Space, most generally, might be described as the boundless container of the universe. Its contents are all physical things that we know of, and more. To describe the contents of space, we use terms of distance, mass, force, motion, energy, and time. The units we use depend on the scale we are considering. Units useful to us on the scale of human life become difficult to use for much smaller domains (such as atoms), and as we describe space beyond our planet Earth.

Consider a distribution of mass at very large distances throughout space and the motion and energy transformation processes going on all the time. While these masses are very distant from each other, they do interact in various ways. These include gravitational force attraction; emitting, absorbing, or reflecting energy; and sometimes, though statistically very seldom, colliding.

Interplanetary Space

Interplanetary space refers to...