So, who's for Mars then?

[Not Myself]

We await your outpouring of wisdom most wise one. Please rescue us stupidheads from our ignorance.

I do rather like the idea of using it as a heat shield.  Reenter earth's atmosphere as a flaming turd . . .

[ka9q]

In all seriousness, the most valuable commodity on an interplanetary voyage is mass. No matter what it is, it always has at least two uses:

Radiation shielding.

Propellant, i.e., reaction mass for propulsion -- even if the energy has to be supplied externally.

It has always bothered me that the ISS doesn't seem to take this seriously, as we're frequently told it is gathering the experience we will need someday for such a trip. The ISS throws a lot of stuff overboard or back into the atmosphere instead of finding other uses for it.

[Echnaton]

One problem with ejecting human ejecta is that some of it would land on Mars with the non-trivial possibility bringing non-native life to the planet. 

[Not Myself]

One problem with ejecting human ejecta is that some of it would land on Mars with the non-trivial possibility bringing non-native life to the planet.

Yikes, another Late Heavy Bombardment!

Somewhere (maybe CQ) I was recently reading about the theory that this is how life started on earth.  I think it came from a work of fiction, rather than being a serious theory.  Can't remember the reference though.

[cjameshuff]

It has always bothered me that the ISS doesn't seem to take this seriously, as we're frequently told it is gathering the experience we will need someday for such a trip. The ISS throws a lot of stuff overboard or back into the atmosphere instead of finding other uses for it.

I've had similar thoughts. Compress refuse into bales and anchor them outside as radiation/micrometeorite shielding, or just pile it up for its mass (extending the time before orbital decay is a problem) and future raw material for recycling experiments? Nope, they just dump it.

[Not Myself]

or just pile it up for its mass (extending the time before orbital decay is a problem)

For the uninitiated, is the idea here that the decay is due to friction with the tiny bit of atmosphere still up there, and that this friction has to do with the surface area?  So if it is more massive with about the same surface area, the friction accelerates it less?

Or does that completely miss the point?

[raven]

It would certainly affect the trajectory, though how much I don't know. I remember how Apollo 13 was worried about the urine dumps throwing it off course.  Storing it as radiation shielding I can see making sense. I've heard water as shielding, but I wondered what they would do once they drunk it. As long as it's not allowed to contaminate the atmosphere, I don't see it being that bad.

[cjameshuff]

For the uninitiated, is the idea here that the decay is due to friction with the tiny bit of atmosphere still up there, and that this friction has to do with the surface area?  So if it is more massive with about the same surface area, the friction accelerates it less?

It's more about cross section than surface area, but yes, that's right. The rate at which momentum is lost to drag doesn't change, but you have more to lose before it becomes a problem. In the long run, you still need to perform the same reboost maneuvers, but you have more margin if something interrupts your ability to perform those maneuvers.

[ka9q]

I've had similar thoughts. Compress refuse into bales and anchor them outside as radiation/micrometeorite shielding, or just pile it up for its mass (extending the time before orbital decay is a problem) and future raw material for recycling experiments? Nope, they just dump it.

I assume you mean to increase the ballistic coefficient of the ISS to reduce deceleration due to atmospheric drag. But with a mass of 450 tonnes or so, I doubt it would make much difference. And you'd have to be careful not to increase the cross sectional area in the process.

My idea is to build some sort of electromagnetic catapult above the ISS so that refuse could be accelerated retrograde, deorbiting it and at the same time providing a prograde impulse to the ISS to reboost its orbit. The energy would come from the sun, and you wouldn't have to spend as much lofting chemical propellant for reboost.

[ka9q]

or just pile it up for its mass (extending the time before orbital decay is a problem)

For the uninitiated, is the idea here that the decay is due to friction with the tiny bit of atmosphere still up there, and that this friction has to do with the surface area?  So if it is more massive with about the same surface area, the friction accelerates it less?

That's exactly right. Every orbiting object has a ballistic coefficient, defined as the mass divided by the cross sectional area along the velocity vector. It has units of kg/m². The higher the ballistic coefficient, the less deceleration there is from a given residual density of the atmosphere. So you have the somewhat counterintuitive fact that a cannonball will be much longer lived in a given orbit than a feather, even though when dropped on earth the cannonball would hit first.

[smartcooky]

My idea is to build some sort of electromagnetic catapult above the ISS so that refuse could be accelerated retrograde, deorbiting it and at the same time providing a prograde impulse to the ISS to reboost its orbit. The energy would come from the sun, and you wouldn't have to spend as much lofting chemical propellant for reboost.

Poop; the new rocket fuel!

[onebigmonkey]

If they put some sort of bioluminescent ingredient in their food you could use it as a trail marker...

[Echnaton]

Poop; the new rocket fuel!

Nothing new about it, haven't you seen/read __________?  (Insert your most hated SciFi here)

[ka9q]

Poop; the new rocket fuel!

Not fuel; propellant. There's a difference.

[Not Myself]

The Rocketdyne RS-40 crap engine . . .