“A
man’s desire to explore the universe has no bounds but nature has its way.”
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| Picture Source: Google |
A gravity
assist manoeuvre, also known as “gravitational slingshot” or “swing-by” is
the technique in orbital mechanics to change the speed (either accelerate or
deaccelerate) or trajectory(having either higher energy or lower energy) of the orbit of a spacecraft by the use of the relative movement(with respect to the
Sun) and gravity of
astronomical objects, without the use of any fuel.
The basic principle of the
gravity assist manoeuvre is to either gain momentum from or lose momentum to a planet
(or any other astronomical body) while moving past it, which changes the spaceship's
trajectory and speed relative to the Sun. At the meantime, the speed and
trajectory of the planet are also affected but the effect is insignificant due
to its huge mass as compared to that of the spacecraft. Depending on the
relative direction of motion of the planet and the spacecraft, the spacecraft can
either speed up, slow down, or merely change the direction.
For
missions to fly to the outer planets and beyond, it is necessary to increase a
spacecraft’s speed. The spacecraft’s speed can be increased by flying it with
the movement of a planet, acquiring some of the planet’s orbital energy in the process. To do so the spacecraft must approach the planet in the direction of
the planet’s orbit around the Sun.
However,
if a spacecraft is to be sent to the inner planets, it’s speed must be
decreased. The gravity assist can be employed to slow down the spacecraft when it
approaches a planet in a direction opposite to the planet’s orbit around the
Sun. In this scenario, the spacecraft would lose momentum (orbital energy) to
the planet.
Moreover,
during the exchange of the angular momentum and orbital energy between a planet
and a spacecraft, the orbital period of both the planet and the spacecraft
changes. This fact can be exploited to give new inclinations to the
spacecraft’s orbit. This may cause the change in the orbital period of the
planet but its massiveness overshadows it.
As
Shadan Ardalan, an orbit
determination lead for NASA's Juno navigation team says "It's smart, it's
cheap, and it's phenomenally clever … it's a bit like interplanetary
billiards,", gravity assist indeed is the nature’s way of playing
billiards and at the same time helping man to explore the universe and beyond.
Hence, gravity assist is a great tool for interplanetary manoeuvre as it can be
employed to speed up or speed down a spacecraft or tweak its orbit without
using fuel. And, this has made mankind’s dream of interstellar commute seem not
so far away.
References
- “How Gravity Assists Work: Asteroid Probe's 'Interplanetary Billiards' Flyby Explained”, September 21, 2017, www.space.com
- “Gravity assist”, September 27, 2013, www.planetary.org
- “Gravity assists explained simply-How the Voyagers escaped the Solar System”, Oct 27, 2017, www.medium.com
- “How Does a Martian-Style Gravity Assist Actually Work?”, March 01, 2019, www.wired.com
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