How do NASA and SpaceX plan to send humans to Mars and return safely – Part 2

Timing is very important

Another challenge closely related to fuel is time.Missions of launching uncrewed spacecraft to outer planets are often following complex orbits around the sun. With the help of the so-called gravitational catapult effect, they use the gravitational field of the planet to accelerate and continuously use the following target point to obtain enough kinetic energy, so as to reach the target planet.This saves a lot of fuel, but it could take years to reach the target planet. Obviously, it is the last thing that human beings want to do.The orbits of Earth and Mars are almost circular. The Hohmann transfer is the best fuel-saving way to travel between the two planets. Only two thrusts on the way can make the spacecraft fly from one planet to another through an elliptical transfer orbit.The Hohmann transfer between Earth and Mars takes about 259 days, that is 8 to 9 months. Due to the different orbits of Earth and Mars around the sun, the time window period will possibly only appear once every two years.Of course, SpaceX claims that the spacecraft can also reach the Mars orbit in six months, but it will consume more fuel.

Safe landing

Assuming that the spacecraft and astronauts can successfully reach Mars' orbit, the next challenge will be landing. Spacecrafts entering Earth can use the resistance caused by interaction with the atmosphere to slow down. This allows the spacecraft to land safely on Earth's surface as long as it can withstand the high temperature generated by the friction with the atmosphere.But the atmosphere of Mars' is 100 times thinner than Earth's. This means that Mars' atmosphere has much less resistance, so it is impossible to land safely without the help of external forces.For example, NASA's Pathfinder mission was landed with the airbag, while Phoenix mission was landed with thrusters. The Phoenix mission also needed more fuel. (Source: Tencent News)