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I was reading Wikipedia which stated

....Voyager 1's current relative velocity is 17.062 km/s, or 61,452 kilometres per hour (38,185 mph).....

It travels away from sun. So sun's gravity must slow it down. What propels Voyager to that speed?

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up vote 8 down vote accepted

Rocket fuels initially, followed by a series of gravitational assists (slingshots): http://en.wikipedia.org/wiki/Gravity_assist The linked article mentions Voyager 1 mission as an example.

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Gravitational force decreases with distance squared. So the deceleration due to the sun is negligible at that distance.

Acceleration due to gravity is given by $\frac{GM}{r^2}$, where $G$ is the gravitational constant $6.67\times 10^{-11} \mathrm{m}^3 \mathrm{kg}^{-1} \mathrm{s}^{-2}$. The mass of sun is $2\times 10^{30} \mathrm{kg}$ and the distance is $2\times 10^{13} \mathrm{m}$.

Plugging those values in gives a slowing down of $3 \times10^{-7} \mathrm{m} \mathrm{s}^{-2}$, or losing $300$ nanometers per second every second.

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i guess some other body in the space is pulling it towards itself and because it has been travelling since very long time, it could have achieved that acceleration. it's just a hypothesis of mine

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