J Rekker of Macandrew Bay, asks :-

When sending spacecraft to distant planets, scientists often use the slingshot effect to speed up a spacecraft by having it pivot around a closer planet. But wouldn't the spacecraft slow down again as it goes away from the slingshot planet?

The late Bill Pickering, a New Zealander who, as director of Cal Tech's Jet Propulsion Laboratory, launched the USA's first satellite and led that country's exploration of the planets, responded.

If a spacecraft flies near a planet the force of gravity of the planet certainly affects its motion. Assuming that there is only the one force acting on the spacecraft. the path it will take is an ellipse, a parabola or an hyperbola depending on its speed and direction of motion. If it does not hit the planet, it will leave the vicinity of the planet at its original speed with only the direction of travel changed.

However in the actual case the sun's gravity is acting on both the planet and the spacecraft. The path of the spacecraft, as seen from the sun is quite different from the simple case above.

As seen from the sun, the spacecraft is travelling at a different speed than the planet. After the encounter, the spacecraft speed may be increased or decreased depending on the way it approached the planet. Thus Mariner X which was flying to the planet Mercury, used a close encounter with Venus to slow it down so that it would fly closer to the sun. Gallileo flew by Venus to speed it up to enable it to travel farther from the sun on its way to Jupiter. Actually it also required two close encounters with earth to have enough speed to get to Jupiter.