GRAVITY ASSIST

     The Universe is all of time and space. It includes all the planets, stars and galaxies. The size of the whole Universe is not known and may be either finite or infinite. We belong to the galaxy named Milky Way in which lies our Solar System. To explore the planets in our Solar system, many space crafts have been sent. Many of these spacecrafts use a technique named Gravity Assist. Today, we shall learn about this very simple and clever maneuvering technique.

What is Gravity Assist?

Gravity assist is a technique used to change speed and/or direction of spacecrafts.

Why is it so special?

Simple, it saves fuel and time.

How does it save fuel and time?

This is the real question that needs to be answered. ….Not so fast though, let us slow down and get started with the basics.

SPACE-TIME

It is a 4D model wherein 3D space is interwoven with time into a 4D continuum (Don’t bother to understand, it isn’t that important.)

GRAVITY

Gravity is a property of space-time geometry (I am sure not many people understood that). A simple well known analogy would bring clarity to the above statement. Imagine space to be a rubber sheet. Now, place a massive ball(this ball is equivalent to a star/planet in space) on the rubber sheet. The ball will create a dent on the rubber sheet. Next, place a small ball(equivalent to space craft) anywhere on the rubber sheet. You will observe that the small ball will fall into the dent created by the massive ball. What you just observed is exactly how gravity works. Massive bodies (like stars and planets) create a curvature in space time. Anything near that curvature has a tendency to fall into that curvature (remember this point).
                                                                  

 

Earth also creates a dent on space-time. Now you might ask why moon is not crashing onto our earth (like the analogy explained above where the small ball falls into the dent created by the large ball).The reason is, the moon has a horizontal velocity which prevents it from crashing on to earth. This is quite similar to riding a bike inside a death well. The bike will not fall into the well as long as it maintains a minimum speed.

FUEL CONSTRAINTS

A spacecraft requires fuel to accelerate. To travel longer distances in the desired time frame, the spacecraft has to travel at higher speeds. This increases the fuel requirement and also calls for a powerful launch vehicle (this is a financial constraint).To overcome this problem, gravity assist maneuver is performed about a planet. It involves transferring an infinitesimal amount of a planet’s momentum to the spacecraft ,which is enough to give it a substantial speed boost. It is very clear, that a massive planet will give a higher speed boost to the spacecraft. Jupiter is the most massive of all planets in our Solar system. Hence, Jupiter has been involved in many gravity assists maneuvers. For exploring planets beyond Jupiter, we need not build extra big spacecrafts with larger fuel capacities. All we need to do is, build a spacecraft powerful enough to reach Jupiter and then perform a Gravity assist maneuver to raise its velocity to the required level.

GRAVITY ASSIST

Let us consider a scenario wherein we need to increase the velocity of the spacecraft. For this purpose, the spacecraft is made to encounter the planet from behind.

Observe the figure to the left.

Let the points I,J,K,L,M indicate the path of the planet around the sun(the orbit is elliptical, but it can be considered fairly straight for a small path). Let the points a,b,c,d,e indicate the path of the spacecraft while performing the gravity assist maneuver.

The circles on the planet orbit indicate the curvature in space time caused by the gravity of the planet (similar to the dent created by the massive ball on the rubber sheet) at different instants. As the space craft approaches the moving planet, it falls into the curvature created by the planet. The important thing to be noted here is that the planet itself is moving around the sun which means the curvature in space time which is created by the planet is also moving. Let us plot the movement of the planet and the space craft at different instants of time.

Observe the space-times of both the planet orbit and the space craft path at different instants. At instants “2” and “3” you will see that the space craft falls into the curvature created by the planet and this is where the spacecraft gains speed. This is quite similar to sea surfing where the surfer gains speed by maneuvering around waves.

Now you know how the spacecraft gains speed. But…..Whats the catch? Nothing is free.

Momentum gained by spacecraft=momentum lost by planet.

This is where the energy comes from. If you perform enough (billions) gravity assist maneuvers around a planet , then the planet will lose its momentum and crash into the Sun.

Today,we had a brief look into gravity assist maneuver and studied how it is used to give spacecrafts the necessary speed boost.No complex equations or laws to remember.That is all about Gravity assist (for now). Even rocket science is as simple as sea surfing.But  some questions are still unanswered.

How to decrease velocity of space craft using gravity assist?

Can gravity assist be performed around the sun?

Think over and let me know. Until then, Adios!