“Full Impulse” is generally considered to be 0.25c.
The force of an impact of a Voyager-sized (700 kilotons) mass at that speed would be many times greater than that of the asteroid that killed the dinosaurs.
And I think its reasonable to assume that 0.25c isn’t a hard limit, but rather an agreed-upon speed limit for starships.
If you could make an object go even faster, that energy goes up by a few orders of magnitude.
0.9c sounds doable. I don’t know about any faster, but maybe?
At that speed, the force of Voyager hitting a planet would be at least hundreds of times greater than the aforementioned asteroid. This sounds like it would completely sterilize the planet.
Which begs the question, why don’t we see weapons like this in star trek? I’d figure the Federation wouldn’t use them, but the Federation isn’t alone.
One argument I tend to see when this comes up is, that the shields would block it. Which then makes you think, if they could block that, then what couldn’t they block? It makes them pretty much invincible. So I don’t think that’s it.
There is an in universe hint of this in ENT 3x10, where the whole NX-01 Enterprise needs a force of 2500 Kilodynes to be pulled from a nebula. Which, if you do the math, results in the whole ship only weighing about 158 kg. Most of the times this is interpreted as a writing error, but could also be the result of some arcane technology such as inertial dampeners or the deflector dish.
You also have Emissary where the deflector is used to reduce the mass of the station.
Which in turn was a redux of Geordi changing the gravitational constant of the universe