So far, as the first Mars probe launched by China, "Tian Wen-1" has completed three technical steps of Mars exploration at one time. These three steps are orbit around Mars, landing and patrol, that is, entering Mars orbit, landing on the surface of Mars and the rover walking and patrolling on the surface. Why did China Space Agency successfully break the so-called "Mars curse" in the first landing mission of Mars probe? How did the various space technologies involved in Zhu Rong's landing develop?
Because the distance between the earth and Mars is extremely far, the surface environment of Mars is very complicated, and the journey of exploring Mars is extremely difficult from the beginning. So far, the success rate of human probes exploring Mars is only 50%, and the success rate of landing missions on the surface of Mars is only about 40%. Therefore, the Mars exploration mission has always been called "the curse of Mars". Before China's "Zhu Rong", only the United States was able to launch the rover safely to the surface of Mars, and the missions of the former Soviet Union, Russia and Europe were unsuccessful. In order to achieve the mission of Mars exploration, China needs a number of key technologies, including but not limited to: global deep space exploration network, heavy rocket launch and satellite orbit control capability, spacecraft atmospheric re-entry and autonomous soft landing technology.
Deep space TT&C generally refers to tracking, telemetry and remote control of satellites flying to the moon and beyond by using ground radio signals. To realize the global deep space exploration network, a country not only needs to establish satellite monitoring stations in its own territory, but also needs to seek international cooperation to achieve full space coverage. As one of the pioneers of Mars exploration, the United States has set up its own monitoring station in its own territory, and deep space exploration facilities in Spain and Australia, and cooperated with the deep space tracking network of the European Space Agency during the flight and landing of the Perseverance probe.
In contrast, China's deep space exploration network started late, but the starting point was very high. Since the 20th century, China has established or perfected seven China space exploration networks with a maximum diameter of 70 meters. They are distributed all over the country to ensure the widest possible airspace coverage. Among them, at the end of 20 12, the TT&C station of Kashi and the TT&C station of Jiamusi, where the TT&C center of Xi satellite was put into use in Xinjiang, successfully completed the sampling return plan of China lunar rover and the moon, that is, the "Chang 'e lunar exploration project".
From July 2065438 to July 2005, when the American New Horizon probe flew over Pluto for scientific observation, Jiamusi TT&C station successfully captured the signal of the probe, with the longest tracking distance reaching 4.76 billion kilometers. From 2065438 to September 2007, China's TT&C network in deep space realized the tracking and measurement of the whole process of American Cassini probe hitting Saturn, which accumulated valuable experience for the subsequent independent deep space exploration mission. In order to further meet the measurement and control requirements of Tianwen-1 Mars probe, China's first deep space exploration antenna array was debugged and tested in Kashgar, Xinjiang on June 5438+065438+1October 8, 2020. Three newly-built 35-meter-diameter antennas and the original 35-meter antenna form four 35-meter-deep beam waveguide antenna array systems. The antenna has the data receiving ability of an equivalent antenna with a diameter of 66 meters, which is suitable for all kinds of deep space missions and has stronger measurement and control support.
In addition to the domestic space station, in 20 14, the governments of China and Argentina signed an agreement to establish the first deep space monitoring and control station in Las Lahas, neuquen province, Argentina. In 20 17, the Argentine deep space station was successfully built and put into use for the development and utilization of outer space between China and Argentina. The geographical position here is roughly symmetrical with that of China along the center of the earth, which can provide support for the tracking of Tian Wen-1 in the western hemisphere and the southern hemisphere. In addition, like perseverance, Tian Wen-1 mission also cooperated with the deep space tracking network of the European Space Agency; Spanish and Australian ESA monitoring stations participated in signal relay and transmission, and the development of China deep space exploration network laid a solid foundation for the Mars exploration mission.
The advanced orbit control and attitude adjustment capabilities of heavy rockets and satellites are very important for a country to realize the detection of Mars probes. As the world's first Mars probe to complete orbit, landing and patrol at one time, "Tianwen No.1" is a giant probe with a total weight of 5 tons, which is about 4.9 times that of the American "Tenacity". The rocket thrust required to launch such a probe consisting of two trucks into the orbit of Mars is extraordinary. Among the rockets in use in the world, only three rockets have more than 5 tons of fire transfer ability to the ground. They are the Delta 4 heavy rocket of NASA, the Falcon heavy rocket of Space Exploration Technology Company and the Long March 5 rocket of China.
"Long March 5" is the first large-scale liquid oxygen, hydrogen and kerosene series launch vehicle in China that adopts the latest technology from the whole to the subsystem. It is the largest space transportation system project developed in China with the largest technical span. Compared with the previous Long March rockets, new technologies account for more than 95%. At the same time, it is the rocket with the largest take-off mass, the largest core diameter and the strongest active carrying capacity in China, with a payload of 25 tons in near-earth orbit, ranking third in the world. Its firepower transfer capacity is 6 tons, which meets the mission requirements of "Tianwen No.1". On July 23, 2020, the Long March 5 rocket carrying Tian Wen-1 detector was launched at Wenchang launch site in Hainan. This is the fourth successful launch of the Long March 5 rocket and the second19 successful launch of the Long March series of launch vehicles.
In recent years, China has launched a large number of rockets with high load and high success rate, which has laid a solid foundation for the mission of Mars probe. Judging from the rocket launch data, the number of rocket launches in China has remained at about 40 times a year in the past two years, which is basically the same as that in the United States. A large number of large-scale remote sensing satellites, Beidou navigation satellites, meteorological satellites and agricultural census satellites are being built every year. It can be said that after decades of technological development, the launch vehicle technology and satellite orbit control technology of China's space industry have matured.
In the mission of China National Space Administration, the "Chang 'e to the Moon" project is a pioneering project for deep space exploration, paving the way for Mars exploration. From the launch of "Chang 'e I" in 2007 to the completion of the lunar soil return mission of "Chang 'e V" in 2020, the deep space orbit control and maneuverability of China spacecraft have been well tested in 13 years. Applying the attitude and orbit control experience gained from Chang 'e Project to the mission of Mars probe, it can be imagined that the orbit control of Mars probe is not the main challenge of China's space flight. During several orbital adjustments of Tianwen-1 in 2020 and 20021,we can see that every orbital maneuver of the probe is accurate.
Due to the high accuracy of the orbit, the midway correction of the orbit originally scheduled for February 12, 20265438 was even cancelled. On February 15, the orbit was directly adjusted, and a 90-degree right-angle turn was made at apogee, from a horizontal orbit around Mars to a vertical orbit, and the probe entered the polar orbit of Mars, achieving the best orbital coverage on the surface of Mars. In fact, as a relatively simple part of the whole exploration journey, after the Cold War, in addition to the United States and Russia, many countries also tried to launch orbiters to Mars, including several probes of the European Space Agency. Indian "Man Garian" and United Arab Emirates "Hope" have successfully cooperated with the international deep space exploration network and made important contributions to human exploration of Mars.
Compared with orbiting Mars, the most challenging aspect of mission to mars is the soft landing of the probe. The landing process of Mars probe is called "nine minutes of terror", and in some cases, it is also called "seven minutes of black". This process is usually called EDL, which is an acronym for entry, descent and landing. The whole process depends on the autonomous control of the lander. In human history, the success rate of Mars soft landing mission is less than half. For China National Space Administration, this is the first attempt to soft land a lander weighing more than one ton on the surface of Mars, which is really a new challenge. Due to the well-known difficulties, the successful landing of Tian Wen-1 shocked some people who didn't pay much attention to China's space industry to some extent. However, if you really understand the development path of China's deep space exploration mission, you won't be particularly surprised by the complete success of Tian Wen-1. After all, opportunities are always reserved for those who are prepared, and the soft landing mission of Mars is no exception.
Due to the unique atmosphere and gravity environment of Mars, there are three key technical actions in the soft landing process:
Similar to the Earth, the surface of Mars is also surrounded by the atmosphere, so the spacecraft will inevitably collide with the Martian atmosphere before landing, and the surface temperature of the spacecraft can even exceed 1000 degrees Celsius. This requires the landing equipment to have good thermal protection measures and aerodynamic shape, so that the electronic equipment of the detector can remain intact in the friction with the Martian atmosphere and avoid overturning.
Re-entry is nothing new to China's space program. Looking back at the historical development track of China spacecraft's reentry, starting from 1999, the Shenzhou series spacecraft will have serious friction with the Earth's atmosphere every time they enter the Earth's atmosphere. Since 2003, 1 1 Chinese astronauts have entered space and returned safely. With the opening of China's?Tiangong?space station, 12 Chinese astronauts will go into space and return to earth in the next two years. The Chang 'e-5 T 1 re-entry flight testing machine launched in 20 14 and the Chang 'e-5 re-entry capsule which recovered lunar soil and returned to Earth in 2020 tested the safety and reliability of the spacecraft re-entry into the atmosphere at the second cosmic speed. It can be said that before Zhu Rong landed on Mars, the key technologies such as aerodynamic shape design, attitude control and thermal protection materials for spacecraft re-entry into the atmosphere have been improved and tested.
During the landing, the tail heat shield and the bottom heat shield played a role in heat insulation. The attitude control nozzle and balance wing outside the heat shield prevent the spacecraft from rotating axially and ensure the attitude stability. In order to further improve the thermal insulation reliability of Tianwen-1 entering the atmosphere of Mars, the 306 Institute of China Aerospace Science and Industry Corporation designed a new thermal insulation material-nano aerogel for the detector, which was used to cope with extremely hot and cold environment. At the same time, the ultra-light characteristics of nano-aerogels also greatly reduce the burden of the rover and make it run faster and farther.
In addition, in order to resist the uncertainty risk brought by the Martian atmosphere, China has adopted a unique ballistic lift-type atmospheric entry scheme based on the so-called balance wing, which is the first time that humans have applied it to Mars exploration. This scheme has long deceleration time and small overload when entering Mars. At the same time, the landing accuracy of the detector can be improved by controlling the lift direction. It is also an ideal way to enter the future Mars sampling, returning and manned Mars landing missions. Previously, the "Chang 'e V" return capsule also successfully landed on the earth using the same technology.
Compared with the earth's atmosphere, an important difference of the Martian atmosphere is that its density is only 1% of the earth's atmosphere. In order to reduce the speed of the probe to a safe range below 100 meters per second before releasing the lander, besides the friction between the Martian atmosphere and the heat shield, the probe also needs the help of a parachute. On earth, due to the high density of the atmosphere, the speed of the detector usually drops below the speed of sound before the parachute is deployed. However, the thin atmosphere of Mars forced the probe to open the parachute in time and slow down during supersonic flight. This poses a great challenge to the supersonic parachute system of the probe, and it is also a key factor that leads to the repeated postponement of Mars exploration plans in Europe and Russia.
As early as 20 16, the wind tunnel test of supersonic parachute was completed in No.508 Institute of the Fifth Academy of China Aerospace Science and Technology Corporation. In 20 18, the new "Tianying" series sounding rockets developed by China Aerospace Science and Technology Corporation were successfully launched at the Korla launch site in Xinjiang, successfully completing the supersonic parachute technology test for Mars probe landing, which laid another technical foundation for landing on Mars three years later. After the supersonic parachute reduced the probe speed from Mach 2 per second to 95 meters, the last step of the soft landing of Mars fell on the lander.
Different from the "crane" scheme of the Mars probe "Tenacity", Tianwen-1 adopted the reverse thrust hovering landing scheme. When the landing platform carrying Mars probe approaches the ground, it will start the anti-thrust system and make the platform slowly descend. Enter the hovering phase at a distance of 100 meters from the surface of Mars. With the help of optical imaging sensors and other measuring equipment, the detector reaches the surface of Mars under the protection of buffer mechanism after completing accurate obstacle avoidance and slow descent. The key component of this process is "7500N variable thrust engine". Through the throttle control of a single engine, the lander can realize a series of complex actions such as deceleration, hovering, obstacle avoidance and slow landing.
China's "7500N Variable Thrust Engine" has been developed since 2008. At present, the verification, design, qualification test, process research and high temperature test of the engine have been completed. 20 13, first used in Chang 'e III. It helped Chang 'e-3 land successfully in reverse thrust hovering mode, making China the third country to successfully achieve a soft landing on the moon after the former Soviet Union and the United States. With this engine, Chang 'e IV in 20 18 made China the first country to make a soft landing on the back of the moon. In 2020, the "Chang 'e V" lander still relied on the stability of the "7500N variable thrust engine" to successfully land on the surface of the moon and return to Earth with lunar soil.
In addition to the variable thrust engine, because the distance between Mars and the earth is as high as hundreds of millions of kilometers, the fully automatic landing procedure and autonomous obstacle avoidance ability required for the probe landing have also been verified in the missions from Chang' e III to Chang' e V. It is not difficult to see that the technical experience of Chang' e mission has increased the confidence of the success of the three goals of "Tian Wen No.1".
From manned space flight, to Chang 'e's lunar exploration, and then to the Mars exploration mission in Zhu Rong, scientists from China National Space Administration are advancing these technologies step by step. Especially in the case of tight budget in the early stage, we are still breaking through key technologies year by year, conducting a large number of technical tests and making steady progress according to the existing project planning and road map. As early as 2004, China's lunar exploration mission set the goal of completing the three-step walk around the moon, landing and returning samples by 2020. From June 5438 to February 2020, the mission of "Chang 'e V" ended and was successfully completed. In addition, as early as 2000, China's space station mission made plans for manned spacecraft launch, astronaut spacewalk, launching space laboratory and establishing a space station by 2022.
Without exception, these plans are gradual, stable and timely. As for Tian Wen-1, it inherited the tradition of China's space program. Although the project was formally established on 20 16, the technologies directly or indirectly used in this project are the accumulation of space exploration by China National Space Administration for more than ten years. Based on these technologies, China's future space programs, such as the super-heavy Long March 9 rocket, the lunar space station, Mars soil sampling and its return to Earth, and the exploration of the edge of the solar system, have attracted the attention of space fans. After all, after the end of the Cold War, the pace of human exploration in space has slowed down slightly, and the rapid development of China's space engineering has also injected a strong impetus into space exploration all over the world. Perhaps the new wave of space exploration that space enthusiasts are looking forward to has just begun.