What are the technical obstacles to human migration to Mars?

1. Radiation in outer space. Some people put forward artificial magnetic field, injecting water into the cabin wall, and even smearing feces on the spacecraft to fight radiation. But there's nothing I can do.

The second is the flight time. For astronauts, it is more prone to psychological problems to face the risks of muscle atrophy and decalcification caused by long-term weightlessness.

Third, the load of the spacecraft, although the scheme of refueling in orbit for many times is relatively novel, whether it can be realized and whether it is economical is a problem.