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Multiple flyby or gravity assist - It seems so easy.....

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killium

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We all know about the gravity assist flyby or simple observation flyby. It is relativly easy to send a probe on a trajectory that will bring it close to some object and do the manoeuver. How about multiple flyby ?<br /><br />For example, a mission recently did a multiple gravity assist to reach its target. It did it with Venus, then the Earth, then again Venus (or the other way around ? anyway...) and then Jupiter. What are the odd that all those planets are correctly positionned for this ?<br /><br />I would have thought that the correct alignment of MANY objects would be very rare but a lot of mission are desing that way.<br /><br />Is it more frequent than i think or there is trajectory manoeuver done during the flight to have to correct alignment on the next pass, or there is something i am missing ?<br /><br />For example, Cassini will do something like 40 moons flyby over 70 orbit.... isn't that high ? <div class="Discussion_UserSignature"> </div>
 
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mrmorris

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<font color="yellow">"It did it with Venus, then the Earth, then again Venus (or the other way around ? anyway...) and then Jupiter. What are the odd that all those planets are correctly positionned for this ? "</font><br /><br />Inner planet orbits are close enough togther that multiple orbital assists are possible (a year for Venus is only 224 days). You'll never see a multiple assist on outer-system planets (i.e. the same orbiter using Jupiter twice).<br /><br />As for 40 moons flybys in 70 orbits for Cassini (it's actually supposed to be over 55 close encounters in 77 flybys per this SpaceDaily article) -- keep in mind how many moons we're talking about. Eight of the 33 moons will receive 'close' flights, so each orbit *can* hit more than one moon.
 
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killium

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What i find fantastic in this ballet is that, launching a probe from here and giving it the right swing to go to Venus is already involving a lot of calculs, predicting the right trajectory to go to Earth after the flyby is IMHO challenging. And they do it for 3 to 4 hops ahead <img src="/images/icons/shocked.gif" /><br /><br />After the initial boost to LEO and away from Earth, the rest of the ride is essentially passive. Its like doing 3 balls and know where the white will end-up (while the other balls on the table are moving). The right alignement (launch window) must be very rare <img src="/images/icons/rolleyes.gif" /><br /> <div class="Discussion_UserSignature"> </div>
 
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mrmorris

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<font color="yellow">"...launching a probe from here and giving it the right swing to go to Venus is already involving a lot of calculs, predicting the right trajectory to go to Earth after the flyby is IMHO challenging. "</font><br /><br />Yes -- well it's not simple (*I* don't know the calculations for sure), but it's not quite the rocket science you think it is (pause for groans). There are several factors that make it much easier than what you're obviously thinking:<br /><br />- Planets travel fixed courses at unvarying speeds. Pick a time 200 years in the future, and someone with the right computer program can tell you exactly where all nine planets will be at that time.<br />- Vacuum simplifies vector calculations <b>immensely</b>! Friction sucks... er is a drag. <br />- Gravititational calculations are relatively simple. It varies with the mass of the two objects and the distance between their centerpoints and nothing else.<br /><br />Because of the above -- there are a relatively small number of variables involved in the calculation. Creating an extremely accurate computer model that can do />99% of the work in computing the proper launch windows and thrust vectors required is possible, and I'm sure it has been done. <br /><br />By contrast to computer models suach as are required to do weather forecasting and airflow dynamics for the shuttle and hypersonic flight (each requiring hundreds or thousands of variables -- many of which are 'best guesses') -- a slingshot model is childsplay.
 
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CalliArcale

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The nice thing is that it's all pure mathematics. <img src="/images/icons/cool.gif" /> Celestial mechanics was for centuries the pinnacle of mathematics research; Lagrange (who came up with the concept of Lagrange Points) was not an astronomer at all, but rather a mathematician. You can do most of it with purely Newtonian physics (though for a mission such as MESSENGER, relativity might become significant because of Mercury's proximity to the massive Sun).<br /><br />The launch windows aren't as tight as for, say, missions to the ISS, which have five-minute windows. This is because the probe usually has a long way to go before reaching its first gravity assist flyby -- that's plenty of time to do a trajectory correction maneuver. Killium is correct that deep space probes carry thrusters to adjust their trajectory to make sure the alignment is perfect prior to the flyby. If you make the ideal launch window, your probe will probably have to expend less fuel to attain the right trajectory for the first flyby, but they usually give them enough fuel to provide for some fudge factor in case the weather is bad or something like that.<br /><br />But there are limits. For instance, ESA's Rosetta probe will not be going to its orginally-planned target because it missed its window due to the temporary standdown of all Ariane V rockets. But a new target was found, so the probe will not be relegated to a museum piece somewhere. And the Voyager probes were originally meant to visit all of the outer solar system, including Pluto, taking advantage of the unique alignment of the planets -- the Grand Alignment, which will not recur for a very long time. But budget concerns and changes within NASA following Apollo delayed the mission too long. Pluto was out of the question. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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mrmorris

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<font color="yellow">"But there are limits. For instance, ESA's Rosetta probe will not be going to its orginally-planned target because it missed its window due to the temporary standdown of all Ariane V rockets. "</font><br /><br />That's sort of an apples and orange comparison. Rosetta missed its target not because of a planetary alignment issue (i.e. gravity assists), but because its target is a comet with a path that stretches from the Oort cloud to the inner solar system.
 
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CalliArcale

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Nope, it's not apples to oranges. It doesn't matter what kind of orbit the target has; you have to get there, and if you want to orbit it, you have to match velocity, and that can require a complex trajectory.<br /><br />Rosetta's originally planned trajectory went like this:<br />Launch: January 12, 2003<br />Mars gravity-assist: August 26, 2005<br />Earth gravity-assist: November 28, 2005<br />Earth gravity-assist: November 28, 2007<br />comet 49P/Wirtanen rendezvous: November 29, 2011<br />nominal comet 49P/Wirtanen orbit insertion: August 22, 2012<br /><br /><br />The actual Rosetta trajectory is this:<br />Launch: March 2, 2004<br />Earth gravity-assist: March 1, 2005<br />Mars gravity-assist: March 1, 2007<br />Earth gravity-assist: November 15, 2007<br />Earth gravity-assist: November 11, 2009<br />comet 67P/Churyumov-Gerasimenko rendezvous: May 1, 2014<br />nominal comet 67P/Churyumov-Gerasimenko orbit insertion: November 1, 2014<br /><br /><br />The relative positions of the planets and the comet were all significant factors, because they are trying to do this with as little fuel expenditure as possible. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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thinice

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<i>"launching a probe from here and giving it the right swing to go to Venus is already involving a lot of calculs, predicting the right trajectory to go to Earth after the flyby is IMHO challenging. And they do it for 3 to 4 hops ahead"</i><br /><br />Almost every flyby requires multiple propulsion maneuvers to clean up errors. They call it 'trajectory correction maneuvers'. Particularly, in case of Cassini, there was also large 'Venus targeting maneuver' between the two Venus encounters.
 
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