Here's why:

* Energy: An object in orbit has a specific amount of total energy, a combination of its kinetic energy (from its motion) and potential energy (from its position in the gravitational field). This energy remains constant unless acted upon by an external force.

* Angular Momentum: An orbiting object also has a specific angular momentum, which measures how much it's rotating around the central body. This angular momentum is also conserved unless acted upon by an external force.

To change an orbit, you need an external force to alter the object's energy or angular momentum. This force could be:

* Gravity from another object: A passing star or planet can tug on the orbiting object and change its path.

* Rocket propulsion: By firing a rocket engine, you can add or remove energy and change the object's velocity, altering its orbit.

* Friction with the atmosphere: This is only relevant for objects orbiting very close to a planet with an atmosphere. Friction slows the object down, reducing its energy and causing it to spiral inward.

Without any external force, an orbit will remain stable. The object will continue to follow its elliptical path, maintaining its constant energy and angular momentum.

Think of it like this: Imagine a ball rolling in a circular track. If you don't touch the ball, it will continue rolling in the same circle. But if you push the ball, you'll change its speed and its path, causing it to move in a different way. The same principle applies to orbits.