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: 
**Probably yes**. To add to the other already excellent answers: There is the very relevant blog [Spaceflight History](https://spaceflighthistory.blogspot.com/) which focusses on "space exploration history told through missions & programs that didn't happen".

It has many [postings regarding Mars](https://spaceflighthistory.blogspot.com/search?q=mars).

Some examples:

* The [NERVA-Electric Piloted Mars Mission](https://spaceflighthistory.blogspot.com/2016/12/nerva-electric-mars-mission-1966.html) from 1966, which was the last great plan by Braun and would put humans on Mars in 23 September 1986. That might be what you are looking for: it's the most realistic plan from 1966, i.e. when Apollo was not yet doomed.
* [After EMPIRE: Using Apollo Technology to Explore Mars and Venus](https://spaceflighthistory.blogspot.com/2015/07/after-empire-using-apollo-technology-to.html) from 1965, which would lead to a manned flyby in 1978. Certainly possible.
* [A New Step in Spaceflight Evolution: To Mars by Flyby-Landing Excursion Mode](https://spaceflighthistory.blogspot.com/2017/09/a-new-step-in-spaceflight-evolution-to.html) from 1966, which describes a manned flyby with an orbiter in 1978. Also quite possible and didn't even need peek Apollo.
* [Humans on Mars in 1995](https://spaceflighthistory.blogspot.com/2015/10/the-martian-adventure-humans-on-mars-in.html) from 1980-1981, which uses the Shuttle and puts humans on Mars in 1995. This is a realistic plan from 1980 with European involvement.
* And if you really want to dream: [Dyna-Soar's Martian Cousin](https://spaceflighthistory.blogspot.com/2015/09/the-martian-adventure-dyna-soars.html) from 1960 which uses a version of X-20 Dyna-Soar to put humans on Mars in 1971. But that's more SF as you need a working X-20 in 1965.
It seems strange to me that no one has mentioned Project Orion (1958-1965), a theoretical study of using atomic bombs to launch rockets and propel them in interplanetary space.

That would have been many times more efficient than chemical rockets and the members of project Orion hoped that it would enable manned lunar and interplanetary missions in the the 1960s and 1970s.

In regard specifically to Mars, in 2003 the BBC aired a documentary about Project Orion titled *To Mars by A Bomb: The Secret History of Project Orion*.

It seems quite possible that if Project Orion had been supported there could have been manned missions to Mars and other planets in the 1970s.