What prevents me to accelerate an object to near light speed in space? As far my limited knowledge go, things in space aren't slow down unless something interferes with them, so what prevents me to build a spaceship powered by nuclear power that will keep accelerating until we get to the limits of physics?
Like the voyager ship that is now outside our solar system, it had by know plenty time to accelerate to be much more faster than it's right now (about 17030 m/s)?
Wouldn't that greatly reduces the time you need to get to a distant star, since you can increase your speed exponentially ?  
 A: 
As far my limited knowledge go, things in space aren't slow down
  unless something interferes with them, so what prevents me to build a
  spaceship powered by nuclear power that will keep accelerating until
  we get to the limits of physics?
Like the voyager ship that is now outside our solar system, it had by
  know plenty time to accelerate to be much more faster than it's right
  now (about 17030 m/s)?

You can accelerate near to light speed, but the nearer you get the more difficult it will be. If the object accelerating has mass $M$ then in order to accelerate from speed 0 to speed $v$ you will have had to supply at least
$$
KE=Mc^2\frac{1}{\sqrt{1-v^2/c^2}} - Mc^2
$$
amount of energy (this expression is the total energy minus the rest energy, i.e., the relativistic kinetic energy). Clearly this expression approaches infinity as the speed approaches light speed and you can not supply an infinite amount of energy. So, the more energy you can supply the closer you can get, but even a nuclear power plant can not supply an infinite amount of energy.

Wouldn't that greatly reduces the time you need to get to a distant
  star, since you can increase your speed exponentially ?

Since the energy involves square roots and quadratic functions of the velocity, the speed can not be an exponential function of the energy. It is much less quickly increasing and indeed approaches the speed of light at infinity.
A: 
Wouldn't that greatly reduces the time you need to get to a distant star, since you can increase your speed exponentially?

In addition to hft's answer, due to relativistic kinetic energy $KE$ , assuming you use high constant thrust your actual acceleration would start tailing off quickly at relativistic speeds, so you certainly couldn't "increase your speed exponentially". Quite the opposite, in fact.
A: 
so what prevents me to build a spaceship powered by nuclear power that
  will keep accelerating until we get to the limits of physics?

Now matter how long the spaceship is able to accelerate (as measured by an accelerometer attached to the spaceship), there is always an inertial reference frame in which the spaceship is instantaneously at rest.
Indeed, at any instant, there are an infinity of inertial reference frames in which the spaceship has a speed arbitrarily close to $c$.
So, it isn't clear to me what you mean by "until we get to the limit physics".
Certainly, if the spaceship accelerated (linearly) long enough, the CMB would become an intense gamma ray beam in front of the spacecraft so that is at least one type of limit.
