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After having visited the L4 leading Trojans ("Greeks") of Jupiter, almost a decade after launch Lucy will spend almost 4½ years moving on to the trailing L5 Trojans, 5 AU away. Is there a special window of opportunity of trajectories that motivates this huge traverse across the inner Solar system in order to fly by another asteroid?

enter image description here

Image is from a document if the Lunar and Planetary Science Conference 2016. The Lucy mission is commented by blogger van Kane here.

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  • $\begingroup$ From that image it looks like they'll just be doing two highly elliptical orbits with perihelion near Earth orbit and aphelion near Jupiter's orbit. $\endgroup$
    – Hobbes
    Jan 11 '17 at 16:31
  • $\begingroup$ @Hobbes Mmm. But how is that better than launching another Jupiter mission (only NASA has ever done it) or visiting more L4-Jovian or Main Belt asteroids during those 4½ years that the trip will take? Jovian L5-asteroids are the most remote asteroids there are as seen from the "Greeks". Saturn Trojans wouldn't be more distant. There must be some kind of clever opportunity here that I am missing. $\endgroup$
    – LocalFluff
    Jan 11 '17 at 17:01
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    $\begingroup$ Using 1 spacecraft instead of 2, the mission cost is much lower. This being a low-cost Discovery mission (cost cap $450M), building 2 probes and having 2 launches would blow the budget. $\endgroup$
    – Hobbes
    Jan 11 '17 at 18:08
  • $\begingroup$ Great - thanks for the links! The blog is quite interesting. The Lucy Flyer has some helpful info also, however it lists the final encounter near L5 as March 2032, while the others show March 2033. planetary.s3.amazonaws.com/assets/resources/NASA/Lucy_Flyer.pdf $\endgroup$
    – uhoh
    Jan 12 '17 at 9:53
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To get from Earth orbit to Jovian orbit takes a lot of energy - You need to turn your nearly circular orbit round Earth into a Hohmann transfer orbit (an ellipse with the other end out by Jupiter's orbit) and then once you are there you need to reduce the elliptical extremes by expending more energy to move your orbit to match Jupiter. If you then wanted to come back in towards the sun you need to expend yet more energy to move the ellipse once more. These changes are your delta-v budget.

However in this case Lucy is not in a Jovian orbit. It is in a highly elliptical orbit, as @Hobbes pointed out, and one end of that ellipse is already within Earth's orbit. So it doesn't require any further delta-v to come back in towards the Sun and out again. All that is required is a much smaller energy cost to aim the outermost end to be at L5 rather than L4.

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