How Gravity Assists Help Spacecrafts Reach their Destinations?

by Carson
How do gravity assists work?

How can a spacecraft get enough energy to explore the far regions of the Solar System? Other than having full of rocket fuel, space probes often need help from other planets. In fact, the furthest spacecrafts from Earth, Voyager 1 and 2, gets to the location today due to gravity assists. How do they work? We’ll talk about that in this article!

Gravity Assists Adjust Orbits

What are gravity assists? In short, they’re energy exchanges that are assisted by gravity. How about a more detailed explanation? We’ll talk about it in the next paragraph!

When two objects intercept in orbit, one of the objects either gives out or acquires energy from the other object. For instance, when a spacecraft encounters Jupiter, either some of the probe’s angular momentum was lost and the spacecraft moves to a lower orbit, or it gains some energy and move to a higher orbit.

But how to determine whether an object loses or gains energy? It’s by looking at who’s “in front”. The one who is in front of the another object provides its angular momentum to another one. Therefore, if a spacecraft wants to escape heliocentric orbit, it would have to go behind a gas giant planet such as Jupiter.

All in all, gravity assists adjust orbits of all objects involved. In fact, they occur as orbits of asteroids and planets intercept, too.

How Does it Help Spacecrafts?

After talking about the physics behind gravity assists, we should now discuss how do they help spacecrafts go to their destinations.

As mentioned above, gravity assists can change orbits. They are useful when we have to give a natural tweak to the spacecraft’s orbit. This action saves a lot of fuel, and can possibly lengthen the probe’s life.

Other than small tweaks that help the spacecraft aim at the target, they can actually do much more. For example, some probes need to escape the orbit of the Solar System to explore interstellar space. Therefore, they need some gravity assists from gas giants to obtain enough speed to reach the escape velocity of the Solar System at that distance from the Sun.


How about proofs and examples of them? Well, there are some, too!

An obvious one involves two space probes — Voyager 1 and 2. Voyager 1 flew by Jupiter and Saturn, while Voyager 2 had gravity assists on all outer planets. Thanks to those flybys, they successfully left the heliosphere in 2012 and 2018 respectively.

The trajectory of Voyager 1 and 2
Image Credit: NASA1

The Cassini spacecraft which headed to Saturn also used this technique. It arrived near Venus twice, Earth once, and Jupiter once. Moreover, it altered its orbit around Saturn by flying by Saturn’s moons, including Titan.

The trajectory of Cassini
Image Credit: NASA4


We talked about how do gravity assists work, how does that help spacecrafts, and two examples of this phenomenon. However, this article is not enough. So, check out in the references below to learn more.

References and Credits

  1. (n.d.). Basics of Space Flight – Solar System Exploration: NASA Science. Retrieved January 31, 2021, from
  2. Ian O’Neill. (2017, September 21). How Gravity Assists Work: Asteroid Probe’s ‘Interplanetary Billiards’ Flyby Explained | Space. Retrieved January 31, 2021, from
  3. Ota Lutz. (2018, December 18). Then There Were Two: Voyager 2 Reaches Interstellar Space – Teachable Moments | NASA/JPL Edu. Retrieved January 31, 2021, from
  4. (2019, March 14). Cassini Trajectory | NASA Solar System Exploration. Retrieved January 31, 2021, from

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