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I was reading this SpaceNews article on the launch, which states:

SLIM won’t be taking a direct route to the moon. After a lunar transfer orbit burn, it will make a lunar flyby, heading into a wide loop away from the Earth-moon system and returning to enter lunar orbit in around four months’ time. This route allows for a smaller burn to enter lunar orbit, saving propellant and mass.

SLIM will then orbit the moon for around a month before making its 20-minute descent and landing attempt, with the aim of demonstrating a lightweight landing capability with high accuracy.

I recall that the Chandrayaan-3 mission did a series of ~6 burns each to raise and lower the orbit on each end, plus a transfer burn, or something along those lines, for better efficiency, which generally makes sense to me. Chandrayaan-3 trajectory depiction from India Today

I managed to find an animation of SLIM's trajectory on Wikipedia

SLIM trajectory GIF from Wikipedia (slightly compressed due to size limitations)

What I don't understand is why the trajectories are so different, if they both seem to be trying to optimize efficiency. The article states that the long loop allows for a smaller burn to enter lunar orbit, but doesn't mention anything about the propellant requirements pre-lunar transfer. Is there negligible impact propellant-wise to get it into that long loop? If so, are there disadvantages to that approach, as compared to Chandrayaan-3?

As a bonus, do we have any information on the delta-V required for each of the two approaches?

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  • $\begingroup$ SLIM has JPL ID -240. Horizons has a preliminary trajectory spanning 2023-Sep-07 00:32 UTC to 2024-Jan-19. ssd.jpl.nasa.gov/api/… $\endgroup$
    – PM 2Ring
    Sep 10, 2023 at 4:26
  • $\begingroup$ I have 3D trajectory plotting stuff here: astronomy.stackexchange.com/a/54240/16685 I recommend a 1 day timestep to get a rough idea of the general structure, then use a 1 hour step over restricted sections. Use the Moon (id 301) as the center for the final part of the trajectory. $\endgroup$
    – PM 2Ring
    Sep 10, 2023 at 4:30
  • $\begingroup$ Sure, Chandrayaan used a more efficient path than manned missions do, but they "optimized" by choosing a path that was as fast as possible while remaining within their fuel budget. That doesn't mean it was the most efficient POSSIBLE path. SLIM may be using close to the most efficient possible path, but the cost is it takes four times longer to get there. $\endgroup$ Sep 12, 2023 at 20:47

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Here's a quick answer, the orbital mechanics and astrodynamicists here may offer a more quantitative analysis.

tl;dr: to get captured into a lunar orbit, you have to do two things - get near the Moon and match the speed of a lunar orbit while you're there. It's the 2nd part where the two techniques differ, and SLIM uses a gravitational assist (flyby) to reduce the fuel needed to match orbital velocity.


The burn that converts the trajectory to a gravitationally bound orbit of the Moon is probably smaller for the 2nd one.

Chandrayaan-3 gets to the Moon and uses a relatively big burn to slow down to be gravitationally captured (at least according to the image) It then uses additional burns several times to lower that orbit

enter image description here

SLIM uses a mostly propulsionless (mostly "free") gravitational assist during a first close flyby of the moon to kick it into an elliptical orbit that brings it back around on an "approach" that may use a much smaller or potentially zero impulse to be at least temporarily captured by the Moon. It may even take a little advantage of Earth's gravity in a three-body sort of way. Once close to the Moon in position and close to the right speed for a bound orbit around it, a smaller impulse can kick it into a bound orbit.

enter image description here

enter image description here

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