Spacecraft need to eject mass to move forward. Essentially they are pushing off parts of themselves, and leaving it behind.

That mass is often burnt exhaust gasses from rockets. For small adjustments they tend to use compressed gas directly since it's easier to control. No need to constantly start and stop a rocket motor.

You push off your propellant. In order to make your ship move, you expell superheated gas out the back at extremely high velocity. Basically, you bring the stuff you push off of with you.

By analogy, if you're standing on a skateboard and throw a ball, you'll move in the direction opposite your throw. You may be resting on the ground, but you didn't push off of the ground. Rather, you pushed off the ball.

You’ve identified why spaceships don’t use propellers or jet engines…there’s no air. Propellers and jet engines push air so they only work in an atmosphere.

So instead you have to push off something. The most common way is to create a controlled explosion and shoot the expanding gases out a nozzle. Smaller releases of gas can be used to make smaller changes, like to point slightly to the left or right, or to start or stop rotation.

Once moving there’s no air to create drag so once you’re going as fast as you want to go you turn your engine off and just coast along. You’ll still be affected by gravity, so you have to account for that. And there is a small amount of gas even in mostly empty space.

For interstellar travel there’s a different proposed solution to the problem. You see in order to use exploding gases to accelerate you fast enough to make traveling to another star achievable within the lifetime of the astronauts you’d have to use a lot of fuel, and fuel is heavy, and takes even more fuel to carry into space. Instead you can deploy a large sail and catch the wind coming off of the sun or fired from a laser on earth, similar to how a sailboat catches the wind. That can get you going to 10% light speed. You’d reach the nearest star in just about 40 years! We’re still a little ways off from being able to construct a sail thin and light enough while still being strong. But we’ll get there soon.

Newton's third law of motion. For every action there is an equal and opposite reaction. If you push an object backwards, then that object will push you forward. When it comes to space travel we use the exhaust from the rocket. The act of pushing gas out the back of the ship pushes the ship forward. You can actually see this effect for yourself. Stand on a skateboard and throw a heavy object. You will accelerate in the opposite direction.

You don't need air to push off of. Rockets, chemical thrusters, and ion engines work in space because they spit out some sort of propellant, spitting out mass in one direction pushes the ship in the opposite direction

nuclear bombs also work in space, you drop nukes behind you and they can push your craft in your desired vector

once you have reached your desired speed, you don't need to do anything to keep going. you WILL need to expend energy or mass to slow down when you reach your destination

Rocket engines work by pushing out exhaust gases behind them at high speed. It's the same principle as the recoil on a gun.

> I’m basically asking how interstellar travel would even be possible.

Well, it would be very difficult. You would need either to wait a long time or to take a vast amount of fuel with you, bearing in mind that the more fuel you add, the heavier the rocket becomes and the more thrust is required to achieve the same acceleration.

And even if you do have a super-powerful rocket and a magical never-ending supply of fuel, you have to worry about the g forces, since presumably you want to travel to this other star safely and comfortably.

Of course, it's possible that someone will eventually discover some new physics that allows us to get around some of these problems. But without that, it is pretty hard to imagine how interstellar travel could ever become feasible (other than Voyager-style probes that get there very slowly and don't carry humans).

When you burn rocket fuel, it makes a bunch of hot gas. By pushing all that gas out of a (moveable) nozzle in one direction, it pushes the ship in the opposite direction.

ELI5:

It's like blowing up a balloon and then letting go of the neck. It flies off because every action has an opposite reaction. The balloon squeezes the gas out the neck, which pushes the balloon off in the other direction. This would happen in space with no outside air around the balloon too! We just use rockets instead of huge balloons because rocket fuel is a convenient way to bring up a large volume of solid or liquid fuel in a small space and then turn it into gas to shoot out the nozzle as needed.

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Spacecraft and astronauts can move in the vacuum of space by using propulsion from rockets/jets of gas.

Newton's second law says that "for every action, there is an equal and opposite reaction".

Basically, that means if you throw something, no matter what, you'll start moving in the opposite direction.

Rockets achieve this by shooting lots of gas very quickly out their rear. By shooting that gas out the back, it's effectively "pushing" the rocket forward.

And, you are indeed correct that, once you're on a path, the only way to stop moving or change direction is to either use more fuel or to use gravity.

As for interstellar travel, even the nearest star would take on the order of decades to centuries to reach. But there are some really cool ideas out there for how we could try to get there faster.

One of them is a solar sail. It's essentially a really big, thin, and super reflective parachute that catches the momentum of light to accelerate.

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You bring propellant with you and eject it in the opposite direction you want to go. Because momentum is conserved that causes you to move. You don’t need to push off anything.