What is slowing down space travel?

For example, as of December 2016, Voyager 1 was travelling at about 17,000 m/s away from the Sun. However, due to the gravity of the sun, by the time it is one light year away from the sun, it will have slowed down by about 400 m/s to 16,600 m/s. It will take about 17,800 years for Voyager 1 to travel that one light year. read more

Gravitational time dilation is at play e.g. for ISS astronauts. While the astronauts' relative velocity slows down their time, the reduced gravitational influence at their location speeds it up, although at a lesser degree. read more

Pioneer 10 is slowing down faster than expected simply based on the gravitational attraction of the sun and planets. The effect is tiny - only about one part in 10bn of that due to the Earth's gravity acting on you right now - but it is definitely there. read more