Why use Earth as a gravity assist?

[JayFlynn]

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

 

[tony873004]

Earth is 10x more massive than Mars.

 

[JayFlynn]

so why can't you just do a really close slingshot around Mars to make up for it's small mass?

 

[Janus]

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.

 

[JayFlynn]

So, it would be more efficient to do a Mars slingshot but they're aligned so infrequently that it's not worth waiting?

 

[Janus]

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.

 

[JayFlynn]

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!

 

[Arkalius]

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.

 

[tony873004]

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.