The moon and a lunar orbital station. The moon is halfway to anywhere in Sol system.
Payloads boost from Earth to ISS. Transfer to a low thrust (ion?) lunar shuttle to the lunar orbital station. Transfer again to a lunar tug. Standardized payload boxes like semi truck boxes (probably a different shape).
The Mars missions will boost from lunar orbit. This stuff was thought out 60-70 years ago when we were barely LEO capable.
The VERY hard part is making the lunar station self-sufficient.
You wouldn't want to do that... ISS is a total distraction and we WON'T be doing ANYTHING substantial or sustainable in deep space until ISS is on the bottom of the Pacific... ISS simply costs TOO MUCH and it's a distraction from actual deep space missions. ISS is in the wrong orbit (inclination) to be of much use for a way-station or assembly or marshaling point for deep-space missions... the 51.6 degree inclination that was necessary for the Russians to be able to access it (well, for ANYBODY to be able to access it NOW) puts it too far "out of plane" to be of much use for deep-space missions... launching to that high of an orbital inclination also substantially reduces the payload capabilities of your launch vehicle as well just to get stuff into orbit and to the station. Any payloads departing from ISS to the Moon or elsewhere in the solar system, which is basically in the "plane of the ecliptic" upon which all the planetary orbits basically lie (with the exception of Pluto) would require an expensive "plane-change" maneuver which consumes vast amounts of propellant to achieve. Therefore, ISS has NO role in deep-space exploration except perhaps for long duration "simulations" or experiments and test over long duration of deep-space equipment and systems... and so long as ISS is flying, we simply DO NOT and WILL NOT have the money to do *anything* beyond perhaps a few "stunts" beyond LEO... it simply soaks up too much of the available funding.
No, if you want to send payloads to the Moon or Lagrange points (which are MUCH better locations to depart for Mars from, BTW... propulsion requirements are MUCH less than LLO. If you want to marshal those payloads in LEO and send them to the Moon via solar electric propulsion (SEP) or something like that (using low thrust long duration systems like Hall thrusters, ion propulsion, etc, which are unsuitable for manned missions due to 1) the long transit times required, eating up too much valuable consumables like food, water, and oxygen, and 2) multiple passes and long loiters in the Van Allen Radiation Belts as the thrusters slowly "spiral out" the spacecraft toward the Moon in an ever-widening Earth orbit, necessary to gain escape velocity over time, exposing the astronauts to too much radiation). If you want to do LEO assembly/launch of payloads to the vicinity of the Moon, then do it in 'stand alone' operations by having the launching payloads from Earth rendezvous and autonomously or remotely dock to the "lunar SEP shuttle" or whatever in LEO... If it's necessary to physically have someone "up there" to handle things, then orbit a SMALL purpose-built man-tended (NOT "permanently manned") space station at the 29.5 degree launch azimuth (orbital inclination) of launches from Cape Canaveral and KSC... better yet, skip the expensive space station and just launch a manned Dragon or other commercial vehicle to rendezvous with the spacecraft, conduct operations, and return once the payload left for the Moon.
As I mentioned, the Lagrangian points are MUCH better points to launch from for Mars or anywhere else in the solar system than LLO. LLO is unstable due to lunar mascons, and requires periodic thruster adjustments to the orbits to prevent spacecraft from crashing into the Moon. While it's true that establishing and maintaining a "halo orbit" around a Lagrange point also requires periodic thruster firings to keep from "drifting off into space", the requirements are IIRC less, as is the propulsive requirements to send a spacecraft from a Lagrange point to any other point in the solar system, compared to LLO. Basically, the proposed "Gateway station" is the way to go... establish a SMALL MAN-TENDED (NOT permanently manned) station at EML-2 (Earth-Moon Lagrange Point 2, high above the far side of the Moon) and send your Mars vehicle elements there for staging and assembly. Depart from EML2 for Mars. Heck EML2 is an excellent waypoint for lunar missions as well... the travel time is about 5-7 days versus about 4 days directly to LLO, BUT, there's a lot of advantages to going to LLO or the lunar surface from EML-2 rather than directly to LLO. For one thing, ANY point on the lunar surface is EQUALLY accessible from EML-2, whereas the propulsion requirements for the plane change from the equatorial lunar orbit from a spacecraft coming directly from Earth to LLO requires huge amounts of propellant. That's why Apollo could ONLY visit certain sites close to the lunar equator... high latitude sites required too much propulsive power for Apollo to be able to access them. If you want to go to the lunar poles, you'll HAVE to go through EML-1 or EML-2. Another benefit is, EML-2 allows "anytime return" from the Moon. This was a 'requirement' for the Constellation program, but it was realized soon after that it was basically an impossible one due to orbital phasing and alignments to get back to Earth from the Moon... it would require either a variable (sometimes substantial) wait time on the surface of the Moon until the landing site, the orbiting Orion hibernating in LLO, and the Earth were all in the proper alignment. It might also require another wait in LLO once the Altair ascent stage and the Orion had linked up, before the orbital alignment was correct for a return to Earth... therefore, by definition, "anytime return" from the lunar surface to Earth is not possible, not unless you're going to EML-1 or 2 first... then you can launch from *anywhere* on the lunar surface to EML-2 at any time, since EML-2 is NOT moving relative to the Moon, and then return to Earth immediately at any time from EML-2, since it is in a constant relationship with the Earth and Moon. (EML-1 is partway between the Earth and Moon, and COULD be used for this purpose, but due to the gravitational interference of both the nearby Earth and Moon, it's harder to maintain position there... Earth and Lunar gravity are constantly perturbing the "halo orbit" around the Lagrange point.)
Anyway, we have to get SMARTER about how we do these things... the "old school" ideas just aren't going to work with the kind of money it's going to cost to do these sorts of missions... we're going to NEED some kind of Lagrange point capabilities, and we're going to need propellant depot technologies and automated rendezvous and docking and automated propellant transfer technologies at a minimum to be PROVEN operational before we can realistically attempt a sustainable lunar OR Mars program...
We CAN do it the "brute force" way, but not sustainably... the sheer costs alone will prevent it... so unless we want "flags and footprints" again, we need to acknowledge that we're going to need a smarter infrastructure base and reference mission plans...
Later! OL JR
