With the growing ambition of mankind, we began to consider sending astronauts to explore Mars-and may even have begun future colonial activities. This requires us to solve the biggest problem in space exploration-it is really difficult to successfully transport heavy payloads and land on the surface of Mars!
Landing on Mars has many challenges, including lack of protective magnetic field, low gravity and cold temperature. The atmosphere of Mars is also high in carbon dioxide. If you don't wear a spacesuit and stand on the surface of Mars, you will be suffocated by hypoxia even if you don't freeze to death.
However, compared with the above difficulties, the biggest problem is that the atmosphere of Mars is too thin, and the air pressure on Mars is less than 1% of that of the earth. It turns out that this thin atmosphere makes it very challenging to safely land a large number of payloads on the surface of the red planet. In fact, so far, only 53% of mission to mars has been operating normally as planned.
In this article, I will review the former mission to mars and describe in detail the specific challenges of manned exploration of Mars.
Historically, mission to mars was basically launched from the Earth during the biennial Mars launch window. During this time, the route of the launched aircraft from the Earth to Mars is relatively shortest, and the time required is relatively minimal. The first part of ExoMars, jointly developed by the European Space Agency and the Russian Space Agency, was launched on March 20 14, 2006, and entered the orbit of Mars on June 10 of that year. The NASA Mars Observer (Insight) was launched on May 5, 20 18, and successfully landed on the surface of Mars on October 26, 20 165438/KLOC-0. NASA's Mars 2020 rover will be launched on July 7, 2020, and it is planned to land on Mars on February 8, 2026. These missions all follow the interplanetary transfer orbit and aim to reach their destinations with the fastest or least fuel.
When the spacecraft enters the atmosphere of Mars, its speed reaches tens of thousands of kilometers per hour. Before landing gently on the surface of the red planet, it must slow down in some way.
On earth, we can use the thick earth's atmosphere to slow down the descent, and use the heat shield to slow down the speed of the spacecraft. The thermal insulation tile of the retired space shuttle is designed to absorb the air friction heat generated during reentry, thus reducing the speed of this 77-ton orbiter from 28,000 kilometers per hour to zero. The same technology is also applicable to landing on Venus and Jupiter, because they have dense atmospheres similar to Earth.
Landing on the moon without any atmosphere is also relatively simple. In the absence of any atmosphere, you don't need a heat shield, just use a propeller to slow down and lower the orbit until you land on the ground. As long as you carry enough propellant, you can land smoothly.
Back to mission to mars. Generally speaking, the spacecraft will enter its thin atmosphere at a speed of about 20,000 kilometers per hour. We can neither use the landing strategy on the earth to slow down the spacecraft through atmospheric friction and resistance, nor use the landing strategy on the moon, relying entirely on the spacecraft propeller to slow down. Frequent sandstorms on Mars will further increase the difficulty of landing.
Traditionally, when the Mars exploration mission begins to land, the fairing of the spacecraft will help reduce the flight speed of some spacecraft. Historically, the heaviest probe sent to the surface of Mars was the "Cuirisity" Mars probe launched by NASA at Cape Canaveral on 20 1165438+1October 26th, with a total weight of 2,200 pounds (about1ton). When it entered the atmosphere of Mars on August 6th the following year, the speed of the spacecraft was 5.9 kilometers per second, that is, less than 22,000 kilometers per hour.
Curiosity is by far the largest aircraft launched to Mars, with a maximum length of 4.5 meters. Its huge fairing has a certain inclination angle, allowing the spacecraft to maneuver when entering the thin atmosphere of Mars, so as to land in the selected landing zone.
At a height of about 13 1 km from the surface of Mars, the thruster of the spacecraft began to ignite and the spacecraft was adjusted to a perfect landing orbit. After about 80 seconds of atmospheric flight, the temperature on the insulation board rose to 2 100 degrees Celsius. In order not to burn the spacecraft, a special material called phenolic impregnated carbon ablation material (PICA) is used for the heat shield. By the way, space exploration technology company used the same material on the manned "Dragon" spacecraft.
When the speed of the spacecraft drops below Mach 2.2, the spacecraft will release the largest parachute ever built for mission to mars, with a diameter of16m. This parachute can produce 29,000 kilograms of resistance, further reducing the descent speed of the spacecraft. The suspension line of parachute is made of Technora aramid fiber and Kevlar, which is the strongest and most heat-resistant material we know so far.
Then it abandoned the parachute and used the rocket engine to further slow down the descent of the spacecraft. When it was close enough to the surface of Mars, Curiosity used an aerial crane to slowly lower the rover to the ground.
The above is a brief version of Curiosity's landing process. If you want to have a comprehensive understanding of Curiosity's landing on Mars, I strongly recommend you to read the book Design and Engineering Construction of Curiosity written by Emily Lakdawala (available in Meiya).
But Curiosity weighs only one ton. We may ask, can heavier spacecraft land on the surface of Mars by using larger aerodynamic fairings, larger parachutes and larger aerial cranes? In theory, the space exploration technology company "Starship" can send people and goods weighing 65,438+000 tons to the surface of Mars.
This is the problem. Curiosity's deceleration method in the Martian atmosphere cannot be effectively amplified for heavier landing spacecraft.
First, let's start with parachutes. To tell the truth, Curiosity weighing 1 ton is probably the maximum weight that a parachute can land. At present, engineers can't find any material that can bear the deceleration load required for the landing of heavier spacecraft.
A few months ago, NASA engineers just celebrated the successful test of the advanced supersonic parachute inflation research experiment (ASPIRE). This is a parachute, which will be used for the Mars 2020 (lander name: Mars 2020 rover) exploration mission.
Technicians put parachutes made of advanced composite materials such as nylon, Technora aramid fiber and Kevlar into sounding rockets and launched them to a height of 37 kilometers, simulating the experience when the spacecraft arrived at Mars. The parachute was released and fully opened in a fraction of a second, during which it experienced a pulling force of 32,000 kilograms. Imagine that if you are in a spaceship, the tension you bear is equivalent to 3.6 times that when you hit the wall with your seat belt at 100 km/h. With all due respect, you will only hang up in this case.
Regardless of the passengers themselves, there is no composite fabric that can make the parachute bear the deceleration pull generated by a heavier spacecraft.
NASA has been trying different ways to land heavier payloads on Mars, such as loads up to 3 tons.
One method is called low density supersonic reducer (LDSD). The idea is to use a larger aerodynamic reducer. When the spacecraft enters the gravitational range of Mars, it will inflate around the spacecraft like an inflatable castle.
In 20 15, NASA actually tested this technology and loaded a prototype spacecraft on a probe balloon. The prototype spacecraft ignited its solid rocket at an altitude of 36 kilometers and pushed it to an altitude of 55 kilometers. When flying upwards, it inflates the supersonic inflatable aerodynamic reducer to a diameter of 6 meters (or 20 feet). Then, the reducer decelerated the spacecraft to Mach 2.4. Unfortunately, in the final stage of the experiment, its parachute failed to be released correctly, and the spacecraft finally fell into the Pacific Ocean.
But it's still a great victory. If the project can really be completed, it is physically proved to be feasible in the Martian environment. Then one day, we can see a 3-ton spaceship landing on the surface of Mars.
The next idea to increase the landing weight of Mars is to use more propellants. In theory, we can carry more fuel. After reaching Mars, start the rocket engine and reduce the speed to zero completely. Of course, the problem is that the greater the mass used for deceleration, the smaller the payload mass that can actually land on the surface of Mars.
It is expected that the "Starship" of Space Exploration Technology Company will use this push-back landing method to land a load of 100 tons on the surface of Mars. By taking a more direct and faster path, the starship will enter the Martian atmosphere at a speed of more than 8.5 km/s, and then use aerodynamics to slow down. Of course, it really doesn't need to be so fast. Spacecraft can slow down by crossing the upper atmosphere many times and using air braking. In fact, this is the deceleration method used by spacecraft in orbit when flying to Mars. But with this method, the passengers on the spacecraft will have to wait for several weeks, let the spacecraft slow down and enter the orbit around Mars, and then descend into the atmosphere.
According to Elon? Musk said that his pleasant and intuitive strategy to deal with this heat is to make a spaceship out of stainless steel. Then the methane fuel is discharged through small holes distributed on the shell to keep the windward side of the spacecraft cool.
Every kilogram of fuel slows down the spacecraft and lands on the surface of Mars, and there is one kilogram less cargo that can be brought to the ground. To solve this problem, we need some new ideas.
Recently, the University of Illinois at Urbana? A new study by the Department of Aeronautics and Astronautics of University of Urbana-Champaign shows that mission to mars in the future can use a thicker atmosphere closer to the surface of Mars. In a paper entitled "Selection of Landing Trajectory of Mars Spacecraft with High Ballistic Coefficient", the researchers suggested that the spacecraft flying to Mars should not be in a hurry to slow down. When the spacecraft enters the atmosphere, it can still generate a lot of aerodynamic lift, which can be used to guide it through the atmosphere.
They calculated that the ideal angle is to let the spacecraft descend vertically and dive directly to the ground. Then at the last moment, it is pulled up by aerodynamic lift and moves sideways in the thickest part of the atmosphere. This will increase the drag and make the spacecraft lose most of its speed before starting the descent engine and completing the power landing.
At present, this is an interesting idea worthy of further study.
If humans want to build a viable future on the surface of Mars, we will need to solve this problem. We need to develop a series of technologies to make landing on Mars more reliable, safe and effective.