To find the correct preposition to fill in the blank in the sentence, it is important to understand the context of how prepositions are used with certain verbs or phrases, especially in the context of movement or change.
The sentence provided is: "Some historians are already saying that thrust ___ space represents a vital turning point in history."
We are given four options: "on," "upon," "in," and "into."
In the context of movement or transition towards something, we typically use "into." The phrase "thrust into space" suggests an action that involves movement from one point into another, emphasizing entering the space environment.
Let's rule out other options:
on: This is typically used to indicate a position or location on a surface, not suitable for motion into a space.
upon: Similar to "on," is more formal and often used in literary context to mean "on top of" or "immediately following," which does not fit the movement context here.
in: Used to indicate being inside a location or condition. However, the sentence suggests a movement 'towards' or 'inside' something, so "into" is more appropriate.
Hence, "into" is the correct answer as it appropriately describes the action of moving from one place or condition to another, especially from the Earth's surface into the realm of outer space.