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I'll keep it short but please read through!

Gravitation's $1/r$ potential means everything in the solar system is pulling on everything all the time.

Gravitational slingshot maneuvers are normally though of as passing fairly close to a massive object producing a large deflection and change in velocity, but there's no cut-off; all bodies are affecting a spacecraft orbits and if you don't take several into account your calculated trajectory will not reflect where your spacecraft ends up.

So there probably isn't a universally agreed-upon objective mathematical test to say if a particular solar-system's body's effect on a trajectory is above or below "slingshot threshold" but there may exist a particularly weak one that was still a deliberate gravitational slingshot maneuver.

Question: What was the smallest intentional, acknowledged slingshot maneuver? Does one stand out as having the lowest delta-v or smallest deflection angle, or perhaps was the least beneficial, and yet was still acknowledged as a deliberate gravitational assist?

Discussion below How fast can an orbit exist in the solar system? first got me started on this.

BrendanLuke15
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uhoh
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    This is way too broad. Do you count a distant Lunar flyby on the way out of the Earth-Moon System? How about New Horizons flyby of 2014 MU69? There doesn't really seem to be a good answer for this... – PearsonArtPhoto Mar 14 '19 at 12:05
  • @PearsonArtPhoto I'm having trouble figuring out the best way to word this. I want to ask only about maneuvers that were chosen because it provided some benefit, and were clearly described as such. I see that that is not so clear; I'll delete and then take a few minutes to fortify the wording... – uhoh Mar 14 '19 at 12:08
  • @PearsonArtPhoto how does that look? – uhoh Mar 14 '19 at 12:15
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    Better, but it still seems a bit tricky to quantify. But good enough I'll leave it open. – PearsonArtPhoto Mar 14 '19 at 12:21
  • @PearsonArtPhoto thanks! I know what you mean. – uhoh Mar 14 '19 at 12:22
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    An interesting source I came accross while researching this is Richard L. Dowling et al: The Effect of Gravity-Propelled Interplanetary Space Travel on the Exploration of the Solar System: Historical Survey, 1961 to 2000. In: History of Rocketry and Astronautics, AAS History Series, Vol 28. Donald C Elder, Page 339. Unfortunately, the document is not OCR'd so text search is impossible, and quickly glossing over it did not reveal a satisfactory answer. Still, the document is a fascinating read so i thought I'd share it. – Polygnome Mar 24 '19 at 09:24
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    I don't like this question because it's more about 'which astrodynamics team was more nitpicky' acknowledging stuff others don't, than the maneuver itself. Asteroid fly-bys will inevitably be minuscule gravity assists. Crossing a planet's orbit while it's somewhat in sync by chance may deflect the angle by a fraction of angle second. They will be accounted for in the mission plan and trajectory, but whether the team calls them "slingshot maneuvers" or just fly-bys, depends strictly on how nitpicky the team is... – SF. Mar 25 '19 at 07:09
  • @SF. I it's not a question of "who's is smaller" only, a maneuver should have some useful purpose like saving a non-trivial amount of propellant or making a future even in the mission possible. Since mission planning will have an established list of maneuvers that are considered integral to the mission description, and those must be pre-approved by a broad range of people well ahead of launch, I think this can be answered in an objective way without having to cast astrodynamics teams in such a dark light. – uhoh Mar 25 '19 at 09:44
  • @SF. here's an example of such a list: If Cassini's final flyby of Saturn's moon Titan is the 127th Titan flyby, why is it called “T-126”? I can't imagine that any deep space mission doesn't also have such a detailed list. – uhoh Mar 25 '19 at 09:50
  • @uhoh: Thing is nobody does "smallest" gravity assists for gravity assist sake. Either it's a good strong slingshot, or the purpose is primarily scientific - take photos, gather science, and inevitably the trajectory will be affected - so aiming the flyby so that it will benefit the trajectory instead of necessitating counter-acting it is no-brainer. So does a minimal gravity assist that happens because you wanted a near flyby (any, for science) but you made it beneficial instead of costly for the trajectory, qualify or not? Where to draw the line between "added benefit" and "useful purpose"? – SF. Mar 25 '19 at 10:09
  • @SF. I don't think this is helping to clarify my question, and you're "I don't like this question..." has been duly noted, thanks. – uhoh Mar 25 '19 at 10:39
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    Subtle gravitational assists have become quite common, for example Cassini and Juno use lots of them. – Hobbes Mar 25 '19 at 10:49
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    @uhoh: Taking your Cassini example: every fly-by (except the last) was a gravity assist that would enable the next fly-by, the exit trajectory carefully calculated to return and meet Titan once again. They were absolutely deliberate, definitely useful gravity assists - but do they qualify in your book? The mission was to observe Titan, not an exercise in astrodynamics on most Titan fly-bys. – SF. Mar 25 '19 at 10:53
  • @Hobbes I think that's exactly the right idea. Traditionally people have thought only in terms of interplanetary trajectories in a heliocentric system, but gravitational assist maneuvers within a planetary system are probably where much of the interesting stuff is happening. – uhoh Mar 25 '19 at 11:05
  • But that means hundreds of candidates, most of which are unlikely to be published in enough detail to answer your question. – Hobbes Mar 25 '19 at 11:31
  • @Hobbes "I can't answer your question right now therefore nobody can ever answer your question" is a little premature. Let's wait a while and see if and how the question is answered? It's only 10 days old, answers can come in for years. – uhoh Mar 25 '19 at 11:34
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    @Polygnome thanks for the link. the site, http://www.gravityassist.com , contains many articles reproducing much of the content of the paper. For anyone wanting to understand the history and nature of gravity assists, and why they are fundamentally required for planet hopping beyond mars or venus, I highly recommend starting at the home page. – Bit Chaser Mar 25 '19 at 15:02

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I'm going to use the definition that the object must have been within 40,000 km of the object, done only a flyby, and visited another object after. Lastly the effect must have been studied with results released to the public, via paper, website, patent, or similar.

The most likely contenders by far are one of the 3 missions to have orbited one of the outer planets. There are also a few lunar to Earth orbit missions that are worth examining.

  • Cassini - ~ 100 m/s from a Titan flyby.
  • Galileo- One with IO with 175 m/s, possibly less.
  • JUNO- No significant gravity assists due to its nearly polar trajectory.
  • HGS-1- The first had a delta-v of about 681 m/s.
  • TESS- Likely higher than HGS-1, because it passed closer, but I can't verify.

It very likely is one of the distant flybys of Titan, although it really depends on your definition. There might have been a smaller one that was used for Galileo, but Titan/ Cassini seems a clear winner.

PearsonArtPhoto
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