Yarrow leans over the puddle to look at the ripples, the stars flickering in and out of resolution on the moving surface. “Really cool.”

“Is it?” I ask. “Why are you showing us this? Does this have something to do with the isotopes you were using up all that processing power for?” I ask, drawing my heavytunic tighter around me.

Dad laughs, gives Father a punch in the upper arm. “See? This wasn’t hidden from them after all. We’re hopeless at keeping secrets.”

“It’s only because you mentioned it earlier, Dad,” I say.

“Oh, did I?”

“Yeah, like right when I woke up.”

Father fixes him a withering look.

“The water is heavy?” Yarrow asks. Sensing our attention on the puddle, Rover stops flicking its rippling pattern on the surface.

“Yes,” Father says. “This water is heavy. Far more deuterium isotopes than we’d expect if this were a planet like Earth.”

I draw my tunic even tighter. “What does that mean? Minerva is radioactive?”

“No,” OS responds. “The level of isotopes in this body of water indicates that its ice is extraminervan in origin. Likely from a comet strike, and a relatively recent one, at that.”

“That’s where Earth probably got its water, too, right?” Yarrow asks. “Ice from comet strikes? I think I remember learning that.”

“We think so, yes,” Dad replies.

“This is the second heavy water puddle we’ve found,” Father says.

“The first one was from a strike seven Earth decades ago. This one is closer to two decades ago,” OS interrupts.

“Quiet. Let me explain this to the kids,” Father tells Rover. I tap my foot impatiently. It’s time he stopped trying so hard to control the flow of information.

“Comets used to strike Earth,” I say. “It’s normal. We know that from our learning reels.”

“Yes, and when they did strike, they caused mass extinctions. Like the K-Pg boundary 66.5 million years ago that killed off most of the dinosaurs. We’d either be vaporized, if we were near the strike, or we’d be thrust into a yearslong winter.”

“It’s actually quite a bit worse than that,” OS says. “For starters—”

“Those two recent strikes could be a coincidence,” Yarrow interrupts. “Maybe there won’t be another one for a very long time.”

“This is so statistically unlikely as not to be worth considering,” OS says.

Father walks away from the puddle, looking up into the night sky, arms crossed over his chest. Dad puts his arms around Yarrow and me. “There’s nothing to be too alarmed about.”

“Dad! Just acomet strike,” Yarrow says.

“We are very likely in the orbital paths of extrasolar objects. We need to get working on a solution as soon aspossible,” Father says.

“Does this mean we might need to find a new home?” I ask.

“That’s partly it. We made rudimentary hovercrafts when we first settled here, until we reallocated our metal supply to better purposes. With algal strains that can produce biofuel, we could slowly move our settlement across Minerva, if we have enough warning of a comet strike, and a good location for it.”

“I can provide moderate warning, even with my current equipment,” OS says. “There is no sign of an incoming comet anytime within my forecasting range. So we have weeks at least.”

“Weeks. That’s good,” Yarrow says. He’s grasping for the positive, like he usually does.

I give him a half hug. I’m not particularly good at them. It’s so nice whenever Dad gives one, but it’s all elbow-y and awkward when I do. “Say the comet strikes on the other side of the planet,” I say. “We’d still need a substantial shelter to live in here, to make it through the aftermath. A... what’s the word for it?”

“A bunker,” Father says.