For the garbage in daily work and life, we will collect it and put it in the garbage bin, which will be collected and disposed of by the municipal department. Since the beginning of human space activities, the waste generated in space has been mostly discarded in situ and left in space. These non-functional man-made objects abandoned in space are called "space debris" by aerospace scientists, and they are better known as "space junk".
Schematic diagram of the space debris situation
Who "threw garbage" into space?
In 1989, the Space Debris Monitor Agency discovered that a satellite called the Cosmic Ray Background Detector produced about 76 pieces of space debris, all of which were larger than 10 centimeters in diameter.
Satellites disintegrate in orbit after failure and are one of the main sources of space junk. Curiously, the satellite is still working properly after the debris was generated, and there has been no disintegration due to abnormal operation. Subsequently, the satellite even successfully completed all the observations during its lifetime, but the cause of the space debris has not yet been found. Although similar incidents are rare, the occurrence of this peculiar phenomenon shows that the mechanism of space debris generation is still not fully understood.
The largest volume of space junk generally comes from the failed spacecraft itself. For example, after reaching the end of its life, a satellite will become a "dead star" stranded in orbit if it does not take deorbital measures.
In addition to failed satellites, the second, third or upper stages of the rocket that propel satellites into orbit are also one of the main sources of space junk. Some of the propellant remains in these spacecraft, and within a day or even decades of launch, the residual propellant can explode, producing space fragments ranging from a few to hundreds of pieces of debris that can travel at varying speeds. According to ESA's annual space environment report published in October 2020, space junk produced by the explosion of the remaining part of the rocket is the most dangerous.
A collision between a failed satellite would turn two large pieces of space junk into massive amounts of space debris. On February 11, 2009, the U.S. communications satellite Iridium-33 and the Russian satellite Cosmos 2251, which had been scrapped for many years, collided violently over Siberia, causing the first major collision between satellites in human history. The collision resulted in up to 2,000 large pieces of space junk, as well as an even larger number of small, untraceable pieces of space junk.
In addition, the parts and paint pieces that fall off the spacecraft, and even the wrenches and tools that astronauts accidentally lose when they go out of the capsule will also become space junk. According to the different sizes, space junk is generally divided into three categories: large debris, dangerous debris and small debris, of which the size of large debris is greater than 10 cm. While the most damage is possible, the debris can be tracked and cataloged by ground-based equipment, and working spacecraft can take circumvention measures against them. Small fragments with a size of less than 1 centimeter, although they cannot be tracked, have less kinetic energy, and spacecraft can cope by strengthening the protective structure. For debris with a size between 1~10 cm, it can not be tracked, but also has considerable destructive power, and the potential threat to spacecraft is the greatest, so it is called dangerous debris. According to ESA data, as of January 2020, there are more than 34,000 large pieces of debris in space, and the number of dangerous pieces is as high as about 900,000.
ESA's space debris monitoring system
In order to "hide from garbage", the International Space Station has urgently evaded
In the early hours of July 16, 2015, the Space Debris Monitoring Agency discovered a piece of space junk approaching the International Space Station in a dangerous manner. The piece of space junk, numbered 36912, is about the size of a dinner plate and comes from a disintegrated Soviet military weather satellite. The satellite ceased to work more than two years after it was in orbit, and in the decades that followed, debris kept falling off the satellite and becoming space junk. No. 36912 may have once been part of the satellite's thermal shield.
Due to the special orbit of "36192", only a very small number of stations in the space debris monitoring network can monitor it. The disk-like shape and low mass of 36192 make its orbit susceptible to changes in the density of the upper atmosphere caused by solar activity. When it reappears in the monitoring range of the station from the blind spot of the monitoring, the track may have changed significantly from before. In the early hours of the same day, after the Space Debris Monitoring Agency updated the orbital data of the debris, it was horrified to find that the debris would approach the ISS within a few hours, and the probability of a collision had increased to a very high one in 1,000.
One in a thousand may be a negligible probability for everyday events in our lives, but because the consequences of a large space junk impact on a spacecraft can be catastrophic, a very low probability is used as a threshold for early warning when assessing the risk of impact. According to the current collision avoidance procedures of manned spaceflight, when the impact probability is greater than 1 in 100,000, the impact risk is medium, and if the normal development of the mission is not affected, the spacecraft needs to perform orbital maneuvers to avoid space junk. And when the probability of impact is greater than 1 in 10,000, the spacecraft must take orbital maneuver in any case. A 1 in 1,000 chance of an impact is already a high-risk threat.
However, because the risk was discovered too late, it was too late for the ISS to avoid the debris.
The ISS capsule has been designed with the protection of space junk in mind, and a Whipple protective cover has been installed on the outside of the capsule. This protective cover consists of one or more thin layers of metal sheets. When space debris hits the shield, the shield absorbs a portion of the impact energy and allows the debris to disintegrate, thereby reducing the impact of the debris on the outer walls of the capsule. The Whipple shield is also the device used by most other spacecraft to protect against space junk impacts. However, after estimating, technicians found that the ISS was not strong enough to withstand the impact of "36912". They were able to allow the astronauts on the space station to take refuge in the Russian Soyuz spacecraft docked on the space station and closed the hatches of some of the cabins. In the event that debris hits the station, such a measure would minimize the impact of the damage, and if the station completely collapses, Soyuz could act as a lifeboat to take astronauts off the station and back to Earth.
A piece of space debris observed on the International Space Station
Fortunately, in the end, there was no danger. The debris eventually swept past the ISS at a relative speed of about 50,000 kilometers per hour, just a few kilometers away from the station. Although the ISS survived this time, the threat of space debris impact will persist and will continue to grow as the amount of space junk continues to grow.
What happens if space junk is left alone?
In recent years, SpaceX's Starlink and other low-Earth low-orbit communication satellite constellations have brought new challenges to the problem of space junk. According to SpaceX's current vision, the total number of satellites used in the Starlink satellite Internet will be as high as 12,000, which is several times the number of satellites currently in orbit in the world. Other commercial space companies have proposed similar plans for a constellation of near-Earth communications satellites, with the number of satellites ranging from a few hundred to thousands. With such a large number of satellites, even a small number of satellites will bring a lot of space junk risks.
Schematic diagram of space junk in low Earth orbit
According to the results of the research of relevant agencies, for a low-orbit constellation system composed of thousands of small satellites, more than 99% of the satellites must be able to be disposed of after completing the mission and deorbited immediately after the completion of the work.
For normal working satellites, fuel is generally reserved, and when the end of its life is reached, the engine is used to control the satellite to lower its orbit, and quickly enter the stage of strong interaction with the atmosphere and crash and incinerate. However, if the satellite fails unexpectedly during the working process, it cannot complete the immediate deorbit, and can only wait for the slow interaction between the satellite and the thin atmosphere in the middle and upper layers, so that the orbit of the satellite will gradually decrease, and it can enter the atmosphere and burn up after a long time. According to a study by the Harvard-Smithsonian Center for Astrophysics in November 2020, about 3% of the 800 Starlink satellites currently in orbit have failed and become new space junk. At present, the "tolerance" of foreign government departments for the failure of commercial satellites to deorbit normally is 10%, which is obviously much higher than the 1% failure limit in the research results.
If space junk continues to grow uncontrollably, it will trigger the "Kessler effect" - space debris enters a state of serial impact in space, orbital resources are depleted due to the pollution of space junk, and humans can no longer launch spacecraft into a specific orbit. Researchers have called on government departments and space companies to pay attention to the new threat of space junk posed by large-scale communication constellations through various channels, so as not to lose the effective use of space in the future.
This article was originally titled "The Growing Problem of Space Junk" and was published in the third issue of Space Exploration magazine in 2021.
Text/Li Huichao
Editor/Yang Sishuang
Audit/Mutan
Executive Producer/Jiang Jun