At this time Ultima Thule is the most distant object that we’ve so far sent a probe past. it took several hundred days, close to three years in fact, from New Horizons pass of Pluto to reach Ultima Thule.
The distance is 1.5 billion kilometers beyond Pluto and it only took three years to get there. It’s somewhat faster than your average car. Of course, it doesn’t have to take quite as many turns.
After it bypassed Pluto NASA decided that it had enough fuel for a slight correction change. Not much of one but enough that New Horizons would be able to pass close to another Kuiper Belt Object (KBO).
The Kuiper Belt is an amorphous amount of space beyond Pluto:
The Kuiper belt (/ˈkaɪpər/), occasionally called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive.Wikipedia
It’s huge, its massive and it’s empty. It’s several times more massive that the well known Asteroid Belt which, despite having millions of objects in it, is actually significantly less massive than the moon. We’re not going to be looking here for the mass necessary to terraform Mars.
Beyond that is the Oort Cloud which is split into the Inner Oort Cloud and the Outer Oort Cloud. We’re not going to be looking at any objects out there in the near future. They’re way too far out and it took Voyager One and Two nearly 50 years to pass through it and, at just under ~15km/s, they aren’t exactly slow either.
Slowly, but surely, we’re getting into interstellar space. Voyager One and Two are there already but they can’t change their course, can’t go and look at any other objects. Their power levels are so low that only the minimum of instruments are operating. The time that until they stop transmitting is measured in months.
At that time they will neither have enough power to keep themselves warm nor enough to transmit back to Earth. Even now, their transmission power is less than 23w. That’s less than 1/4 that of your cell phone but they’re transmitting over interstellar distances.
At this time we should be giving a huge shout-out to the engineers who designed and built them. There’s no household white-ware that will operate nearly flawlessly for better than 40 years.
There is so much out there, so much we haven’t seen even in out own solar system. Space is vast. Our solar system extends for light years and we’ve barely even scratched the surface on the closest light hours.