The perception that there is only an up and down direction on Earth, as opposed to in space, is primarily due to the influence of gravity and our frame of reference. Allow me to explain further.
On Earth, gravity pulls objects toward its center. As a result, we experience a gravitational force that gives us a sense of weight and a clear distinction between up and down. When we stand on the surface of the Earth, we feel the pull of gravity acting downwards, which we interpret as "down." Conversely, we perceive the opposite direction as "up."
In space, away from the strong gravitational influence of celestial bodies like planets, the concept of up and down becomes less meaningful. In the absence of significant gravitational forces, there is no universally defined "down" direction. Astronauts in space experience microgravity, where they float freely, as the gravitational forces acting on them and their spacecraft are nearly balanced.
Without a dominant gravitational force, the sense of up and down becomes subjective. Astronauts can orient themselves relative to their spacecraft, fellow astronauts, or celestial objects. In space missions, communication and navigation systems use different frames of reference, such as celestial coordinates or relative positions of spacecraft, to establish directions and orientations.
It's important to note that even in space, where the concept of up and down is less relevant, astronauts and spacecraft still use the terms in a relative sense based on their specific context or mission requirements. However, the conventional notion of up and down that we have on Earth becomes less meaningful in the vastness of space.