[__MatrixMan__]

Perhaps buoyancy could be a decent substitute, at least for the solid waste part. I imagine being waist deep and flushing the entire bathroom after each training session. Maybe some kind of spatula/squeegee might assist with separation, coupled with a robotic spatula cleaner and sanitizer. There would be a monitor and cameras so you could calibrate your aim. What an odd workday that would be.

[throwaway173738]

buoyancy only applies in gravity. The buoyant force on an object is equal to and opposite of the weight of the displaced fluid. No gravity, no weight.

[pulvinar]

The goal here is neutral buoyancy when in gravity so that it behaves as though there were no gravity. Put a bag of water in water and it floats like the rest of the water, gravity or no.

[throwaway173738]

So you’re strapping yourself into a material with the same density as poop and then pooping into it? How is that cleaner than pooping in a bag or over a vacuum?

[malfist]

Neutral buoyancy is achieved with very specific densities. You can either make the astronaut buoyant, or you can make the poop, but not both at the same time.

[__MatrixMan__]

Do you need both? I assumed the astronaut has a handle or strap or something to fix their reference frame to the toilet. They can be only partially submerged.