I could make a small centrifuge. Something big enough for the smallest lab scale I have. Rotate at some constant rate with the scale inside. Measure something I know the mass of and then measure the ball. I could calculate the mass of the ball from the ratio of the two measurements.

But I’d have to build a consistent centrifuge. How would I do that? I can spin something in the zero-g environment of the lab easily enough, but how do I spin it at a constant rate across multiple experiments?

Oooh! I don’t need a constant rate! I just need a string with a mark in the center!

I fly into the Hail Mary. Rocky will forgive me for running off. Heck, he can probably “observe” me from wherever he is on his ship anyway.

I bring the ball down to the lab. I get a piece of nylon string and tie each end around a plastic sample canister. I now have a string with little buckets at each end. I put the canisters next to each other and pull the now-folded string taut. I use a pen to mark the farthest point. That’s the exact center of this contraption.

I wave the ball back and forth with my hand to get a feel for its mass. Probably less than a pound. Less than half a kilogram.

I leave everything floating in the lab and kick my way down to the dormitory.

“Water,” I say.

“Water requested,” says the computer. The metal arms hand me a zero-g “sipper” of water. Just a plastic pouch with a straw on it that only lets water through if you unlatch a little clip. And inside is 1 liter of water. The arms always give me water a liter at a time. You have to stay hydrated if you want to save the world.

I return to the lab. I squirt about half of the water into a sample box and seal it. I put the half-depleted sipper into one of the buckets and the metal ball into the other. I set the whole thing spinning in the air.

The two masses clearly aren’t equal. The lopsided rotation of the two connected containers shows the water side is much heavier. Good. That’s what I wanted.

I pluck it out of the air and take a sip of water. I start it spinning again. Still off-center but not as bad.

I take more sips, do more spins, take more sips, and so on until my little device rotates perfectly around the marked center point.

That means the mass of the water is equal to the mass of the ball.

I pull out the sippy. I know the density of water—it’s 1 kilogram per liter. So all I need to know is the volume of this water to know its mass and therefore the mass of the metal ball.