Tsiolkovsky didn't publish a "proof" that space travel is possible, but rather he published the fundamental equation of rocketry, which laid the foundation for understanding the principles of space travel. This occurred in 1903, when he published his paper "Investigation of the Space with Reaction Devices."

This paper was pivotal as it outlined the mathematical relationship between the mass of the rocket, the exhaust velocity of its propellant, and the attainable velocity of the rocket itself. This equation, now known as the Tsiolkovsky Rocket Equation, is a cornerstone of rocket science and is still used today to calculate the necessary fuel requirements for space travel.