The Eridian homeworld is the first planet in the 40 Eridani system. Humans actually spotted it a while ago, obviously not knowing there was a whole civilization there. The catalog name for it is “40 Eridani A b.” That’s a mouthful. The planet’s actual name, from the Eridians, is a collection of chords like any other Eridian word. So I’ll just call it “Erid.”

Erid is extremely close to its star—about one-fifth as far as Earth is from our sun. Their “year” is a little over forty-two Earth days long.

It’s what we call a “super-Earth,” weighing in at eight and a half times Earth’s mass. It’s about twice Earth’s diameter, and a little over double the surface gravity. Also, it spins very fast. Absurdly fast. Their day is only 5.1 hours long.

That’s when things started to fall into place.

Planets get magnetic fields if the conditions are right. You have to have a molten-iron core, you have to be in the magnetic field of a star, and you have to be spinning. If all three of these things are true, you get a magnetic field. Earth has one—that’s why compasses work.

Erid has all of those features on steroids. They are larger than Earth, with a larger iron core. They are close to their star, so they have a much stronger magnetic field powering their own field, and they spin extremely fast. All told, Erid’s magnetic field is at least twenty-five times as strong as Earth’s.

Plus, their atmosphere is extremely thick. Twenty-nine times as thick.

You know what strong magnetic fields and thick atmospheres are really good at? Radiation protection.

All life on Earth evolved to deal with radiation. Our DNA has error-correction built in because we’re constantly bombarded with radiation from the sun and from space in general. Our magnetic field and atmosphere protect us somewhat, but not 100 percent.

For Erid, it’s 100 percent. Radiation just doesn’t get to the ground. Light doesn’t even get to the ground—that’s why they never evolved eyes. The surface is pitch-dark. How does a biosphere exist in total darkness? I haven’t asked Rocky how that works yet, but there is plenty of life deep in Earth’s oceans where the sun doesn’t shine. So it’s definitely doable.

Eridians are extremely susceptible to radiation, and they never even knew it existed.

The next conversation took another hour and added a few dozen more words to the vocabulary.

Eridians invented space travel quite a while ago. And with their unparalleled materials technology (xenonite) they actually made a space elevator. Basically a cable leading from Erid’s equator up to the synchronous orbit with a counterweight. They literally take elevators to get into orbit. We could do that on Earth if we knew how to make xenonite.

Thing is, they never left orbit. There was no reason to. Erid has no moon. Planets that close to a star rarely do. The gravitation tidal forces tend to rip would-be moons out of orbit. Rocky and his crew were the first Eridians to leave orbit at all.

So they never found out that Erid’s magnetic field, which extends well beyond its synchronous orbit, had been protecting them all that time.

One mystery remained.

“Why did I not die, question?” Rocky asks.

“I don’t know,” I say. “What’s different? What do you do that the rest of your crew didn’t do?”

“I fix things. My job is to repair broken things, create needed things, and keep engines running.”

Sounds like an engineer to me. “Where were you most of the time?”

“I have room in ship. Workshop.”

I’m getting an idea. “Where is workshop?”

“In back of ship near engines.”

That’s a sensible place to put your ship’s engineer. Near the engines, where things are most likely to need maintenance or repairs.

“Where does your ship store Astrophage fuel?”

He waves a hand generally around the rear of the ship. “Many many containers of Astrophage. All in back of ship. Close to engines. Easy to refuel.”

And there’s the answer.

I sigh. He’s not going to like this. The solution was so simple. They just didn’t know it. They didn’t even know the problem until it was too late.

“Astrophage stops radiation,” I say. “You were surrounded by Astrophage most of the time. Your crewmates weren’t. So the radiation got to them.”

He doesn’t respond. He needs a moment to let that sink in.

“Understand,” he says in low notes. “Thank. I now know why I not die.”

I try to imagine the desperation of his people. With a space program far behind Earth’s, no knowledge of what’s outside, and still making an interstellar ship in a bid to save their race.

No different from my situation, I guess. I just have a little more technology.

“Radiation is here too,” I say. “Stay in your workshop as much as you can.”

“Yes.”

“Bring Astrophage to this tunnel and put it on the wall.”

“Yes. You do same.”

“I don’t need to.”

“Why not, question?”

Because it doesn’t matter if I get cancer. I’m going to die here anyway. But I don’t want to explain that I’m on a suicide mission right now. The conversation’s been pretty heavy already. So I’ll tell him a half-truth.

“Earth’s atmosphere is thin and our magnetic field is weak. Radiation gets to the surface. So Earth life evolved to survive radiation.”

“Understand,” he says.

He continues working on his repairs while I float in the tunnel. A random thought occurs to me. “Hey, I have a question.”

“Ask.”

“Why is Eridian science and human science so similar? Billions of years, but almost the same progress.”

It’s been bugging me for a while. Humans and Eridians evolved separately in separate star systems. We had no contact with each other until now. So why is it that we have almost identical technology? I mean, Eridians are a little behind us in space technology, but not a ton. Why aren’t they in their stone age? Or some superfuturistic age that makes modern Earth look antiquated?

“Has to be, or you and I would not meet,” Rocky says. “If planet has less science, it no can make spaceship. If planet has more science it can understand and destroy Astrophage without leaving their system. Eridian and human science both in special range: Can make ship, but can’t solve Astrophage problem.”

Huh. I hadn’t thought of that. But it’s obvious now that Rocky says it. If this happened when Earth was in the Stone Age, we would have just died. And if it happened a thousand years from now, we’d probably work out how to deal with Astrophage without breaking a sweat. There is a fairly narrow band of technological advancement that would cause a species to send a ship to Tau Ceti to look for answers. Eridians and humans both fall into that band.