What's the fastest a human can travel without dying?

Changes in velocity are expressed in multiples of the gravitational acceleration or “G”. Most of us can withstand up to 4-6 G. Combat pilots can handle up to 9G for a second or two. But sustained G-forces, even 6G, would be fatal.

Undoubtedly, micrometeoroids are not the only obstacle to future space missions, where higher human travel speeds are likely to come into play. Eric Davis, senior research physicist at the Austin Institute for Advanced Studies and collaborator of NASA's Innovative Propulsion Physics Program, a six-year research project that ended in 2002, describes three of the most promising ways — assuming conventional physics — to achieve reasonable interplanetary travel speeds. In fact, it's the fastest thing that exists, and a law of the universe says that nothing can move faster than light. Marc Millis, a propulsion physicist and former director of NASA's Innovative Propulsion Physics Program, warns that this possible speed limit for traveling with humans remains a distant concern.

The Orion spacecraft is destined to take astronauts to a low Earth orbit, and it's a good bet for the vehicle that will break the 46-year record for being the fastest we've ever traveled. Scientists are investigating many other ways to go fast, including high-speed travel, the faster-than-light trips popularized by Star Trek. These G forces are mostly benign G from front to back, thanks to the intelligent practice of holding passengers to space in seats facing their direction of travel. Speculative dangers could also arise if humans manage to travel faster than light, either by taking advantage of gaps in known physics or through paradigm-shattering discoveries.

He and his father roughly estimated that, barring some kind of conjectural magnetic shielding to deflect the deadly hydrogen shower, starships could not go at more than half the speed of light without killing their human occupants. Therefore, to achieve significantly faster travel speeds for humans to Mars and beyond, scientists recognize that new approaches will be needed. However, shortening travel times would mitigate these problems, so taking a faster approach is highly desirable. Light travels at 186,000 miles per second (300,000 kilometers per second) and can go from Earth to the Moon in just over a second.