On the third anniversary of the landing on Mars, let's review the thrilling 9 minutes of Tianwen-1's landing on Mars

At 7:18 a.m. on May 15, 2021, China's Tianwen-1 probe landed steadily on the Utopia Planitia in the northern hemisphere of Mars. So far, my country's first Mars exploration mission has successfully passed the most dangerous stage - the "thrilling 9 minutes" from entering the Martian atmosphere to landing on the Martian surface. The successful landing of "Tianwen-1" means that my country has become the second country in the world to successfully land a spacecraft on Mars.

Schematic diagram of Tianwen-1 falling into fire

What preparations did Tianwen-1 make for landing?

At around 4 a.m. on May 15, 2021, above Mars, the "two brothers" in the Tianwen-1 probe system said their final farewell - after 295 days of companionship, the orbiter and the lander carrying the Zhurong Mars rover officially separated.

Simulation diagram of the separation of the Tianwen-1 lander and orbiter

The orbiter then raised its orbit and embarked on a new mission - to build a communication bridge between Mars and Earth.

At this time, the lander set out on its own. It adjusted its attitude, with its heat shield facing forward, and glided along the trajectory of entering the Martian atmosphere, aiming at the narrow "corridor" of entering the Martian atmosphere. Three hours later, it entered the Martian atmosphere, ushering in the thrilling moment of the entire Mars exploration mission. After the test of life and death, it will represent China in completing the feat of landing on Mars for the first time.

Adjusting attitude before entering the Martian atmosphere

Tianwen-1 is 320 million kilometers away from home, making it the farthest Chinese traveler. It set off from Wenchang, Hainan on July 23, 2020, and has traveled for 295 days. During these days, it has raised its selfie stick to take photos of deep space to report safety to the motherland; it has also looked back at its homeland and taken photos of the earth and the moon to express its homesickness. Most importantly, it has corrected its forward trajectory many times and is heading towards Mars more accurately.

On February 24, 2021, it stepped on the "hardest" space brake in history - after three near-Mars brakes, it successfully entered the orbit of Mars and flew to Mars smoothly. How difficult is it to grasp the degree of these brakes over Mars? Scientists say it is like hitting a golf ball from Paris and letting it fall into a hole in Tokyo.

After reaching Mars, Tianwen-1 did not land immediately, but flew around Mars for several months. For China's first exploration of Mars, the most important thing is to succeed, so Tianwen-1 chose the most reliable mode - "orbit first and then land".

Du Yang, deputy chief designer of the Tianwen-1 orbiter, said that before the Tianwen-1 mission, my country did not have first-hand information on the Martian environment and was not familiar with the topography, weather and other aspects of Mars. In this mission, based on publicly available data from around the world, a preferred landing area and an alternative landing area were first selected in the mission planning, and the pre-selected landing sites were explored by orbiting Mars.

For nearly three months, Tianwen-1 flew in circles in its parking orbit on Mars, constantly taking bird's-eye views of its landing site, Utopia Planitia. With the help of its cameras and spectrometers, it saw the topography of Mars, monitored the seasonal trends of Martian dust storms, and observed the slopes and pits of the landing area.

Based on the latest observations, researchers continuously improved the Mars landing plan, avoided the severe Martian weather, and chose a "sunny" day to implement the Mars landing plan.

During the landing process, how did Tianwen-1 independently overcome dangerous obstacles ?

At the Beijing Aerospace Flight Control Center, as the two brothers of "Tianwen-1" separated, the researchers also entered the final busy time. Soon, the lander will face the challenge of landing on Mars. During this period, the signal will be cut off by the plasma sheath, and "Tianwen-1" will lose contact for a short time. All difficulties can only be overcome by the lander autonomously.

At this moment, aerospace science and technology workers are like a husband whose wife is about to enter the delivery room and he is waiting outside the door. There is little he can do, but he cannot suppress his excitement and nervousness.

The Mars landing is known as the "black seven minutes", which originated from the US Mars exploration mission, revealing that the landing process is full of dangers, but the specific time taken is different. Geng Yan, director of the Deep Space Exploration Department of the Lunar Exploration and Space Engineering Center of the National Space Administration, introduced that in China's Tianwen-1 mission, the probe is expected to take 9 minutes from entering the atmosphere to landing on Mars.

The thin atmosphere of Mars has caused the death of several human probes. Diving into the unknown atmosphere is the main reason for the failure of more than half of the world's Mars probes to land. Before Tianwen-1 wins the honor, it must first withstand the cruel test rarely seen in the world.

In fact, although my country has not landed on Mars, it has a similar technical foundation. my country's Shenzhou series of spacecraft have successfully returned to Earth from space many times, and the Chang'e series of probes have also achieved soft landings on the moon several times, which means that my country has mastered the technologies of parachute deceleration and power reverse thrust deceleration.

But can these technologies be copied to Mars landing? The answer is no.

This is mainly due to the special nature of the Martian atmosphere. Mars has a thin atmosphere, which means that compared with landing on the moon, landing on Mars cannot only use power reverse thrust, but also requires aerodynamic deceleration and parachute deceleration, making landing control more complicated. Although the Earth also has an atmosphere, it is different from Mars. According to the information currently available, there are often sandstorms and dust storms on the surface of Mars, which will cause fatal interference to landing on Mars, but little is known about its specific operating rules.

Another difficulty is that when the Mars exploration reached the most dangerous step of landing, the probe completely lost contact with the Earth. Wang Chuang, chief designer of the Mars probe, said that first, when the probe reentered at high speed, it was compressed with the thin atmosphere to produce shock waves, forming a plasma sheath, which led to communication interruption; second, because the distance between Mars and Earth is 320 million kilometers, it takes 35 minutes for radio signals to go back and forth, and the landing process only takes a few minutes, so people have no time to intervene.

How did Tianwen-1 slow down from 20,000 kilometers per hour to zero in less than 10 minutes?

At about 7 a.m., the Tianwen-1 lander flew into the Martian atmosphere, and the "thrilling 9 minutes" kicked off. In the previous human attempts to land on Mars, only the United States achieved 9 successes, and the Soviet Mars-3 lost contact 20 seconds after landing, and all other attempts failed. Now, the Chinese team has arrived at the track.

The lander is rushing towards Mars at a speed of 20,000 kilometers per hour, which is about 70 times the speed of the Harmony high-speed rail. It takes less than 10 minutes to reach the surface of Mars from the sky above Mars. How to reduce the speed from 20,000 kilometers to zero in such a short time? The ultimate means of landing on Mars is to reduce the speed.

The landing of the Tianwen-1 mission on Mars is divided into four stages: aerodynamic deceleration, parachute deceleration, power deceleration, hovering and obstacle avoidance, and slow descent. "The landing process on Mars is complex, with many actions, each one closely linked and each step thrilling. If one move goes wrong, the whole process will fail," said Geng Yan.

The first stage is the aerodynamic deceleration stage, where the existing thin atmosphere of Mars creates friction with the entry capsule to achieve deceleration. The lander is installed in the entry capsule, which is divided into a back cover and a large bottom. The large bottom is a shield-shaped structure. When flying into the atmosphere, the large bottom is tilted downward to resist impact and ablation. The research and development team of the China Aerospace Science and Technology Corporation conducted a large number of wind tunnel tests, starting with the accumulation of basic data, and finally completed the research and design of the aerodynamic shape and thermal protection of the entry capsule.

Entering the Martian atmosphere and using the atmosphere to slow down

Geng Yan introduced that one of the steps in the aerodynamic deceleration stage is to deploy the trim wings, the purpose of which is to reduce the shaking of the probe and create better conditions for the subsequent opening of the parachute.

The aerodynamic deceleration process overcame the high temperature and attitude deviation. After about 5 minutes of deceleration, the descent speed of the lander was reduced by about 90%, down to about 460 meters per second. At this time, the lander was about 11 kilometers away from the surface of Mars, and the landing entered the second stage - the parachute deceleration stage.

Parachute system deceleration section

"If the parachute can be opened successfully, the Mars landing will be 90% successful," said Geng Yan.

The parachute opening process is very clever. The parachute is installed in a parachute tube, which is the size of a single-door refrigerator and is installed directly below the back cover of the entry cabin. After receiving the ignition command from the control system, the gunpowder in the parachute tube ignites, and the high-pressure gas generated pushes the parachute bag to break through the tube cover and fly out of the entry cabin. Then, the parachute is unfolded in a reverse pull. This parachute is a zigzag-shaped seam belt parachute that debuted in my country for the first time, with an unfolding area of ​​200 square meters. Soon, it was fully inflated, and the 48 yellow parachute ropes were pulled straight.

Huang Wei, chief engineer of the 508 Institute of the China Academy of Space Technology under the China Aerospace Science and Technology Corporation, introduced that to land on Mars, my country will have to open its parachute under supersonic speed, low density and low dynamic pressure conditions for the first time. Previous spacecraft parachutes are not suitable for use on Mars, either because they cannot complete the stable inflation of the parachute at supersonic speed, or because they cannot meet the stability requirements under low-density and low dynamic pressure conditions.

For the Mars exploration mission, parachute designers designed and used a zigzag-shaped seam parachute for the first time, adopted a newly developed special textile material, and innovated the parachute rope splicing process to improve the strength and reliability of the parachute.

As the giant red and white parachute unfolded, the lander's descent became slower, and then the bottom of the entry cabin was thrown away, and the landing platform carrying the Zhurong Mars rover was the first to come face to face with Mars.

"How did the Zhurong land safely on Mars?"

The parachute reduced the speed to about 95 meters per second, and the lander was 1 to 2 kilometers away from the surface of Mars. At this time, the parachute carried the back cover of the entry cabin and separated from the lander. "Zhurong" emerged and breathed the fresh air on Mars.

But the landing has not stopped, and the thrilling moment has not ended. At this time, the propulsion system begins to take the lead, and the Mars landing enters the third stage - the power deceleration stage.

Power reduction section

Han Quandong, chief designer of the propulsion subsystem of the Tianwen-1 lander, introduced that the lander is equipped with a 7500N variable thrust engine and 26 attitude control engines. Among them, the 7500N variable thrust engine is installed directly below the landing platform, providing the main power for deceleration at this stage; the 8 250N engines are in the same direction as the main engine, cooperating with the main engine to implement deceleration; the 8 horizontally installed 250N engines can be used for lateral translation; the remaining attitude control engines are used for precise attitude control.

Under the action of multiple engines, the landing descent speed was reduced from about 95 meters per second to about 1.5 meters per second.

When it was 100 meters away from the surface of Mars, the lander hovered, but it was not time to rest. The Mars landing process entered the final stage - hovering obstacle avoidance and slow descent.

The deceleration methods used by humans to explore Mars are roughly similar, but when it comes to the final step, when they are very close to Mars, the methods begin to diversify. Looking back at the past, the final step can be roughly divided into airbag bouncing, reverse thrust landing legs, and aerial crane.

According to aerospace science expert Pang Zhihao, the airbag bouncing type is suitable for the landing requirements of small-weight probes, the reverse thrust landing leg type can meet the soft landing requirements of heavier probes on Mars, and the aerial crane type can meet the soft landing requirements of even heavier probes.

my country's Tianwen-1 lander and rover used a reverse-thrust landing leg method to softly land on the surface of Mars, which is similar to the way my country's lunar exploration projects Chang'e-3 and Chang'e-4 landed on the moon.

Wang Chuang introduced that after hovering in the air, six instruments carried by the lander, including the microwave ranging and velocity sensors and optical cameras, were turned on at the same time to observe and analyze the surface of Mars to determine where the surface of Mars is flatter and where it is safer to "land".

This step is also critical. If the landing site is not chosen well, all previous efforts may be wasted. "At an altitude of 100 meters, we can accurately judge the landing terrain and see if there are big pits and big rocks. If there are, we need to avoid them."

At this time, under the action of the continuously adjustable axial thrust and strong lateral thrust provided by the propulsion system, the lander rover searches for the optimal landing point, selects a flatter terrain, confirms the final landing position, and implements a slow descent.

At 7:18, the four landing legs made their first close contact with the surface of Mars. After touching the ground, the four landing legs with cushioning devices effectively withstood the impact force at the moment of landing. Working together with the propulsion system, the 1.3-ton lander landed steadily in the designated landing area in the southern part of the Utopia Plain on Mars.

After "thrilling 9 minutes", China's first Mars probe safely arrived on the surface of Mars.

"Tianwen" landed steadily on the surface of Mars