In terms of theoretical physics, there is no biological limit to how fast humans can travel through space, as long as the acceleration is gradual. According to Einstein's theory of relativity, speed is relative; as long as you aren't accelerating or hitting something, you wouldn't "feel" high speeds, even if you were traveling at 99% the speed of light. Currently, the fastest humans have ever traveled was during the Apollo 10 mission, reaching roughly 24,791 mph (39,897 km/h) relative to Earth. In the future, with technologies like nuclear thermal propulsion or solar sails, we could reach significantly higher fractions of light speed. However, practical limits are imposed by interstellar dust and radiation: at even 10% the speed of light, a collision with a tiny grain of sand would release energy equivalent to a bomb, potentially vaporizing the spacecraft. Furthermore, traveling at near-light speeds would cause "blueshifting," where normal starlight is compressed into deadly X-rays or gamma rays, requiring massive, currently non-existent shielding to protect the human crew from being irradiated.