Science Puzzle
The Gravity Slingshot
The Voyager probes were launched in 1977 with rockets that could not carry enough fuel to reach the outer planets directly. Instead, engineers used a "gravity slingshot" or gravitational assist manoeuvre.
The spacecraft flies close to a moving planet, curves around it under gravity, and flies away much faster than it arrived. No extra fuel is burned. Where does the extra speed come from?
The Answer
The spacecraft steals momentum from the planet. In the reference frame of the moving planet, the spacecraft approaches, curves around, and leaves at the same speed it arrived, just in a different direction. But the planet is moving through space.
Switching back to the sun's reference frame, the spacecraft has gained the component of the planet's velocity in its direction of travel. The spacecraft speeds up; the planet slows down. The total momentum of the system is conserved.
The planet is so enormously more massive than the spacecraft that its slowdown is completely unmeasurable. But the spacecraft, being tiny, gains a significant velocity boost.
Voyager 1 and 2 used gravitational assists from Jupiter and Saturn to achieve speeds that no rocket of the 1970s could have provided directly. Voyager 1 is now the most distant human-made object, more than 23 billion kilometres from Earth.
The principle: Conservation of momentum in orbital mechanics. A spacecraft gains momentum from a moving planet during a gravitational flyby. The planet slows imperceptibly; the spacecraft gains a significant velocity boost.