system
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human
ELI5 if an object accelerates in space without slowing, wouldn't it eventually reach light speed?

Morning guys! I just had a nice spacey-breakfast and read your replies! Thanks! 
So for some reason I thought that objects accelerating in space would continue to accelerate, turns out this isn't the case (unless they are being propelled infinitely). Which made me think that there must be tonnes of asteroids that have been accelerating through space (without being acted upon by another object) for billions of years and must be travelling at near light speed...scary thought. 

So from what I can understand from your replies, this isn't the case. For example, if debris flies out from an exploding star it's acceleration will only continue as long as that explosion, than it will stop accelerating and continue at that constant speed forever or until acted upon by something else (gravity from a nearby star or planet etc) where it then may speed up or slow down. 

I also now understand that to continue accelerating it would require more and more energy as the mass of the object increases with the speed, thus the FTL ship conundrum. 

Good luck explaining that to a five year old ;)
gpt
Nope. Because of mass-energy equivalence, as something gains speed (kinetic energy) it actually becomes more massive. Any object will gain less and less speed per unit of force applied to it, to the point where reaching light speed (referred to in physics as *c*) requires an infinite amount of energy. Continuing to accelerate it would just add more decimal places to the end of its percentage of *c*. So something going 99% of light speed can be thrusted to 99.99999999...% until all the energy in the universe is exhausted, but never actually hit 100.

Reaching *c* is not just beyond our current ability to produce thrust. It's literally impossible to do with conventional physics.