1. Gravity: This is the force of attraction between any two objects with mass. It's what keeps planets orbiting stars, moons orbiting planets, and even the stars clustered together in galaxies.
2. Inertia: This is the tendency of an object to resist changes in its motion. Once an object is in motion in space, it will continue moving in a straight line at a constant speed unless acted upon by an external force. This is why spacecraft can stay in orbit without constantly firing their engines.
It's important to note that other forces also play a role in space, such as:
* Electromagnetic forces: These forces can be significant, especially in the case of charged particles and magnetic fields.
* Nuclear forces: These forces are responsible for holding atomic nuclei together, and can be important in some astronomical phenomena.
However, gravity and inertia are the two most fundamental forces that determine the motion of objects in space.
