Unlike the Earth, Mars and the Moon don’t have strong directional magnetic fields, which means traditional compasses don’t work. So how did the Apollo rovers and current Mars rovers navigate their way around? By using manually set directional gyroscope and wheel odometers.

While current un-crewed rovers don’t have to return to the comfort of a lunar module, some aspects of the Apollo systems live on in their design. Four U.S. Martian rovers have used wheel odometers that account for slippage to calculate distance traveled. They’ve also employed gyroscopes (in the form of an inertial measurement units) to determine heading and pitch/roll information.

One of the fun things about reading The Martian is you get to learn a little bit about this sort of thing. Here’s a passage about navigation on Mars where astronaut Mark Watney is trying to get to a landmark several days’ drive away.

Navigation is tricky. The Hab’s nav beacon only reaches 40 kilometers, so it’s useless to me out here. I knew that’d be an issue when I was planning this little road trip, so I came up with a brilliant plan that didn’t work. The computer has detailed maps, so I figured I could navigate by landmarks. I was wrong. Turns out you can’t navigate by landmarks if you can’t find any god damned landmarks. Our landing site is at the delta of a long-gone river . NASA chose it because if there are any microscopic fossils to be had, it’s a good place to look. Also, the water would have dragged rock and soil samples from thousands of kilometers away. With some digging, we could get a broad geological history. That’s great for science, but it means the Hab’s in a featureless wasteland. I considered making a compass. The rover has plenty of electricity, and the med kit has a needle. Only one problem: Mars doesn’t have a magnetic field. So I navigate by Phobos. It whips around Mars so fast it actually rises and sets twice a day, running west to east. It isn’t the most accurate system, but it works.

I wonder why the rovers in the story weren’t outfitted with directional gyroscopes and wheel odometers? (See also the operations manual for the lunar rovers.) (via @JaredCrookston)