On May 15, 2021, tianwen-1, China's first Mars exploration mission, successfully landed on the Martian Utopia plain; On May 22, its Mars rover zhurong began to explore Mars. Over 300 days and nights, based on the scientific research data obtained by tianwen-1 and zhurong, scientists are producing a series of original scientific research achievements. Now, the Martian winter has arrived, and the zhurong spacecraft has begun to "hibernate".
What should we do if the zhurong ship gets grey? Designer: don't worry
Exploring the Chinese footprint of Mars
Human beings' exploration of the universe is endless. A strong thirst for knowledge and curiosity is a powerful driving force to promote human progress. Among them, space exploration is of great significance to the study of the origin and evolution of the solar system, the origin and evolution of life and other major scientific issues. It is in this powerful driving force that human beings started their exploration of Mars. Mars is the fourth closest planet to the sun. It is one of the four earth like planets in the solar system. Because it is as bright as fire, its position and brightness often change, making people unpredictable. In ancient China, it was also called "fluorescence".
With the development of space technology, the diversification of exploration means, the continuous improvement of detection accuracy, and the gradual deepening of human understanding of Mars, the enthusiasm and frequency of Mars exploration by space powers have also gradually increased. Exploring Mars, exploring its water and life information, and exploring its habitability have gradually become the mainstream of international deep space exploration.
However, the road to "fire" is startling step by step. So far, countries have carried out Mars exploration more than 40 times, with a success rate of about 50%. Therefore, Mars is also known as "spacecraft cemetery".
Tianwen-1, China's first Mars exploration mission, won its first battle. It can orbit, land and patrol Mars through one mission. The tianwen-1 probe was launched on July 23, 2020. On May 15, 2021, the landing patrol successfully landed at the scheduled landing site (109.9 ° e, 25.1 ° n) in the south of the Martian Utopia plain. On May 22, the zhurong Mars rover left the landing platform that it had been accompanying for 303 days and took the first step of 0.522 meters on the Martian land. On May 26, the Mars rover took and sent back photos of the landing platform. The five-star red flag on the landing platform was dazzling and clearly shone on the surface of Mars, leaving a "Chinese footprint" on Mars.
The tianwen-1 probe consists of a orbiter and a landing inspector. The orbiter is equipped with 7 loads, including medium resolution camera, high resolution camera, subsurface detection radar, Martian mineral spectrometer, Martian magnetometer, Martian ion and neutral particle analyzer and Martian energy particle analyzer. Carry out global and comprehensive remote sensing exploration of Mars through orbit exploration. The landing inspection vehicle is composed of an entry module and a Mars rover. The entry module completes the tasks of Mars entry, descent and landing (EDL). The zhurong Mars rover is equipped with 6 scientific payloads, including a multispectral camera, a subsurface detection radar, a Mars surface composition detector, a Mars surface magnetic field detector, a Mars meteorological measuring instrument and a terrain camera. Through inspection and exploration, it carries out high-precision and high-resolution fine inspection and exploration in key areas of the Mars surface.
By May 15, 2022, the tianwen-1 lander had successfully landed on Mars for one year, and the orbiter had been in orbit for 651 days, 240million kilometers away from the earth; The zhurong Mars rover has worked on the surface of Mars for 347 Martian days, traveled a total of 1921 meters, completed the established patrol and exploration tasks for 90 Martian days, continued to carry out scientific exploration on the surface morphology, structure, material composition, magnetic field, etc. of Mars, and accumulated 940gb of original scientific data.
Why "Zhu Rong" should "hibernate"
Winter goes and spring comes and goes. Like the earth, Mars also has revolution and rotation, and there is a yellow red angle. The day on Mars is 24 hours and 37 minutes, which has almost the same day and night rotation time as the earth. The sun seen on Mars also rises in the East and sets in the West. The inclination angle between Mars' orbital plane and Mars' equatorial plane (rotation axis inclination) is 25.19 °, which is very close to the earth's inclination angle of 23.5 °, which makes them also have the alternation of day and night and the rotation of spring, summer, autumn and winter.
The average distance between Mars and the sun is about 1.5 astronomical units, about 230million kilometers. The eccentricity of the elliptical orbit around the sun is greater than that of the earth, which is 0.093 (the earth is only 0.017), which leads to quite different lengths of Mars in different seasons. A year on Mars is equivalent to 687 Earth days, and its season length is about twice that of the earth; Because its aphelion is close to the summer solstice in the northern hemisphere, the spring and summer in the northern hemisphere of Mars are about 40 days longer than those in autumn and winter. Similarly, due to the larger eccentricity of Mars, the temperature difference on the surface of Mars at perihelion and aphelion can reach nearly 160 ℃, which has a great impact on the climate of Mars, and the seasonal differences between the northern and southern hemispheres of Mars are more significant. The four seasons on Mars are basically cold and dry. The average temperature on the surface of Mars is about -55 ℃, with a large temperature span. The average temperature in summer is about +27 ℃ and the average temperature in winter is as low as -133 ℃.
The landing site of tianwen-1 probe is located in the south of Utopian plain, with a latitude of 25.1 °, which is roughly equivalent to the latitude range of Guiyang and Fuzhou in China. At present, the region where the zhurong Mars rover is located has entered winter. Similar to the earth, after the northern hemisphere of Mars enters winter, the illumination altitude angle decreases, the sun's illumination tilt has less heat, and the illumination duration also shortens. At the same time, there are sandstorms on Mars, which will be more frequent in winter, resulting in further weakening of light intensity. Compared with the vacuum environment and the atmospheric environment on the Martian surface, it will also increase the consumption of electric energy for the Mars rover. Due to the unpredictable dust storms on the Martian surface, it will block the solar radiation, also affect the Martian atmospheric temperature and surface temperature, and reduce the average temperature of Mars. The engineers also wore a special "cotton suit" made of aerogel gel. When Mars winter comes, this "cotton suit" will play a role in keeping zhurong warm.
So, do Mars rovers in other countries also need to "hibernate"? Up to now, a total of 6 Mars rovers have successfully set fire and carried out patrol and exploration, including sojourner, spirit, courage, curiosity, perseverance and zhurong in China. Except curiosity and perseverance, which use radioisotope heat source (Rhu) for power generation, the other four Mars rovers are all powered by solar wing power generation. The energy is affected by the intensity of solar light, so it is necessary to "hibernate" when Mars comes to winter.
According to the measured data of the first generation Mars rover, SOJINA, its solar wing is covered by Martian dust, resulting in a loss of 0.3% of the efficiency of the solar cell every day. On May 24, 2011, the National Aeronautics and Space Administration (NASA) of the United States officially announced the end of the Mars spirit mission. Spirit and its sister rover Opportunity arrived on the surface of Mars in 2004. In the following years, they collected a large number of scientific data and sent back numerous beautiful and spectacular photos of the Martian landscape. However, in March2010, after the Spirit sent back its last signal, there was no message. NASA did not give up and insisted on calling every day, but there was no result. More than a year later, it had to announce the end of the mission.
Scientists judged that the loss of the spirit was due to the lack of sunlight on the surface due to the arrival of Martian winter, and the spirit's solar cells could not collect enough power. In the case of lack of power, the temperature inside the spirit dropped rapidly, and the electronic equipment was likely to be damaged and unable to work. Even if the temperature picked up in the next spring, spirit never "woke up" and has been "sleeping" in the sand and wind on Mars forever.
When does Zhu Rong wake up
The average distance between the earth and the sun is about 150million km, the average solar constant is about 1378w/m2, the average distance between Mars and the sun is about 230million km, and the average irradiation intensity is about 589w/m2, only 43% of that of the earth. Complex and harsh environmental conditions have an important impact on the design of the rover.
The solar wing on the rover converts solar thermal energy into electric energy, which is the only energy source of the rover. At the same time, Mars is far from the sun and is affected by the atmosphere. The solar radiation intensity reaching the surface of Mars is only 0.2 solar constants, about 20% of the lunar surface. The weak light intensity leads to low power generation of the rover, which poses a great challenge to the energy of the rover.
According to the environmental characteristics of low temperature, low pressure, weak light, strong wind and dust on the Martian surface, and considering the engineering constraints, the designers of zhurong put forward the basic design idea of "opening up sources and reducing expenditure" with the goal of controlling energy transmission. They designed two windows like telescopes on the roof of Mars, with a layer of film bound on the windows, so that the sun can shine in when the lighting conditions are good. The principle of this layer of membrane is similar to that of a vegetable greenhouse, which can make energy only enter and leave. That is, the energy supply problem of equipment insulation in the Mars rover cabin can be solved by using solar energy in situ collection, storage and utilization technology, so as to realize "open source" of energy. How can we "cut down" the expenditure? The designers put ten "wine bottles" under the film, and put a phase change material in it - when the light conditions are good, the phase change material in the "wine bottle" will change from solid to liquid, and at night, the phase change material will change from liquid to solid, releasing heat during the solidification process, so as to ensure that the temperature inside the rover does not drop at night.
After the zhurong Mars rover goes into sleep, its electronic equipment will be shut down. The solar energy efficiently collected during the day will be converted into heat energy and stored to uniformly insulate the electronic equipment in the cabin, so as to solve the energy security problem of the Mars rover after encountering bad weather and ensure that the Mars rover zhurong has enough energy to complete its autonomous wake-up.
So how long is the Martian winter? When the zhurong Mars rover just landed, Mars moved near the aphelion point. The direct point of the sun was in the northern hemisphere, which coincided with the summer in the northern hemisphere and the winter in the southern hemisphere. The surface temperature of Mars in the northern hemisphere was relatively high. Now zhurong has been working on Mars for one year. Mars currently runs near the perihelion, the direct point of the sun is in the southern hemisphere, and the northern hemisphere of Mars is heading for winter. In the next two months, the direct point of the sun will continue to move to the south of Mars and reach near the Tropic of cancer of Mars in mid and late July. At that time, the northern hemisphere of Mars will enter the coldest season in a year. Then the direct point of the sun will start to move northward, and it will take more than two months to return to the current latitude. Therefore, it will take 4 to 5 months, about the autumn and winter season around October on earth, and the northern hemisphere of Mars will usher in spring.
At present, the zhurong Mars rover has worked on the surface of Mars for 347 Mars days, far exceeding the established life design of 90 Mars days, and is currently in "extended service". It is expected that around December this year, the zhurong inspection area will enter the early spring season. After the environmental conditions improve, it will resume normal work. The zhurong Mars rover has brought us countless surprises. We have reason to look forward to receiving the news of zhurong again when the Martian spring comes.
In the vast alien continent, zhurong is great enough as a pioneer. From taking off in Wenchang, Hainan to landing in the Utopia plain on Mars, China's tianwen-1 probe has crossed countless difficulties and dangers in space. Domestic scientists are using the electromagnetic waves that shuttle back and forth between the Martian continent and the earth during their inspection to carry out relevant research with new pictures and valuable scientific exploration data. It is believed that in the near future, a large number of major scientific achievements will also bring us new understanding.
The stars are always shining. Our journey is a sea of stardust. Human exploration of space will never stop. China's first Mars exploration mission started the pace of deep space exploration beyond the moon in China. Although it started late, it started from a higher starting point - through one mission, it realized Mars' orbit, landing and inspection, and realized the landmark achievement of independent innovation and leapfrog development of China's aerospace industry. We look forward to zhurong getting more scientific data and bringing us new understanding and new cognition!
(author: lvbohan, project director of the General Department of lunar and deep space exploration, Chinese Academy of Sciences; jiayingzhuo, senior engineer)