(Photo: US Army)
The story goes that on April 12, 1961, Soviet cosmonaut Yuri Gagarin was about to launch into space, becoming the first human being to do so. At the Baikonur Cosmodrome in Kazakhstan, Gagarin took a bus to the launch pad. When he arrived, he sensibly decided to relieve his bladder one last time. So he peed on one of the bus's tires.
Specifically, Gagarin peed on the back right tire.
Ever since, Soviet and Russian cosmonauts faithfully peed on the back right tire of the shuttle bus before blasting off into space.
This is one of many unusual traditions that astronauts of the spacefaring nations have picked up. You can read about the rest at Atlas Obscura.