Pi is the ratio of a circle's circumference to its diameter. And this deceptively simple bit of math is a critical tool for keeping our space probes running.
NASA's upcoming Mars lander Insight will need pi along every step of its journey. Mission planners use it to calculate everything from the fuel left in Insight's tanks, to the orbits of Earth and Mars, to the path Insight will take to get from one to the other.
Once it arrives, Insight will measure "Marsquakes." Its instruments are so precise it can detect seismic waves as they circle the planet. Its calculations — which include pi — will show exactly where and when these quakes happen.
And to talk to Earth, Insight will have to go through the circular radio dishes of the Deep Space Network. Thanks to pi, NASA knows the area of the signal versus the area of the receiver — so it can line them up and ensure messages from Mars get through loud and clear.