Zero distance
After falling into the fire, where did the "other half" of Tianwen No.1 go?
On May 15, 2021, China successfully landed on Mars. This feat was achieved by the Tianwen-1 probe, my country's first Mars exploration mission.
What is less known is that the Tianwen-1 probe includes two parts: the orbiter and the landing patrol.
The landing rover that landed on Mars on May 15th, so where did the "other half" of Tianwen-1-the orbiter go?
Yes, it returned to the orbit of Mars and continued to soar over Mars.
At 4 o'clock in the morning on May 15, the Tianwen-1 probe successfully implemented the "two-device separation" between the orbiter and the landing patrol. Three hours later, the landing patrol successfully passed through the Martian atmosphere and achieved a soft landing on the surface of Mars; two About 30 minutes after the separation of the vehicle, the orbiter will ascend and return to the Mars mooring orbit, becoming a communication relay station between the landing patrol vehicle and the earth, while continuing to perform Mars orbiting exploration.
Interstellar "special car"
According to Zhu Qinghua, deputy chief designer of Orbiter of the Eighth Academy of China Aerospace Science and Technology Corporation, in order for the landing patrol to accurately enter the orbit of Mars, the orbiter needs to first carry the landing patrol to enter the orbit of Mars, and after the two devices are separated. , The orbiter needs to quickly raise the orbit, and the landing rover enters the Martian atmosphere.
The control before and after separation takes 7 hours.
As an interstellar "special car" equipped with a landing patrol, the orbiter needs to sequentially complete a series of actions such as the orbit lowering of the engine ignition and shutdown, the establishment of the two-device separation attitude, and the orbital raising of the engine ignition and shutdown after the two devices are separated.
"This is a series of critical attitude and orbital maneuvers. With a little carelessness, the probe may be pulled towards the surface of Mars by the gravitational force of Mars. Due to the communication delay, we have no way to know the state of the probe in real time. Intervene in abnormal situations." Zhu Qinghua said.
In his view, the process of separating the device is the most sufficient test for the accuracy of the control algorithm, the reliability of the product, and the thoroughness of the failure plan.
In fact, after the accurate landing point of the landing patrol is clarified, a series of maneuvers of the probe will be determined accordingly.
Three hours before the probe's first de-orbit ignition, the aerospace designer had already noted all control strategies, which included responses to possible situations.
During separation, the orbital control accuracy and attitude control accuracy of the orbiter are the prerequisites for whether the landing patrol can enter the predetermined landing zone.
These need to rely on the accuracy of sensors, actuators, computers and algorithms, which is a "big test" for the detector's guidance, navigation and control system.
Solution designer Wang Weihua used an analogy: This is like shooting outdoors at a distance of 980 meters from a standard basket. The angle, timing, pitching strength of the basketball must also be considered in advance, as well as the basketball's own rotation, wind speed and wind direction, and the external environment. The influence of various factors.
At the same time, the designers also made plans and countermeasures for different situations.
When the orbiter finds that it has not completed the predetermined action through its own sensor, it will autonomously take the landing patrol to quickly raise the orbit to avoid hitting Mars, and again choose to perform a series of actions that separate the two devices at the right time.
Communication "relay station"
In this Mars exploration mission, the orbiter is not only an interstellar "special vehicle", it is also a powerful communication "relay station", which builds a communication bridge between the Mars surface patrol and the earth, and is responsible for remote sensing of the surface of Mars. At the same time, choose the right time to "relay" the data from the patrol to the earth.
Accurately pointing to the earth on an orbit of 293 million kilometers away from the earth is equivalent to aiming at the embroidery pinhole at a distance of 2 meters, and maintaining the aiming state at all times when the orbiter is in flight.
Zhu Qinghua said that the orbiter carries two solar cell arrays, one high-gain data transmission antenna, and one pair of patrol data relay antenna.
When the orbiter performs data relay missions, it needs to drive the solar array to aim at the sun to ensure its own power supply. At the same time, it needs a high-gain antenna to track the earth and the relay antenna to point to the patrol to establish a data "magpie bridge".
"At this time, the orbiter needs to simultaneously achieve high-precision and synchronized pointing control of the three targets of the patrol, the earth, and the sun. It can definitely be called'exquisite and smooth'." Guidance, navigation and control system chief designer Nie Qinbo Say.
According to him, near-Earth satellites are usually stable to the ground for a long time, using omnidirectional antennas, there will be no problem of communication link interruption, and the orbiter is far away from the earth when flying around the fire. Due to the limited antenna beam angle, the designer We need to make plans for possible communication link interruptions while ensuring high-precision control of the antenna pointing.
"We have designed an autonomous recovery strategy after the communication link is interrupted. Once the communication link is interrupted, the detector will'autonomously spin down', and in this process, the antenna will be swept to the earth and the communication link will be restored. Road. This process is also realized independently by the surround device." Software designer Zhou Zhiyuan said.
Today, the orbiter is still flying around the fire in a stable manner, and has become a communication bridge between the landing patrol and the earth. After completing the data relay mission, it will also fully start its own remote sensing journey of the fire.
China Youth Daily·China Youth Daily reporter Qiu Chenhui Source: China Youth Daily