Space junk is increasing day by day. How should we deal with it? (Part 1)

Computer generated image of space junk

The crew of Shenzhou 19 entered the space station on October 30, 2024. One of their important tasks during this trip is to continue to install the space station debris protection device outside the cabin through spacewalks. The crew of Shenzhou 18 previously installed the space debris protection device outside the space station cabin.

Space debris, also known as space junk, refers to the fragments of various man-made objects orbiting in the Earth's orbit. The problem of increasing space junk is currently getting worse and will become more and more serious in the future.

The more space junk there is, the more likely it is that they will collide with each other, thus forming more fragments, and there is even the possibility of a domino effect. Therefore, they will pose an increasing threat to spacecraft in orbit, including threatening the lives of astronauts.

01The source of space junk

With the widespread application of aerospace technology, the number of spacecraft launches has increased, and space debris has also increased accordingly. For example, on October 19, 2024, the Intelsat-33E satellite unexpectedly disintegrated in the geostationary orbit, adding at least 500 pieces of debris to the geosynchronous orbit region.

According to statistics, there are currently at least 3,000 tons of space junk flying around the Earth, and the amount is increasing at a rate of 2% to 5% per year. It is mainly generated from three parts:

The first part comes from artificial objects such as various retired artificial satellites, as well as fragments produced by collisions between artificial objects and between natural celestial bodies and artificial objects.

The second part comes from the upper stage of the launch vehicle.

The third part comes from the garbage generated and thrown into space by astronauts during their space activities, as well as items accidentally left in space during space walks (such as nuts and tools).

The destructive force of man-made collisions is also related to the angle of collision. For example, on February 10, 2009, the Russian satellite 2251 collided with the American satellite Iridium 33 at a relative speed of 11.64 km/s, generating 2,201 pieces of space junk that could be monitored and catalogued.

Schematic diagram of the orbital intersection of the US and Russian satellites colliding

According to the size of space debris, space debris is generally divided into three categories internationally:

① Large fragments are larger than 10 cm, and there are currently tens of thousands of them, which can be monitored; ② Small fragments are between 1 and 10 cm, and there are currently more than 100,000 of them; ③ Tiny fragments are between 1 mm and 1 cm, and there are currently hundreds of thousands of them. Fragments no larger than 1 mm are currently numbering in the hundreds of millions.

From the perspective of spatial distribution, the density of spacecraft and large debris in medium and low orbits is about 10 per cubic kilometer on average. Because they are extremely fast, and the destructive power is proportional to the square of the speed, that is, the impact energy of a space debris weighing only 10 grams is no less than the impact energy generated by a car traveling at a speed of 100 kilometers per hour, so the event it causes is a "low probability, high risk" event.

Space debris in low orbit will gradually fall under the influence of atmospheric drag and then return to the atmosphere to burn up. If the orbit of space debris is very high, it will remain for a long time. Generally speaking, the lifespan of a space object 300 kilometers above the earth is about one year, 600 kilometers above can fly for decades, more than 1,000 kilometers above can fly for hundreds of years, and more than 4,000 kilometers above can fly for thousands of years. Fragments higher than this will almost always be in space.

0 2 Space junk is very harmful

The impact of space debris on the normal operation of spacecraft is mainly determined by factors such as the spacecraft's on-orbit time, the protected area, orbital altitude and orbital inclination, among which the first three factors are the most important.

The flying speed of low-orbit space debris is about 7.9 kilometers per second. If it hits the surface of a spacecraft, it will leave a dent at the very least, and it will penetrate the spacecraft and cause some system functions to fail, and even have catastrophic consequences, including threatening astronauts performing extravehicular activities.

Due to their large number, extremely small space debris can change the surface performance of spacecraft; slightly larger space debris can damage the surface material of spacecraft, cause impact craters, and damage surface devices; when large space debris collides with a spacecraft, the spacecraft's attitude may change, and even the spacecraft's orbit may be changed; space debris that impacts at hyperspeed may vaporize itself and the surface material of the spacecraft that is hit into a plasma cloud, causing the spacecraft to fail; when the energy of space debris is large enough, it can penetrate the surface of the spacecraft, damage the control system or payload inside the spacecraft, causing the spacecraft to explode or break up the entire structure.

In addition, when space debris re-enters the atmosphere, if it is not completely burned, it will pose a serious threat to the safety of life and property on the ground, especially when a nuclear-powered spacecraft falls, the consequences are particularly serious.

For example, in the 1970s and 1980s, a Soviet nuclear-powered satellite crashed in Canada due to loss of control, causing great panic. The U.S. Skylab space station also crashed due to loss of control and damaged the houses of residents on the ground. Too much space debris can also seriously affect space astronomical observations. When debris hits a space telescope, it will damage the mirror.

Space shuttle window hit by space junk

Since the impact of sand-sized space debris is equivalent to that of a bowling ball traveling at 160 kilometers per hour , small space debris with a diameter of less than a few millimeters can cause damage to operating spacecraft. This damage is divided into two categories: the first is damage to the surface of spacecraft subsystems, and the second is the impact on spacecraft operation. For example, the windows of the US space shuttle have been hit by space debris.

Space debris has caused serious damage to the International Space Station. For example, in 2016, metal debris with a diameter of several microns left a 7 mm diameter crater on the porthole of the International Space Station. In 2021, a piece of debris that was not within the monitoring range hit the Canadarm-2 of the International Space Station, tearing its outer protective layer and leaving a perforation with a diameter of nearly 10 mm. In 2022, a small hole of 0.8 mm appeared in the outer shell of the Russian Soyuz spacecraft docked at the International Space Station, causing 44 kg of coolant to leak.

To this end, the International Space Station currently has a collision avoidance program. When the relevant equipment detects that a space object is approaching the station and there is a possibility of a collision, the program will instruct the station to start the engine to change the orbit to avoid a collision. If the International Space Station does not have time to adjust the orbit, then for safety reasons, the astronauts must hide in the spacecraft so that they can escape at any time.

Manned space flight is a matter of life and death. In order to ensure the safety of astronauts on the space station, China has also taken corresponding measures. At the Shenzhou-19 press conference held on October 29, Lin Xiqiang, spokesperson for China's manned space program, said that in response to the threat of leakage caused by space debris hitting the space station, the emergency response plan has been continuously optimized. Compared with the early stage of the space station operation, the time available for astronauts to deal with emergencies has increased by 5 times , and the safety of the space station and astronauts has been greatly improved.

0 3 How to deal with old spacecraft

When artificial satellites, space stations and other spacecraft reach the end of their life or die prematurely, they become space junk. It may affect other spacecraft in orbit, so it is important to properly handle waste spacecraft.

There are many ways to deal with scrapped spacecraft, depending on the specific situation , because scrapped spacecraft are divided into two categories: controllable and uncontrollable. Even when dealing with controllable scrapped spacecraft, different methods must be used based on the height of the orbit, the size of the spacecraft, etc.

When a satellite in geostationary orbit fails or reaches the end of its life, it is usually changed by remotely controlling the satellite's engines to make the satellite fly to a higher, useless orbit. One purpose for this is that the geostationary orbit is very valuable and is currently overcrowded, so obsolete communication satellites must be vacated; the second purpose is that obsolete geostationary orbit satellites have become space junk, so they must be stored in a safe location so that they do not threaten other geostationary orbit satellites.

The same is true for abandoned large satellites in medium orbit. However, the prerequisite for remote control of abandoned satellite orbit change is that the satellite can be controlled and has excess fuel.

SpaceX claims that its low-orbit Starlink satellites are equipped with electric propulsion systems, which can actively lower their orbits to re-enter the atmosphere faster after their lifespan expires, meeting the 25-year re-entry requirement. However, on September 2, 2019, the European Space Agency's Aeolus satellite performed a dangerous maneuver to avoid a collision with a Starlink satellite.

When small spacecraft operating in low orbit are scrapped, they are generally ignored because they will continue to lower their orbits due to the influence of residual atmosphere and eventually be burned up when they enter the atmosphere.

When a large spacecraft in low orbit is scrapped, the best way to deal with it is to crash it into an uninhabited area through ground remote control, because it is not easy to burn up completely when re-entering the atmosphere. For example, the world's heaviest satellite, the US "Compton" gamma-ray telescope (17 tons) and the Russian Mir space station weighing hundreds of tons, were both manually controlled to crash into a place in the South Pacific known as the "spacecraft graveyard" after their service life ended. my country's "Tianzhou" series, Russia's "Progress" series and other cargo spacecraft are also handled in this way after completing their missions.

At 21:25 Beijing time on November 17, 2024, the Tianzhou-7 cargo spacecraft re-entered the atmosphere in a controlled manner. Most of the spacecraft's components were burned and destroyed during the re-entry process, and a small amount of debris fell into the designated safe waters in the South Pacific.

There is no successful experience with large spacecraft that lose control in low orbit. There are several options:

One is to use a large space shuttle-like shuttle to recover the spacecraft in orbit, but the cost is too high and it is rarely used now, and the space shuttle has been retired. The second is to use anti-satellite weapons to break up large spacecraft so that they will burn up when they re-enter the atmosphere, but this plan is very controversial because if they do not re-enter the atmosphere after breaking up, it will easily form a large amount of space debris. The third is to determine the crash site of large spacecraft through precise tracking so that relevant personnel can be evacuated in advance.

Author: Pang Zhihao

Chief Scientific Communication Expert of National Space Exploration

Editor: Dong Xiaoxian

Reviewer: Liu Kun and Li Peiyuan