What ISS seems to have demonstrated is the technology that we DON'T have for a manned Mars mission.
We don't seem to have flight capable, lightweight, cost effective radiation shielding.
We don't seem to have water recycling efficient enough for supplies to last for a three year mission without starting with an obscene volume/weight at the beginning of the trip.
We clearly don't have thrust/propellant technologies that can get a ship there in a short amount of time and we don't have the capability to generate oxygen/breathable atmosphere without constant resupply.
The idea of generating a habitable/breathable atmosphere by growing plants/algae remains solidly in the realm of science fiction.
From recent events, it also seems that the present state of technology used to build coolant pumps is considerably lacking in reliability. I mean, we keep replacing these things in LEO, just how many spares would a Mars mission need to take along?
Since each crew is limited to a six month "rotation" (more or less) do we really understand what happens on really long duration flights (at two and three years of continuous low - or zero - gravity)?
To prepare for an interplanetary mission(s) it would seem (to me) that it would be more helpful to build a small station at a LaGrange point.
It would also seem to be useful to begin the development of the technology to actually build larger things (ships, stations) in orbit instead of just replacing pumps, etc.
At the risk of asking a stupid question, I have always wondered at just how much space we would have available in orbit if we could assemble all the supply ships (end to end, Lego style) that have carried supplies to ISS over the last ten years. It seems like a huge waste to just deorbit them, especially when most of them have their own solar panels, etc.
Good points... the main thing ISS has taught us is how not to build a space station-- out of lightweight 20 ton modules orbited by a super-expensive and complex manned vehicle. As for what it will teach us about going to Mars-- well, I guess all that health research will be useful, if only to prove they have a handle on the issues and things aren't as bad as they all assumed.
On the radiation problem-- it depends on what kind of radiation your talking about-- there's two primary types and the solutions are different. First is solar particle events, aka solar flares, which spew high energy protons through space which can cause damage or even kill a crew at high enough levels. The good thing with this radiation is that 1) it's short lived, usually lasting only a few days at most, and 2) it's from a SINGLE SOURCE, ie the Sun, so we need only put shielding between the sun and the hab to minimize the exposure and reduce it to safe levels, more or less. Putting the mass of the vehicle's propulsion system and propellant tankage in a direction pointing toward the sun would alleviate a lot of the radiation. For extremely severe events, a "storm shelter" surrounded by water is sufficient protection, especially coupled with putting the "heavy parts" of the vehicle between yourself and the sun. Related to solar particle radiation is radiation from the Van Allen belts, which girdle Earth with rings of trapped by Earth's magnetic field of high energy solar particles from the solar wind, which eventually drop along the field lines into the upper atmosphere at both poles as the northern and southern lights (auroras). The solution to exposure here is to NOT hang out in the Van Allen belts... crossing them is fine, as the radiation exposure rate isn't too high (but high enough that you don't want to stick around) and if one uses a high thrust (usually chemical rocket engine) escape stage to accelerate the manned vehicle to escape velocity, it traverses the Van Allen belts in minutes to hours... not long enough to really get any real "dose" from the radiation there... the clincher comes in when you start talking about "low energy trajectories" or "advanced propulsion" which usually implies by its very nature low thrust, requiring a vehicle to 'spiral out' to escape velocity by slowly building up velocity over a single extremely long (but extremely efficient) burn of various electrical or plasma propulsion systems, be they powered by solar or nuclear power sources. Nuclear thermal rockets (NTR's) can produce high thrust at high efficiency, but those aren't even on the table-- the Prometheus program for a nuclear engine was the first thing axed to free up money for Constellation. Low-thrust high-efficiency systems like ion thrusters or Hall-effect thrusters and such (Solar Electric Propulsion or Nuclear Electric Propulsion, SEP or NEP) are fine for unmanned cargoes-- one need only radiation harden the electronics to live through multiple passes through the Radiation Belts while the vehicle slowly builds speed to escape velocity over a period of weeks or months, but for a manned vehicle this is simply unacceptable. I guess one could launch their transit vehicle to EML-2 or something and rendezvous with it there, but you'd still need a high-thrust rapid-acceleration short-transit-time chemical propulsion to get you past the Van Allen Belts with your manned portion of the mission as quickly as possible.
The second radiation issue is Galactic Cosmic Radiation, or GCR... heavy ions of iron and other heavy elements accelerated to relativistic speeds by supernovae... this is a far more pernicious form of radiation because 1) it comes from ALL DIRECTIONS and 2) it is EVER PRESENT. It is far more difficult to shield against. The metallic aluminum and alloy structures of spacecraft present a special danger, because the heavy ions sometimes collide with the atoms in the metallic structure, causing a spray of lighter high energy particles as the ion/metal atoms disintegrate-- a tiny "shotgun blast" of highly charged and accelerated particles that can then penetrate the bodies of the astronauts and cause all sorts of radiation damage to cells. Making the metallic shell of the spacecraft heavier only makes this effect worse, but the present solution is pretty cheap and it works, though not extremely effective-- if the spacecraft is lined with a layer of long-polymer plastic like polyethylene, this material is very good at trapping the particles and reducing the radiation exposure. Unfortunately, most of the radiation shielding that was designed into Orion was stripped out, during the attempts to make the spacecraft light enough to be lifted by the anemic Ares I booster rocket before it was cancelled. Of course this reduces the radiation protection for the astronauts when they're in the Orion for prolonged periods, like say on a lunar mission... As it turns out, inflatable habs are probably THE best way to minimize exposure to this 'secondary radiation' because the outer hull of the module is not metallic, but consists of many layers of Kevlar and insulation and load-bearing polymers and such, which are actually good shielding materials for this type of radiation (carbon-dense materials IIRC).
As for longer missions, GCR is going to be the main thing to deal with... a lot of it comes down to intelligent design-- if we can intelligently design the spacecraft so that the water and food and other materials are stored toward the outside, with the crew living and working behind all that stuff acting as shielding, that will go a long way to solving the problem. Of course this complicates matters if you want to build a spacecraft with centrifugally induced artificial gravity, since that effect is greatest out around the perimeter of the spacecraft and least in near the rotational hub, but again, creative spacecraft design can work wonders with this... a hab spinning on one end of a beam with the lander/Orion at the other for a counterbalance, for instance, or the propulsion stage for a counterbalance, so that the hab module consisted of a series of "floors", each being arranged with the living/working area at the center and the stores/provisions, water, and equipment arranged all around the core area along and on and in the walls, for instance... it would require climbing down the core from one level to another, but that would be good exercise, and it would result in lower gravity in the upper levels than the lower ones, but it could be made to work... I'm sure someone smarter could come up with a MUCH better solution though. BTW I think AG is going to be important, if for no other reason than simplifying things like hygiene and housekeeping a bit... even 1/3 or 1/6 gravity would allow the use of a "normal" (though of course highly modified compared to Earth) design for the toilet and shower... which will be important items for morale on a long mission... Who's gonna want to bathe with wet-naps or a washcloth or scrape their poop down the hole of the space toity for two years?? Being able to take a limited shower (without having to vacuum the walls like in Skylab) and being able to flush (rather than tape a bag to your butt or scrape the crap down the hole with a spatula after you're done and vacuum the toity enclosure after every BM) is going to be really important for morale and to keep folks from going nuts in the long run...
As for the thing about all the resupply modules and stuff-- well, sure, some of them could be used for "extra space"... they kept one of the ISS resupply modules from the shuttle up there for just that reason IIRC... or maybe it was a European ATV, can't recall ATM... but they're not equipped with the systems needed, like thermal control, air scrubbers, etc. to make them habitable volume in large quantities... IOW if they're small and connected to something else providing services, they're fine... on their own, they're little use unless they can be connected to something else. Plus, most of them aren't designed for any long duration sort of lifetime on orbit anyway, or to be reused for much of anything else... Another thing is most of these modules serve as "trash dumpsters" for the space station... Progress is basically a modified Soyuz, without the Descent Module in which the cosmonauts ride-- basically just the "Orbital Module" which on a Soyuz is discarded to burn up before reentry. In place of the DM is an adapter section housing propellant and water tanks, with lines which can automatically connect to the Russian docking system on their module, to transfer propellants over to the stations maneuvering (reboost) thrusters... Not sure if they connect the water up for transfer that way or if they use a hose from inside the pressurized module, connected to the tanks below... At any rate, the fluids are pumped over to the station (just as they did with Salyut and Mir, for which Progress was originally invented) while the crew opens the pressurized OM and removes the packed supplies. The vehicle is then filled with trash, junk equipment and used-up waste experiment material, dirty clothes, etc, and when it's full or they run out of garbage, it's sealed up the airlock closed and the thing is jettisoned with the load of trash, to reenter the atmosphere and burn up. Remember no space station since Skylab had it's own "septic tank" (the unused and unpressurized LO2 tank from the S-IVB stage of the original S-IVB that Skylab was constructed from-- they installed a "trash airlock" in the common bulkhead between the pressurized habitat in the LH2 upper tank and the lower unpressurized LO2 tank, and then "flushed" all their garbage down into the LO2 tank as stuff was used up and thrown away, including all their clothes-- all the astronaut clothing is worn once and thrown away on the stations, as it has been since the first space stations in the early 70's... another HUGE logistical problem to overcome before a Mars mission-- they're going to have to invent a space washer and dryer to minimize mass and space requirements for clothing! That's one of the biggies on the resupply issue with ISS...
The just-launched Cygnus resupply craft is just a cheap knockoff of Progress, without the tanker capability. It's a pressurized can into which supplies are packed, berthed at the station, unpacked, and garbage and junk stuffed in to be disposed of. Cygnus has no heat shield, so it can only serve as a trash dumpster, just like Progress, after its delivery mission is complete. It separates from the station, maneuvers away, and deorbits to burn up with its load of garbage and dirty clothes and junk. The Japanese HTV resupply vehicle is the same way, as is the European ATV resupply craft.
The only vehicle that's different (and so far, unique) is the SpaceX Dragon, which IS equipped with a heat shield and designed for reentry and return of materials to Earth from space, as well as transporting them upstairs from the ground. This is basically doing the job shuttle used to do, transporting equipment and completed experiments (or materials from them) back to the ground for further analysis, study, or use (or reuse). Due to the small size and limited capabilities of Soyuz, basically anything much over pocket size cannot be returned via Soyuz with a crew... and launching an unmanned Soyuz with a DM simply to return hardware or experiments would be incredibly expensive and inefficient. Hence the unique role Dragon plays in the ISS program-- at least/until some other company duplicates that capability, be it Sierra Nevada with their Dream Chaser, or Boeing with the CST-100 crew vehicle...
Your idea sort of reminds me of the early days of shuttle, when it was thought that the ET could be kept in orbit for conversion into "wet workshops" and stuff like that... they were certainly bigger than Skylab and I've seen artwork of even RINGS of ET's joined nose to tail like a parade of elephants going in a circle, creating a 2001-like ring station, but it wasn't realistic-- conversion would add enormous costs, and besides, the foam coating on the ET's was never designed for prolonged exposure to the space environment-- it "popcorned" off into bits and fragments that would float around like a debris cloud and drift off slowly into other orbits, presenting a space debris hazard to other spacecraft (granted, probably a short-lived one given the mass/area ratios and drag effects at the normal altitudes shuttle flew to, but still, a hazard nonetheless). No real solution to this was forthcoming, so the idea was just dismissed out of hand.
Later! OL JR
