There are many lunar exploration orbits, and the key is to choose wisely

Recently, NASA launched the first lunar exploration satellite under the framework of the "Artemis Project". It will try a special lunar orbit for the first time, but it lost contact during the flight, which aroused concern. In fact, there are many types of lunar exploration orbits selected by spacecraft to meet different mission requirements. As human exploration and development of the moon gradually enters a climax, the selection, design and maintenance of lunar exploration orbits are becoming more and more important. So what are the basis behind the selection of lunar exploration orbits? What are the advantages of different lunar exploration orbits?

Tailor-made for the lunar space station

On June 28, the US Capstone satellite was launched from New Zealand on an Electron rocket. The cube satellite weighs about 25 kilograms and will carry out technical operations and navigation tests of the lunar autonomous positioning system. Its main purpose is to test the orbit of the future lunar space station. Specifically, it plans to arrive near the moon in about four months, enter a near-rectilinear halo orbit, and carry out testing.

Schematic diagram of the American CubeSat heading to the moon

The design of the near-rectilinear halo orbit is very sophisticated. The spacecraft does not orbit the moon, but orbits the Earth-Moon L2 point. The Earth-Moon L2 point is located on the extension line from the Earth to the Moon, 65,000 kilometers away from the Moon. There, the gravity of the Earth and the Moon on the spacecraft reaches a balance, and less fuel is used to maintain the position.

However, due to external forces, it is difficult for the spacecraft to stay stably at the Earth-Moon L2 point, and it is blocked by the moon and cannot communicate directly with the earth. In response to this, scientists designed a halo orbit specifically for spacecraft.

The halo orbit is a complex shape, a non-coplanar three-dimensional irregular curve, which makes it difficult for spacecraft to control. From the earth, the trajectory of the spacecraft looks like a halo, hence the name orbit.

There are many types of halo orbits, and spacecraft can set them specifically according to mission requirements. According to public information, during the Chang'e-4 mission, the Queqiao relay satellite entered a halo orbit with a Z-axis amplitude of about 13,000 kilometers, which helped Queqiao see both the Earth and the far side of the moon, successfully established a communication connection, and laid a solid foundation for the first soft landing of a human probe on the far side of the moon.

In the Artemis Program, the Gateway lunar space station plays a very important role. It is a future communication relay station, a lunar surface exploration transit station, a relay station for traveling between Mars and deep space, and a test site for deep space exploration technology. NASA has put forward many requirements for its orbit, including that it should be conducive to lunar landing, convenient for astronauts and cargo to travel back and forth, easy to enter deep space, and convenient for the establishment and measurement and control of the space station.

An image of the future U.S. Gateway lunar space station

How to achieve so many requirements? Scientists specially designed a nearly straight halo orbit, with the perigee at about 4,000 kilometers and the apogee at about 75,000 kilometers.

On the one hand, the round trip from here to Earth orbit and Moon orbit requires only a small speed increment, and the cost of maintaining the space station orbit is also relatively small.

On the other hand, the thermal environment of this orbit is stable, which is convenient for the long-term operation of the space station. The orbital plane is basically perpendicular to the Earth-Moon line, completely unobstructed, making it very easy to communicate with the Earth, and it covers the lunar polar regions well, which is conducive to the landing mission at the lunar South Pole.

Previously, this special orbit had never been used by a spacecraft, and it was necessary to select a specific launch date and master precise flight control technology. One of the main tasks of the Capstone satellite is to demonstrate the maneuvers required to reach this orbit and to collect test data for 6 months to explore the way for the "Artemis Project."

There are many different lunar exploration orbits

The lunar orbit is a very important space resource, which is of great value for lunar exploration and resource development. On April 3, 1966, the Soviet Union's Luna 10 probe entered the lunar orbit for the first time. Since then, the probe no longer only takes snapshots during the brief period of passing over the moon, but can fly around the moon stably, continuously acquire images, and continuously improve the detection resolution.

Depending on the center of the spacecraft's orbit, the lunar orbit can be divided into a lunar orbit, an orbit near the Earth-Moon equilibrium point, and an Earth-Moon cycle orbit.

The lunar orbit is centered on the moon and includes low lunar circular orbit, high lunar circular orbit, lunar large elliptical orbit, etc.

Low lunar orbit is the most commonly used in lunar exploration missions. It is 100 to 300 kilometers away from the lunar surface. Spacecraft can achieve high-resolution imaging and high-precision detection of the lunar surface. It is also an important transition orbit for lunar landing missions. my country's "Chang'e 1" operates in a circular orbit at an altitude of 200 kilometers, and my country's "Chang'e 2", Japan's "Kaguya", and India's "Chandrayaan-1" all operate in circular orbits at an altitude of 100 kilometers.

Japan's "Moon Goddess" lunar exploration renderings

It is worth noting that the gravitational field of the moon is much more complex than that of the earth. If the orbit of the probe is low, it will soon evolve from a circular orbit to an elliptical orbit.

After two months of operation, the lunar polar circular orbit probe at an altitude of 200 kilometers will drop in perigee altitude by about 20 kilometers. The probe needs to consume fuel and maintain its orbit altitude regularly, otherwise it will plunge into the moon.

The high lunar circular orbit is similar to the low lunar circular orbit, but higher, reaching thousands of kilometers. Its advantage is that it is less affected by the non-spherical gravitational perturbation of the moon, and the orbit will not deform during long-term flight of the probe. It is expected to be an ideal place to establish a lunar navigation and communication constellation in the future.

The lunar elliptical orbit runs near the lunar equator, with an orbital period of about 14 hours. Its advantage is that the speed increment required to enter the low lunar circular orbit and return to the Earth is not much different, and it can also take into account the future Mars transfer mission.

In addition to the lunar orbit, the nearly linear halo orbit where the US Capstone satellite and the future lunar space station are located belongs to the orbit near the Earth-Moon equilibrium point, and the center of the orbit is the Earth-Moon equilibrium point.

The Earth-Moon cycle orbit is a large elliptical orbit with the perigee hundreds or even thousands of kilometers away and the apogee hundreds of thousands of kilometers away. The orbital center is the Earth and the Moon. The probe periodically travels back and forth between the two, and can take into account the scientific exploration of both the Earth and the Moon.

The track design has a secret

There are many types of lunar orbits, each with its own uses. The key is to design a suitable spacecraft orbit based on mission requirements to achieve twice the result with half the effort.

According to public information, the orbit design of Chang'e-1 took many factors into consideration, including the choice of lunar polar orbit to achieve full lunar observation, and the use of a circular orbit at an altitude of 100 to 200 kilometers to obtain lunar images of higher resolution and equal resolution. At the same time, taking advantage of the long rotation period of the moon, the maximum distance between adjacent orbits of the probe is 35 kilometers, and it takes 27 days to cover the entire moon.

Taking into account the abnormal lunar gravitational field, Chang'e-1 adopted an orbit at an altitude of 200 kilometers, adjusted it every two months, and operated for one year and four months, exceeding its expected lifespan. It can be said that Chang'e-1 has achieved technological innovation in my country's lunar orbit design and laid a solid foundation for subsequent missions.

When it comes to the Chang'e-2 mission, in addition to obtaining higher-resolution images, it is also necessary to prepare data and technology for the Chang'e-3 soft landing on the moon.

Chang'e-2 was designed with two lunar orbits: 100 km and 100/15 km. The probe first acquired high-resolution full-moon images in the 100-kilometer circular orbit, then performed three near-moon brakings and entered an elliptical orbit, with the 15-kilometer perigee just passing over the Moon's Bay of Rainbows. Using the short flyby time, Chang'e-2 acquired 1-meter resolution images of the pre-selected landing area.

China's Chang'e 5 ascender rendezvouses and docks with orbiter

In the Chang'e-5 unmanned sample return mission, the probe first entered a 210-kilometer circular orbit, and then the lander performed two orbit-lowering maneuvers to enter a 200/15-kilometer elliptical orbit, and initiated a soft landing procedure on the moon at the 15-kilometer perigee position.

After the sampling is completed, the ascender first enters an elliptical orbit of 180/15 kilometers, then flies to an altitude of 210 kilometers, rendezvouses and docks with the orbiter waiting there, and transfers the lunar samples. Through this series of complex orbital operations, the lunar sample return mission is completed.

In the future, with the continuous development of aerospace technology, the moon will become a transit station for flying into deep space, and more sophisticated lunar orbits will be developed to meet the needs of human exploration of the solar system and even outer space.

By then, we will be able to see spacecraft of various purposes running in an orderly manner in their respective orbits, observing the starry sky or heading into deep space. What a magnificent picture it will be.