What is space wave propagation

A space wave travels in a straight line from transmitting antenna to the receiving antenna. Space waves are used for line-of-sight (LOS) communication as well as satellite communication. At frequencies >50 MHz, communication is essentially limited to line-of-sight paths because waves cannot travel along earths surface as they get absorbed by earth,  so propagation cannot be like ground wave propagation. Likewise propagation cannot be like sky waves as they are not reflected by ionosphere.
At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth. If the signal is to be received beyond the horizon then the receiving antenna must be high enough to intercept the line-of-sight waves.