• Question: How do spacecrafts gain power from orbiting around a planet to catapult off? Wouldn't leaving the orbit consume a lot of energy?

    Asked by anon-244767 on 30 Apr 2020.
    • Photo: Dipendra Mistry

      Dipendra Mistry answered on 30 Apr 2020:

      Hi cBond, what a great question. So yes, as the spacecraft leaves the earth’s orbit it would consume a lot of energy to overcome the force of gravity, hence all the rocket fuel required for a typical shuttle launch. But in the case of a gravity assist (catapult) you are not actually leaving the planet’s gravitational field straight away. But instead you are using it to accelerate (increasing speed) by going in a circle around the planet. Once you have reached the desired speed you can as you say catapult off. A catapult can also be used to change the direction of a spacecraft and also slow it down, by going in a different direction around a planet.

      Hope this helps.
      Dipendra

    • Photo: Roan Haggar

      Roan Haggar answered on 30 Apr 2020:

      That’s a good question — you’re right that it doesn’t appear to make sense!
      –
      This process is usually called a ‘gravity assist’, and it allows spacecraft to change their speed / direction by passing nearby to a planet. The reason why this works is that the planets are orbiting around the Sun.
      –
      If a spacecraft approaches a planet from ‘behind’ (so that the spacecraft and planet are moving in the same direction), then it will be pulled along by the planet’s gravity as the planet orbits, kind of like a trailer being pulled along by a car. This causes the spacecraft to accelerate, until it catches up with the planet. By this point the speed of the spacecraft has increased, so it can easily be flung away from the planet rather than going into orbit around it.
      –
      In doing this, the spacecraft is taking a bit of energy from the planet, so this causes the spacecraft to speed up, but it actually decreases the momentum of this planet’s orbit! However, because planets are so much larger, this is a tiny effect, far smaller than we could measure. Hope this helps!

    • Photo: Susan Cartwright

      Susan Cartwright answered on 30 Apr 2020:

      This is a very good question! The first thing to say is that they don’t gain speed (not “power”) from ORBITING a planet, they gain speed from PASSING CLOSE TO a planet (but not entering orbit). However, your question still stands: you would expect the spacecraft to gain kinetic energy when approaching the planet (because it is losing gravitational potential energy) but lose it again when leaving the planet (because it is lost gravitational potential energy), and wind up with the same amount of energy it had when it started (just as a perfectly elastic ball would bounce back to the same height from which it was originally dropped). And this would be true if the planet were stationary. The critical point is that the planet is NOT stationary: it is orbiting the Sun. Therefore, the kinetic energy is conserved in the reference frame of the planet, but not in the reference frame of the Sun. This is like bouncing a ball off a moving wall – a cricket bat, for example. The cricketer moves his bat TOWARDS the ball when he hits it, and the result is that the ball bounces off with much more speed (relative to the fielders) than it had when it hit the bat. Similarly, if the spacecraft approaches the planet from the opposite direction to the planet’s direction of motion, then from the planet’s point of view it is moving faster, and has higher kinetic energy, than it does from the Sun’s point of view. It swings round the planet and leaves in the same direction as the planet is going: from the planet’s point of view it has the same kinetic energy that it had when it arrived, but from teh Sun’s point of view it’s going faster. This is not violating any conservation laws: the momentum and energy gained by the spacecraft are actually lost by the panet, but as the planet is so much more massive this has no measurable effect.

      There is an animation of this at

    • Photo: anon

      anon answered on 30 Apr 2020:

      As the spacecraft approaches the planet, it is pulled in and gains velocity. But when the spacecraft is leaving the planet, it is pulled back and loses velocity, just as you say.
      The net gain in velocity is due to the planet moving in its orbit. So to speed up, we direct the spacecraft so that they fly “with” the planet as it orbits the sun, and to slow down, we direct the spacecraft so they fly “against” the planet’s orbital motion.

      I think “slingshot” is a confusing terminology, because we can’t straightforward connect the gravity assist with a literal stone shot from a sling. A much better analogy is hitting the ball with a tennis racket. If you move the racket forward as you hit the ball (so, in the direction you are sending the ball) – the ball will have more velocity than before you hit it. If you move the racket backward (against the direction you send the ball) – it will lose velocity.

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