ok so not near to light speed & not 90%.
& lets say we will find a way to do it without a fuel tank & get the energy from outside in some way...
will we get to... say... 70%?
B. lets make it simpler. why a comet can't get to 70%? it just accelerating & accelerating (no friction)...
The fastest speed a human-built spacecraft has ever achieved is about 70 kilometers per second, which is a measly fortieth of a percent of lightspeed. And that was a probe launched inward toward the Sun, so it got added acceleration from the Sun's gravity.
oh... wasn't voyager the fastest?