Hopeless said:
How difficult would it be to establish a modular space station in Mars orbit?
Would it be easier to tether it to one of Mars moons?
Right now and for the foreseeable future, it'll be obscenely expensive. The cost and difficulty arises from transporting stuff to Mars. It doesn't matter if that stuff remains in Mars orbit or lands on the ground, the problems remain the same - building the equipment here on Earth, getting it into orbit, then sending it to Mars.
As for the expense, despite NASA's lofty plans and regardless of who is in the American White House, I don't think any plan to go to Mars is going to pan out at the moment. Meanwhile other countries are looking at a more step-by-step path to Mars by going to the Moon first.
Outside of national actors, I've heard that a fellow named Elon Musk has some ideas about this. You might want to look him up on the internet.
Hopeless said:
It's just they mentioned the difficulty of slowing down from supersonic speeds in Mars atmosphere so would it be easier to dock with a station in Mars orbit and they drop supplies from there?
Braking to a reasonable speed to achieve orbit isn't the issue, iirc. The issues are more about what you do once you're in orbit and want to drop things to the surface.
Mars' gravity isn't anywhere near as strong as Earth's, but there's little in the way of atmosphere to provide drag to slow things down, either. This means traditional drop methods like like parachutes are much more limited to combat the inevitable acceleration that gravity will cause when you drop things down from orbit - you have to use very large parachutes to slow things down. Parachutes have that sticky issue that the bigger the parachute becomes, the more difficulty you have in packing it and getting it to deploy right. The same issue exists even dropping things from an orbital station or a moon station - even if you want to build a base on one of Mars' moons and hope to manufacture fuel there (assuming there are things like water ice), you would still have to get things to that moon safely, then down to Mars itself.
This is not a hurdle that cannot be overcome - probes and similar things have successfully landed on Mars before - it can be done again. It just makes it much more difficult, particularly if humans want to drop things that are very heavy, such as construction equipment or prefabricated colony modules.