為甚麼太空是沒有摩擦力?

When an object moves in air, it has to push away the air in front of it in order for it to continue moving. Since air has mass, the pushing away of the air requires force and expenditure of energy. This is air resistance.

為甚麼太空是沒有摩擦力?
我知道是因為太空是沒有空氣
Not exactly. It should be " There is NOTHING in the space. "

為甚麼沒有空氣就是等於沒有摩擦力?
Not exactly.
If there is some other substances, there is friction too. Say, water, dust (even the concertration is very low).

Friction is the force resisting the relative lateral motion of two solid surfaces in contact. Friction is often subdivided into dry friction and fluid friction in which the surfaces are actually separated by a layer of gas or liquid.
Therefore, it need something in contact with the Object to create friction.