Because as it travels, it is working against the gravitational field of the Sun. This causes its speed to constantly decrease as it gets farther away. If it did not have escape velocity from our solar system, it would eventually come to a stop (meaning its radial velocity would be zero), and then begin to fall back into the solar system.

In 1998, the speed of the Voyager I was 39,000 miles per hour or 3.5 AU per year. In 2003 the outward speed of Voyager 1 was about 38,500 miles per hour.

Meanwhile, at these distances the solar wind has slowed from 700,000 miles per hour to just under 100,000 miles per hour. In the case of the solar wind, it is decellerating because it is approaching the heliopause region where it collides with the interstellar medium.

Return to Ask the Astronomer