What happens when an astronaut throws something in outer space?

Answer 1

By newton's Third law of Motion, every action has an equal and opposite reaction, the astronaut would be thrown back. The tennis ball would however keep on going in the direction until another force acted upon it(planet's gravity, meteor, etc.) by Newton's Third Law

Answer 2

Assuming there were no large celestial bodies, like the earth or sun, near by to capture the object in its gravitational field, the lack of friction in the vacuum of space would allow any object thrown to travel at the same speed indefinitely, or until it came in contact with another object.

Consequently, if an astronaut threw an object in space without standing on something like a space station or space ship, according to Newton's third law, he would tumble, or spin in a circle because of the equal, opposite force exerted upon him by the thrown object, as well as the rotation of the astronauts arm used to throw the object.