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The Voyager 1 spacecraft is the most distant man made object from earth, having left the solar system in 2004. It continues on outwards at a constant speed without propulsion. Why does it just keep going? It is because there is nothing to stop its motion. It is completely isolated from all other objects. Space is empty so there is no air resistance. The spacecraft is a long way from any other object so gravity is (almost) zero. Hence it just keeps moving at a constant speed in an unchanging direction. Likewise, an ice skater, once in motion, continues moving at a constant speed. Here, again, there is nothing to change the skater's motion. The friction between the skates and the ice is (almost) zero. Air resistance is also (almost) negligible.