Now any general understanding of the dependencies of physical states would require something like Aristotles well-known four causes. Restricting ourselves to the temporal order, however, we find, among other things, that every physical state, no matter how inclusive, has a necessary condition in some specific type of state which immediately precedes it in time and is fully existent prior to the emergence of the state which it contains. This means that for any given state---for example, Voyager II being past Triton----all of the necessary conditions of that state must be over and done with at that state or at the event of which the state is the ontic residue. The series of efficient causes, to speak with Aristotle, is completed for any given event or state that obtains. At the state in question, we are not waiting for any of these causes to happen, to come into being.
Moreover, this completed set of causes is highly structured in time and in ontic dependence through relationships which are irreflexive, asymmetric, and transitive. Thus, no physical state is temporally or ontically prior to itself, and if one, a, is prior to another, b, b is not prior to a. Further, if a is prior to b and b to c, then a is prior to c. This rigorous structure of the past is eternally fixed, and it specifies a framework within which every event of coming into existence and ceasing to exist finds its place. Most importantly for present interests, since the series of causes for any given state is completed, it not only exhibits a rigorous structure as indicated, but that structure also has a first term. There is in it at least one cause, one state of being, which does not derive its existence from something else. It is self-existent.
If this were not so, Voyagers passing Triton, or any other physical event or state, could not be realized, since that would require the actual completion of an infinite, an incompletable series of events. In simplest terms, its causes would never get to it. (As in a line of dominoes, if there is an infinite number of dominoes that must fall before domino x is struck, it will never be struck. The line of fallings will never get to it.) Since Voyager II is past Triton, there is a state of being upon which that state depends but which itself depends on nothing prior to it. Thus, concrete physical reality implicates a being radically different from itself: a being which, unlike any physical state, is self-existent.