We have had the technology since the 1960's to make a successful mars trip. There and back. It would be very expensive, and without pressure from some sort of cold war space race, there was very little political will to persue it.
Forget "and back" that's a @#%^ing pipe dream.
No one has any clue how to land a craft large enough to support human space travel on Mars. There are multiple, significant, technological challenges. We're trillions of dollars and decades away from having a *theory* of how to do it, never mind being able to execute it. The failure rate of landing smaller craft that have heat and G tolerances that humans would die within is still in the 40% range. What you're talking about is landing a craft that's conservatively 10 times larger significantly more gently, with a method that works 99.9999% of the time.
The failure rate of previous mars expeditions have all been due to human error, Little things like metric vs english units, etc. If you take away those and look only at failures due to hardware design, you are left with a pile of russian failures and a 95% success rate amongst US missions that weren't lsot due to some underpaid lab student swapping a decimal point. The Mars observer being the one I would attribute to hardware failure outside of design paramaters. The polar lander and the climate orbiter were due to crappy design and testing.
Trillions of dollars? Eh, I'll give you 600 billion. Thats roughly 6 times what the entire full on constellation program would have cost. and 10 years to design build and launch if we started right this second wouldn't be unreasonable. Maybe 15. That being said, if there was a large enough pile of cash, and we broke out our existing designs, we could get there today, using existing technology, without any problems,m assuming we had a heavy lift capability. The challenges are not insurmountable by any means. Lets examine them.
1. Can we get there?
Orbital mechanics is a known quantity. given values for thrust, mass, gravitational influence, and the ability to alter course along the way, we could easily get to mars. We have done so with probes. Orbital insertion is nothing new, and assuming someone doesn't horribly botch the numbers again, that part of the process is the easy part.
2. Can we land there?
Sure. Mars has an atmosphere. Less dense than earths, but still significant. Parachutes, albiet larger than required here would work. Soyuz style direct rocket landing was used with great success during the viking missions. Need to land more mass? simply scale them up. You land things in pieces, and then either land robots or humans to attach the pieces together. Robotic telemetry at that range is a known constant, and is quite doable. Aside from the angry martians shooting at our mars probes, there is nothing to prevent using current existing technology to land there. Hell, a modified lunar lander with significantly uprated engines could do it. Easily. Its a simple question of thrust to weight ratios. The G forces encountered during landing are determined by the type of rentry method used and the amount of money spent on them. If you could somehow have sat a human on one of the viking probes, that reentry would have been quite survivable. Furthermore G stress has not played a factor in any of the loss of US mars mission hardware. The high G airbag bounce landings were utilized by the rovers simply because it was inexpensive compared to more expensive options such as the so called "sky crane" that will be landing the multi thousand pound large scale "Curiosity" mars rover here in a few months. A rover incidentally that masses about the same as that of oh, lets say a Lunar Lander. We can land sh*t on mars. Yes, its rocket science, but we have good rocket scientists. A landing craft 10 times larger than that which landed sprit and oppertunity is easily done. Landing it significantly more gently, also easily done. works 99.99999 percent of the time? We'd have to try it to find out for sure, but if we get the math right and do proper flight testing and trial runs, then yes. that too is easy.
3. Can we break orbit once we get there?
Yup. we have rockets that will lift sufficient mass to exit the atmosphere of mars. We know they will work in the martian atmosphere. Fueling them is an issue that needs to be solved, but we could either land fuel ahead of time, or land near known ice deposits and make our own fuel from water using electrolosys. the chemical process works in space, so it should work on mars, and we would know either way before we got there. Thats one of those things that we would need to research further if we didn't simply land the fuel, which has its own risks.
4. Can we make it there in the first place alive? What about cosmic rays, radiation, low light, low gravity?
Not everyone is going to be physically able to make the trip to mars. It would take a dedicated, specially conditioned individual with experiance in microgravity. I hear some countries call them "astronauts" these days. The cosmic radiation issue is a big issue, but one that is solvable by a combination of magnetic shielding and lead plates, possibly combined with strategic placement of water tanks. The effects of a low light environment on humans would be detrimental, but any theoretical mars mission would need some sort of nuclear power source onboard anyways, so electricity and artificial lighting would not be an issue. Low gravity exposure over the duration of a mars mission is a problem, but we have had people in orbit (the russians on MIR and us on the ISS) sufficient in duration to have made it to mars. Addition of a centrifuge module to any theoretical mars ship would help alleviate the detremental effects, though the return trip could have complications. Its another area that would need to be researched further, but not insurmountable. What about food, water, heat and air? The nuclear plant provides sufficeint power for electrolosys and running a Sabatier reaction CO2 reclamation operation. Heat and methane are byproducts, with hydrogen being required as reaction mass. Large water tanks, doubling as radiation shielding would provide hydrogen and oxygen via simple electrolysis. Water would also be recycled. The hardware for such processes has already been flight tested on smaller scales. Psychological isolation effects would be difficult, but given high bandwidth transmission capability, aside from the transmission lag, communications should be easily accompleshed, and as the lag increased, the excitement of nearing the goal of a landing on mars would fill some of that gap. Social effects from people living in close quarters for over a year would also need careful monitoring, but if you build the ship large enough in the first place, possibly using a BA330 transhab module per person to provide sufficeint space, then that too should be manageable. Medical emergancies en route would be problematic, but if one of the crew were a trained physician with access to sufficient medical gear and the rest of the crew was trained to assist, that too should be manageable, especially given that the selection process in the first place would weed out anyone with likely medical issues that would crop up along the way. Possibly with pre removal of appendicies, etc.
We have the structural engineering capability. Once we have a heavy lift rocket, we will be able to place suitible components in orbit to build a mars ship. The basic designs already exist. Transhabs have been flight tested several times now, and both the sundancer modules have been in orbit for over a year now I believe, and are still maintaining pressure, temperature, and air quality. Scaling them up is not a problem. Even building an entirely rigid ship is within our capabilities once we have heavy lift capability again. Add a frame and shielding plates to ISS style cylenders in place of the keel module, strap a reactor and a bunch of water tanks to it, put a command capsule on the front, point it at mars, and sit back and wait for it to get there. Sending a few tanks to orbit mars in advance with water and gasses onboard for a return trip is also easy, though keeping the gasses and liquids in a non frozen state would be somewhat of a challenge unless you incorporated some significant form of climate control. But if you can do it for humans, you can do that for their water.
The lander part is easy. they already launched the damned thing. The failure rate is achievable with sufficeint testing and design, no more of this "faster cheaper hope it is better but it isn't" bullsh*t. Theory is already done. hardware designs are already in existance on paper. Its the "find large pots of money to throw at it as a project to make it happen" portion of the equation that remains out of reach.
So, which multiple, significant technological challenge did I miss here?