - carbon, nitrogen, etc are all important. On earth there's an abundance of those things. On one of Saturn's moons, not so much. $\endgroup$
    – David Rice
    Dec 4, 2018 at 21:59
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    $\begingroup$ One (minor) plot point in Heinlein's The Moon is a Harsh Mistress is that by farming crops on the moon which are then sent to Earth, the Lunar colony is spilling the lifeblood of its own ecosystem and gradually killing itself. $\endgroup$
    – Wildcard
    Dec 5, 2018 at 3:53
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The problem is energy

Since you're on the moon orbiting the gas giant, both you and the trash are moving at the moon's orbital speed around that gas giant. And it takes a lot of energy/fuel to slow the trash down and get it out of the planet's orbit, allowing it to fall into the atmosphere. As long as you have cheap and plentiful energy resources and disposable engines, this shouldn't be a problem. In fact, the trash will be incinerated as it enters the atmosphere.

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Yes, but for a different reason.

This is basically what happened with the Cassini probe, which was sent to crash into Saturn in 2017. However, the reason the probe was successfully disposed of wasn't because Saturn's a gas giant; it's because the spacecraft burned up upon entering Saturn's atmosphere. The same things happen with meteors, and would happen with spacecraft returning from Earth if they didn't have heat shields. They'd be burnt to a crisp.

Now, the waste might not burn up completely. Skylab didn't, in the 1970s; some pieces of debris hit Australia. Fortunately, Saturn's interior is hot (at the center, it's twice as hot as the surface of the Sun) and under a lot of pressure, and anything that survives entering the atmosphere will not survive for a whole lot longer.

That said, as folks have pointed out, this might not be phenomenally appealing. There are reasons for and against sending your rubbish to a gas giant.

Reasons against doing it:

  • It takes time and energy and money and labor, and those are all in short supply in space.
  • Who wants to waste rockets?
  • Can't you just dump it elsewhere?
  • Some of it can be reused.

Reasons for doing it:

  • Let's try to not contaminate the moon we might be trying to study.
  • Nobody wants nuclear waste in their backyard space colony.
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    $\begingroup$ A reason against: stuffing your waste in a fast-moving, firey, potentially crashy explosive device and sending it hurtling above your homeworld opens up the possibility of a very messy cleanup if something goes wrong. $\endgroup$
    – Clinton Pierce
    Dec 5, 2018 at 17:27
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    $\begingroup$ To expand on the reuse aspect: even nuclear waste could represent a resource. After some cooldown and breeding, the radionuclids could be useful for RTGs, medicine, and other technologies. Also, the hazards of radioactive contamination are much less significant since outside the colony there is neither an atmosphere nor a biosphere that could be polluted. Radiation from outer space and a gas giant's radiation belts are much larger and more immediate dangers. $\endgroup$
    – MauganRa
    Dec 5, 2018 at 22:14
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The solution you are looking for is a mass driver. In essence, they are a giant electromagnetic gun that would accelerate a mass past escape velocity and send it to space. They were proposed decades ago as a cheaper method of sending cargo to space. The main benefit against rockets is that, as most of the energy is provided by the mass driver, the ship won't require as much fuel. The main drawbacks would be:

  • Friction against the atmosphere. However, the only moon on the Solar System to have a significant atmospherical pressure is Titan at 147 kPa (1.45 atm, denser than Earth!). Triton has a pressure of about 2 Pa, most other moons are on the order of micropascals.
  • The escape velocity. But even the biggest moon in the Solar System, Ganymede, has a relatively low escape velocity of 2,741 m/s, which is easily attainable with current technology.
  • Related to that, if you want to reach the escape velocity in a short distance, you need a huge acceleration. For example, to reach 2,741 m/s on a 1 km long track you would need an acceleration of 3,755 m/s2 over a time of 0.73 s, or 383 g. Again attainable with current technology, even if you have to spend an enormous amount of energy, but such an acceleration would destroy any delicate machine, and would reduce a living being to a bloody paste. But not a problem for waste that you want to get rid of.

So all in all, I would say that a mass driver is your solution. Also, if you can make it several kilometres long and keep the acceleration low enough, you could use it to launch both manned an unmanned ships.

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    $\begingroup$ This is probably the most practical solution. Note that it's essentially just a maglev train. There are several in use already. $\endgroup$
    – JollyJoker
    Dec 5, 2018 at 8:49
  • $\begingroup$ Plus, you don't even have to aim it at Saturn. You can fling it into deep space where it will orbit the Sun for eternity. $\endgroup$
    – Shawn V. Wilson
    Dec 5, 2018 at 18:57
  • $\begingroup$ You do have to waste some iron or similar magnetic material to encapsulate your actual waste material though. And that doesn't seem like a great tradeoff, unless you have a surplus of iron from nearby mining anyway. (And maybe not even then.) $\endgroup$
    – Miral
    Dec 5, 2018 at 23:44
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    $\begingroup$ The problem here is that you're forgetting the delta-v needed to actually hit Jupiter's Atmosphere. Even once you're in orbit of the moon, the fuel needed to slow down enough to hit Jupiter is MASSIVE. Worse, it would be very hard to do this with a railgun. Still a good thought though! $\endgroup$
    – Bert Haddad
    Dec 6, 2018 at 1:13
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    $\begingroup$ But you don't need to go into orbit, shoot it fast enough so you escape orbit and go directly into the planet. $\endgroup$
    – José Franco Campos
    Dec 6, 2018 at 2:14
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What you've described is the most expensive way possible to rid a world of waste, but yes, it can be done.

However, human nature (with regrettably rare exception) is to use the simplest, cheapest solution possible. So unless your story includes an explanation of either...

  • Why it's cheaper and more efficient to ship waste to Saturn over, say, dumping it down an incredibly deep pit with flowing magma at the bottom, or

  • Why the particular waste is of a nature that it's an absolute requirement to get it off the planet no matter what the cost,

...it might not be particularly believable.

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There have been mentions of railguns/electromagnetic mass drivers, but while these would be more efficient than rockets, they have their own problems:

  • A small railgun has to be operated constantly to get rid of waste. That requires a lot of valuable ferrous metal (for each slug cartridge) and a lot of energy (since the gun is being fired constantly).
  • A big railgun only needs to fire once a day/week/month, but then it has a much larger volume of waste to get rid of, which requires a lot of metal (larger surface area and thicker cartridge) and a lot of energy (to accelerate the much larger mass).

A much simpler and more efficient solution, in my opinion, is essentially a catapult system that functions much like the US Navy's EMALS carrier aircraft launch catapult, except you replace the aircraft with a disposable container holding the waste you need to get rid of. There is no requirement for the container to be ferrous, or even regularly-shaped.

In fact, you can go one further: build an L-shaped "cart" that is attached to the accelerating part of the catapult, where the vertical part of the L is the "back" of the cart (i.e. opposite the direction of travel). You load up the cart with waste and accelerate it to escape velocity; when the cart reaches the end of its track, it stops, while the waste's momentum propels it out of the gravity well and into the gas giant.

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A hybrid solution:

  1. Put up a space elevator for surface-to-orbit transit. Elevators are intended to solve the expense of getting stuff off-planet, whether trash or treasure.(https://en.wikipedia.org/wiki/Space_elevator#Economics) Send the waste up the elevator as needed, and you'll avoid the cost of overcoming escape velocity with rockets/mass drivers.
  2. The terminus platform could have a (relatively) small mass driver to nudge the waste in the right direction toward the gas giant. Just slap a hazard beacon on the payload so system-local traffic can avoid it. :)
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Would it not be true that the only necessary energy would be that which would lower the velocity of the trash to just below orbital velocity?

Then, it would eventually decay into the planet. The problem of almost infinitely orbiting trash would only be a problem, if there were some reason for valuable cargo to transit the space that the trash is orbiting in (towards it's inevitable doom).

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    $\begingroup$ No, you'd have to get it out of the gravity well of the moon. Like here on Earth, we can't get stuff to the inner planets or the Sun by merely having a person throw it against our orbit, it just crashes back into the surface. $\endgroup$
    – user71659
    Dec 4, 2018 at 19:41
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Make use of it instead.

The amount of true waste that a closed system colony like that produces is going to be truly miniscule. In the process of figuring out how to establish colonies that far out, we're also going to have to figure out how to economically reuse everything that we normally think of as waste. Face it, every gram of stuff that we send up the gravity well to a colony is horribly expensive to transport. There's going to be some way to reuse it--even running it through acid baths to break it out into constituent elements is going to be cheaper than firing something new up from Earth.

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No. It is far too expensive to reach escape velocity for really useful stuff already. Have any idea of how much it costs to send 1 kg of very sophisticated electronics out of earth's gravity grip? Then you say you want to pay those amounts of money for many many kilos of waste?

Better solution is to not produce any waste at all. Make the products recyclable instead of having them turn to waste.

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As others have noted, the first big problem to solve is getting the payload to escape velocity of whatever moon you're on, but that is only the first problem. Escape velocity of Ganymede is roughly 2.7km/s, but that's nothing compared to the problem of getting out of Ganymede-height orbit and on a collision course with the planet. Ganymede orbits at about 11km/s, so you've still got over 8km/s to lose, and that's without considering gravity losses from the launch from Ganymede.

I have not worked out the math, but a likely good technique is to burn Ganymede-retrograde to get an encounter with Europa and get a gravity assist from it that lowers your Jupiter periapsis into the atmosphere. Remember, you'll want to pass in front of Europa in its orbit such that your projectile's course is altered in the Europa-retrograde direction. Try it out in your favourite orbital mechanics simulator and see what you can come up with.

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If you have the tech to ferry waste to a Gas Giant, then you have more than enough tech to turn waste into useful material.

The concept of actually throwing all sorts of valuable mass away is so type-zero civilization, whereas ability to ferry it to a nearby gasgiant is at least tech-1

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