HSC Physics: The Slingshot Effect

The slingshot effect (also known as ‘gravity assist’) is one of the most misunderstood topics in HSC Physics. I hope this explanation will give you a strong grasp of the topic and the ability to answer relevant questions in your exams.

We should always begin by reading the relevant dot point:

Identify that a slingshot effect can be provided by planets for space probes.

What is the slingshot effect?

The slingshot effect is used by a spacecraft to increase its velocity. To do this, the spacecraft travels close to another moving object such as a planet or moon. As the craft moves away from the planet, it is traveling faster than when it started. However, it is vitally important to realize that its speed relative to the planet is unchanged, only its speed relative to the sun (and other planets orbiting the sun). Regardless, this is a useful maneuver which is frequently utilized by probes moving to the outer planets (or Pluto).

Motion relative to the planet


The diagram above shows the velocity of a craft relative to the planet it is using for the slingshot effect. As we know from our understanding of projectile motion, this flight will be symmetrical in the shape of a hyperbola, which means the magnitude of the velocity at A and B will be equal (although the direction is clearly different).

Motion relative to the sun

To appreciate the usefulness of gravity assist, we must remember that the planet itself is moving with great velocity relative to the sun. When we add this to the diagram, it becomes clear why the probe gains velocity. As it comes close to the planet, it is dragged in the direction of that planet’s orbit by the force of gravity and gains this velocity relative to the sun. This is seen in the diagram below – the velocity relative to the sun at point B is the vector sum of the velocity relative to the planet and the orbital velocity of that planet.


Where does the energy come from?

The probe has gained velocity relative to the sun, so its kinetic energy must have increased. However, the law of conservation of energy means that this energy must have been transferred from somewhere else. In reality, it comes from the kinetic energy of the planet itself. This means that every time gravity assist is used, the orbit of the planet is slowed slightly. But don’t worry! Because of the huge difference in mass, it takes only a very small reduction in orbital velocity to greatly increase the velocity of a passing probe.

Can you simplify that into a single gif?

The best way to understand the slingshot effect is to visualize it, which we can do quite easily thanks to this gif from The Planetary Society:


The slingshot effect in the HSC

The HSC dot point only uses the verb ‘identify’, which suggests that you will not be assessed on the topic in great detail. The key points which are frequently assessed are as follows:

  • The slingshot effect is used to increase velocity relative to the sun and other planets.
  • Velocity relative to the planet used is not increased.
  • The gain in kinetic energy of the probe is due to a corresponding loss of kinetic energy by the planet.

In higher level questions, you may be required to relate this concept to the logistics of long-distance space flight and escape velocity. In this case, the key point to remember is that a slight reduction in the necessary launch velocity equates to an exponential reduction in costs due to lower fuel load. Any mechanism which can be utilized to improve efficiency is highly advantageous.

Although it is less relevant to the HSC, I like to draw attention to the most impressive (in my opinion) use of gravity assist – that of the International Cometary Explorer. In order to observe two special comets as they passed close to the earth, a satellite was retasked from an existing orbit between the Earth and Sun. As the probe carried very little fuel, an intricate maneuver involving 5 separate lunar gravity assists was used. The path of the probe is shown below:


I hope that you found this article useful! If you did, you might also like our recent article on Gravitational Potential Energy. If you have any further questions or you’d like to suggest a topic for the future, comment below!