Why use Earth as a gravity assist?

In summary: The reason for this is that the Earth's gravitational field is very strong at that distance, so any change in velocity will cause the spacecraft to swing around the Earth's gravitational field.
  • #1
JayFlynn
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I am doing research for my aerospace engineering dissertation to plan a mission to a Jupiter trojan asteroid. I was researching the type of outgoing trajectories used in missions such as JUNO, Galileo and Europa and noticed they all use Earth as a gravity assist in getting to deep space. Surely be more efficient and save time if a Mars slingshot was used rather than performing deep space manoeuvres way beyond Mars' orbit just to come back to Earth for a slingshot? Can anyone explain the mechanics of why NASA do it like this?
https://upload.wikimedia.org/wikipe...x-Juno's_interplanetary_trajectory_en.svg.png
 
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  • #2
Earth is 10x more massive than Mars.
 
  • #3
so why can't you just do a really close slingshot around Mars to make up for it's small mass?
 
  • #4
With a Mar's flyby you would have to wait until all three bodies were in their proper relative positions to make it work. With an Earth fly-by, only Earth and Jupiter have to have the proper relative positions.
 
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  • #5
So, it would be more efficient to do a Mars slingshot but they're aligned so infrequently that it's not worth waiting?
 
  • #6
JayFlynn said:
So, it would be more efficient to do a Mars slingshot but they're aligned so infrequently that it's not worth waiting?
I wouldn't go that far. There are a lot of factors that going into determining what the optimal flight path is. For example, not only is Earth 10 times more massive than Mars, velocity changes made when the probe is at or near its perihelion are more effective than those made at other points of its orbit if you are trying to increase the aphelion.
 
  • #7
Janus said:
I wouldn't go that far. There are a lot of factors that going into determining what the optimal flight path is. For example, not only is Earth 10 times more massive than Mars, velocity changes made when the probe is at or near its perihelion are more effective than those made at other points of its orbit if you are trying to increase the aphelion.

Cheers Janus. This is really helpful!
 
  • #8
The two things affecting the value of a body for a slingshot are its mass and its velocity relative to the central body. Earth is not only more massive than Mars, but it's moving faster as well. Mars is, generally speaking, not a great slingshot target.
 
  • #9
Earth is also easy to get to. Any object launched from Earth is automatically on an Earth-crossing trajectory. Giving your spacecraft an eccentric solar orbit with an semi-major axis of 1 AU will return you to Earth in 1 year. Only minor deep space burns are required to fine-tune the slingshot.
 

1. Why is Earth used as a gravity assist?

Earth is used as a gravity assist because it is the most readily available and convenient source of gravitational energy for spacecraft. Its proximity to other planets and its well-known orbital patterns make it an ideal choice for navigating through the solar system.

2. How does a gravity assist work?

A gravity assist works by using the gravitational pull of a planet to change the speed and direction of a spacecraft. The spacecraft approaches the planet and is pulled by its gravity, which increases its speed. As it moves away from the planet, the spacecraft's speed decreases, but its direction changes. This allows the spacecraft to conserve fuel and travel farther distances in the solar system.

3. What are the benefits of using Earth as a gravity assist?

Using Earth as a gravity assist offers several benefits for spacecraft. It allows for faster and more efficient travel through the solar system, as the spacecraft can take advantage of Earth's orbital speed and gravitational pull. Additionally, it helps save fuel and extends the lifespan of the spacecraft.

4. Are there any risks associated with using Earth as a gravity assist?

While there are some risks associated with using Earth as a gravity assist, they are minimal. The most common risk is the possibility of a spacecraft colliding with Earth, but this is highly unlikely due to precise calculations and control by ground teams. There is also a risk of changes in the spacecraft's trajectory due to the gravitational pull of other objects in the solar system, but these can be accounted for and adjusted for in the spacecraft's flight path.

5. Can any spacecraft use Earth as a gravity assist?

In theory, any spacecraft can use Earth as a gravity assist. However, certain factors such as the spacecraft's size, speed, and trajectory must be taken into consideration to ensure a successful gravity assist maneuver. Additionally, the spacecraft must be equipped with the necessary technology and instruments to accurately navigate and adjust its flight path during the gravity assist.

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