Conservation of angular momentum is the principle on which spacecraft orientation is based. Angular momentum is a fundamental concept in physics that describes the rotational motion of an object. It is defined as the product of an object's moment of inertia and its angular velocity.

Explanation:
When a spacecraft is in motion, it possesses angular momentum due to its rotation. According to the principle of conservation of angular momentum, the total angular momentum of a system remains constant unless acted upon by an external torque. This means that the spacecraft will continue to rotate at a constant rate unless a torque is applied to change its angular momentum.

In the context of spacecraft orientation, the conservation of angular momentum is crucial for maintaining the stability and control of the spacecraft. Here's how it works:

1. Initial angular momentum: When a spacecraft is launched into space, it has an initial angular momentum that is determined by its moment of inertia and angular velocity.

2. Action and reaction: In the absence of external torques, the spacecraft will continue to rotate at a constant rate due to the conservation of angular momentum. Any action taken by the spacecraft to change its orientation will result in an equal and opposite reaction, as required by Newton's third law of motion.

3. Reaction wheels: To control the spacecraft's orientation, reaction wheels are used. These wheels are motor-driven devices that can change their rotational speed. By spinning the reaction wheels in different directions or changing their speeds, the spacecraft can exert torques on itself and alter its angular momentum.

4. Principle application: By carefully controlling the angular momentum of the reaction wheels, the spacecraft can achieve the desired orientation. For example, if the spacecraft needs to rotate along a particular axis, it can spin one or more reaction wheels in the opposite direction to generate a torque that counteracts the existing angular momentum and causes the spacecraft to rotate.

In conclusion, the conservation of angular momentum is the principle that governs spacecraft orientation. By applying torques through reaction wheels, the spacecraft can change its angular momentum and achieve the desired orientation in space.