Can't satellites just "fly"? How should space "traffic rules" be established?

Space collision, how to avoid danger?

Once upon a time, space debris collisions were still bridges in science fiction movies. Now, the danger is real — in July and October 2021, the Starlink satellite launched by the American Space Exploration Technology Company approached the Chinese space station twice, causing the Chinese space station to take emergency collision avoidance measures. And that's not an isolated case. Space is so vast, why is this happening? Can't satellites just "fly"? How should space "traffic rules" be established?

The root of the collision

Although space is vast, the laws of nature cannot be violated, and the close encounter of space objects cannot be avoided

In the process of understanding from the center of the earth to the Milky Way, human beings found that celestial bodies generally have the characteristics of rotation, and Newton could not explain the reason for its occurrence, so he said that it may be "the hand of God pushed it". According to the law of "universal gravity" learned in high school physics, space objects in the Earth's orbit operate in their respective circular or elliptical orbits under the influence of gravity. In the absence of external interference forces, the orbit of objects in orbit space will not change, let alone collide.

Today, we understand that the Earth is not a regular spherical object, that the equatorial radius is about 21 kilometers longer than the polar radius, and that there are slight uplifts on the equatorial plane. The earth can be figuratively likened to a squashed basketball, and two pieces of "mud" can be pasted at the symmetrical position of the equator. These two pieces of "mud" are one of the mysteries of the east-west drift of synchronous orbit satellites, and synchronous orbit satellites need to regularly spend a certain amount of fuel to "resist" interference and maintain orbital position. Due to the limited fuel to be carried, there are constraints on the life of the orbit.

However, there is no "but" - under the action of celestial bodies such as the Earth's apheric shape, ocean tides, atmospheric damping, sun and moon, the orbit of space objects orbiting the Earth is always in a slow change. Depending on the nature of the interfering force, orbital changes are divided into periodic changes and long-term changes. Orbits change slowly, and there is an approximate periodic approximation of space objects with similar heights, called "close rendezvous events".

Things always have two sides, both opposites and unity. Just as the Earth's flat rate perturbation is harnessed, a sunsynchronous orbit can be designed to facilitate satellites such as remote sensing to acquire images under the same lighting conditions. At the same time, the presence of jamming forces brings difficulties and challenges to accurately predicting orbits. Especially under the disturbance of solar activity, it is difficult to accurately predict the atmospheric density environment in low Earth orbit, resulting in false alarms and leakage of warnings in space debris early warning work. The so-called "false alarm" is to overestimate the risk of the intersection of two space objects, trigger unnecessary collision avoidance work, waste the valuable fuel of the spacecraft, and affect the normal satellite observation tasks. "Missed alarms" refers to underestimating the risk of rendezvous of space objects, putting the spacecraft or astronauts in extreme danger.

Status of space debris

Space has become crowded, vast spaces are gone, and the consequences of collisions are unimaginable

The Inter-Agency Space Debris Coordination Committee (IADC) defines space debris as a space object produced by human space activities that is inactive in orbit. Since October 4, 1957, when humans launched the first artificial satellite Sputnik-1 into space, to date, 5,775 space launches have been recorded, sending a total of 12,803 spacecraft into space.

According to data released by the US Space Surveillance Network, as of December 2021, the number of cataloged space objects is 50,454, and the number of space objects that are still in orbit is 24,687. Of the 50,000 space objects, nearly 40,000 originate from in-orbit disintegration events such as collisions and explosions. In 1977, scientists at NASA's Johnson Space Center, Kesselr, and others predicted that space debris generated by human activities would soon pose a huge threat to satellites in low-Earth orbit. After in-depth research, Kesselr published the article "Collision Cascade Effect: The Limit of the Number of Debris in Low Earth Orbit" in 1990, which described the rapid growth of space debris, which will eventually produce cascading collisions between fragments, and the vast space will never return...

Growing space debris has overwhelmed the Resources of Earth's orbit and threatened the safe operation of spacecraft in orbit. Impacts of space debris in size in centimeters and above can cause spacecraft to perforate or even disintegrate until they are completely damaged. Impacts of space debris in the centimeter range and below can cause some of the spacecraft to function or fail, and damage to key components may also cause the failure of the entire star.

In 1992, the US Space Shuttle Atlantis, which operated at an altitude of 505 kilometers for 11 months, returned and found that there were more than 2,000 collision points in the solar panels, and the debris left impact holes in the portholes, some of which had penetrated the aluminum partitions. But fortunately, there was no catastrophic event that caused the space shuttle. Other satellites were less fortunate – the gravitational gradient rod of the French satellite Cerise in 1996 was damaged by debris from the explosion of the Ariane satellite, and the satellite's attitude was out of control and eventually scrapped; on February 10, 2009, the two satellites of iridium 33 and the Russian universe 2251 collided in orbit, and the disintegration of the satellite produced more than 2,000 pieces of space debris of more than 10 centimeters.

How risks are assessed

On the basis of space debris monitoring, there are two methods of collision risk identification

So, how do you monitor so much debris? Many space objects are operating in orbit around the earth, and in order to grasp their operating status, including orbital information, it is necessary to track and observe space objects with the help of ground stations, spaceborne radars and telescopes, which is called cataloging work.

The cataloguing of space objects can be compared to the household registration management of public security organs, and each space object will be given a unique identification number from "birth". The space surveillance departments of space powers, such as the Space Surveillance Network of the United States and the Russian Space Surveillance System, are performing the responsibilities for the management of space objects. Limited to the detection capabilities of monitoring equipment, the current cataloging work usually only stably tracks space objects with dimensions greater than or equal to 10 cm, periodically updating their orbital data. With the rapid growth of the number of space objects, the large number of similar orbital space objects produced by disintegration events and giant constellations have brought great challenges to the cataloguing work, and it is necessary to simultaneously improve tracking detection and data processing capabilities to meet the challenges.

When debris is monitored, how do you assess whether there is a risk of collision?

Some people have proposed to remove space debris, but from the perspective of protecting space assets and space orbit resources, collision avoidance is a cost-effective and immediate means compared with the cost of hundreds of millions of dollars of debris removal. The daily collision warning work of spacecraft is the basis for early warning and avoidance, and when the collision risk of important space assets is identified to exceed the avoidance threshold, orbital maneuvers are usually taken to avoid collisions.

The collision risk identification of space objects is often described by two methods: "collision probability" and "box method". Orbital forecast error is not terrible, after mastering the statistical law of error distribution, you can use the error ball to describe the possible position of the space object, such as the use of 3 times the standard deviation, so that the probability of the space object falling in the error ball will be controlled at 99.73%. If the error ball where the two space objects are located does not intersect, the collision probability of the two is zero, and if the two error balls overlap, the probability density of the overlapping area is integrated, and the collision probability is obtained, which is the description method of collision probability. Just as the density integral of an object with an uneven density is obtained, the result is the mass of the object. Because the satellite runs in three directions of speed, pointing to the center of the earth and the vertical orbital surface, the force model has different degrees of accuracy, and the prediction error of the corresponding three directions is also different, so the error ball is generally described by an ellipsoid. The box method also uses this law of prediction error, centered on the spacecraft, takes different lengths in three directions, and draws a box box to indicate the intersection risk level.

The challenges ahead

Collision alarms appear frequently, and space disorderly "horse racing" will harm others and harm themselves

The development of space requires orderly planning and observance of order. Is it necessary to plan for giant constellations with tens of thousands of stars? Should the race in space be restricted?

Taking the 42,000-satellite satellite constellation and the 48,000-satellite constellation as an example, let's outline the risks and challenges it brings. Since the implementation of Starlink at the end of December 2021, 35 batches have been launched, sending a total of 1942 satellites into Earth orbit, and 1794 are still in orbit. Starlink satellites are mainly distributed in a space area with an inclination angle of 53 degrees and an operating altitude covering 200 to 550 kilometers. Since the implementation of the first-network satellite constellation, 11 batches have been launched, sending a total of 358 one-network satellites into earth orbit, and 358 are still in orbit. These satellites are mainly distributed in a space area with an inclination angle of 88 degrees and an altitude of 600/900/1200 km. In order to improve the delivery efficiency and save costs, Iron Man Elon Musk's starlink satellite usually uses a one-arrow 60-satellite method to send a batch to an orbit of about 300 kilometers, which takes more than two months to climb to a running altitude of 550 kilometers.

During orbital climbing, to cross the near-circular orbital area of the space station, below, we randomly select the close proximity events with the sky and the space station in one month of 2021 for statistics. The results showed that the total number of rendezvous meetings within 10, 30, 50, 70 and 100 kilometers was 28, 468, 1452, 2680 and 6034 respectively, and the number of rendezvous with starlink satellites was 4, 134, 428, 722 and 1232 respectively.

It can be seen that in the close rendezvous event, starlink satellites account for the highest proportion of nearly 31%. What is more worrying is that the orbit climbing process of the Starlink satellite is in the mode of electric propulsion. Without mastering maneuvering strategies and maneuver models, orbital forecast errors for sustained maneuvering targets with small or variable thrust are difficult to estimate. The forecast error can reach tens of kilometers or hundreds of kilometers, which is determined by the prediction time, and the probability density distribution of this error does not conform to statistical laws, and it is difficult to accurately assess the collision probability. To protect important space assets "foolproof", only the radius of the error ball can be enlarged to identify potential collision risks. However, the amplification error ball also has a negative effect, that is, the number of false alarms increases, resulting in propellant wear. At the same time, the collision avoidance maneuver process will destroy the zero-gravity environment of the space station's experimental module and affect the development of scientific experiments.

Traveling on Earth requires traffic rules, as well as in space, and scientific and technological research, especially exploration in public areas such as space, should also be carried out within a certain rule framework. As the old saying goes, "no rules, no squares", and space transportation also has corresponding "rules". The United Nations has developed international laws on the conduct of outer space, such as the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, which entered into force on 10 October 1967, referred to as the Outer Space Treaty, and the Convention on Registration of Objects Launched into Outer Space, which entered into force on 15 September 1976. The Treaty provides that any State Party to this Treaty which discovers any phenomenon in outer space, including the Moon and other celestial bodies, that may pose a danger to the life or health of astronauts, shall immediately notify the other States Parties to this Treaty or the Secretary-General of the United Nations. Apparently, Space Exploration Technologies is not complying with this regulation.

We hope that with the rapid development of space technology, we can revise and improve the laws and regulations on space exploration at the United Nations level, so that everyone can have a law to follow. It was also hoped that all States would be able to carry out scientific research and space activities within the framework of international law, otherwise it would harm others and harm themselves. Exploring and using space is the common cause of mankind, and we need to join hands and move forward together.

(Author: Liu Wei, Associate Professor, National Space Science Center, Chinese Academy of Sciences)

Source: Guangming Daily