Every night, I sleep in the tunnel. He watches. I don’t know why. We haven’t talked about it yet. We’ve been too busy with other stuff. But he really doesn’t want me to sleep without him watching. Even if I just want to catch a quick nap.

Today I want to work on an extremely important scientific unit that’s been eluding us. Mainly because we live in zero g.

“We need to talk about mass.”

“Yes. Kilogram.”

“Right. How do I tell you about a kilogram?” I ask.

Rocky produces a small ball from his satchel. It’s about the size of a ping-pong ball. “I know mass of this ball. You measure. You tell me how many kilograms ball is. Then I know kilogram.”

He thought it through!

“Yes! Give me the ball.”

He hangs on to several support poles with various hands and puts it in the mini-airlock. After a few minutes of waiting for it to cool, I have it in my hands. It’s smooth and made of a metal. Fairly dense, I think.

“How will I measure this?” I mumble.

“Twenty-six,” Rocky says out of nowhere.

“What about twenty-six?”

He points to the ball in my hand. “Ball is twenty-six.”

Oh, I get it. The ball weighs twenty-six of something. Whatever his unit is. Okay. All I have to do is work out the mass of this ball, divide by twenty-six, and tell him the answer.

“I understand. The ball is a mass of twenty-six.”

“No. Is not.”

I pause. “It isn’t?”

“Is not. Ball is twenty-six.”

“I don’t understand.”

He thinks for a moment, then says, “Wait.”

He disappears into his ship.

While he’s gone, I speculate on how to weigh something in zero g. It still has mass, of course. But I can’t just put it on a scale. There’s no gravity. And I can’t spin up the Hail Mary’s centrifugal gravity. The tunnel is connected to her nose.

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.

I get a large plastic syringe from the supplies. It can pull a maximum of 100 cc of volume.

I attach the syringe to the sippy and unclip the straw. I draw out 100 cc of water, then squirt it into my “wastewater box.” I repeat this a few more times. The last syringe is only about a quarter full when I empty the bag.

Result: 325 ccs of water, which weighs 325 grams! Therefore Rocky’s ball also weighs 325 grams.

I return to the tunnel to tell Rocky all about how smart I am.

He balls a fist at me as I enter. “You left! Bad!”

“I measured the mass! I made a very smart experiment.”

He holds up a string with beads on it. “Twenty-six.”

The beaded string is just like the ones he sent me back when we talked about our atmospheres—

“Oh,” I say. It’s an atom. That’s how he talks about atoms. I count the beads. There are twenty-six in all.

He’s talking about element 26—one of the most common elements on Earth. “Iron,” I say. I point at the necklace. “Iron.”

He points at the necklace and says, “♫♩♪♫♫.” I record the word in my dictionary.

“Iron,” he says again, pointing at the necklace.

“Iron.”

He points to the ball in my hand. “Iron.”

It takes a second to sink in. Then I slap my forehead.

“You are bad.”

It was a fun experiment, but a total waste of time. Rocky was giving me all the information I needed. Or trying to, at least. I know how dense iron is, and I know how to calculate the volume of a sphere. Getting to mass from there is just a little arithmetic.

I pull a pair of calipers out of the toolkit I keep in the tunnel and measure the sphere’s diameter. It’s 4.3 centimeters. From that I work out the volume, multiply by the density of iron, and get a much more precise and accurate mass of 328.25 grams.

“I was only off by one percent,” I grumble.