Author Topic: Why did Cassini use Venus for its gravity assists?  (Read 871 times)

Offline CitizenVeen

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Hey there space enthousiast,

Hopefully I am posting in the right subforum, my apologies if it isn't.

Ive got an orbital mechanics question:
Cassini used the gravity assist of 2 Venus flyby's to get the right orbit to have an encounter with Saturn.

If I understand it correctly Cassini Hohman transferd to Venus, so decreasing its orbit (relative to the sun), afterwards increasing it's ap twice on the Venus flyby's. (Is this correct?).

My question is: how come Venus was the most efficient planet to use for its gravity assist? First decreasing your speed relative to the sun (lowering ep), to later increase it, seems counter intuative. Why is it not more efficient to use mars for example? So far I havent been able to clear this up with my limited googling skills. Can someone explain this to me?

Would be very much appreciated.

Thanks!











Offline Phil Stooke

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #1 on: 11/12/2017 01:12 AM »
It used Earth too.  Mars is too small to help much.  Venus and Earth gave the push necessary to get to Jupiter and then to Saturn. 

Offline Augustus_

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #2 on: 11/12/2017 01:14 AM »
Mars is too small to give any kind of meaningful gravity assist, and Venus is so close to Earth that the drop in velocity relative to the Sun required is minimal compared to the gains from the gravity assists.

Online IanThePineapple

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #3 on: 11/12/2017 01:14 AM »
Venus is commonly used for gravity assists, along with Earth and sometimes Jupiter, Mars and Mercury. Venus and Earth are great for gravity assists because orbits near them don't take a long time, and they're big enough to make a big difference to the spacecraft.
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Offline Bob Shaw

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #4 on: 11/12/2017 01:29 AM »
Mars is too small to give any kind of meaningful gravity assist, and Venus is so close to Earth that the drop in velocity relative to the Sun required is minimal compared to the gains from the gravity assists.

Mars is still good for orbit-shaping, just not for velocity changes - as Rosetta demonstrated, and as was considered for Galileo at one point. Mars is also a great resource for asteroid belt missions.

Online ugordan

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #5 on: 11/12/2017 10:44 AM »
Technically, they could have designed a trajectory that doesn't take it into the inner solar system, but that would have increased flight time if you only limited yourself to injection orbits with semimajor axes larger than Earth's. You'd still need probably 2 Earth gravity assists to eject toward the outer solar system and by that time (higher orbits take more time to complete) Jupiter would probably have been too far along its orbit to allow its own gravity assist toward Saturn.

The Cassini trajectory was actually quite remarkable in that it took advantage of a Venus-Earth alignment so that it took less than 2 months from the 2nd Venus flyby to the Earth flyby. Such VVEGA gravity assists are I believe more rare than Galileo-like VEEGA trajectories.

Venus is more desirable for gravity assists than Mars  (if you can cope with higher thermal environment) because it's significantly more massive AND its heliocentric velocity is higher so it gives you more "bang per buck" in terms of available heliocentric delta-V than Mars.
« Last Edit: 11/12/2017 10:46 AM by ugordan »

Offline dchill

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #6 on: 11/13/2017 01:49 PM »
The Cassini trajectory was actually quite remarkable in that it took advantage of a Venus-Earth alignment so that it took less than 2 months from the 2nd Venus flyby to the Earth flyby. Such VVEGA gravity assists are I believe more rare than Galileo-like VEEGA trajectories.

My recollection from being the Centaur flight software lead at the time was that it was a VVEJGA trajectory.  Did you mean to leave out the Jupiter leg in your discussion?  (Once it separated from Centaur, and Centaur had to do a blow-down maneuver to avoid contaminating Venus my job was done, so I can't tell you what the benefits were for including Jupiter.)

Online ugordan

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Re: Why did Cassini use Venus for its gravity assists?
« Reply #7 on: 11/13/2017 02:43 PM »
Did you mean to leave out the Jupiter leg in your discussion? 

Yes, because the discussion above focused on the inner solar system gravity assists.

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