On Humanity’s Lunar Day, how to protect the lunar heritage?

In order to take a photo of the banner on the lunar spacecraft, an "astronaut" who landed on the moon accidentally stepped on the first footprint left by Armstrong on the lunar surface. The "first footprint of mankind in space", the lunar heritage, was destroyed. This hilarious scene comes from the promotional advertisement of a mobile phone brand.

(Astronaut: I'm in trouble on the moon!

Screenshot from https://www.bilibili.com/video/av35593926)

In fact, in reality, this risk does exist. With the continuous advancement of human space technology, it will soon become a reality for humans to return to the moon and conduct more comprehensive and in-depth exploration of the moon: my country's Chang'e 4 has achieved the first landing exploration of humans on the back of the moon, and NASA's planned deep space gateway space station will build a forward base for landing on the moon in orbit around the moon, and continuously release probes to the lunar surface to conduct systematic exploration of the moon.

In order to prevent increasingly frequent exploration from destroying human history and scientific heritage on the moon, NASA has issued a corresponding recommendation report.

Descent/Landing Boundary: Avoiding Spacecraft from Destroying Heritage

There is quite a lot of human heritage on the moon.

In 1959, the Soviet Union's Luna-2 controlled impact with the moon left behind a man-made object on the lunar surface for the first time. The Apollo moon landing program in the 1960s and 1970s left behind a large number of man-made objects of various purposes on the lunar surface. Among them are Saturn V rocket S-IVB upper stage, lunar module, lunar rover and other spacecraft that completed their missions, as well as the "Fallen Astronauts" sculpture created to commemorate the astronauts who died in the line of duty, the American flag, astronaut medals and other spiritually valuable objects.

To protect these lunar heritage sites from being destroyed by future space activities, NASA has established two "protected area" ranges for these artificial objects: the descent/landing boundary and the human activity boundary.

The descent/landing boundary is designed to protect the spacecraft "artifacts" from the effects of flying spacecraft. NASA recommends that both the trajectory of a spacecraft descending close to the lunar surface and the trajectory of a spacecraft flying low should be outside a circular area with a radius of two kilometers centered on the lunar heritage.

Through theoretical calculations and actual observations during the Apollo lunar mission, scientists are convinced that the high-speed gas ejected by the spacecraft engine onto the lunar surface can blow most of the lunar soil and gravel on the lunar surface to a speed of 300-2000 meters per second. The smallest lunar soil dust can even be blown to more than 2.37km/s, exceeding the lunar escape velocity and entering the orbit around the sun. If the spacecraft is too close to a lunar heritage when landing or flying over the lunar surface, these lunar soil and gravel may hit these lunar heritages and cause damage. For example, the landing site of Apollo 12 is 155 meters away from the previously landed Surveyor 3 unmanned probe. When Apollo 12 landed, the gravel raised by the engine jet hit the surface of Surveyor 3, causing minor damage. Fortunately, Surveyor 3 is located in an impact crater, and its geographical elevation is 4.3 meters lower than the landing site of Apollo 12, so it actually luckily avoided most of the gravel attacks. Subsequent analysis showed that if the geographic altitude of Explorer 3 was the same as that of Apollo 12, the impact it suffered would have been several orders of magnitude greater, likely causing more serious consequences such as damage to the probe.

(Apollo 12 astronaut Pete Conrad performs a sampling mission near the Surveyor 3 probe. It is not difficult to see from the picture that the Apollo 12 spacecraft lunar module in the distance has a higher geographical elevation than the Surveyor 3. Image source: NASA)

As the distance increases, the damaging effect of the sand and gravel blown up by the engine will gradually weaken. However, if the protection range is too large, it will bring considerable restrictions to future exploration activities. Fortunately, the small radius of the moon provides a possible solution to this contradiction. Calculations show that although the sand and gravel blown up by the engine can fly to a distance of 1.8 kilometers from the spacecraft, due to the curvature effect of the lunar surface, most of the sand and gravel are already far away from the lunar surface here and can fly over the lunar heritage. Expanding the descent/landing boundary to 2 kilometers can further deal with risks such as the spacecraft falling out of control.

Artificial activity boundaries: Avoiding astronauts from getting into trouble by clocking in

If a lunar spacecraft landed on the lunar surface more than 2 kilometers away from the lunar heritage, reckless astronauts might still find ways to "check in" near the heritage, which could lead to the tragedy of accidentally stepping on Armstrong's footprints as shown in the mobile phone advertisement.

In the entire Apollo program, there are two missions with special significance: one is of course the Apollo 11 mission, which was the first human landing on the moon, marking the beginning of the moon landing era. The other is the Apollo 17 mission, which marked the end of the craze for moon exploration.

In order to preserve the legacy of these two iconic events forever, NASA set an artificial activity boundary for them: astronauts and man-made objects such as the lunar rover should not enter the 50-meter range around the descent stage of the Apollo 11 spacecraft, which still remained on the moon. For Apollo 17, this distance was increased to 225 meters because the lunar rover increased the astronauts' range of movement.

(A memorial plaque on the descent stage of the Apollo 11 lunar module reads: "In July 1969, people from the planet Earth set foot on the moon. We came for the peace of all mankind." This memorial plaque remains on the moon with the descent stage of the Apollo 11 lunar module. Image source: NASA)

The concentration of the lunar atmosphere is almost negligible, and there is no strong intrinsic magnetic field to protect it from charged particle bombardment. Spacecraft on the lunar surface will be directly attacked by solar wind particles and cosmic rays, and the changes in the Apollo spacecraft and its affiliated detection instruments left on the lunar surface over the past few decades may bring new discoveries and understandings to scientists. For Apollo 12 and Apollo 14-16 spacecraft (Apollo 13 encountered danger during the flight and failed to land on the moon), NASA has given a smaller protection range of 1-3 meters, hoping to detect the conditions of these old spacecraft in the future when possible, while protecting the heritage as much as possible.

Although more than 40 years have passed, the laser transmitters placed on the lunar surface by Apollo 11, 14 and 15 are still playing a role in scientific research. After scientists aim at the position of the transmitter on the moon and fire a laser, they can capture the laser photons emitted by the reflector, calculate the time it takes for the laser to go back and forth, and multiply it by the speed of light to get the distance between the earth and the moon. It is as difficult to accurately illuminate a reflector with a small surface area with a laser as it is to shoot a moving coin 3 kilometers away with a gun. Due to the inevitable divergence effect of the light beam, even for a telescope with a diameter of up to 3 meters, only one photon is received after an average of 1017 photons are emitted by the laser. At the same time, when determining the propagation delay time, some disturbances and changes that occur when the laser passes through the earth's atmosphere must be taken into account, and high-precision data on the distance between the earth and the moon can only be obtained after performing quite complex calculations.

(Apollo 11 textile reflector on the lunar surface. Image courtesy NASA)

The longer the detection data is accumulated, the more scientific information it can reflect. However, once future space activities accidentally contact the reflectors on the moon and change some of their indicators, there will be a "gap" between the accumulated data and the data to be obtained in the future, and the scientific value will be greatly reduced. Therefore, the artificial activity boundaries defined by NASA for the lunar heritage will also help the lunar missions that have become history to continue to produce scientific results.

However, in recent years, observations have shown that the ability of these reflectors to reflect lasers is decreasing. Scientists believe that deformation caused by sunlight, erosion by cosmic rays or solar wind particles, or deposition of dust on the lunar surface on the reflectors may be the culprits. Therefore, scientists also hope to conduct field detection and investigation of the conditions of these reflectors in the future. The 1-meter artificial activity boundary set by NASA for these reflectors can make these detections possible while ensuring the safety of the reflectors as much as possible.

(The Goddard Space Flight Center in the United States is using lasers to measure the distance between the Earth and the Moon. Source: Wikipedia.)

Ethical standards for space exploration

In fact, the exploration recommendations given by NASA through this report have no legal effect even in the United States, and have not been concluded through international agreements such as the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS) and the Committee on Space Research (COSPAR), and have no de facto binding force.

If there really is some reckless person who breaks the protection boundaries set by NASA and steps on Armstrong's footprints, NASA seems to have no other means except beating its chest and stamping its feet and severely condemning him.

However, any country or company that has the ability to send a spacecraft to the moon must be fully aware of the hardships and difficulties of the lunar exploration journey, and will therefore respect the previous lunar heritage and follow NASA's protection recommendations. After all, although Armstrong is an American, his step is indeed a big step for mankind. Respecting and protecting the lunar heritage should be the ethical code of space exploration that space activities should be subject to.

(Cover of the Moon Heritage Protection Report. Source: NASA)

Reference content: NASA's Recommendations to Space-Faring Entities: How to Protect and Preserve the Historic and Scientific Value of US Government Lunar Artifacts, NASA, 2011