Voyager 1 and 2

It always seemed impossible for spacecraft to escape Sun’s gravity and explore beyond the solar system. Travelling vast distances in space is limited by some factors like fuel, time and money. All of us know that space flights are highly expensive and also take a lot of time. The most important factor that limits space travel is fuel or propellant. One might wonder more fuel will let the spacecraft fly further and faster. However, it is not that simple. Adding fuel also means adding mass to the spacecraft This means more fuel needs to be added to the rocket to launch the now-heavier spacecraft. Since including extra fuel also increases the rocket’s mass, more fuel is needed to carry that fuel, and so on. Thus, adding fuel is not a proper solution or in other words we are limited to the amount of fuel to use.

Also check out this post to learn about how an aircraft flies

Introduction to gravity assist:

When traveling among the planets, it’s a good idea to minimize the propellant mass needed by your spacecraft and its launch vehicle. That way, such a flight is possible with current launch capabilities, and costs will not be prohibitive. The amount of propellant needed depends largely on what route you choose. Trajectories that by their nature need a minimum of propellant are therefore of great interest.

But how can we achieve this? The answer to this question lies within the nature. We can take help of gravity to alter the path and speed of a spacecraft, typically to save propellant and reduce expense. This is called gravity assist. This idea was first proposed by astronomical pioneers Yuri Kondratyuk and Friedrich Zander in their papers published in 1925 and 1938, respectively. It was first implemented in 1959 by the Soviet probe Luna 3 to photograph the far side of Earth’s moon. Since then, notable successful implementations include space probes Mariner 10, Voyager 1 and 2, and Galileo.

Voyager 1-gravity assist
voyager 1

How does Gravity Assist work?

Simply put, a gravity assist around a planet changes a spacecraft’s velocity (relative to the Sun) by entering and leaving the gravitational sphere of influence of a planet. When the spacecraft approaches the planet its speed increases and while it leaves the planet its speed decreases (which is approximately same). However, this maneuver is also affected by the orbital motion of the planet around the sun. So, the spacecraft can move in the direction of planetary motion to accelerate and in the opposite direction to slow down. This happens because the spacecraft acquires planet’s orbital energy while it flies with the movement of the planet and transfers some of its to the planet while flying in opposite direction.

gravity assist-how does it work-space travel

It should be noted that in both types of maneuver the energy transfer compared to the planet’s total orbital energy will be negligible and The sum of the kinetic energies of both bodies remains constant (see elastic collision). A slingshot maneuver can therefore be used to change the spaceship’s trajectory and speed relative to the Sun.

path of voyager 1 and voyager 2- gravity assist
Flight path of voyager 1 and voyager 2

Yuri Kondratyuk also suggested that a spacecraft traveling between two planets could be accelerated at the beginning and end of its trajectory by using the gravity of the two planets’ moons. Also even higher speed can be achieved by utilizing the rocket burn. When there is need of higher speed than what gained from gravity assist, then the most economical way is to do rocket burn near the periapsis (closest approach). A given rocket burn always provides the same change in velocity (Δv), but the change in kinetic energy is proportional to the vehicle’s velocity at the time of the burn. So to get the most kinetic energy from the burn, the burn must occur at the vehicle’s maximum velocity, at periapsis. This additional maneuver is called Oberth maneuver which is the modification of gravity assist.


Gravity assist does have some limitations. The planets are hardly at the desired position for the gravity assist to work and the spacecraft to reach its destination. For example, the Voyager missions which started in the late 1970s were made possible by the “Grand Tour” alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century. That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits. Similarly, interplanetary slingshots using the Sun itself are not possible because the Sun is at rest relative to the Solar System as a whole. However, thrusting when near the Sun has the same effect as the powered slingshot described as the Oberth effect. This has the potential to magnify a spacecraft’s thrusting power enormously, but is limited by the spacecraft’s ability to resist the heat.


Despite having some limitations, gravity is indeed one of the greatest discoveries in the field of space travel. To think that nature is offering free help to humans to explore the universe even further, it really seems as if nature wants to be explored. Gravity assist took space travel beyond its limit. Based on gravity assist, other manuevers like Oberth manuever, aerogravity effect (theoretical) have been developed or being developed to mitigate its limitations. Hopefully, we are going to travel even further and further into the deep space.

By Nishchal Poudel

Nishchal is currently studying Bachelor in Aerospace Engineering at IOE Pulchowk Campus, Nepal.

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