The challenges of building a computer for space

HostMost of us get a new phone every few years because the old one gets slow or the screen cracks. We just head to the store and grab a new one. But when we send a rover to Mars or a probe out past Pluto, that gear has to work for decades without anyone ever touching it. Why is it so hard to build a computer that can handle the trip into the stars?

GuestIt really comes down to how mean space is to hardware. On Earth, we have a thick blanket of air and a magnetic field that protects us. Up there, you're basically standing in a shooting gallery of tiny, invisible bullets. These are high-speed bits of atoms flying through the dark. When one of those hits a normal computer chip, it can flip a tiny switch inside. A zero suddenly becomes a one. If that happens in the middle of a big calculation, the computer might think up is down or just freeze up entirely.

HostThat sounds like a disaster, but can we not just wrap the computer in a big lead box? That seems like a simple way to block the junk flying around.

GuestYou would think so, but weight is the big enemy here. Every extra pound you send up costs a fortune in rocket fuel. But even if money wasn't an issue, thick metal walls can actually make things worse. When those fast-moving bits of atoms hit a heavy metal shield, they can shatter. They create a spray of even more junk that flies into the chip. It's like a car hitting a wall and sending shards of glass everywhere. Instead of one hit, now you have a hundred. To get around this, we have to change how we make the chips themselves. We build them with wider wires and bigger parts so that a tiny hit doesn't break the whole path.

HostSo we're making them tougher on the inside rather than just adding armor. But what about the heat? I know my laptop gets hot just sitting on my desk. If a computer is out in the sun in space, does it just cook?

GuestHeat is a massive headache. Down here, your computer has a tiny fan. It blows air over the hot parts, and that air carries the heat away into the room. In space, there's no air. There's no wind. If a chip gets hot, that heat has nowhere to go. It just sits there and builds up. It's like being in a small room with no windows and no vents while you're running a space heater. Eventually, the electronics would just melt. To fix this, we have to build long metal paths that let the heat crawl through the body of the ship and shine away into the dark as light. It makes the whole computer much bigger and heavier than it would be on Earth.

HostIt still feels like we're settling for less, though. I have read that the computers on the Mars rovers are way slower than the phone in my pocket. If I'm flying a billion-dollar ship, I want the best, fastest brain available. Using a chip that's twenty years old sounds like a bad plan.

GuestIt feels that way, but it's a trade-off. Modern chips are fast because they're tiny. The paths inside are so small that it doesn't take much to break them. A single hit from a cosmic ray can bridge the gap between two wires and cause a short. The older chips are much bigger. Their wires are fat and spread out. You can hit them with a lot more junk before they fail. We choose the slow, steady turtle because the fast rabbit would likely break the moment it left our air. Those older chips have also been tested for years. We know exactly how they behave when things go wrong. In space, being predictable is much more important than being fast.

HostWhat if we just sent a bunch of cheap, fast computers instead? If you pack twenty smartphones in a box, it doesn't matter if five of them break. You still have fifteen left to do the work.

GuestPeople are starting to try that for small missions close to Earth, but for the big stuff, it's very risky. The main problem is figuring out which computer is telling the truth. Let's say you have three computers figuring out which way to fire the rockets. If one of them gets hit by radiation and says go left, while the other two say go right, you need a system that can listen to the majority and ignore the broken one. This is called a voting system. Setting that up so it never fails is incredibly hard. If a big solar flare hits, it might scramble all twenty of those cheap phones at the very same moment. Then you're left with a dead ship and no way to talk to it.

HostSo you have to build a brain that can think for itself, keep its cool without any air, and survive a constant rain of tiny invisible bullets.

GuestMost of the chips we send up are still based on designs from the nineteen nineties because those fat wires are the only things we can trust to stay alive for a ten-year trip through the void.

HostThe next time my phone acting up makes me want to throw it across the room, I should probably be glad it only has to deal with a little bit of dust and a drop of water.

GuestA single bit of dust on a chip in deep space would be a dream compared to the heavy hits those machines take every second they're out there.

HostComputers in our pockets have it easy compared to the old, rugged scouts we send out to explore the dark.