Typically, when guiding a spacecraft on its trajectory and modifying its path, acceleration, and speed, a propellant is used. However, it is costly, and therefore, to reduce the expenses of extensive space programs, aerospace engineers have developed an alternative method of spacecraft maneuvering. Commonly known as a gravity assist, or sometimes as a gravitational slingshot, it is a technique that consists of harnessing the gravitational pull of planets in the vicinity of the spacecraft’s trajectory to assist its motion. More specifically, during the probe’s flybys of planets, their trajectory is tailored to momentarily be trapped in orbit around it and leave it shortly after on a redirected path and at a modified speed.

Technicalities and purpose of gravity assist

Like many other physical phenomena, the foundation of gravity assist relies on the concept of conservation of energy where the initial and final sums of the kinetic energy of the planet and the spacecraft will remain constant like in elastic collisions. Indeed, both the spacecraft and the planet have certain amounts of energy. Therefore, this slingshot effect relies on the transfer of energy either to the planet or to the spacecraft, depending on if it is aimed at a decreasing or increasing velocity, by making both bodies interact for a short period of time.

Gravity assist explanation
An illustration of gravity assist. Concept by Charles Kohlhase, based on artwork by Gary Hovland | Image: NASA

Therefore, the first element that affects the spacecraft is the gravitational field of the planet, which depends on its mass. As the spacecraft will approach the planet, it will feel a stronger force and experience an increased speed that will decrease again shortly after escaping the gravitational pull. However, if this were the sole factor, the spacecraft would have gained a net amount of energy of zero. Still, we must not forget that the planet, just like the spacecraft, is also moving (in most cases around the sun), affecting the spacecraft.

As such, to gain energy and thus velocity, the spacecraft will fly into the gravitational speed in the direction of movement, acquiring in the process some of its orbital energy. On the other hand, if the spacecraft wishes to see its velocity decrease, it will approach the planet opposite its movement. On the whole, gravity assist is possible because the energy transferred to or from the planet is negligible compared to its total energy.

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One of the purposes of such procedures, other than sparing fuel and thus reducing costs, is to maximize the energy efficiency of the reaction engines. Indeed, the Oberth Effect states that engines generate greater change in mechanical energy at higher speeds than when operating at lower speeds. Therefore, by falling in the gravitational well of a planet from the direction of its movement, the spacecraft can gain energy and speed and thus operate at higher speeds and greater efficiency.

Gravity Assist: A Clever Technique To Speed Up A Spacecraft Using A Planet. 1
Diagram illustrating the two important components of Juno’s gravity assist. Earth’s point of view is represented on the left, and the sun’s point of view is represented on the right | Image: NASA/JPL-Caltech.

Look at the illustration above. From the planet’s (it could be any planet) point of view (left), the approaching spacecraft is like a cyclist heading downhill into a perfectly symmetrical valley – the speed increases as the cyclist approaches the planet (or goes down into the valley). As the cyclist emerges from the valley, the speed is reduced (because they are going back up the valley hill).  Their speed (relative to the valley) is the same as it was before going into the valley, but the direction has changed. The spacecraft’s direction, but not speed relative to the planet, will similarly be altered by the gravity assist.

But from the Sun’s point of view (Sun’s frame of reference), the planet is a very massive object moving around the Sun. Its orbital speed around the Sun can be considered as momentum, which is something that any moving massive object carries (for example, a speeding truck has a lot of momentum). When a spacecraft flies closely past the moving planet, the planet’s tug on the spacecraft has a profound effect. The planet loses an immeasurable amount of momentum to the spacecraft. Because it has much less mass compared to the planet, this small amount of momentum results in a relatively large boost to the spacecraft.

The history and uses of gravity assist

In 1959, when the Soviets launched their third space probe to the moon’s neighborhood – Luna 3 – to photograph its far side, engineers inaugurated the first use of gravity assist. Indeed to send the probe there and back, the trajectory was designed to change orbit and orbital plane when interacting with the moon’s gravitational field. Indeed, Luna 3 entered the moon’s gravitational well from south to north, using the pull as a turnaround maneuver to reach Soviet ground once again.

Gravity Assist: A Clever Technique To Speed Up A Spacecraft Using A Planet. 2

Subsequently, the notable Voyager 1 space probe acquired a considerable fraction of its energy to reach far space from such slingshot maneuvers around Jupiter and Saturn. The most impressive instance of the use of gravity assist was possibly the Cassini-Huygens spacecraft which had to increase its transit time to up to six years passing by Venus twice, then the Earth, and finally Jupiter before reaching the target planet – Saturn.

Indeed, such manipulations of trajectory allowed for prowesses that would not have been enabled by propulsion capacities of even the most powerful engines. In effect, the detour allowed the spacecraft to reduce its extra velocity to a mere 2 kilometers per second in contrast to an exorbitant 15.7 kilometers per second otherwise. Therefore, Gravity assist is one of the many creative solutions humankind has developed to bypass and overcome its technological shortcomings to harness the properties of the Solar System we evolve in.

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