We began this module by discussing that, even today, Voyager 1 is travelling at more than 60,000 km/h, without using any fuel. The concept of Inertia explains how this is possible.
As Voyager 1 is in mostly empty space, there is very little force acting on it. It was launched at a high speed, and it initially burnt a lot of fuel to increase its speed. When it passed a few planets, their gravity increased its speed further. But, after that, Voyager 1 has experienced very little external force. So, it can continue to travel at such high speeds until it collides with another object, or experiences the gravitational force of some star/planet.
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Check out the next module in this series! Newton's Laws Of Motion 2