In front of Mars, how difficult is Tianwen-1 to "brake"
Near-fire braking is one of the important nodes in the process of Mars exploration and has certain risks.
The risks mainly include several aspects. For example, there is only one opportunity for braking to capture, and the opportunity cannot be missed; the timing, duration, and strength of braking must be very accurate.
Yang Yuguang Researcher of the Second Academy of China Aerospace Science and Industry Corporation
On February 5, the National Space Administration released the first Mars image taken by the Tianwen-1 Mars rover.
In the picture, iconic landforms such as the Asidalia Plain on Mars, the Kruse Plain, the Meridian Plateau, the Skiapareli Crater, and the longest canyon, the Vale of Sailors, are clearly visible.
When this photo was taken, the distance between Tianwen-1 and the Earth was more than 180 million kilometers, and about 2.2 million kilometers from Mars.
According to the National Space Administration, Tianwen-1 started the engine in the evening of the same day and successfully completed the fourth midway correction of the ground-fire transfer section to ensure the accurate implementation of subsequent actions.
After that, it will complete the near-fire braking, be captured by Mars, enter the ring-fire orbit, become a satellite of Mars, and observe Mars in preparation for landing on Mars.
Yang Yuguang, a researcher at the Second Academy of China Aerospace Science and Industry Corporation, told a reporter from the Science and Technology Daily that near-fire braking is one of the important nodes in the process of Mars exploration and has certain risks.
The risks mainly include several aspects. For example, there is only one opportunity for braking to capture, and the opportunity cannot be missed; the timing, duration, and strength of braking must be very accurate.
In addition, the detector is far away from the earth when it arrives on Mars, the communication delay is long, and the ground cannot be monitored and controlled in real time. The entire braking process must be completed by the detector autonomously, which puts high demands on the autonomous control of the detector.
The timing and duration of braking need to be the same
According to the laws of orbital dynamics, when a spacecraft approaches a celestial body, if no measures are taken, it will pass by the celestial body, forming a hyperbolic orbit throughout the process.
Yang Yuguang said that during the approach, the spacecraft will be affected by the gravitational force of celestial bodies, and its speed will become faster and faster, reaching a peak when the distance is the closest.
If the spacecraft does not brake at this time, it will gradually move away from the celestial body, its speed will slowly decrease, and finally get rid of gravity and fly away from the celestial body.
This law determines that if you want to enter the orbit around the celestial body from the transfer orbit, the spacecraft must implement braking and deceleration.
Pang Zhihao, the chief science communication expert of national space exploration technology, introduced that when the detector implements braking and deceleration, it must first adjust the flight attitude, turn the engine nozzle toward the front, and ignite when the time is right to reduce the detector's speed.
The control of the entire braking process must be very precise.
Yang Yuguang introduced that the probes of various countries basically use the Homan transfer orbit to fly to Mars, and the same is true for Tianwen-1.
For the gravitational field centered on the sun, this is an elliptical orbit connecting the earth and Mars.
When the rover arrives near Mars, if there is no braking or insufficient braking force, it will deviate from the gravity of Mars and continue to orbit the sun. It will be unknown when it will meet again next time.
This is the uniqueness of the brake capture opportunity mentioned earlier.
On the contrary, if the force is too strong, it will not work.
Yang Yuguang said that the detector has a principle of braking and capturing. The lower the track, the more energy-saving the engine, and the higher the braking efficiency.
But for Mars that is hundreds of millions of kilometers away, errors may occur in the position measurement and control of the probe. If the orbital height of the braking point is too low, the probe may crash into the surface of Mars.
At the same time, the detector does not reduce the speed at a certain point, but continues to decelerate within an arc of flight. The time of braking and ignition requires precise calculation and precise control.
If the braking time is too long and the braking force is too large, the detector will also face the risk of crashing.
"Only when the timing and duration of braking are not bad every second can form an ideal capture track." Pang Zhihao said.
Braking is different from "Chang'e"
In the previous lunar exploration missions that my country has successfully implemented, the Chang'e series of detectors have successfully completed near-month braking and accumulated rich experience.
However, the near-fire brake faced by Tianwen No. 1 is different from that of "Sisters Chang'e".
The most significant difference comes from distance.
The average distance between the earth and the moon is about 384,400 kilometers. For measurement and control communication, the delay is only about 1 second.
When Tianwen-1 implemented near-fire braking, the distance between the earth and Mars exceeded 180 million kilometers, and the one-way communication delay reached more than 10 minutes.
Yang Yuguang said that in this case, the ground cannot monitor the detector in real time, and instructions need to be uploaded in advance so that the detector can execute it autonomously.
In theory, the autonomy of the detector can be achieved by design.
For example, how much the speed needs to be reduced during braking can be calculated based on the engine thrust and the time required to start the machine, and control accordingly.
But this is not the case.
Yang Yuguang said that there will be many uncertainties during the mission. For example, there may be slight deviations in engine thrust. The position, speed, and attitude of the aircraft can only be obtained by measurement. Various variables make autonomous control more complicated.
Therefore, autonomous control does not rely solely on ground preparation instructions, which can be completed by detector execution, but relies heavily on measurement methods.
In order to ensure the accuracy of the measurement, there are multiple sensors on the aircraft that perform key actions, and through multiple channels and different means, combined with ground measurement data, determine whether the measurement results are accurate.
When executing instructions, sensors are also used to feedback the execution status.
When encountering an unexpected situation, it is too late to wait for ground processing, the detector will also judge by itself, and then respond according to the plan.
In addition, Tianwen-1 weighs more than 5 tons, not only surpassing the "Chang'e" series of probes, and even ranking first among planetary probes in the world.
And it is equipped with a 3000 N engine, is it a little "small horse-drawn cart"?
You know, the 3.78-ton Chang'e-4 has a main engine thrust of 7,500 Newtons.
In this regard, Yang Yuguang said that Chang'e-4’s engine must take into account both near-moon braking and lunar landing, and therefore requires greater thrust.
For Tianwen No.1, the 3000N engine is indeed too small, but this is the choice after weighing the pros and cons of all parties.
Yang Yuguang said that the most important indicator to measure engine performance is specific impulse. The higher the specific impulse, the greater the increase in speed that the propellant can produce under the same conditions.
But on the other hand, the higher the specific impulse, the greater the mass of the engine. In other words, under the same specific impulse, the greater the thrust of the engine, the greater the volume and mass.
But for Tianwen-1, it is hoped that the engine will be as small as possible, so that more weight and space can be reserved for the load.
The designers made a trade-off between two contradictory indicators.
The current plan can not only meet the needs of the load, but also meet the requirements of braking, it is nothing more than extending the braking ignition time.
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Those detectors that failed to brake
Due to the risk of near-fire braking, some Mars probes in the past have experienced various failures when they entered the orbit of Mars.
The Mars 4 probe launched by the Soviet Union on July 21, 1973, flew over at a distance of 2,200 kilometers from the surface of Mars in February 1974, but did not cut into the orbit of Mars due to the failure of the brake engine.
On September 25, 1992, the US Mars Observer probe took off.
It flew in space for about 11 months. On August 21, 1993, three days before it was planned to enter the orbit of Mars, it malfunctioned and lost contact with the ground.
It is judged that the reason for the failure of the detector may be the rupture of the fuel transportation pipeline of the propulsion system.
The Nozomi Mars orbiter launched by Japan on July 3, 1998, has been malfunctioning since its launch.
According to the plan, Nozomi should reach a position of 894 kilometers from the surface of Mars on December 14, 2003, and then enter the ring fire orbit. However, due to the failure of its circuit system affected by the solar storm, the orbital engine cannot be started and the mission failed.
The most unjust failure came from the US Mars Climate Orbiter.
On September 23, 1999, the probe was supposed to enter the Martian atmosphere at an altitude of 80-90 kilometers, but during the orbit cut operation, ground personnel made a fatal low-level error.
In the probe flight system software, the imperial unit "pound" is used to calculate the thruster power, but when the ground personnel input the direction correction amount and thruster parameters, the metric unit "Newton" is used.
The confusion between the two units caused navigation errors, causing the probe to slow down and cut in until it was only 57 kilometers away from Mars, which eventually led to the probe's disintegration.
Our reporter Fu Yifei