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All of the descriptions I've seen online describing Chandrayaan-2's path to the Moon either explicitly or implicitly describe it involving a "gravity assist":

The spacecraft will make use of the Earth's gravity to amp-up its own velocity using a technique popularly referred to as 'gravity assist', to conserve fuel.

But every illustration I've seen that attempts to show the relent information about the orientation of the spacecraft's path, shows the craft heading the wrong direction to achieve a gravity assist:

enter image description here

My (admittedly rudimentary) understanding of a gravity assist is that it relies on a much more massive body tugging a spacecraft along the larger body's orbital path on closest approach; but these illustrations (like the one above) show Chandrayaan-2 heading in the opposite direction at periapsis.

Does Chandrayaan 2 use gravity assist? If so what aspect of gravity assist have I got wrong. If not, what is the correct characterization of Chandrayaan-2's fuel saving maneuver?

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  • $\begingroup$ @JCRM So the Earth-orbit-raising maneuver is not gravity assist, but a burn in a gravity well taking advantage of the Oberth effect? $\endgroup$
    – orome
    Commented Jul 23, 2019 at 18:24
  • $\begingroup$ That diagram looks pretty sketchy, geosynchronous is misspelled. Seems like the confusion may be bad diction through bad journalism ;). $\endgroup$
    – Magic Octopus Urn
    Commented Jul 24, 2019 at 17:10

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(Placeholder answer: somebody should be able to do a better write-up. Posted because it contradicts the other answer, putting it here allows it to be voted down if it is incorrect)

It's taking advantage of the Oberth effect to save fuel. By raising the orbit in several stages the Oberth effect is increased. In addition, raising the orbit in several steps allows a smaller engine, which in turn saves weight, and so fuel (the limiting criteria on the engine size is the burn required for the Lunar orbit injection).

A gravity assist usually refers to exchanging momentum with a body as one passes through its sphere of influence. The spacecraft here remains inside the Earth's SoI at all times, so this is not a gravity assist (if it didn't use its engines it wouldn't raise its orbit at all).

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  • $\begingroup$ No worries, added again. $\endgroup$
    – orome
    Commented Jul 23, 2019 at 18:40
  • $\begingroup$ Thanks @orome a definite improvement. $\endgroup$
    – user20636
    Commented Jul 23, 2019 at 18:48
  • $\begingroup$ @JCRM Ohh. Ok I get it now. You're right. Thanks for the clarification. Going to delete my answer now. $\endgroup$
    – Star Man
    Commented Jul 23, 2019 at 19:17
  • $\begingroup$ Just to round this out and make sure I get the whole picture:.The other neat trick is that because the mission is taking its time, a point that is initially just another spot on a very eccentric earth orbit, eventually becomes a (basically) lunar capture spot, as the Moon encroaches into that orbit. Right? $\endgroup$
    – orome
    Commented Jul 23, 2019 at 20:30
  • $\begingroup$ I don't know, but I had assumed the orbit will be increased until it becomes a TLI, and a burn will be needed to enter a lunar orbit. $\endgroup$
    – user20636
    Commented Jul 23, 2019 at 20:35

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