Spaceships and other objects can move in space despite the absence of matter because motion in space is governed by the principle of inertia and the application of external forces.
The principle of inertia, as described by Newton's first law of motion, states that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity unless acted upon by an external force. In the vacuum of space, where there is no significant air resistance or friction, objects will naturally maintain their state of motion.
To initiate or change the motion of a spacecraft in space, external forces are applied. There are several methods by which spacecraft can generate these forces:
Chemical Propulsion: Most commonly used by spacecraft, chemical rockets expel high-speed exhaust gases in one direction, creating an equal and opposite reaction that propels the spacecraft forward. This is based on Newton's third law of motion ("for every action, there is an equal and opposite reaction").
Electric Propulsion: Electric thrusters, such as ion thrusters or Hall effect thrusters, use electric fields to accelerate and expel charged particles. While they provide lower thrust than chemical rockets, they can operate for extended periods and achieve higher velocities over time.
Gravitational Assist: Spacecraft can utilize the gravity of planets or other celestial bodies to gain speed or alter their trajectory. By executing carefully planned flybys, spacecraft can use the gravitational pull of a planet to "slingshot" around it and gain momentum.
Solar Sails: Solar sails harness the pressure of photons from sunlight to generate a small but continuous force. By reflecting or absorbing photons, these large, lightweight sails can generate propulsion over time without consuming propellant.
Reaction Wheels and Thrusters: Small adjustments in a spacecraft's orientation or trajectory can be achieved by using reaction wheels, which spin to counterbalance angular momentum. Thrusters can also be used for fine adjustments by expelling small amounts of gas or propellant.
It's important to note that while space is mostly empty, there are still extremely low densities of matter and particles, such as interstellar gas and dust. However, the primary means of spacecraft propulsion in space relies on the principles of motion and the application of external forces rather than interactions with matter.