What are the biggest obstacles currently preventing humans from landing on Mars?

The fragility of the human body contributes greatly to both: launching unmanned vehicles is expensive, but it is cheap compared with sending people and life-sustaining equipment and supplies. A one-way trip to Mars would expose the human body to close to safe levels of radiation. Extra radiation shielding for the spacecraft? That'll cost more. A lot more. Getting a kilogram of material into Orbit around Mars currently costs about $45,000, and landing will become more difficult as the size of the payload increases, requiring more funding for research and development. If you were to send astronauts to Mars and back in a short time, you would need to launch many times as much mass to fuel the return journey. If you're going to live on Mars for a while, produce your own methane to fuel spacecraft for return to Earth, or have astronauts build bases on Mars, you're going to have to deal with the Martian environment and fragile human bodies for a long time. Mars' surface gravity is about 37.5 percent that of Earth's, and living in such a low-gravity environment for a long time could cause irreversible damage and possibly damage human fertility. The intense radiation on the surface of Mars is deadly to humans. Radiation damage could be mitigated by living underground or building heavy bases with radiation shielding, but cancer rates would increase exponentially over time on Mars. The lava tubes left behind by ancient Volcanic activity on Mars are ten times as wide as those on Earth due to low gravity. Some scientists plan to put astronauts inside them, but they're complicated blind tubes that you have to manually ventilate. They're full of volcanic rock. Whether humans can reproduce on Mars is an open question. The intense radiation from Mars is of no benefit to human embryos. Although sperm can move, the developing embryo will be elevated in the womb due to low gravity, which can put pressure on a pregnant woman's diaphragm and cause breathing difficulties. Low gravity conditions can also lead to abnormalities in pregnancy and a large number of premature births. However, if you have the money to send more supplies, we can simulate earth's gravity with a rotation loop. For the foreseeable future, people in so-called Mars bases will have to live in underground facilities or thick bunkers with artificial light, out of sight for months at a time, to the maximum extent possible. Living in a closed environment for long periods of time can lead to other health problems: depression, vision loss, high blood pressure, lack of responsiveness and concentration due to lack of external stimulation. Of course, we can also make the astronaut space huge and have a variety of recreational facilities, so the design of the construction schedule, the construction machinery, the nuclear batteries and lots of solar panels to power them, the tools to clean up the dirt and so on, and their freight, please. In addition, the lack of microbial diversity on the Mars base, like the space station, is detrimental to maintaining the balance of the human microbial community. Moving soil in large quantities from the earth? Freight per kilogram, please. Musk has repeatedly claimed that the cost of operating SpaceX Starship will eventually be reduced to just fuel and maintenance, like passenger jets, for a full-scale Mars mission. In this case, it would cost about $100 to send a kilogram into Orbit around Mars. There's a huge technological gap in between, and SpaceX is filling it in. But if you have 450 times SpaceX's budget injection, you can do all the things he promised with today's rockets... It's a question of money.