Humans have been exploring the moon for more than half a century, so why is it still difficult for spacecraft to softly land on the moon?

On October 3, the Russian State Space Corporation announced the preliminary results of the investigation into the crash of the Luna 25 probe on the moon, which attracted attention from the outside world. What key points can the outside world interpret from the information released by the Russian side? Humankind has been exploring the moon for more than half a century. What difficulties does the spacecraft still face in softly landing on the moon? What new technologies and solutions are expected to help the mission succeed?

Exploring the details behind the failure

The Russian State Space Corporation issued an announcement stating that on August 19, when the Luna 25 probe was transitioning from lunar orbit to lunar orbit, the engine ignited and ran for 127 seconds, instead of the planned 84 seconds, causing the probe to enter a lower non-target orbit and eventually collide with the lunar surface.

The failure of the Luna 25 mission was a pity because Russia inherited the Soviet Union's technical background in the field of lunar exploration, and this lunar exploration program was launched as early as the 1990s. If all goes well, Luna 25 will softly land in the high latitudes of the southern hemisphere of the moon and use 8 instruments and "the most complex space robot arm in Russian history" to collect and analyze lunar samples.

Schematic diagram of the Russian Luna 25 probe's orbit change

The power system of Luna 25 is similar to that of many Soviet lunar probes. It does not use a deep variable thrust engine or multiple variable thrust engines, but a combination of a large thrust main engine and two floating engines. During the powered descent of the probe, the engine group will ignite twice. The first ignition is to "eliminate" the horizontal speed to ensure that the probe safely switches to vertical descent, and then the second ignition is to "eliminate" the vertical speed to ensure that the probe achieves a soft landing on the moon.

In fact, the risks of this lunar landing plan cannot be ignored, and the tolerance for equipment failure is lower, but the technology required is relatively simple. Some people believe that after experiencing two failed Mars exploration missions, the Russian space agency has recognized its own technological weaknesses, so it tries to refer to the Soviet experience.

However, during the process of lowering the orbital perigee from 100 km to 18 km, the engine group of Luna 25 malfunctioned, resulting in a rare lunar landing failure in the lunar orbit phase in recent years. The Russian side pointed out that the most likely cause of the accident was that a large number of commands were sent at the same time, causing some instruments to malfunction, which in turn caused the integrated control system to operate abnormally, and failed to shut down the propulsion system in time when the probe reached the predetermined speed.

Specifically, the accelerometer in the inertial navigation unit of Luna 25 did not work properly, so the probe could not measure its own orbital changes based on the accelerometer data during braking, and could not shut down the engine in time. Instead, it was forced to keep the engine running for the maximum allowed time, causing the descent orbit to be too low. Similar situations had occurred briefly before, but the Russian side did not pay enough attention and continued to lower the probe's orbit, which eventually led to the crash.

Further analysis of the details shows that the problem of the Luna 25 probe is not only in the accelerometer, but also includes the control system bus priority problem, which delayed the accelerometer start command, and the error of the bus transceiver scheduling software caused the command to be "lost". In addition, the communication protocol of "Luna 25" did not respond, causing the flight control software to fail to detect that the accelerometer was not turned on.

It is reported that the general contractor of "Luna 25", the Lavochkin Scientific Research and Production Association, did not cooperate with Russia's specialized institutes for spacecraft flight control software, resulting in insufficient experience of the flight control program writers and posing hidden dangers.

Solving the "high-risk" problem

Luna 25 is the fifth lunar probe to fail in the past five years. More than 50 years ago, humans landed on the moon many times, and many technological fields have made great progress, but the success rate of spacecraft landing on the moon in the new century is still less than 50%, which shows that it is not a simple task to land a spacecraft safely on the moon.

In fact, if you just want to land on the moon, the difficulties faced by spacecraft with sufficient redundant design and sufficient testing are not that great. For example, the Japanese miniature lunar probe that failed to land on the moon at the end of last year was essentially a 12U cubic satellite, weighing only more than 10 kilograms, and chose a semi-hard landing method. However, the scientific value of this probe is better than nothing. The new generation of lunar landing spacecraft often needs to carry enough scientific instruments and lunar rovers, be equipped with a dual-component power system, and form a complete three-axis stable control platform. The risks and challenges are naturally "rising with the tide."

Launching a spacecraft and entering lunar orbit are the "basic skills" of lunar exploration missions. Among the five failed lunar landing missions in this century, the powered descent phase is the "high-risk" moment for most probes, which can be regarded as the core difficulty of the lunar landing mission. To solve this problem, the key is not to blindly pursue advanced large-range adjustable variable thrust engines and increase system risks, but to have inertial navigation elements with high enough accuracy and reliable and applicable flight control software. In reality, small problems in the power system are often "infinitely magnified" by the latter two deficiencies, resulting in mission failure.

In this way, the key solution to the problem of landing on the moon is to recognize the country's technical foundation, choose an appropriate solution, and test the subsystems and the probe. However, some lunar exploration teams lack engineering experience in landing on celestial bodies with greater gravity. They should have added sufficient redundant designs, but due to factors such as funding and rocket performance, they chose smaller spacecraft, resulting in limited spacecraft quality and insufficient redundancy.

Some lunar exploration teams also failed to strictly control the design and production of spacecraft, lacking sufficient fault analysis and isolation research on locations prone to single-point failures. Mentality problems also led to lax software testing, resulting in failures not being fully exposed during ground testing of spacecraft. As a result, hidden dangers of spacecraft are inevitable in the harsher environment of the Earth-Moon space.

In the early stage of the probe's powered descent, the ground team can intervene in the control work, but in the later stage of this stage, the probe will mainly make autonomous measurements and decisions. Since the moon has basically no atmosphere, the entire powered descent phase is very dependent on the probe's power system, which has high requirements for flight control and low fault tolerance.

Especially when the probe is about to land on the moon, the high-temperature and high-speed airflow ejected by the engine will blow up the lunar dust, which is likely to cover the ground measurement instruments and cause large abnormal data fluctuations. For this reason, the instruments need to take isolation and filtering measures, or choose special instruments that can penetrate the lunar dust for measurement, in order to prevent the moon landing mission from failing. In addition, the terrain on the lunar surface is complex and changeable. When the instrument monitors special geographical features, it may cause large-scale data fluctuations. It is necessary to do a good job of data analysis and processing.

New challenges call for new approaches

The new round of international lunar exploration has made the lunar South Pole region a key target, and the mysterious back side of the moon has also attracted the attention of scientists. When the probe lands on the back side of the moon or the South Pole region, due to the problem of signal shielding, the probe must keep tracking the signal of the relay satellite. In addition, the terrain of the lunar South Pole region is complex, and the area suitable for landing is small, which requires the probe to be able to process terrain data better, achieve high-precision landing, and make emergency landing plans in unknown areas.

Many countries are promoting the construction of navigation constellations to serve the Earth-Moon space

At present, in order to further improve the accuracy of lunar landing, researchers have mainly tried two methods.

The first is to apply image matching technology. In September this year, Japan's SLIM lunar lander was launched. Early next year, it plans to use data from lunar orbiters from the United States, Japan, South Korea and other countries to conduct image matching guidance, strive to achieve a 100-meter landing accuracy, and explore potential lunar base alternatives such as lunar lava tubes.

However, image matching technology still has shortcomings when used in lunar South Pole missions. Due to the severe fragmentation of the terrain in the lunar South Pole and the presence of a large number of perpetual night zones, the detection efficiency of traditional lunar probes is limited and it is impossible to generate images of the interior of the perpetual night zone craters. For this reason, lunar probes from the United States, South Korea and other countries are equipped with infrared radiometers, specialized cameras, etc., and are improving the imaging effect of the lunar perpetual night craters, which is expected to play a greater role in assisting probes to land on the lunar South Pole.

The second method is to build a lunar navigation constellation as soon as possible. Although the Lunar Atmosphere and Dust Environment Probe launched in 2013 tried to use the beam sidelobes of GPS navigation satellite signals to initially achieve positioning and navigation of the lunar orbit, the average distance between the earth and the moon is 380,000 kilometers, and it is very difficult for the probe to land on the moon relying on the signals of low-Earth orbit navigation satellites. Currently, many countries are preparing to build a lunar navigation constellation and plan to deploy multiple satellites to achieve a lunar positioning accuracy of 10 to 50 meters.

In the future, the lunar navigation constellation is expected to significantly reduce the demand for measuring instruments for spacecraft landing missions on the moon, improve reliability, and increase payload. Among them, a navigation constellation with signal coverage of the lunar south pole is a "necessity", and multiple satellites in different orbits will provide communication, navigation, remote sensing and other services.

At present, these two methods have been initially verified in the lunar exploration missions of various countries and are expected to be gradually put into practical use in the next few years. As the lunar exploration plans of various countries advance, more and more unmanned and manned spacecraft will land on the moon, prompting humans to continue to deepen their understanding and development of the moon. It is believed that as more lunar landing sites are selected, more advanced, reliable and adaptive technologies will significantly benefit mankind. (Author: Zhang Chen Image source: Russian State Space Corporation Check Expert: Jiang Fan, Deputy Director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)