Why do multiple Mars rover plans to launch in July and August at the same time period?

　　Multiple Mars rover plans to lift off in July and August

　　At present, the US "Fortitude" Mars probe and the UAE's "Hope" Mars probe, which are scheduled to be launched in July-August, have entered the "countdown" for launch. The launch of my country's first Mars exploration mission, "Tianwen-1", has also received much attention. Not long ago, Bao Weimin, academician of the Chinese Academy of Sciences and director of the Science and Technology Commission of the Aerospace Science and Technology Group, revealed in an interview with a CCTV reporter that from July to August this year, "Tianwen No. 1" will be launched.

　　This multi-country "fire detection" drama is waiting to be seen.

　　Launch in the window period, save time, effort and money

　　Including China, the countries that plan to "fire detect" this year coincidentally chose the rocket launch date in July-August. Why is this?

　　"July-August this year is the "window period" of Mars exploration." Liu Yong, a researcher at the State Key Laboratory of Space Weather at the National Space Science Center of the Chinese Academy of Sciences, said that Mars and the Earth revolve around the sun on the same orbital plane, and the Earth's revolution period is 1 Year, Mars revolution cycle is about 2 years. If the Earth and Mars were to race on a circular runway, Mars, which was "slow" after the gun was fired, would meet Earth about every two years, at which time the two planets are closest. Before and after this moment, launching a Mars rover from the earth will allow humans to "flight" the journey in a shorter time, with less fuel and lower cost.

　　However, even for this shortest flight distance, the probe still has to experience a "long journey".

　　Zhang Wei, a researcher at the Space Application Engineering and Technology Center of the Chinese Academy of Sciences, told Science and Technology Daily that Mars is about 400 million kilometers away from the earth and most recently about 56 million kilometers. Taking the Mars probe that China will launch this time as an example, it takes more than 200 days to fly To reach distant Mars.

　　During the "hurry" of the probe, the earth and Mars always moved relatively. Therefore, there are university questions hidden in the design of the probe's flight path.

　　"Mars probes often fly along the Horman transfer orbit between ground fires." Liu Yong said.

　　The Hörmann transfer orbit was proposed by the German physicist Walter Hörmann. It is a general term for a method of changing the orbit of a spacecraft. It only needs two engine propulsion on the way, which is relatively fuel-efficient.

　　Liu Yong pointed out that after the Mars rover is launched, it first accelerates around the earth and enters the elliptical Horman transfer orbit. It does not need to consume its own fuel; when it reaches the "brake" near Mars, it decelerates and is finally captured by Mars. . This requires that the relative position of Mars and Earth must be calculated in advance when the probe is launched, so that the probe's Hörmann transfer orbit can be tangent to the Martian orbit at a certain moment. The tangent point of the two orbits is the space position where the probe will be captured by the orbit of Mars. This specific moment is the moment when the probe is truly put into the "hug" of Mars.

　　Combining the revolution periods of the Earth and Mars, and the relevant content of the orbit design, scientists have calculated through detailed and careful calculations that the best time to launch a Mars rover every 26 months on Earth is the "window period". Come.

　　If you miss this "window period" from July to August this year, the next good time for Mars exploration can only wait until 2022. For example, the Mars probe developed by the European Space Agency (hereinafter referred to as ESA) and the Russian Space Agency was originally scheduled for launch in July this year, but due to the epidemic and other technical factors, it had to be postponed until two years later.

　　The destination is the same, the time for "rush"

　　If you look through historical data, you will find that even though all previous Mars explorations in the world have been flying along the Homan shift orbit, the time to "get on the road" has been long and short.

　　The "Marine 4" Mars probe launched by the United States captured and transmitted the first picture of the surface of Mars from an altitude of 10,000 kilometers away. This photo full of craters shocked the scientific community. The probe was launched at the end of November 1964 and flew over Mars in mid-July 1965 at an interval of 8 and a half months.

　　"Sailor No. 9", the same series as "Sailor No. 4," is less time-consuming on the way to "rush." The probe was launched at the end of May 1971 and arrived in mid-November of the same year, becoming the first artificial Mars satellite sent to Mars by humans. The flight time is only 5 and a half months.

　　With the passage of time, the flight time of subsequent Mars probes has increased.

　　According to previous reports from the People’s Daily Overseas Edition, my country’s first Mars exploration mission is scheduled to fly for 7-8 months. Of course, there is a longer "fire detection" trip. For example, the "Pirate 1" launched by the United States in 1975 took 10 months to mars; the "Mangalrian" launched by India in 2013 took 11 and a half months to enter Mars orbit.

　　The destinations are the same, why are there different times for "rushing"?

　　Liu Yong told Sci-Tech Daily that the flight time of the Mars probe is quite different. In addition to the actual rocket carrying capacity of the project and the quality of the Mars probe itself, there are two factors.

　　First, the orbits of the Earth and Mars are not true orbits in the true sense. Therefore, even when the two are closest, the shortest distance may have its own length. In the same way, if the Mars rover was not launched during the "window period" of the same year, its flight distance will be slightly different, and the flight duration will naturally be different.

　　"Every'window period' Mars probe flight distance varies, the next "window period" with the shortest flight distance of the probe is in 2035, which is called'Mars Chong' in astronomy." Liu Yong said.

　　Secondly, the flight time is also related to different detection targets such as flyby and orbit. Flying over Mars is only necessary to be close, which is equivalent to a "one-day trip". During the journey, I took a glimpse and took a few photos to go away. Around Mars, you need to "brake and brake" at a specific time, enter Mars orbit to become an artificial Mars satellite, which is equivalent to a "deep tour", and then take root around Mars until the end of life. Therefore, when the probe takes orbiting Mars as the mission target, it carries more fuel, orbit changes are relatively complicated, and the flight time is relatively longer.

　　In addition to varying flight times, Mars exploration is a systematic project, and there are many intriguing scientific issues worth exploring. As Liu Yong said: "Aerospace engineering is the ultimate challenge of human science and technology and has a very high risk. I look forward to the successful completion of my country's first independent Mars exploration mission."

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　　Martian charm makes the "fire detection" army grow stronger and stronger

　　International Mars exploration began in the 1960s. Since the 1990s, Mars exploration has ushered in another climax, and related scientific research missions are in full swing, including the United States, Russia, Japan, Europe, India and other countries. So far, Mars exploration missions have been carried out more than 40 times, including the United States more than 20 times, followed by the Soviet Union/Russia, Japan, ESA and India have also conducted "fire detection" missions. The charm of Mars can be seen.

　　So why do countries so "love" Mars?

　　"Mars is the planet with the closest characteristics to the earth in the solar system, and the first choice for human migration in the future." Zhang Wei said that the gravity on Mars is about 2/5 of the earth, with a relatively thin atmosphere-the atmospheric composition is 95% Carbon dioxide, 3% nitrogen, very little oxygen, water vapor, etc., and there may be water resources, through reasonable transformation, it is hopeful that it will become an extraterrestrial living space where humans can reside in the future.

　　"Furthermore, the scientific value of Mars exploration is very outstanding." Liu Yong said in an interview with a reporter from Science and Technology Daily.

　　What caused Mars to lose water? How does it change from a blue planet to a deserted red planet? How has its atmosphere and environment evolved? Is it possible that Mars today is the future of the earth? These are the questions that scientists want to answer through relevant detection.

　　In addition, Zhang Wei pointed out that Mars exploration is a system engineering involving many high-tech fields. It should solve orbit design, autonomous attitude determination and control, landing navigation, thermal control, radiation protection, remote operation and remote analysis, deep space measurement and control and other technologies. The problem can promote the rapid development of many fields such as system engineering, automatic control, energy, materials, communications, remote sensing, etc., and is also an important manifestation of a country's scientific and technological strength and innovation capabilities. "In today's increasingly fierce international competition, Mars exploration has become the main'arena' for the competition of major international space powers for a long time now and in the future." Zhang Wei said.