Why can't astronauts return to Earth from Mars?

If you just want to go to Mars, come back without thinking! However, it is a bit unrealistic or even impossible to bring humans back to Earth from Mars with current technology. Although rocket propulsion, manufacturing and hardware reuse have made great progress in recent years. However, scientists and various space agencies are still bound by the same problems that NASA faced in the 1960s, namely gravity and energy. For human beings, returning to Earth from Mars is more challenging and dangerous than a one-way mission to the moon or Mars.

First of all, the gravity of Mars is stronger than that of the moon. The gravity of Mars is more than twice that of the moon (about one-third that of the earth). At present, if humans want to go to Mars, the fuel cost of launching rockets and getting rid of the gravity of Mars is prohibitive. Every kilogram of mass rising from the surface of the moon needs to launch 632 kilograms of hardware, fuel tanks and fuel from the earth. The weight of 1 kg emitted from the surface of Mars is more than 10 times that of the moon. It is proposed to use the natural resources on Mars to build a rocket launching base or produce fuel. But the mining equipment is very heavy and must be transported to Mars first. Even if it is not mined, all mineral resources will be piled up neatly on Mars in advance and used directly, but the manufacturing equipment needed to convert these resources into alloy materials and usable rocket fuel is heavier than carrying hardware and fuel directly from the earth. Although it is feasible to produce fuel on Mars in theory, these methods are beyond the current technology.

Mars is farther away. It took Apollo astronauts three days to travel 380,000 kilometers from the earth to the moon. The shortest distance between the Earth and Mars appears about once every two years, about 78 million kilometers, which is 203 times the travel distance of the moon. However, Mars is also moving in the process of orbit change, so applying the principle of Homan orbit change, we can know that the shortest path from the earth to Mars is actually 593 million kilometers without extremely high energy (such as extremely high speed), that is, it is 1543 times longer than the distance to the moon.

The limit speed of a rocket. We can achieve great speed through continuous acceleration for a long time. But during all acceleration, you must bring a heavy engine, enough fuel and a fuel tank that can hold fuel. In addition, in order to land safely on Mars, this heavy equipment fuel will need almost the same amount of fuel to slow down. One way to solve this problem is to leave a lot of fuel (and maybe engines) on the earth and only transport the payload. One way to achieve this goal is to transfer energy to a moving payload in the form of light. But like nuclear engines, ion drives and magnetic propulsion, these have not yet been realized. In addition, even an engine with 100% efficiency needs a lot of energy to cross this distance.

Crew replenishment According to Articles 2 and 3 above, the shortest time for any technology to go to Mars is 6-8 months. During this period, the materials needed to maintain a sailor's life made the already serious weight problem worse. We should also consider maintaining a large number of supplies and shelters needed by astronauts on Mars. Replenishing goods from Mars is more troublesome, because you need to bring more fuel and fuel tanks to accelerate the rocket to leave Mars orbit and return to Earth. If you want to be self-sufficient and produce food on Mars, it will take time to build and more equipment to set up and maintain, which will increase the payload quality launched by the earth. Inducing astronauts into deep sleep can reduce the total supply needed, but this technology is risky and untested at present, which is not part of our current technology.

Mars Manned Plan The 2009 NASA report "Severe Human mission to mars" determined that the mission of four people returning to Mars would require assembling multiple hardware and fuel payloads in low Earth orbit, which would be carried by the proposed Ares V rocket (larger than Saturn V). Ares V needs to be launched 13 times in order to send all hardware, equipment, fuel and crew into low earth orbit first. This will be the total number of Saturn V launches in the entire Apollo program. Four other smaller rockets will push independent payloads of hardware, fuel, supplies and personnel from Earth orbit to Mars. Some of these payloads will eventually descend and land on the surface of Mars, while others will wait in the orbit of Mars. After completing the surface work, the smallest and most expensive launch will bring the crew and soil samples into Mars orbit, rendezvous with the fuel and supply payload waiting in Mars orbit, and then fly out of Mars orbit and return to Earth.

Without the pressure of the cold war, the public or private sector would not have enough money or interest to fund such a large-scale project. But there will definitely be a manned mission to mars in the future, and there may be one soon. But at present, manned mission to mars is just a one-way trip, or it needs to develop technologies that don't exist today.