In a vacuum, spacecraft travel primarily by using the principle of momentum conservation and employing rocket propulsion. Here's a general overview of how spacecraft move in a vacuum:
Rocket Propulsion: Spacecraft typically carry onboard rocket engines that expel high-speed gases in one direction, according to Newton's third law of motion: "For every action, there is an equal and opposite reaction." The expelled gases create a thrust force that propels the spacecraft in the opposite direction.
Newton's Laws of Motion: The spacecraft obeys Newton's laws of motion. The first law states that an object at rest or in motion will continue in that state unless acted upon by an external force. In a vacuum, where there is negligible friction or air resistance, a spacecraft will continue moving at a constant velocity until acted upon by another force.
Conservation of Momentum: When the rocket engines expel gases, the spacecraft gains momentum in the opposite direction. The total momentum of the system (spacecraft plus expelled gases) remains constant. By expelling gases at high speeds, the spacecraft experiences an equal and opposite force, propelling it forward.
Trajectory Corrections: To control the spacecraft's path, propulsion systems are used to adjust its velocity and direction. This enables the spacecraft to change its trajectory, perform course corrections, or enter and exit orbits around celestial bodies.
Orbital Mechanics: Once a spacecraft reaches space, it can take advantage of gravitational forces to enter and maintain orbits around celestial bodies. By achieving the right combination of speed and direction, a spacecraft can enter a stable orbit, where the gravitational pull of the celestial body balances the centrifugal force of its motion, allowing it to travel in a circular or elliptical path around the body.
It's important to note that in a vacuum, there is no air or other medium to provide resistance, so spacecraft can continue moving without the need for constant propulsion (assuming no other external forces act on them). This allows spacecraft to travel long distances in space.