Inertia. If you were going at 10% of the speed of light you would need 600 years to reach a star system 40 light-years away. 200 years of acceleration, 200 years of cruise speed, and 200 years of deceleration.

Contrary to what you see in movies, in real life, you can't accelerate from rest to near light speed in a matter of minutes. Even if the propulsion system was capable of that, no pilot would do it because the G force would crush him to paste.

Whenever you accelerate, decelerate or seek to turn the trajectory of a moving object, you need to apply force. And there's a sharp limit to how much force a human body can take. You can go to the wiki pages of any modern fighter aircraft and you'll find that the maximum g-limit is 9g.

Because over 9g the pilot tends to blackout. The maximum g a human body can take is 30g. A US Lt. Colonel in the air force volunteered for the experiment and he could take 30g at max. Even then it caused his eardrums to burst and he suffered internal bleeding and organ damage.

And even a sustained force of 8g would kill the crew overtime. Many pilots get stiff necks and other muscle problems despite rarely ever hitting 9g and only flying for 150 hours a year.

And the real problem in space comes from debris. At .1c, even a small pebble-sized object would hit you with the force of a mini nuclear bomb. Imagine it this way: If you were walking slowly and bumped into a wall, you wouldn't get hurt. If you were running and hit the wall, you will be bleeding. If you crashed into the wall at 1000 km/hr, you'd be reduced to paste.

And space is littered with such debris and there are asteroid fields always lying about. A spaceship would need to maneuver around these things. It can't ram into them at .1c speed. The problem is that due to the law of inertia, it would take enormous amounts of force to decelerate or change trajectory.

Even if your radars picked up the asteroid belt 100,000 km away, at .1C, you'd only have 3.33 seconds of warning. At half the speed, it would only be 6.66 seconds. In 6.66 seconds, if you were to decelerate from .1c to a ''mere'' match 1 at 352 m/s, you'd need to apply a 459,637 g force.

Forget your body, your ship would be torn to bits. And due to the law of angular momentum, the faster your speed, the longer your turn radius is. You'd notice this with your car. The faster it is, the longer it takes to turn around.

So for these reasons, it's completely impractical to try to build a space empire or for humans to try to reach potentially habitable worlds outside the solar system. If humans ever set foot on a habitable world outside the Sol system, it will most likely be due to a fleet of generation ships run by an AI that travels over hundreds of thousands or millions of years. Upon reaching the planet, the ships would then terraform the planet and once it was suitable, then culture live humans from embryos.

But all that is pointless from the perspective of Earth humans. Why would you spend tens or hundreds of trillions of dollars on a project that has a minuscule chance of success and brings you zero benefits? European countries for example colonized the new world because it allowed them to jettison excess population, extract resources for trade, and trade with new outposts of their nations.

No such benefit awaits space colonization. And as far as settling mars, I've written an extensive article here before.