Increasing speed with slingshot works because you leave vicinity of the planet in the same direction planet travels. Basically this allows you to add planet’s s

Increasing speed with slingshot works because you leave vicinity of the planet in the same direction planet travels. Basically this allows you to add planet’s speed to your own. Within solar system you cannot increase speed by slingshotting around the sun. Total energy within system stays the same, some energy is transferred from the planet to the spacecraft.