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Our spacecraft have rarely visited Mercury, for reasons obvious to those who know orbital mechanics. The Mariner 10 visit to Mercury was revolutionary by using Venus for the novel slingshot maneuver, placing it in an eccentric solar orbit to enable the probe to "visit" Mercury every other orbit. It didn't have enough fuel to orbit the planet.

The MESSENGER craft used two Venus and one Earth slingshots to get to Mercury. That was surely a lot of work. It took a few "visits" to Mercury before orbital insertion around Mercury in 2011, about 7 years after launch.

I thought perhaps that Mercury's high (solar orbital) speed would enable a slingshot to be rather effective for other purposes. Instead of a craft trying to "visit," suppose it was used for blasting away to the outer planets or beyond. Would this be effective?

My question is this. If you send a probe on a slingshot around Jupiter, for example, back to Mars or Venus or Earth, for example, could you point it to Mercury as a better accelerating slingshot to the outer solar system? Or is coming back sun-ward to the small, inner planets just a waste of time and resources? Calling this a "momentum transfer" seems to indicate that Jupiter, Saturn, and the outer planets are the best, but other factors are at play like atmospheres: Mercury was approached to "16 miles above the surface" in 2015, whereas that's impossible for the outer planets. And I see an advantage to being near the sun with a high inbound velocity.

So I am leaning toward Venus being a better slingshot candidate than Mercury based solely on its mass.

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Using Mercury for a slingshot has 3 major problems:

  1. Low energy transfer: Mercury is moving relatively quickly but it's low mass means you don't get much benefit
  2. You have to slow down to get to it. Mercury is close to the sun, and getting close to the sun means you have to slow your orbit down. Messenger used its Venus and Earth flybys to slow down, not speed up to the outer planets
  3. Any spacecraft getting that close to the sun will need beaucoup de shielding to avoid being fried. Shielding means a lot of extra weight which offsets any benefit from a Mercury flyby
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    $\begingroup$ On the "low mass" comment, I noticed the gravity assist equation here doesn't include a term for the mass of the the planet, just its velocity, which I think has to do with the point here that the initial and final speed of the ship would be identical in the rest frame of the planet. So is the mass of the planet only relevant because the angle by which the trajectory is shifted (which is part of the eq.) is greater for more massive planets? Or were you also talking about powered maneuvers? $\endgroup$
    – Hypnosifl
    May 23, 2023 at 21:29

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