What’s the Bare Minimum Number of People for a Mars Habitat?
stopthatgirl7@kbin.social to 
universetoday.com
A recent preprint paper examines the minimum number of people required to maintain a feasible settlement on Mars while accounting for psychological and behavioral factors, specifically in emergency situations. This study was conducted by a team of data scientists from George Mason University and holds the potential to help researchers better understand the appropriate conditions …
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I recall a similar study years ago. They concluded 32 was minimal viable, assuming a strict breeding regiment over several generations, with 8 men and 24 women. They also concluded about 500 would be the smallest practical size, given people aren't robots and losing even a couple people before leaving the breeding pool would be very bad. That was a fundamentally different study though, looking at long term, self sufficiency. This one seems more focused on an Antarctica like outpost that would be able to cycle people in and out, and not establishing a full on colony.
Thank you for pointing out this detail of possibly returning!
We might be able to travel to Mars in a few years. But it will take many more years before anybody can travel back from there.
Mars has a gravity similar to earth. In order to leave the planet we need to launch rockets from there, about the same size as we launch from earth. And therefore we need to build lots of stuff there and operate it properly.
The first 'colonists' will have to go with the expectation of never returning.
I don't think Mars colonies are realistic, but not for this reason. Mars has about one third the gravity of earth, and a much thinner atmosphere, so you can return on a significantly smaller rocket than you launched with. It's true that manufacturing a space rocket of any size would require basically an entire civilization, but there's no reason you couldn't bring the return vehicle with you, and only require manufacturing fuel or propellant on site.
The top answer to this stackexchange post goes into a lot more detail on the practicalities https://space.stackexchange.com/questions/2820/how-big-would-a-manned-ascent-stage-for-mars-need-to-be
LOL. For example simply it's weight is a reason. A vehicle for landing and a vehicle for starting may be the same, or may be two very different things because of their weight.
And then bringing the vehicle is one thing, but starting it is quite something else again.
Most of the weight is fuel/propellant, which is why most Mars mission plans have you manufacture propellant on-site. An empty fuel tank and some engines isn't that heavy. Especially if, as you say, you're able to reuse your lander. Anyway, everything you bring has weight. The issue is, how much and can you budget for it?
If your looking for somewhere to save weight, imo start by getting rid of the astronauts and all their associated life support and living space. Bonus - robots don't even need frivolous luxuries like getting to return home.
I mean, they will probably be relying on many unammed missions that deliver payloads to deliver all the construction material for the outpost before sending any people. While you're at it you could send the return craft too.