It then occurred to me that all the spacecraft launched by humanity in its first century of space exploration travel at similar speeds of tens of kilometers per second. This speed limit is set by the sound speed of chemical propellants and the tyranny of the rocket equation, which asserts that the fuel mass must grow exponentially in order to elevate the terminal rocket speed further.
But imagine our second century in space exploration during which we might develop a faster propulsion method, like the light sail in the Starshot Initiative, where the payload does not carry the fuel with it and can in principle reach the speed of light.
Human interstellar travel will never be practical until we unlock the secret to warping spacetime. To read more, click here.