Question: In an "alternate universe" where NASA continued to receive a mandate, funding and public support at say peak Apollo levels, could another ten or twenty years have gotten boots on Mars, with astronauts in those boots?

Or would there be some clear technical challenge that really needed several more decades of development before this would have been possible?

**Ideally:** a bit of math or some supporting links should be presented and not just an opinion or a list.

This question is motivated in part by [this thoughtful answer](https://space.stackexchange.com/a/27968/12102).

Answer: 
I second what Edlothiad suggested, Mars Direct would have been perfectly feasible. Mars Direct was created and proposed mainly by Dr. Robert Zubrin. His approach to a human Mars mission was precisely to find a way to do it using the available technology and not using technologies that are still in development or experimental, such as Nuclear Thermal Rockets, advanced propulsion, on orbit assembly, which would all drive up the costs. His research was initiated due to the 90-day report\* and the very high costs associated with a more extensive and elaborate plan for going to Mars. He also suggested a mission that is as lean as possible, requiring the least amount of mass to be launched to make this possible with the class of launch vehicle technology available.

Some of the key points to his approach:

* Use available Saturn V/Shuttle class launch vehicle
* Launch, transfer, land and verify an unfueled return vehicle prior to crewed transfer
* Create propellant for return voyage on Mars with the Sabatier reaction using small amount of hydrogen and a small nuclear reactor brought from earth(ISRU).
* Transfer crew on direct orbit to Mars, no on orbit rendezvous, use a slower orbit that is safer due to it having a free return in case of emergency
* Land all crew on mars(no astronaut stays on orbit)
* Stay on mars for an extended time (due to conjunction orbit)

Zubrin has been pushing this approach for decades without any real movement from NASA. The main reason for this is that, due to multiple reasons, NASA is no longer driven to a singular goal, such as mars, and has many technology driven goals instead. To do humans to mars with the available budget even in this cost effective way would mean giving up on other programs such as ISS.

Some of the excuses given by opponents for why it would not work (or any other plan using available technology) are:

* We do not know how to deal with radiation, so more radiation research needed
* We want to go to Mars faster, so advanced propulsion needed
* ISRU has never been done so we cannot rely on it, so we need to bring fuel, so we need a huge ship and advanced propulsion
* The mass margins are too slim

Which can all be disproved, and have been, by Zubrin.

You can find multiple publications of this plan and the details such as:

<http://www.marspapers.org/paper/Zubrin_1991.pdf>

\*Report of the 90 Day Study on Human Exploration of the Moon and Mars, 1989
There's always the thought that if you put enough money/resources behind a project, you can do anything, and while this is true in many cases, the amount of resources needed without key innovations can be astronomical and ultimately prohibitive. An example I often cite is the Pyramids- they were built without the wheel, but the resources needed to do them without the wheel was an immense human-powered task!

Much in the same way, if we funneled a virtually limitless volume of resources into NASA, we could have any number of major advances using the technology we have available, and the funding to fuel new, incremental breakthroughs. Would these breakthroughs ultimately lead to an easier path to Mars is impossible to speculate, but consider this- if money were no object, a nation could incrementally build an immense space-faring habitat that could, in time, make it's way to Mars. Whether peak Apollo-era funding would have been sufficient to achieve something of that scale is hard to say- but consider this:

One Saturn V Rocket could move 310,000 pounds into orbit at the cost of roughly 190 million dollars. The modern International Space Station weighs 500 tons, or just about 1,000,000 pounds. Supposing the habitat that would travel to mars weighed a conservative double of that, or 2,000,000 pounds (1000 tons), you are looking at no less than 7 successful Saturn V rocket launches, totaling $1.3 Billion in launches alone, not counting the craft, support, development, labor, assembly, and other costs- all in 1970's dollars. At those dollar amounts, you're beginning to look at taking a huge slice out of the United States total budget pie-chart at the time.

So, in summation, while I think will and determination can overcome pretty much any technical hurdle, the economic factors make the possibility of this very, very slim, even with peak Apollo-level funding.