Comets return to the solar system because they are in orbit around the Sun!
Due to the way orbital mechanics work, the closer a comet gets to the Sun, the more speed it gains. As a result, it gets "flung" outward, gradually losing speed.
This continues until it reaches the maximum possible distance, after which it is once again pulled back by the gravitational force of the Sun and returns!
Let's take Halley's Comet as an example.
During its closest approach to the Sun (perihelion), it travels at a quite high speed of about 55 km/s. However, 40 years later, when it reaches its farthest point (aphelion), it is only moving at a speed of 1 km/s.
At that point, it doesn't have enough energy to "escape" the gravitational attraction, so it returns back.
The Voyager probes are currently traveling at a constant speed of about 17 km/s, and with these speeds, they are well above their escape velocity from the solar system, considering the distance they are from the Sun.
Unless they experience a change in speed or are influenced by external forces that alter their direction, they will never return back!
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