At 19:52 on February 10th, Beijing time, the Tianwen No. 1 detector implemented the near-fire capture brake. The 3000N orbital control engine of the orbiter ignited for about 15 minutes, and the detector successfully entered the near-fire altitude of about 400 kilometers. About 10 Earth days and a large elliptical orbit with an inclination of about 10o, it became China's first man-made Mars satellite, achieving the goal of "orbiting, orbiting, and patrolling" as the first step and successfully orbiting Mars.
Drawing: Li Xueyao
After China’s first Mars exploration mission entered the Mars capture phase from the ground fire transfer phase, the medium-resolution cameras, high-resolution cameras, magnetometers, mineral spectrometers, ion and neutral particle detection and detection carried by the Tianwen-1 orbiter Loads such as energy particle detectors will begin to work one after another to conduct multi-dimensional exploration of Mars.
Since its successful launch on July 23, 2020, the Tianwen-1 probe has flown for 202 days, completing one deep-space maneuver and four midway corrections.
When it arrived on Mars, the Tianwen-1 probe had a flight distance of about 475 million kilometers and a distance of about 192 million kilometers from the earth. The one-way delay of the communication between the device and the ground was about 10.7 minutes, and the systems were in good condition.
Tianwen-1 sent back the first image of Mars.
Photo courtesy of China National Space Administration
The "Space Brake" of Tianwen No. 1 must succeed once.
How did the scientific researchers make a plan to make the Tianwen-1 probe accurately step on the "brake" and successfully implement Mars capture?
The reporter interviewed relevant experts to find out.
Must succeed once
When approaching Mars, the Tianwen-1 orbiter activates its engine to decelerate its thrust and reduce its speed until it can be captured by the gravitational force of Mars and become a satellite orbiting Mars. Otherwise, the probe will fly over Mars and continue to orbit the sun.
For Tianwen No. 1, there is only one opportunity for near-fire braking. Once it is missed, the next suitable window will have to wait a long time.
For example, the ground fire transfer orbit is like an elliptical closed-loop high speed centered on the sun. Mars is only an exit on this high speed. Once the probe cannot brake in time and get off the high speed from the Martian exit, it can only be more. Make a circle until you pass the exit next time.
In December 2010, Japan's "Daybreak" Venus probe failed to complete the Venus capture and braking in time due to engine failure, and flew directly over Venus.
It was not until December 2015 that it returned to the vicinity of Venus again, at which time it was approaching the end of its lifespan. Fortunately, it was successfully captured later, otherwise it would "be a memory".
In this critical braking section, the China Aerospace Science and Technology Group’s R&D team has designed a very reliable “brake” program, which can not only accurately determine whether the target speed is reduced, but also fully autonomously control the engine during the engine thrust deceleration control process. The magnitude and direction of thrust are judged in real time, and braking parameters and corresponding control algorithms are independently updated to ensure reliable and accurate braking.
Fully automatic "brake" at 192 million kilometers away
"For track design, the brake force of near-fire braking is extremely elegant. If you step on it too lightly, you will fly away from Mars; if you step on it too hard, it will have a huge impact on the subsequent flight sequence." Tian Question No. 1 track designer Gao Shan uses a vivid metaphor to describe the near-fire brake.
Data map: At 12:41 on July 23, 2020, China successfully launched the first Mars exploration mission Tianwen-1 probe with the Long March 5 Yaosi carrier rocket at the Wenchang Space Launch Field.
Photo by Luo Yunfei
The "capture window" of Mars' gravity is limited, requiring the probe to reduce its speed from 28km/s (kilometers per second) to about 1km/s within 15 minutes.
Unlike conventional satellites that can be controlled in real time by the ground, during the "braking capture" process, the Tianwen-1 probe is about 192 million kilometers away from the earth, and the one-way time delay of data communication between the earth and the probe exceeds 10.7 minutes. It is necessary to completely rely on itself to complete engine ignition and shutdown, overcome disturbances during engine ignition, and achieve precise control of ignition direction and ignition duration.
Zhu Qinghua, deputy chief designer of Tianwen No. 1 orbiter, said, “Without real-time measurement and control on the ground, we can only implement a precise orbit fully automatically by designing a scheme that fully considers the deviation of the engine thrust and the continuous change of the center of mass of the detector. Control, and then through multi-factor combination testing and simulation analysis to make the control scheme more reliable."
Aerospace "Sky Mission" escorted throughout
The Tianwen-1 probe set off from the earth on July 23, 2020. After a scorching summer, a rusty golden autumn, and a bleak winter, the long-distance journey lasting nearly 7 months finally reached the ring fire orbit.
Data map: On October 1, 2020, the China National Space Administration released the flight images of China's first Mars exploration mission Tianwen-1. The five-star red flag on the map is dazzling, showing a bright Chinese red.
This is the first deep-space “self-portrait” of China's Tianwen-1 probe.
Photo courtesy of China National Space Administration
From the day of the rush to the fire, the Chinese aerospace researchers have carried out detailed interpretations of its state day and night, and made four midway corrections along the way, one deep-space maneuver, and two state self-checks. The journey is more solid.
In the near-fire brake, 159 scientific researchers worked in coordination at the Beijing Aerospace Flight Control Center and tracked continuously for 24 hours; there was also the "strongest heaven group" of the Fifth Academy of China Aerospace Science and Technology Corporation flight control. Among them was the Chinese Academy of Sciences. Academician Ye Peijian and other 16 technical experts and consultants, 5 "two generals", 61 flight control personnel in various positions.
As the interviewed experts said, as the Chinese New Year approaches, the Tianwen-1 probe enters an orbit around Mars as it wishes. It also gives the people of the country an early year and sends blessings for the Year of the Ox in advance.
"Sun Moon An? Lie Xing An Chen?" From the beginning of Huanhuo, the Chinese-made spacecraft Tianwen-1 has officially become a satellite of Mars.
In the follow-up, Tianwen-1 will undergo an orbital adjustment and two near-fire braking. By then, Tianwen-1 will be only 265 kilometers away from Mars.
After that, the Tianwen-1 probe will complete the photographing mission while surrounding it, and carry out the pre-selected landing area detection. It is planned to choose an opportunity to land on Mars from May to June 2021 and carry out inspections.
Author: Guochao Kai, Zheng Yingying