Remember the colourful iMac computers from the late 1990s? The same processor that powered those is being used to run NASA’s Mars Perseverance rover.
This processor, which is also being used in the Curiosity rover, has just 10.4 million transistors – even affordable smartphones now have more than 1000 times as many. So why is such old technology used in a cutting-edge space exploration mission?
It all comes down to radiation. Earth’s atmosphere protects us from most of the harmful rays from the sun, but in space or on Mars there is little to stop them.
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James LaRosa at BAE Systems in the US, which makes the chip, says: “You have this multi-billion-dollar spacecraft going out to Mars and you’re entering the atmosphere and you’re about to start this 7 minutes of terror, where a very complex sequence of commands have to happen in a very specific timeline. If it has a hiccup, you’re going to lose the mission.”
One cosmic ray from the sun could wreck the whole thing. “A charged particle that’s racing through the galaxy can pass through a device and wreak havoc,” he says. “It can literally knock electrons loose, it can cause electronic noise and signal spikes within the circuit.”
Devices designed for use on Earth have no radiation shielding and would suffer regular crashes or catastrophic, permanent failure if used on Mars. Specialist devices need to be designed for use in space.
In the case of Perseverance, it uses a RAD750 processor. These chips are built under licence from the designers of the original, but reengineered from the ground up to add physical radiation shielding and extra logic to detect and correct memory corruption. Each one costs over $200,000.
Perseverance has three computers on board, each with two gigabytes of flash memory (about as much as a small USB stick) and 256 megabytes of RAM. One computer handles the main functions of the rover, a second acts as a backup and a third is used to analyse images for navigation. By taking regular photos of the surface underneath the rover, software can calculate exactly how far it has travelled. This is more accurate than relying on measuring the rotation of the wheels because they occasionally slip in the dusty surface.
“We just had our landing, and for those 7 minutes it was critical that the computing had to be active and responsive the entire time because if it wasn’t, we could lose the whole mission,” says Mark Maimone at NASA’s Jet Propulsion Laboratory in California. “That’s what drives the need for this very specialised hardware. We knew that it would be a good choice because we’d already used it.”
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The InSight mission that landed a seismometer on Mars in 2018 uses the same processors. There are also long-serving RAD750 processors on around 100 satellites orbiting Earth, including those providing GPS, imaging and weather data. LaRosa says this processor is powering various military satellites too. And with hundreds of years of combined service, not one has failed, he says.
Maimone hopes future rover missions will have more capability. Perseverance has a fixed gear ratio for simplicity, but a gearbox would have allowed it to travel faster, and higher-powered computers would support that. At higher speeds, navigation computers would have to respond to “dynamic events” such as losing contact with the ground over bumps. “It would be nice to get up to higher speeds,” he says. “It would be nice to push the envelope.”
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