Disclaimer
My answer only assumes circular orbits for the ease of explanation. Everything I've said applies to elliptical orbits; but I chose to omit their existence for the purpose of keeping things simple.
TL;DR
- The Endurance could have orbited Gargantua at any orbital height. Although the relative distance between the Endurance and planet Miller would fluctuate, it would always be possible to know where the Endurance should be (relative to planet Miller, at a given point in time) and fly there, assuming there is enough fuel of course.
- Due to time dilation, it is theoretically possible for the Endurance to create a synchronous orbit with the perceived orbit of planet Miller (i.e. as perceived when looking at planet Miller from the Endurance, not as experienced when standing on planet Miller's surface).
- Although this special scenario is not explicitly mentioned in the script, it makes a lot of sense to do this as it comes with many benefits, including both communication availability and ease of returning to the ship.
What if we take a wider orbit around Gargantua, parallel with Miller's planet outside of this time shift, to here?
From this part of the quote you added, I read that the ship never orbited around planet Miller, but rather around Gargantua (the black hole itself).
As an analogy:
- The Sun (Gargantua) has several planets orbiting it
- Earth (planet Miller) orbits the Sun
- Mars (the Endurance) also orbits the Sun, but it's orbit is wider (larger radius) than Earth's.
Because they have different orbital heights, Mars and Earth experience a year (one orbit around the sun) differently. This means that Mars and Earth continually change position relative to each other (due do different orbital speeds), but they catch up to each other after a given cyclus.
As an analogy: the hour hand and the minute hand on a clock go around at different speeds, but they still match up with each other every time the minute hand "laps" the hour hand.
This means that the travel distance can vary wildly (depending on their current relative distance), but you can always calculate where the other object is supposed to be (because orbital periods never change so long as the orbit itself never changes).
Assuming the Ranger has enough fuel to travel to the Endurance at any point in time (meaning that they do not need to wait for the Endurance to be close enough to planet Miller), then it makes sense that they could take the Ranger up to the Endurance whenever they wanted.
So the direct answer to your question is that while the Endurance did not orbit around Miller's planet, it did orbit around Gargantua; which means that it would still have been reachable many years later (but requires more travelling compared to when the Endurance would have orbited planet Miller).
There is another interesting point, which has mainly come forward from the comment discussion in the other answer. First, let me explain how orbits work in a normal situation. I'm resuming the Earth/Mars example.
Earth orbits the Sun. Mars orbits the Sun at a higher orbital height than Earth. Because Mars has an increased orbital height, this has some consequences:
- The speed of a satellite is influenced by the orbital height. For a higher orbital height (Mars), the satellite will move slower (compared to Earth).
- Because Mars orbits at a higher orbital height, it has to travel a longer distance to make a trip around the sun. This is somewhat obvious, as the circumference of a circle increases as its radius increases (and orbital height = radius of the orbit).
Relative to Earth and its orbit, Mars travels slower along an orbit that has a larger circumference. This is a double whammy: two independent reasons why Mars takes longer to orbit the Sun, compared to Earth.
Now, the interesting part is the logical consequence:
Two satellites that orbit the same planet at different orbital heights, can never orbit the planet at the same orbital period.
The closest satellite (Earth) will always orbit much faster than the furthest satellite (Mars).
This is a normal situation. I consider it normal, because it does not contain a special scenario where Earth and Mars are experiencing a different flow of time (to a point where it is meaningfully measurable).
However, when we consider planet Miller and the Endurance, we do need to take this into account!
Let's say that, forgetting time dilation, if the Endurance looks at planet Miller orbiting Gargantua, it sees planet Miller complete an orbit in 100 seconds. Anyone standing on the surface of planet Miller could also measure 100 seconds for a single orbit.
However, if we now impose time dilation on planet Miller (e.g. time moves at 50%); then the observer on planet Miller's surface will still measure 100 seconds, but an observer on the endurance would measure 200 seconds.
Conclusion:
If planet Miller's time passes time at X% of "normal time", then an outside observer will observe planet Miller's orbital period to pass at X% of its undilated (expected) orbital period.
"Normal time" is the observer's frame of reference, which in this case is the Endurance.
The slower time progresses on planet Miller, the slower that planet Miller will appear to orbit around Gargantua. Since we know that time passed really slowly on planet Miller, that means that it is orbiting Gargantua at a really slow pace (as observed from the Endurance).
Without time dilation, because the Endurance takes a wider orbit than planet Miller, planet Miller should orbit around Gargantua faster than the Endurance. This is unavoidable, assuming unpowered orbits.
However, because planet Miller experiences time at an incredibly slow pace, it also orbits around Gargantua at an excessively slow pace.
This means that it is possible for the Endurance to pick an orbital height which gives them the same orbital period as the perceived orbital period of planet Miller (as experienced by an outside observer, not an inhabitant of planet Miller).
What if we take a wider orbit around Gargantua, parallel with Miller's planet outside of this time shift, to here?
Although Cooper never explicitly specifies it, it makes sense that the Endurance chose the specific orbit that synchronizes its orbital period to planet Miller's orbital period, for the purpose of keeping communications open (not blocked by other bodies) and having a fixed travel destination from planet Miller back to the Endurance (ignoring planetary rotation, which is not an obstacle if you already travel using orbits)
Because of the different time dilation, a unique opportunity presented itself to synchronize orbits while maintaining different orbital heights.
