Space junk refers to pieces of machinery or debris left in space by humans. It can refer to objects of any size or type and occurs both intentionally and unintentionally, such as dead satellites or remnants of failed missions. A majority of this debris is in low Earth orbit, which is within 2000 kilometers or 1200 miles of Earth’s surface. Space junk has the ability to fall back to the Earth’s surface. The time it takes to fall back depends on the object’s initial altitude. For example, debris below 600 kilometers from the Earth's surface will fall back after orbiting for a few years, while objects over 1000 kilometers from the Earth’s surface can stay in orbit for hundreds of years.
According to the European Space Agency, space debris can include satellites that are dead and/or not in use, satellite fragments, remnants, fragments of early stages of rockets, and dropped tools. Space junk is categorized by size into three groups: below 1 cm, 1 cm to 10 cm, and above 10 cm. There are over 128 million pieces of debris over 1 cm and most are not detectable. Around 900,000 pieces are between 1 to 10 cm, and anything above 10 cm consists of anything from old tools to satellite parts.
Debris in space is a major threat to both manned and unmanned spacecraft. Because the force of an object's impact depends on both its mass and acceleration, even if an object is relatively small, it can still have a significant impact solely due to the speed it is traveling at. This is exactly the case for space junk. Pieces of debris orbit the earth at around 8 kilometers per second, meaning that even small debris can damage spacecraft. For example, the windows of space shuttles were replaced often after collisions with objects smaller than 1 millimeter. Space junk damaging satellites is especially worrying because commerce, communication, travel, and safety systems all depend on functioning satellites. If satellites become significantly damaged, GPS, timing synchronization for finance, banking, and power, and military technologies all get severely impacted. This has serious implications for everyday life.
The United States Department of Defense (DOD) maintains an accurate satellite catalog of objects in Earth’s orbit. Around 27,000 things are officially cataloged, with most of them being larger than 10 centimeters in size. There are guidelines set by the National Aeronautics and Space Administration ( NASA) that are used to assess the threat posed by a certain piece of debris, and to determine if evasive action or other precautions are necessary. These guidelines are in place to ensure the safety of the International Space Station (ISS) and its members onboard.
Using these guidelines, an imaginary boundary area is “drawn” around the spacecraft, with the dimensions generally 4 x 4 x 50 kilometers. If it is indicated that an object will pass close enough to the ISS and that the tracking data is sufficient, mission control in Houston and Moscow will determine the appropriate course of action. For example, if the encounter is known in advance, the ISS will maneuver slightly in order to keep the approaching object outside of its boundary area. Otherwise, if the tracking data is not precise enough, a separate spacecraft is used to transport the crew to and from the station in order to isolate the station from the spaceship.
One proposed solution to limit the amount of space junk that enters Earth’s orbit is the implementation of orbital use fees. An orbit use fee is a fee placed on countries or companies to charge them every time a satellite is put into orbit. This would limit them from launching many new satellites into orbit. According to a new study, this method of space junk mitigation is more effective than capturing debris or deorbiting old satellites. The fee would also increase over time to account for the value of a cleaner orbit increasing. However, for this solution to be effective, all countries that currently launch satellites, as well as any countries that are planning to launch them in the future, need to come to an agreement. In addition, this solution would lead to an increased value of the satellite industry, from a current $600 billion to a future $3 trillion. This increase stems from reducing collisions and therefore costs that arise from collisions, such as costs to replace satellites and satellite parts.
To study and analyze the effectiveness of particular removal and/or prevention techniques of space junk, long-term forecasts are needed to determine future trends based on individual mitigation actions. The European Space Agency uses a tool called the Debris Environmental Long-Term Analysis (DELTA) to perform this analysis. Using DELTA, a user can investigate the evolution of space debris and its environment, as well as collision risks in low, medium, and geosynchronous Earth orbit. DELTA is also able to analyze how space debris mitigation will affect the current amount of space debris and the space debris environment.
In summary, space junk, which is debris left in orbit caused by satellites or rockets, poses serious issues to spacecraft and satellites. It has the ability to damage or destroy satellites, which can lead to expensive repairs as well as an impact on commerce, travel, communication, and security. Measures must be taken to address this issue, including the use of orbital use fees, with tools such as DELTA used to analyze the effectiveness of mitigation.
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