# How can satellites change direction without any medium in space? [duplicate]

How can satellites change direction without any medium in space? How do spaceships move in space if there is no medium?

How does Newton's third law of motion work in space?

How satellites can change direction without any medium in space? How do spaceships move in space if there is no medium?

It works by the principle of conservation of (linear) momentum which is derived directly from the third Newton's law of motion. Effectively, the rocket ejects fuel at high velocity in the opposite direction to where it wants to go. Remember that the momentum is a vector quantity

$$\vec{p} = m \vec{v}$$

Conservation of momentum is applied to the system of rocket and fuel

$$\vec{p}_\text{rocket} + \vec{p}_\text{fuel} = \text{const.}$$

Simply put, velocity of the center of mass of the system of rocket and fuel stays constant. You can try this principle at home with roller skates and some (modestly) heavy object that you can throw. Throw the object in front of yourself while standing at rest in roller skates and watch what happens - in addition to breaking your parents furniture, you will start moving backwards due to the conservation of momentum. Roller skates are used to minimize friction with the surface, and the object you throw is like the fuel in the rocket.

See the following Wiki article for maths: “Variable-mass system”

• No Ether in play here? Commented Mar 24, 2022 at 16:52
• Just kidding. I think this is a good answer. Commented Mar 24, 2022 at 16:55
• @jwh20 Thanks, I appreciate it ;-) Commented Mar 24, 2022 at 16:55
• I've found that for many people, the idea that a rocket's exhaust doesn't have to push against anything to be hard to grasp. I like your example of throwing an object while on roller skates as someone could actually perform that experiment. Commented Mar 24, 2022 at 17:00
• @jwh20 It is a good sign that people do not easily accept ideas - it indicates they actually think about it. That is why it is good to do these little experiments, to develop proper intuition. Commented Mar 24, 2022 at 17:03

If you are in space, floating at zero g, and you throw a heavy ball in one direction, you'll float in the opposite direction (action/reaction, conservation of momentum). You don't need any medium. Once you're moving, you don't need to do anything to keep moving (law of inertia). If you want to change direction or speed (or if you want to stop), you need to throw another ball (or a couple of them).

Now, instead of throwing balls, make a tube with an open end (maybe attach a nozzle), and eject a whole bunch of particles (like, gazillions of them - a fuel of some sort) at high speed. Now you have something like a thruster, or if you really engineer it up, a rocket engine. Take one big engine as your primary propulsion, and arrange smaller thrusters around the rocket so that you can change direction, and there you go.

You do not need a medium to push against. Several principles are used to change direction of satellites.

• Newton's third law is used by thrusters and cold gas engines. Every action (particles leave thruster of the satellite) has a reaction. E.g. The satellite moves the opposite direction. $$\vec p_r=-\vec p_a = m_a \cdot \vec v_a$$ follows conservation of momentum. We want to save fuel. Hence we want to reduce mass $$m_a$$. As a result the fuel has to be leave the satellite a high velocity by e.g. combustion or acceleration by electric force.
• Magnetic torquers leverage Lorentz force of a magnetic field of e.g. the Earth. An electric coil creates a magnetic moment $$\vec m$$. The interaction with Earth's magnetic field of $$T_e\approx 50\,\mu T$$. A torque $$\tau = \vec m \times\vec B$$ is created.
• Reaction wheels are as well not based on Newton's law and stabilise the orientation of the satellite. The physical principle is a gyroscope (spinner).