Reading about missions that have made or will make heavy use of gravity assist (Galileo, Cassini, MESSENGER, BepiColombo...) made me wonder about what if a heavy unmanned ship (BFS?), with all the necessary fuel to do orbital insertion and landing on Callisto or Mercury (for the purpose of establishing a base, perhaps using the same approach as SpaceX plans to do on Mars), follows a similar trajectory to them, and at the appropriate time a much smaller manned capsule docks with it (that is; at the last earth flyby for a Jovian-bound ship / a quick Mariner-10-like trajectory to dock with the mother ship after its last mercury flyby).

This would make the manned part of the journey considerably shorter, perhaps 2 years rather than the 6 required in the case of Jupiter, and a number of months rather than over 7 years for Mercury. The big ship would be relied for all the fuel-consuming tasks of matching velocity with the target, orbital insertion, and landing.

Now, it seems clear that this totally depends on whether the velocities and trajectories of both vehicles can be matched. I don't seem to be able to find the relative velocities at which Galileo passed Earth in 1992, Cassini in 1999, and Mariner 10 approached Mercury in march 1974. This would help to clarify my question.

This is purely a numbers question, so it's clearly not "too broad" or impossible to answer. Thank you.

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    $\begingroup$ Is it more helpful to think of this the other way round? We plan to launch the humans onto the quickest trajectory to their destination that we can manage. Before that, we launch additional supplies and equipment on a longer (but cheaper in terms of delta-V) trajectory which will match that of the humans shortly after their launch. $\endgroup$ – Steve Linton Apr 19 '18 at 10:41
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    $\begingroup$ If we include habitation in the bulk vehicle, @SteveLinton, then the fast crew vehicle can be much smaller - a similar principle exists for cyclers. $\endgroup$ – user20636 Apr 19 '18 at 11:20
  • $\begingroup$ Agreed. That's some of the "equipment". $\endgroup$ – Steve Linton Apr 19 '18 at 13:02
  • $\begingroup$ The mean and maximum temperatures on Mercury are too hot for a manned landing anyway. There is intense radiation too. There was no robotic landing yet. A manned fly by mission does not make sense. $\endgroup$ – Uwe Apr 19 '18 at 13:22
  • $\begingroup$ @uwe The sense or lack of thereof of a manned landing on Mercury is off topic. My question is purely about orbital mechanics. Robotic landers have been planned in the past (Mercury surface element cancelled in 2003, Mercury-P..), temperate regions exist, so it is within our capabilities. Yes, this is similar to docking to a cycler. In the case of Callisto, this would be very much like when in "the martian" the hermes flies by Earth and a capsule of supplies is sent to them, basically hitching to it. For Mercury, hunch is this'd be harder to do, orbits going so deep into the sun's gravity well $\endgroup$ – we'll see Apr 19 '18 at 23:57

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