The crucial missing thing is in-space refueling.
Spacex will start testing on the ground within weeks.
I think it is reasonable to believe that - if there are no blockers uncovered, just steady development and testing - they could be testing it in space by mid-year and have it working by year-end.
Then - as long as fuel storage in space works - I do not know of any other major impediments to launching an unmanned Starship mission to Mars.
Sticking the reentry and landing will be hard, but Starships seem to handle tough reentries well, and Starships have already landed on legs on Earth.
I am a pessimist on a manned Starship landing on the Moon within three years, let alone on Mars.
There are many milestones to go before in orbit refueling is possible, among them
:
1. What efficiency are you expecting without ground-based fuel chilling?
2. Have you solved for lack of gravity? Your fuel is sloshing around weightless.
3. The necessary tempo of refueling flights hasn't been reached with test flights, and is far greater than the maximum tempo reached by Falcon 9, impressive though it is.
These are just the big ones I can think of off the top of my head. Digging into the way that fuel tanks are prevented from crumpling while they rapidly empty of liquid methane probably reveals that that becomes much more complicated in micro gravity in orbit with multiple refueling connections to tanker flights. There are dozens of gotchas like that, and I don't think "they must've already planned for that" is an assumption supported by historical performance.
I think it is reasonable to believe that - if there are no blockers uncovered, just steady development and testing - they could be testing it in space by mid-year and have it working by year-end.
Then - as long as fuel storage in space works - I do not know of any other major impediments to launching an unmanned Starship mission to Mars.
Sticking the reentry and landing will be hard, but Starships seem to handle tough reentries well, and Starships have already landed on legs on Earth.
I am a pessimist on a manned Starship landing on the Moon within three years, let alone on Mars.