What does space traffic management involve?

The so-called space traffic management is, of course, the management of various objects flying in the Earth's orbit. However, in the space environment, what is an object is not an easy question to clarify. Different documents and laws and regulations have different descriptions of what is a space object. Although they are generally the same, there are some differences.

Objects of space traffic management

In a broad sense, space objects include naturally formed celestial bodies and artificial celestial bodies. Natural celestial bodies can be considered as a general term for various celestial bodies in the universe, including star clusters, galaxies, nebulae, interstellar matter, stars, planets, satellites, comets, meteorites, asteroids, meteoroids, etc. Of course, most of them are not the objects of space traffic management we are going to discuss. Only near-Earth asteroids and micrometeors will affect space traffic.

The object of space traffic management is space objects in a narrow sense. In the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, "space objects" are described as objects launched into outer space, their parts, vehicles and parts of vehicles. Is the name of this treaty too difficult to remember? In fact, it is the well-known "Outer Space Treaty". According to the treaty, objects launched into space by various countries must be registered with the United Nations Committee on the Peaceful Uses of Outer Space.

Article 2 of my country's "Space Object Registration Management Measures" issued in 2001 states: Space objects as referred to in these Measures refer to artificial satellites, manned spacecraft, space probes, space stations, launch vehicles and their components, and other man-made objects that enter outer space.

In the report "Orbital Traffic Control Report" written by SAIC for NASA, it is proposed that "space objects" in the field of space traffic management should be limited to orbital debris and spacecraft, that is, only man-made objects should be considered.

However, some other scholars do not agree with such a narrow division, and believe that natural objects should also be considered as "space objects". We all know that the impact of the natural environment on transportation cannot be ignored. On Earth, whether it is aviation, land transportation or navigation, it will be seriously affected by nature, including weather disasters and geological disasters. So, are there these two types of disasters in space? Of course there are, and the influencing factors of space weather are no less than those in the atmosphere. Not only that, space also has its own geological problems, that is, asteroids and micrometeorites. In the short term, near-Earth asteroids are unlikely to cause any destructive impact on transportation in Earth orbit, but the impact of micrometeorites cannot be ignored. The solar cells of the International Space Station have been damaged by micrometeors many times. Although it did not affect the actual work, holes were often made.

In related research abroad, although concerns about space collisions have been raised, threats to orbital flight safety are more often discussed under the topic of space junk.

How Much Space Junk Do We Know?

When it comes to space junk, the definitions of different organizations are not much different.

Space junk imagination

The United Nations Committee on the Peaceful Uses of Outer Space and the International Inter-Agency Space Debris Coordination Committee define space debris as: non-functional man-made objects in orbit or re-entering the atmosphere, and their fragments and components. The "White Paper on the Implementation of the European Space Traffic Control System" jointly issued by the German, Swiss, and Austrian Space Agencies and some space research institutions defines space debris as: man-made objects located in various Earth orbits that no longer work properly, including fragments and parts. Based on this, the white paper further gives the concepts of "trackable objects" and "untrackable objects." "Trackable objects" are defined as objects that can be discovered, and their orbits have been determined and can be predicted. "Trackable objects" include orbital debris and normal satellites. "Untrackable objects" cannot be discovered, and the space situational awareness system cannot perceive them.

The Space Traffic Research report of the International Academy of Astronautics defines orbital debris as objects placed in orbit by humans that no longer have any function. These objects include discarded rocket stages and parts, as well as materials, garbage, debris, and other objects that are intentionally or accidentally discarded or generated.

In the above definition, the so-called "non-functional" and "no longer working normally" are mentioned many times, which requires a discussion of the specific behavior of space objects.

The most important behavior of a spacecraft is to fly in a normal orbit and work normally. For example, a communication satellite can forward network information normally, a remote sensing satellite can obtain ground images normally and send them to the ground station, and a navigation satellite can broadcast navigation information normally. Moreover, all these satellites can correctly maintain their orbital parameters and attitudes without deviating too much. In this process, it stands to reason that the satellite will not collide with any other spacecraft.

The second behavior is orbital entry, that is, after separation from the launch vehicle, the satellite enters the working orbit under its own power. During this process, the satellite may cross the orbits of other spacecraft, and at this time, it is necessary to consider avoiding collisions and interference.

The third behavior is the opposite, which is orbit reduction. When the satellite ends its working life, it needs to return to the atmosphere to burn up, so as not to remain in orbit and become garbage. In the process of orbit reduction, the satellite may also cross the orbit of other spacecraft, which also poses a risk of collision. However, some high-orbit satellites do not need to return to the atmosphere to burn up. They will enter some specially designated graveyard orbits, and there is the same risk in this maneuvering process.

The threat posed by Starlink satellites to China’s space station is the second and third types of behavior.

The last behavior is a complete loss of control. The satellite can neither work nor return to the atmosphere, let alone enter the graveyard orbit. It can only fly in space by inertia like a zombie, which is very dangerous. The early satellites launched did not take into account the problem of orbit reduction, so most of them became zombies. So, how much garbage is there in space? In the more than 60 years of human space history, a total of more than 30,000 tons of matter have been launched into the universe. Except for the spacecraft that entered deep space and fell to the earth, a large part of it has become space debris.

According to data from the U.S. Space Surveillance Network, by 2017, the United States had tracked 42,000 space objects in Earth orbit, with a total mass of 7,500 tons of space debris, of which 2,700 tons were in the low-orbit region (LEO, orbital altitude 200-2,000 kilometers). In Earth orbit, the number of space debris with a scale of more than 10 centimeters has reached 23,000; the number of debris with a scale of 1-10 centimeters is about 500,000; the number of debris with a scale of 1-10 millimeters is about 100 million, and the number of debris below 1 millimeter is in the tens of billions. With the continuous launch of several super low-orbit constellation satellites, the above numbers will definitely get bigger and bigger, and the debris will increase.

The average mass density of space debris is 2.7 grams per cubic centimeter, and the main components are aluminum/aluminum alloy (44%), composite materials (37%), stainless steel (12%), titanium alloy and copper, etc. The main geometric shapes of space debris are plates, blocks, rods, and sheets, and most of them are irregular shapes.

The distribution of space debris in space is not uniform. The proportion of catalogued space debris in low orbit, medium orbit and high orbit is 75.2%, 8.3% and 9.4% respectively. Among them, the most debris is in the orbit between 700 km and 1100 km.

Since most spacecraft do not have the means to leave orbit, the disintegration of retired spacecraft in orbit and the collision between non-functional spacecraft have become the main reasons for the increase in the number of space debris. As early as December 2018, it was reported that 19 of the 64 Starlink satellites deployed by SpaceX at one time had not yet been identified and could not establish communication with ground operators. Although the failure probability of Starlink satellites will become lower and lower with technological advances, considering the huge number of launches, the absolute number of out-of-control satellites is still considerable, which brings new challenges to the sustainable development of space.