In movies and on TV, science fiction spacecraft are often shown firing their engines all of the time and travelling in whatever direction they please. They can do this because science fiction spacecraft get to ignore things like fuel mass. In real life, a spacecraft sent out of Earth’s orbit has very little capability and capacity to create thrust. The spacecraft is too small to carry large propellant tanks. So, in order to ensure the spacecraft can reach its destination, we have to have the upper stage of the rocket that launched the spacecraft propel the spacecraft to such a velocity that even with the constant depleting force of gravity, the spacecraft will still keep moving outwards until it reaches its target.
When Voyager left Earth’s orbit, it entered solar orbit with a total heliocentric velocity (sun centered speed) of the orbital velocity of Earth (about 30 km/s) plus an additional velocity (about 6 km/s) provided by the Centaur upper stage of the launch vehicle. So, Voyager began its journey, traveling in a curved path about the Sun, constantly moving outwards, climbing out of the Sun’s gravity well. That gravitational acceleration from the Sun constantly decelerated Voyager by a small amount every second. By the time its path intersected the orbit of Jupiter, Voyager had lost about 26 km/s of that speed, and was traveling at around 10 km/s.
The smart guys at NASA designed the trajectory such that as they passed Jupiter, they gained some speed by being dragged along by Jupiter. This is called a gravity assist. Voyager 2 picked up about 18 km/s of velocity from that Jupiter gravity assist. By the time it reached Saturn, Voyager 2 has dropped back down to a little over 16 km/s. It received another gravity assist from Saturn, climbing back up to around 34 km/s. It did this again at Uranus and at Neptune. When it left Neptune, it was traveling at just shy of 29 km/s. Voyager 1 had a different trajectory and did not rendezvous with Uranus or Neptune, it moved outwards at a faster pace.
That was 29 years ago. Every day, since then, the Voyagers have continued traveling in curved paths that takes them ever farther from the Sun and every day, since then, they have lost a little energy and slowed, as the Sun continues to pull on them. But, as they get farther away, that pull from the Sun gets a little weaker and weaker, reducing the deceleration experienced by the Voyagers. Today, Voyager 2 is traveling at about 15.4 km/s and Voyager 1 is traveling at about 17 km/s.
Here’s a diagram showing the velocity change of Voyager 2, since launch. Imagine the Voyager one as similar, but without the Uranus and Neptune diversions.
The yellow line shows the velocity of Voyager. The green line shows the solar system escape velocity at that distance. Since Voyager is traveling faster than escape velocity, the Sun will never cause Voyager to drop to zero and fall back into the solar system.
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